Reprocessing of High Resolution Crustal Seismic Reflection ... J... · Crustal Seismic Reflection...
Transcript of Reprocessing of High Resolution Crustal Seismic Reflection ... J... · Crustal Seismic Reflection...
Reprocessing of High Resolution
Crustal Seismic Reflection Data
from the
Abitibi Greenstone Belt
b y
Jan Oxbow Kozel
Geophysics Laboratory
Department o f Physics
University o f Toronto
A thesis submitted in conformity with
the requirements for the degree of
Master of Science
at the
University of Toronto
© by Jan Oxbow Kozel
1990
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Abstract
Deep crustal reflection seismic data were collected in the Abitibi Greenstone
Belt during the winter of 1987-8 as part of the Lithoprobe Project. Although much
reflection energy was visible after initial processing, it was apparent that the stacked
section could be improved. In particular, very little reflection energy was imaged
in the upper 1 s, making it difficult to correlate deeper reflections with the mapped
surface geology. The high resolution line 12A was reprocessed with a particular
effort to obtaining a better image of the shallow data.
Spectral equalisation of the upper 3 to 4 s and an accurate, complete static
solution were found to be the most important steps in the reprocessing. Ground
roll proved difficult to attenuate, with velocity filtering showing some promise, al
though not entirely satisfactory. Neax offset, low frequency vibrator noise was only
partially attenuated through spectral equalisation, and may have been caused by
subhaxmonic distortion in the vibrator signal. The dip moveout effect was substan
tial for shallow data in the southern part of the line.
Reflection energy is apparent at all travel times in the unstacked data, although
most of it does not stack coherently because its amplitude and phase characteristics
vary with offset. Reflection energy above 100 Hz could not be imaged as coherent
reflection energy after stacking.
Reprocessing resulted in great improvement throughout the stacked section,
with considerable reflection energy being imaged to 0.5 s. The results of this thesis
illustrate ihe value of detailed processing of deep crusted reflection data in Archaean
cratons.
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Acknowledgements
I am most of all grateful to my supervisor, Prof. Gordon tyv>>f. His keen
interest in this project as well as his able help have committed these year? fondly
to my memory. I admire his extremeljr broad knowledge and int?i'«*ts, and he has
been a fine example to me.
Next I would like to thank Dr. Kris Vasudevan of the Lithoprobe Seismic Pro
cessing Facility (LSPF) in Calgary. Not only was he an eager participant in many
discussions about my work, he also worked very hard to expedite my processing
on the Lithoprobe computer. The rest of the staff at the LSPF also deserve my
thanks for all of their work in keeping the facility operating smoothly. They are
Glen Lines, Angela Dutoit, and Todd Clark. I especially thank Todd for patiently
wrapping and sending to me many scores of plots, keeping them remarkably free of
footprints. There are also the many (unfortunately) nameless and faceless operators
\vh i mounted so many tapes for me.
Prof. Luck Bailey and Dr. Gerhard P ratt were the readers of this thesis, and I
f canK ' ■ ~ for their careful reading and insightful comments.
Claire Samson has been of much help to me, having read portions of this thesis
thoroughly, and has been a participant in many discussions, both seismic and non-
seismic in nature. Peter Hurley was most helpful in guiding me through my first
experiences with seismics both in the field during the chilly December of 1987 and
in processing.
I appreciate Shell Canada’s allowing me to use in my thesis some of the results
of my work there during the summer of 1990. Gary Billings was very kind in
helping secure permission to do this, as well as in sticking around after work to
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discuss aspects 01 chis thesis.
For helping to put my work into geological perspective, both specifically and
in general, I thank: Drs. Richard Sutcliffe and Steve Jackson of the Ontario Geo
logical Survey for showing me how geologists think (and behave) in the field during
the summer of 1988; John Varsek at the University of Calgary, for many eclectic
discussions; and Prof. Fred Cook, also at Calgary for opening a global geological
perspective to me through his course in 1989.
I would like to take this opportunity to thank the many who have been impor
tant to me personally during my days (and nights) at school: my family, Dianne,
Rob, Geoff, Norrie, Hans the Guide, my longstanding officemate Wank, my various
tennis partners and hockey players, the Unmade Beds, Rudolf Pez, and the Saigon
Palace Restaurant. My guitar thanks Neil Young. My piano thanks Mozart, Bach,
and Gershwin.
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Table of Contents
Chapter 1 : Introduction
1.1 B ackground to the Seismic Reflection S tudy ............................................. 1
1.2 Geology of th e A bitibi G reenstone B e l t .......................................................... 2
1.3 C haracteris tics of the D a t a .................................................................................5
1.4 O bjectives ................................................................................................................7
1.5 O u tline .................................................................................................................... 7
Chapter 2 : A cquisition and Preliminary Processing
2.1 A c q u i s i t i o n ................................................................................................................8
2.2 P relim inary P r o c e s s in g ..........................................................................................9
Chapter 3 : Reprocessing o f Line 12A
3.1 Frequency C ontent of the D a ta ......................................................................15
3.2 A m plitude Com pensation and CM P B inning ........................................... 17
3.3 A tten u a tio n of the G round R o l l ......................................................................19
3.4 V ib ra to r Noise .....................................................................................................26
3.5 S ta tic C o r r e c t io n s ................................................................................................ 29
3.6 Velocity Analysis ................................................................................................ 36
3.7 V ariability o f Reflection Energy w ith O f f s e t ............................................... 37
3.8 Inclusion of High Frequencies in the Processing ...................................... 39
3.9 S tacked S e c t io n s .................................................................................................... 40
Chapter 4 : Conclusions
4.1 S u g g e s t io n s ............................................................................................................. 42
4.2 E valuation and In terp re ta tion of the Reprocessed S e c t i o n ....................44
Appendix
D ISCO Jo b Decks ..................................................................................................... 47
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Chapter 1
Introduction
1.1 B ackground to th e S eism ic R eflection S tu d y
The use of reflection seismology to explore the crystalline basement of the
continental crust dates to the 1950s and 60s (Junger [1951], Richards and Walker
[1959], Widess and Taylor [195:1], Dix [1965], Dohr and Fuchs [1967]). These initial
results were most encouraging, showing that coherent reflections could be obtained
from within crystalline igneous and metamorphic rocks (which had been in doubt),
and eventually led to the establishment of COCORP (Consortium for Continental
Reflection Profiling) in the United States in 1974. The goal of this program is
to conduct and interpret deep crustal reflection surveys throughout the U. S. The
successes of this program resulted in Em increased pace of scientific exploration
around the world in the early 1980s, with deep crustal programs being established
in many countries. Canada joined the scene in 1984 with the institution of Phase I
of Lithoprobe.
The results achieved with the seismic imaging of the subduction zone beneath
Vancouver Island (Clowes et al. [1987]) led to the establishment of phase II of
Lithoprobe: the probing of the continental crust throughout Canada, concentrating
on a number of “transects” , each focussed on particular geological environments and
problems. The program’s philosophy is to coordinate studies using a great variety of
geophysicEd and geological methods (seismic reflection and refraction, potential field,
electro-magnetic, radio-isotopic, structural, and geochemical studies), integrating
the results to give a coherent tectonic interpretation of each region.
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Chapter 1 Introduction 2
The Kapuskasing Structural Zone (KSZ) Transect, initiated in 1987, was the
first of phase II. Five hundred and seventy three line kilometres of vibroseis data
were acquired in the central Canadian shield during the autumn and winter of 1987-
8 by Veritas Geophysical Ltd., with nine lines being acquired in the KSZ, and four
lines (1 2 ,12A, 14, end 14AB) in the Abitibi Greenstone Belt (AGB; see figure 1.1).
The data studied in this thesis are from line 12A in the AGB. Although collected
as part of the KSZ Transect, the AGB lines are actually preliminary studies for
the Abitibi - Grenville Transect. The reflection data for this transect me being
acquired in late 1990. Lithoprobe’s primary goal in acquiring the data in the AGB
was to determine optimal acquisition and processing methods, with a secondary
goal of evaluating existing tectonic interpretations of the AGB.
1.2 G eology o f A b it ib i G reen sto n e B elt
Archaean cratons constitute the bulk of the continental crust (Kroner [1981]).
Having experienced very little deformation since Precambrian times, they are cru
cial to our understanding of crustal formation during the early history of the earth.
These cratons are predominantly composed of two types of terrains, granite - green
stone and granulite - gneiss, with these two types likely being formed in different
tectonic environments (Kroner [1981], Windley [1981]). Greenstone material, which
comprises approximately 20% of the granite - greenstone terrains, is characterised
by mafic volcanic material which has undergone regional low pressure, greenschist
facies metamorphism (Goodwin [1981]). These greenstone belts are common in
Precambrian terrains, but axe often considered to be different from Phanerozoic
volcanic material, although m odem volcanic arcs and back axe basins show some
similarities to Archaean greenstone belts (Windley [1981]). This suggests that at
least some aspects of the Archaean tectonic regime differed from the modern Wilson
Cycle.
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W'i>\m
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M H K r-Vi i i i i !
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■V&fii! i ! i ! i ! i ! i ! i ! i ! V - iCWiffi,.i.i.i.i.i,i i i i i l i i i i■i!i!i!i! iWi!i?i!i?i?i?ili?i
Figure 1.1 Simplified geological map of Central Abitibi Greenstone Belt (AGB). The seismic lines acquired as part of the Kapuskasing Transect in 1987-88 are marked. LCF = Larder Lake - Cadillac Fault Zone; PDF = Porcupine - Destor Fault Zone; BR = Blake River Group; Ki = Kinojivis Group; Ti = Timiskaming and lithologically similar groups; PO = Pontiac Group; HM = Hunter Mine Group; LL = Larder Lake Group; SK = Skead Group; PA = Pacaud tuffs; CA = Catherine Group. From Green et al. [1990].
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99
Chapter 1 Introduction 3
A number of settings have been proposed for the formation of greenstone belts
based on some similarities with Phanerozoic structures. These include various
rift environments, both in continental and oceanic crust, volcanic arcs, back arc
marginal basins, and crusted shortening regimes (Garson and Mitchell [1981]). One
of the purposes of the Lithoprobe seismic program is to evaluate hypotheses regard
ing the formation of the AGB.
The AGB is located in the Superior province of the Canadian shield, which
is composed of a number of east - west trending granulite - gneiss and granite -
greenstone subprovinces. Since the time of the formation of the AGB circa 2.7 Ga
ago (e. g. Ludden et al. [1986], Jackson et al. [1990]), there has been relatively
little deformation, intrusion, and erosion, making the AGB not only the largest,
but also one of the best preserved greenstone belts in the world (Goodwin [1981]).
Moreover, the presence of notable economic mineral deposits has prompted many
studies of the region, so the geology is relatively well explored. Therefore, this is a
prime research area, and we can expect the seismic data to reveal structures extant
from the formation of the AGB.
Earlier studies have shown that the AGB is composed of regions of varying
composition and style of emplacement. Ludden and Hubert [1986] divided the
region into four zones as shown in figure 1.2: the northern volcanic zone (NVZ),
the centred granite - gneiss zone (CGG), the southern volcanic zone (SVZ), and the
southern granite - gneiss ;:one (SGG). Dimroth et al. [1982] have divided the AGB
into an internal zone (towards the interior of the craton, corresponding to the NVZ
and CGG), and an external zone (towards the margin of the craton, corresponding
to the SVZ). Greenstone material from the internal zone was possibly emplaced on
continental crust, whereas material from the external zone appears to have been
emplaced on oceanic crust (Jackson and Sutcliffe [1990]). The SGG corresponds to
the Pontiac Metasedimentary Belt (PMB). Dimroth et al. [1982] interpret the PMB
as being the sedimentary foreland of the AGB.
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ONTARIO
(M A P 'j AREA* J r\
^ g g g = O P A T IC Ar f r - . ® C h ib o u g a m a u -
Q U E T IC O
f t ***’
P O N T I A d ^ GRANITOID ROCKS
MAFIC INTRUSIONS
ULTRAMAFIC LAVAS
SEDIMENTSPOST
ARCHEAN| | MAFIC-FELSIC
£ MASSIVE GNEISSIC ' - TONAUTE100 km
............. rf > TECTONIC FRONT
PARAGNEISS- _ 3 MIGMATITE
Figure 1.2 Map of the Abitibi Greenstone Belt, composed of the folowing: 1 = Northern Volcanic Zone; 2 = Southern Volcanic Zone; 3 = Central Granite - Gneiss Zone; 4 = Southern Granite - Gneiss Zone; dashed lines separate these zones. KSZ = K v puskasing Structural Zone. Box indicates area detailed in figures 1.1 . From Ludden and Hubert [1986].
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Chapter 1 Introduction 4
1.2.1 Volcanic Material
Volcanic rocks of the AGB were erupted in two cycles. The following description
of this volcanic material is summarised primarily from Dimroth et al. [1982, 1983]
and references therein.
Despite limited exposure of the first cycle, it is known to consist of interdigi-
tating rhyolite and basalt flows. The age of the top of this cycle is 2711 +6/-3 Ma
as determined from U - Pb dating of zircons. The bulk of the southern AGB is
composed of material from the second cycle, which unconformably overlies the first
cycle and is subdivided into three stages. The lowest is an extensive ultramafic (ko-
matiitic) lava plain of 7 km thickness, formed in a deep (~ 2 km) sea environment,
and underlies much of the southern AGB. The second stage involved the eruption
of the Kinojevis Group, composed of uniform and differentiated tholeiites. This
material is 5 to 7.5 kilometres thick and was erupted as deep marine lava plains in
the west, and central volcanic complexes in the east. These complexes were built up
nearly to sea level in some areas. The upper stage involved the eruption of the Blake
River Group (BRG) from central volcanic complexes, with material building up to
sea level. The maximum thickness of the BRG is 10 km in the west, with it lensing
out to the east. This group has a complex interned stratigraphy and very diverse
composition (mafic to felsic), being formed of alternating tholeiitic and calc-alkalic
differentiation suites. The age of the top of this group is 2700 +4/-3 Ma, so the
entire 15 to 25 kilometres thickness of the second cycle erupted over approximately
10 Ma. The emplacement of these large volumes of volcanic material may have
resulted in large scale subsidence and a synclinal structure of the BRG (Jensen and
Langford [1985]). Isotopic evidence indicates that this volcanic material does not
have a significantly evolved crustal component, indicating that this materiel was
not emplaced on evolved (continental) crust (Jackson and Sutcliffe [1990]).
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Chapter 1 Introduction 5
1.2.2 Deformation Structures
The AGB is transected by two regional steeply dipping mineralised deformation
zones, the Destor - Porcupine Deformation Zone (DPDZ) and the Larder Lake -
Cadillac Deformation Zone (LCDZ). These have been interpreted in a number
of ways. Dimroth et al. [1982] believe them to be normal faults resulting from
subsidence during the eruption of the ultramafic basalts at the beginning of the
second cycle. Hubert et al. [1984] and Ludden et al. [1986] have interpreted them
to be sinistral strike slip faults caused by oblique convergence between different
terrains to the north and south, resulting in rifting and the eruption of the second
cycle volcanics. This was cotemporal with or followed by north - south compression,
resulting in thrust movement along the the DPDZ and LCDZ. Jackson and Sutcliffe
[1990] infer north side down motion along the LCDZ after 2680 Ma, followed by
south side down motion in the early Proterozoic (24S0 to 2460 Ma). Thus, these
zones appear to have been active at a number of stages with composite dip and
strike slip movement. The presence of such regional deformation zones throughout
the Abitibi - Wawa subprovince with an apparent absence in the Kapuskasing Uplift
suggest that these may not extend to the lower crust — they either become listric
with depth, or are overprinted (Jackson et al. [1990]).
1.3 C h aracteristics o f th e D a ta
All of the data collected as part of the KSZ Transect was initially processed
by Veritas Seismic Ltd. using relatively basic procedures to give a preliminary set
of seismic profiles. This processing scheme will be described in section 2.2 . From
these stacks, we shall now make general observations about the characteristics of
the data.
One is immediately struck by the number and quality of reflections in the line
12A preliminary section (see plate 1). It is apparent that there are two styles of
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Chapter 1 Introduction 6
reflectors in these data, which are typical of all of the data from the KSZ Transect:
1) in the mid crust (2 to 5 s), reflecting zones are continuous over long distances and
are relatively short in time duration (see figure 1.3(a)); and 2) in the deeper crust (>
5 s), there is higher overall reflectivity with thicker reflective zones being composed
of many interweaving reflections of short spatial extent (see figure 1.3(b)). It is
worthy of noting that throughout the KSZ dataset, there are no strong indications
of a reflection Moho. In the AGB, there is a decrease in reflectivity at 11.0 to 11.5
s which has been interpreted to be the Moho (Green et al. [1990]), but this is not
a sharp, clear boundary. This is in agreement with Meissner's [1986] observation
th a t the Moho is less prominent or not at all detectable from reflection data in
old cratons. The lower crust in such areas is of high velocity (~ 7 km /s), with a
gradual transition from crustal to mantle velocities. Meissner attributes this velocity
structure to higher heat flow in the Archaean, which could have resulted in more
melt being extracted from the mantle, forming a thicker, less differentiated, higher
velocity crust.
There is good penetration of the energy, as is demonstrated by the high reflec
tivity up to 8 s. Only in the upper 1 s of the section is there very low reflectivity.
This may not be a true reflection of the geology because there axe penetrative
structures visible at the surface which one might expect to be capable of producing
reflections. Some notable examples are 1) the Crosby Sill, 2) the contact between
the Kinojevis and Blake River Groups, and 3) a nearby deformation zone, all of
which are possible candidates for the dipping reflection noted in figure 1.4 (Jackson
et al. [1990]). If not due entirely to the low reflectivity of the rock, then the appar
ent low reflectivity in the shallow data is likely the result of problems in the data
processing.
This zone of apparent low reflectivity causes problems in the interpretation of
the section. Direct observations of the geology have only been made near the surface,
whereas reflections from the seismic section cannot generally be traced to shallower
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300
500
700
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Figu
re
1.3(
a)
A ty
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300
500
700
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Figu
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1.3(
b)
Typ
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fro
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de
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with perm
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reproduction prohibited
without
permission.
VP 400£ KINOJEVIS GROUP
CROSBY
SILL
600 800BLAKE R IV ER GROUP
0.0 8
1.0
2.0
Figure 1.4 Reflection projecting to the surface location of the Crosby Sill.
Chapter 1 Introduction 7
than 1 s ( 3 km). Thus, there is a gap of several kilometres between the two. The
situation is further complicated because many structures at the surface are steeply
dipping (Jackson et al. [1990]), whereas many of the reflections are sub-horizontal,
causing difficulty in correlating surface geology with deeper structures.
1 .4 O b jectives
The primary objective of this thesis is to reprocessing line 12A, thereby evalu
ating the need for detailed processing of crustal seismic data. Therefore, the bulk
of this thesis will involve an analysis of processing methods used in obtaining an
improved stacked section. Due to the importance of the shallow data in interpreting
the section, special effort was spent in reprocessing the upper 1 to 2 s.
An evaluation of the acquisition is beyond the scope of this work. However,
certain aspects of the acquisition method will be examined where they may have
contributed to difficulties in the processing (for example, problematic noise which
could be attenuated through design of the survey).
1.5 O u tlin e
In chapter 2, work performed by the contractors will be reviewed (the acquisi
tion method in section 2.1, the preliminary processing in section 2.2). Some weak
nesses in the processing will be identified which will guide efforts to reprocess the
data, detailed in chapter 3. Also in chapter 3, some aspects of the acquisition will
be examined where these aspects have significant effects on the processing. In chap
ter 4, the results of the reprocessing will be examined and evaluated in comparison
with the results of the preliminary processing. Conclusions about the processing of
seismic data in the AGB will also be detailed, hopefully guiding future seismic work
in the Lithoprobe Abitibi - Grenville Transect.
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Chapter 2
Acquisition and Preliminary Processing
2.1 A cq u isitio n
Two sets of parameters were used in acquiring the KSZ reflection data: “re
gional” , used for most of the data, and “high resolution” , used in selected areas of
special interest (lines 12A and 14AB in the AGB — see figure 1.1). The subject of
this study is the high resolution line 12A. The two sets of parameters are detailed
in table 2.1, with the most significant aspects being contrasted in the following
paragraphs.
Vertical motion recordings from 240 receiver groups were acquired using two
DFS-V recording systems and OYO McSeis III (14 Hz) geophones. Most of the data
in the AGB were collected using an asymmetric receiver spread with 60 channels to
the north and 180 to the south of the vibrator point (VP). Six (sometimes seven)
stations on each side of the VP were not recorded for all lines except 12A and 14A,
where this gap was increased to eight stations on each side. At the ends of the
lines, this shot gap was removed, and the recording spread was stationary as the
VP advanced along the line (known as “roll-on” or “roll-off”). The spacing between
receivers was 20 metres for the high resolution, and 50 metres for the regional.
The source array consisted of two Mertz 18 vibrators (each with 20 072 kg
peak force) for the high resolution data, and four for the regional (although much
of the regional data was acquired using only three vibrators due to phase control
problems). For the high resolution, an 8 s, 20 to 120 Hz upsweep was used, with
a 14 s, 12 to 52 Hz upsweep for the regional. The toted record length of the high
resolution records is 8 s, and 16 s for the regional. There was a VP at every station
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Regional High Resolution
Number of Vibrators 4 2
Sweep 12 - 52 Hz 20 - 120 Hz
Sweep Length 14 sec 8 sec
Record Length 16 sec 8 sec
Vibration Point (VP) Spac 100 m 20 m
ingNumber of Sweeps per VP 8 8
Number of Receivers 240 240
Receiver Spacing 50 m 20 m
Subsurface Coverage 60 Fold 120 Fold
Table 2.1 Acquisition parameters for regional and high resolution surveys.
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1) STACK OF ADJACENT SHOT GATHERS
2) AUTOMATIC GAIN CONTROL
3) TRACE EDITING
4) CROOKED LINE GEOMETRY
5) REFRACTION STATIC CORRECTIONS
6) CMP SORT
7) VELOCITY ANALYSIS
8 ) NMO CO RRECTIO N
9) FIRST BREAK MUTE
10) NON SURFACE-CONSISTENT RESIDUAL STATIC CORRECTIONS
11) STACK
Table 2.2 Preliminary processing sequence.
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Chapter 2 Acquisition and Preliminary Processing 9
for the high resolution, resulting in 120 fold data with 10 metre common mid-point
(CMP) spacing. For the regional, there was a VP at every second station, resulting
in 60 fold data and 25 metre CMP spacing.
2 .2 P ro cessin g
Veritas Seismic Ltd. processed the data to obtain initial seismic sections using
the processing sequence listed in table 2.2 . There were two main reasons for
producing preliminary seismic sections: 1) preliminary geological interpretations
may be made, which would, in turn, guide future work, and 2) as an intermediate
result, they can guide further processing.
In the following section, significant procedures will be examined with possible
weaknesses being highlighted. In section 2.2.2, possibilities for improvement during
reprocessing will be outlined.
2.2.1 Preliminary Processing Scheme
11 Stacking of Adjacent Shot Records
The KSZ seismic data form an enormous dataset with a very high CMP fold
of 120. There were 1530 shot records of 8 s length with 2 ms sampling for line
12A alone. Thus, the processing of the data is a formidable undertaking, consid
erably more costly and problematic than standard processing in industry which
often involved 4 s of data with a CMP fold of 24 or 48 at that time. In order to
alleviate the cost, adjacent shot records were summed before further processing.
This has two detrimental effects on further processing: 1) the quality of the first
arrivals is degraded due to interference effects, resulting in poorer estimates of the
static corrections using a refraction static routine (see step 3), and 2) the quality of
the signal is degraded in the shallow data where the differential moveout between
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Chapter 2 Acquisition and Preliminary Processing 10
summed traces could be significant. Thus, the stacking of adjacent shot gathers is
likely a partial reason for the low apparent reflectivity in the shallow section.
21 Crooked Line Geometry
The definition of the geometry of the survey involves the specification of the
ground surface geometry (the location and elevation of each station, and the record
ing spread configuration), and the sub-surface geometry (the locations and sizes of
the CMP bins — reflection points which lie outside these bins are not included in
further processing). Because the seismic data were acquired along existing minor
roads, the seismic lines are crooked, resulting in a wide scattering of CMPs wher
ever there were substantial bends in the line, spanning a width as great as several
kilometres in certain areas. Thus, the choice of a CMP line is important, with many
traces falling outside of the CMP bins and being dropped from further processing.
The line defined for preliminary processing was selected to maintain as high a fold
as possible (i. e. so that as many traces as possible fall within the defined CMP
bins).
31 Static Corrections
Due to the varying elevation of the sources and receivers along the line, elevation
static corrections must be applied to reposition the data to a datum level. The
surface conditions along the line are variable, ranging from minimal overburden
to over ten metres of loose glacial till. This results in varying travel time delays
in addition to the elevation static correction. Refraction static corrections were
applied to account for this varying overburden thickness and for lateral bedrock
velocity variations. However, the accuracy of the corrections is dependent on the
accuracy of the picking of the first arrivals. From a visual examination, the picks
made during preliminary processing were found to be unsatisfactory. Partly, this
is because of a relatively high noise level, which is known to cause difficulties for
automated picking algorithms (Mayrand et al. [1987]). This is aggravated by the
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Chapter 2 Acquisition and Preliminary Processing 11
variable surface conditions which may result in variable source - ground coupling
and a variable wavelet. Furthermore, as was mentioned in step 1, the stacking of
adjacent shot gathers causes a deterioration in the quality of the first arrivals. Due
to the unsatisfactory quality of the first arrival picks, one would expect that the
refraction static corrections Eire also unsatisfactory.
Refraction static corrections do not form the complete solution to the statics
problem; the method cannot determine very short wavelength statics (less than one
receiver spread). For this reason, non surface - consistent residual static corrections
were also determined for the data. These corrections are not well founded in physical
reality since Ein independent correction is determined for every trace in the dataset
in order to align reflections across a CMP gather. No regard is given to the surface
locations which correspond to the static corrections. To be more physically valid,
the residual corrections would have to be constrained to be surface - consistent
(i. e. each surface location would have an associated static correction representing a
near surface travel time delay). This method will be summarised in section 3.5.3 .
41 Velocity Analysis
An EuiEilysis must be performed to select stacking velocities which are used
to remove the normal moveout (NMO) effect. By selecting the highest coherency
of events on a number of stacks made using different stacking velocities, a velocity
function for the line was defined. One would expect deep reflections to be insensitive
to the choice of velocity in this survey, but shallower reflections were surprisingly
insensitive as well. This leads one to believe that the pre-stack processing was not
optimal. In particular, a poor solution to the statics problem and the reverberatory
character of the wavelet could lead to velocity insensitivity. The latter suggests the
need for deconvolution or another method of spectral balancing. Furthermore, there
Eire few coherent reflections above 2 s, resulting in poor shallow velocity control.
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Chapter 2 Acquisition and Preliminary Processing 12
S'! Stack
The stacking of CMP gathers is meant to increase the signal to random noise
level (S/N). In addition, coherent noise which does not have hyperbolic moveout
corresponding to the stacking velocity should also be attenuated. There are two
forms of coherent noise which are common to the entire KSZ dataset: 1) ground
roll, and 2) neax offset, low frequency source generated noise, termed “vibrator
noise” . The stronger of the two is the ground roll — a high amplitude, low velocity
wavetrain found in the upper 1.0 to 1.5 seconds in all shot records (see figure 2.1).
Vibrator noise, appearing as ghost images of the ground roll (see figure 2.2), is not
as pervasive as the ground roll, but often dominates the near offset traces.
Since both types of noise are of much higher amplitude than primary reflections,
the reflection energy is obliterated by the noise, even after stacking. Moreover, the
autom atic gain control (AGC) reduces the amplitudes of shallow primary reflections
in the vicinity of the ground roll, forming an “AGC shadow”. Thus, the coherent
noise can degrade data quality, especially in the shallow data.
2.2.2 Strategy for Reprocessing
Having outlined the preliminary processing scheme, it is clear that reprocessing
should improve reflection energy on the seismic section. Modifications to the steps
which were taken and additional procedures which could be used are summarised
in the following sections.
1) Stacking of adjacent shot records
This was done solely to reduce the data volume for processing. Due to the
number of processing problems which resulted from this, this data reduction should
not be done during reprocessing.
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TRACE 50 100
REFRACTED P WAVE ARRIVAL
GROUNDROLL
Figure 2.1 Ground Roll.
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TRACE 50 100
0.0 s
1.0
2.0
Figure 2.2 Near offset, vibrator related noise. Trace spacing is 20 m. The vibrators were located between traces 60 and 61.
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Chapter 2 Acquisition and Preliminary Processing 13
21 Spectral equalisation
Higher frequency reflection energy is attenuated more than low frequency en
ergy. The amplitude spectrum of the data should be equalised in order that the
weaker high frequency information is boosted, resulting in broader signal bandwidth.
3) Crooked line processing
Since the CMP scatter can be as large as severed km, there is freedom in the
choice of a CMP line. The choice made during preliminary processing should be
reexamined to determine if a better CMP line could be defined.
41 Attenuation of coherent noise
As was mentioned in the preceeding section, stacking is not a satisfactory means
of attenuating coherent noise where it dominates the reflection energy. Thus, par
ticular effort should be spent in attenuating the ground roll and vibrator noise.
51 Static corrections
A new refraction static solution should be calculated after more accurate pick
ing of first arrivals. Then surface - consistent residual static corrections should be
determined rather thai. the less physically valid non surface - consistent method
which was used during preliminary processing.
61 Problems particular to the shallow data
Why might we expect poor imaging of the shallow data after preliminary pro
cessing when it is clear that signal is strong enough to image deep reflectors? 1)
High amplitude ground roll energy obscures signal in the upper 1.0 to 1.5 seconds of
the record. Thus its attenuation should lead to improved imaging of the near sur
face. 2) Shallow data has a greater sensitivity to NMO velocity than does deep data.
After reprocessing, there will likely be much better velocity control on the shallow
reflections. 3) Standard CMP processing assumes that the earth has a laterally
homogeneous, horizontally layered velocity structure. Then the surface projection
of the poir/ where the seismic energy is reflected (the common reflection point,
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Chapter 2 Acquisition and Preliminary Processing 14
or CRP) for a particular shot - receiver pair will coincide with the geometric mid
point, the CMP. Where the velocity structure deviates from this ideal structure, the
CRP and CMP do not coincide, with the discrepancy being greater for shallower
structures. In particular, where reflecting horizons are dipping, the shape of the
reflection hyperbola is changed, with the curvature decreasing and the reflection
points being moved updip (Yilmaz [1987]). This necessitates that a dip moveout
(DMO) correction be applied to accurately image the sub-surface. Since dipping
structures are visible in the surface geology, one would expect that DMO removal
would be necessary during reprocessing.
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Chapter 3
Reprocessing of Line 12A
The resources at the Lithoprobe Seismic Processing Facility (LSPF) at the
University of Calgary were available for use during the course of this thesis work.
The computer hardware consists of a CYBER 835 computer with a MAP-V array
processor; the available software is the DISCO seismic processing package.
The intended reprocessing sequence is listed in table 3.1 . In many cases, results
had to be determined using the bulk of the processing flow in order to determine
the results of individual processes. In the following sections, we shall examine the
individual steps.
3.1 Frequency C on ten t o f th e D a ta
The first step in processing seismic data should be an examination of the fre
quency content. It is most useful for further processing to know the frequency
characteristics of the signal and of the noise.
3.1.1 Reflection Energy
Can reflection energy over the full sweep bandwidth t f 20 to 120 Hz be imaged?
The amplitude spectrum in figure 3.1 shows that most of the reflection energy is
between 25 and 50 Hz. Above 70 Hz, amplitudes are over 20 dB below the peak.
Even if the high frequency components were boosted, it would still be very difficult
to enhance through stacking whatever weak signal that may be present at high
frequencies due to its sensitivity to static corrections, stacking velocities, and the
presence of dipping reflectors. Limiting the frequency bandwidth of the data to 20
- 1 5 -
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1) NEW CROOKED LINE GEOMETRY
2) TRACE EDITING
3) ATTENUATION OF GROUND ROLL
4) ATTENUATION OF VIBRATOR NOISE
5) REFRACTION STATIC CORRECTIONS
6) SPECTRAL EQUALISATION
7) AUTOMATIC GAIN CONTROL
8) CMP SORT
9) SURFACE-CONSISTENT RESIDUAL STATIC CORRECTIONS
10) VELOCITY ANALYSIS
11) NMO AND DMO CORRECTION
12) STACK
T able 3.1 Intended reprocessing sequence.
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0
oowauwQ
-20
-40
0 80 160
FREQUENCY (HZ)Figure 3.1 Amplitude spectrum of da ta between 1 and 4 s from a shot record with good reflection quality, i. e. the spectrum indicates reflection energy and background
240
Chapter 3 Reprocessing o f Line 12A 16
to 60 Hz would likely result in little loss of reflection energy. The data could then be
resampled from 2 ms to 4 ms, reducing the data volume by half, making subsequent
processing less cumbersome and time consuming. Since low frequency energy still
dominates within the 20 to 60 Hz bandwidth, the amplitude spectrum of the data
should be balanced over this range.
The spectral equalisation method which was used operates as follows: 1) the
data are filtered using three zero phase band-pass filters (20-30, 35-45, 50-60 Hz);
2) a 150 ms AGC (see section 3.2) is then applied to each of the resulting band-
limited records; 3) the 35-45 and 50-60 Hz band records are scaled to have equal
root-mean-square (RMS) amplitudes, with the 20-30 Hz record scaled to one half
of the amplitude of the other two (to reduce contamination by low frequency noise
— see section 3.1.2 below); and 4) these band-limited records are stacked.
To show the effect of spectral equalisation, similarly stacked data with and with
out the application of this process are compared in figure 3.2 . Clearly, the imaging
of some shallow reflections has been improved. For data below 3 s, the spectral
equalisation process was found to make very little improvement, suggesting that
the higher frequency reflection energy within this bandwidth has been attenuated
to below noise levels. Therefore, since it is an extremely time consuming process,
spectral equalisation was only applied to the upper 3 to 4 s of data.
S.1.2 Noise
There are three primary types of noise evident in the pre-stack data: 1) ground
roll; 2) near offset, low frequency vibrator noise; and 3) miscellaneous noise which
contaminates entire traces (e. g. 60 Hz noise, cultural noise, or poor receiver cou
pling). The third type of noise can be removed quite simply by trace editing and
applying a 60 Hz notch filter. The first two types of noise require further examina
tion.
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400
500
600
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Figu
re
3.2(
a)
Stac
ked
sect
ion
afte
r re
proc
essi
ng,
with
out
spec
tral
equa
lisat
ion.
400
500
600
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Figu
re
3.2(
b)
Stac
ked
sect
ion
afte
r re
proc
essi
ng,
with
spec
tral
equa
lisat
ion.
Chapter 3 Reprocessing o f Line 12A 17
The ground roll was found to be of a bandwidth comparable to that of the
reflection energy, suggesting that there will be difficulty in attenuating this noise.
This issue will be examined in detail in section 3.3 .
On shot gathers where vibrator noise is significant, it was found to be of pre
dominantly low frequency (20 to 40 Hz). This noise will be discussed further in
section 3.4 .
3 .2 A m p litu d e C o m p en sa tio n an d C M P B inn ing
One has two options with regard to amplitude compensation in the process
ing of seismic data: 1) to preserve the relative amplitudes by applying gain to
compensate for spherical divergence and varying coupling between the ground and
the sources and receivers, or 2) to lose relative amplitude information by applying
non-deterministic scaling (AGC). By preserving the relative amplitudes, different
reflectivity levels throughout the stacked section may constrain lithological inter
pretations and correlation of reflection events across the section. However, this
requires more careful editing to remove noise from the data, which is extremely
time consuming and beyond the scope of this thesis.
A pre-stack AGC was applied; in the upper 3 s, a 150 ms window was used in
order to minimise the size of the AGC shadow of the first arrivals and ground roll,
with a 750 ms window used below 3 s. By balancing the amplitudes both within
and across traces, the level of incoherent noise becomes comparable to that of the
reflection energy and will not dominate after stacking. Nevertheless, traces with
obviously low S/N were killed at an early stage during the processing. Anderson
and McMehan [1989] and Mayrand and Milkereit [1988] have found that cursory
trace editing provides an increase in S/N comparable to that provided by very
careful trace editing provided that the fold is not low and that an AGC is applied.
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C hapterS Reprocessing o f Line 12A 18
The choice of a CMP line and bin sizes determines which traces will be dropped
from further processing, and may be particularly important where CMP scatter is
great. In such areas, the CMPs corresponding to different offsets will trace different
lines. By stacking traces within various offset ranges, the effect of using different
CMP lines is approximated. In this way, the data need not be sorted into different
CMP gathers a number of times, which is an extremely time consuming process.
Sections stacked using only very long offsets (over 2 km) or very short offsets
(up to 500 m) showed poorer reflection quality than those using only medium offsets.
At long offsets, S/N is lower because an array of vibrators emits most of its energy
vertically downwards, with less energy being emitted at shallow angles (Sheriff and
Geldart [1982]). At short offset, the data Eire often contaminated by vibrator noise.
Reflection energy w eis not sensitive to offset within the range of 500 to 2000 m,
in part due to the need for more comprehensive processing in order to image the
weaker reflections (especiEilly in the shallow data). Therefore, an intermediate offset
value of 750 m was selected in order to form the new CMP line; the line formed by
CMPs corresponding to traces with this offset was defined to be the new CMP line.
It now remEiins for the bin sizes to be selected. During preliminary processing,
the bins were 10 m (in-line, equal to the CMP spacing) by 800 m (cross-line). After
processing, every three adjacent traces were stacked to give one final trace. For this
reason, a CMP spacing Eind in-line bin length of 30 m was selected. This makes
reflection energy on CMP gathers more easily observed — since there are more
traces within a narrow offset range, reflection energy which is intermittent with
offset (see section 3.7) is consistent over a greater number of traces.
The selection of the cross-line dimension of the CMP bins is only important in
areas cf large CMP scatter. When bins are very narrow (less than 400 m), the fold
in such areas becomes very low, resulting in much poorer imaging of reflections.
When bins of greater than 800 m width are used, there is slightly poorer imaging of
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Chapter 3 Reprocessing o f Line 12A 19
reflections because reflection information from very widely varying reflection points
is stacked, resulting in some destructive interference. Deterioration in reflection
quality is more marked if the bin sizes are extremely small than if extremely large.
In addition, if bins are wider than 800 m, the residual static solution becomes
unstable (see section 3.5.3). This prompted the selection of 30 x 800 m CMP bins.
3 .3 G round R oll
S.3.1 Characteristics
The upper one second of the shot gathers is dominated by coherent, source
generated surface energy which is known as “ground roll” (see figure 3.3). Any
shallow reflection energy present is obscured within the “AGC shadow” of this
noise. Even after stacking using an AGC window of 150 ms, reflection signal in
the shallow data is obliterated. Thus, the attenuation of this noise before AGC is
im portant in improving the image of shallow structures.
In the Abitibi data, the ground roll is characterized by a high amplitude, broad
band (20 to 60 Hz) wavetrain of roughly 100 ms duration which shows linear moveout
corresponding to a velocity of 3.5 to 4.0 km /s . Its amplitude spectrum is very sim
ilar to that of the reflection energy (see figure 3.4). It is composed of surface wave
energy, shear wave energy (probably converted from P wave energy at the interface
between the bedrock and the overburden), or a combination of the two.
In general, ground roll comprised of surface wave energy may be dispersed, i. e.,
different frequency components travel at different phase velocities. This can occur if
the medium shows anelastic attenuation, or if layers of varying velocity axe present
near the surface, with dispersion being greater in the latter case (Al-Husseini et
al. [1981]). Strongly dispersed ground roll can obscure a large portion of the record
because different frequency components arrive at different times. There exist a
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TRACE 50 100
Figure 3.3 Ground Roll.
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ission of the
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ner. Further
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permission.
0
coWPQHHU
8
-20
-40
0 40 80 120
FREQUENCY (HZ)
Figure 3.4 Amplitude spectrum of ground roll (dashed line) compared with tha t of d a ta between 1 and 4 s (solid line).
Chapter 3 Reprocessing o f Line 12A 20
number of methods which distinguish such dispersive surface noise from reflections
on the basis of dispersion characteristics, thereby allowing one to attenuate this
energy. See, for example, Beresford-Smith and Range [1988], and Saatgilar and
Carntez [1S38].
In the Abitibi data, the ground roll is not significantly dispersive over the
bandwidth of the d ata as is verified by means of bandpass filter panels (figure 3.5).
Note that neither the arrival time nor the phase velocity of the wave train varies
with frequency content, the former indicating that the group velocity is constant
with frequency. Note also that the energy (or the envelope of the ground roll) is
travelling at the same apparent velocity as the individual peaks (the phase velocity),
indicating that the group and phase velocities are equal. Although the duration
of the ground roll is substantial (100 ms), this is not because the wavetrain is
dispersive, but rather it is due to the energy reverberating between the surface and
the bedrock-overburden inte-face. This can be seen by noting that the wavetrain
is comprised of a series of parallel, similar wavelets. If the near-surface velocity
structure leads to significantly dispersive propagation, then the wavelet broadening
can be much more pronounced; for example, Al-Husseini et al. [1981] found ground
roll in Saudi Arabia with an onset time of 0.5 s to have a duration of half of one
second. Since the data do not show dispersion over the bandwidth of the data, and
DISCO does not contain procedures for rejecting dispersive waves, such methods
were not attempted.
3.3.2 A ttenuation through Processing
A number of traditional methods were attempted in order to attenuate the
ground roll prior to stack:
1) low cut frequency filtering
2) spectral equalisation
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Further reproduction
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ithout perm
ission.
2 0 - 3 0
FREQUENCY BAND
(HZ)
3 0 - 4 0 4 0 - 5 0 5 0 - 7 0
10 .0 s
0.5
Figure 3.5 Filter panels of ground roll. Note tha t the arrival time does not vary with frequency content, indicating th a t dispersion is not significant over the data bandwidth.
Chapter 3 Reprocessing o f Line ISA 21
3) minimum phase deconvolution
4) velocity (f-k) filtering
5) surgical muting in the t-x domain
Of course, these axe not the only methods which exist. The possibility of muting
the data in the r-p domain can be investigated. Another possibility is to use a
multi-channel median filter, which is commonly used in vertical seismic profiling to
remove unwanted coherent noise (Stewart [1985]). The latter method would involve
applying time shifts so that the ground roll is horizontal, passing only horizontal
energy using the median filter, then subtracting the resulting record from the origi
nal data, and removing the time shifts. However, software for only the five methods
listed above was available, so the other possibilities must be left for future research.
11 Low cut frequency filtering
The ground roll has a very broad amplitude spectrum, with a shape similar to
that of the reflection energy (figure 3.4). This indicates that low cut filtering will
attenuate signal as much as noise, and so will not be helpful.
21 Spectral equalisation
This process equalises the spectrum by applying an AGC to band-limited
records as described in section 3.1 . The net effect is similar to that of a zero
phase deconvolution process. Again, since the spectrum of the ground roll is similar
to that of the reflection energy, this method does not significantly attenuate the
noise (figure 3.6).
31 Minimum phase deconvolution
This process also equalises the amplitude spectrum of the data, and so was
tested for the attenuation of ground roll. There are two notable differences from 2)
above: 1) this method is a deconvolution process, i. e., it determines an inverse filter,
which is then applied to the data, whereas the above method balances the spectrum
in a non-convolutional, non-linear way; and 2) this method assumes that the data
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TRACE 50 100
Figure 3.6 Shot after spectral equalisation.The bandwidths used were 20-30 Hz (weight=0.5), 35-45, 50-60 Hz (each with weight=1.0). A 750 ms window length was used.
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Chapter 3 Reprocessing o f Line 12A 22
axe minimum phase. Although the vibroseis wavelet is zero phase, minimum phase
deconvolution can work well with careful selection of parameters.
Spiking deconvolution was attempted with a range of parameters, but the
ground roll could not be attenuated (figure 3.7). Moreover, reflection energy has
been decreased throughout the record. The spectral equalisation method described
in section 3.1 was preferred as a method of balancing the spectrum.
4) Velocity (f-k) filtering
Two methods of velocity filtering exist in the DISCO processing package: 1) a
multiplicative method, operating in the f-.k domain, and 2) a convolutional method,
operating in the t-x domain. These methods were tested using a series of cut-off
slopes and tapers. The best results will be presented here.
The f-k spectrum of the upper one second of a shot gather is shown in figure
3.8 . The ground roll can clearly be seen to dominate, being 12 to 24 decibels
above the reflection energy. Figure 3.9 shows the result of applying the f-k domain
filter; the ground roll has been somewhat attenuated, but still shows considerable
amplitude. Note that around channels 80 to 90, the ground roll flattens due to a
bend in the line, resulting in a greater amount of energy leaking through the filter.
Results from the t-x domain filter (see figure 3.10) were found to be less satisfactory
than the f-k domain filter.
Although the ground roll was partially attenuated, the data have been contam
inated with filter artifacts. This limited success did not warrant the large amount of
computer time required in applying the f-k filter before stack (to process the entire
line would be extremely problematic, and would take over 24 hours of CPU time,
or four days of elapsed time at best).
In order to determine the reason for the limited effectiveness of the velocity
filter, a number of tests on synthetic data were run using the t-x domain filter.
Ground roll was simulated as a 40 Hz sine wave of 100 ms duration. This “event”
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TRACE 50 100
0.0 8
1.0
2.0
Figure 3.7 added).
Shot after spiking deconvolution (56 ms filter length, 1 % white noise
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G R O U N D R O L L
3 . 9 k m/ s
0 dB
60
40
20
- 0 .5 0.5WAVENUMBER ( t r a c e )
Figure 3.8 f-k spectrum of the upper 1 second of one side of a shot record, containing the ground roll. Trace spacing is 20 m.
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FREQ
UEN
CY
(HZ
)
TRACE 50 100
S t e m
0.0 8
1.0
2.0
Figure 3.9 Velocity filter (applied in the f-k domain), rejecting slownesses between 3.5 and 10 m s/trace. A 60 Hz high-cut filter was applied afterwards.
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Figure 3.10 Velocity filter (applied in the t-x domain), rejecting slownesses greater than 3.5 ms/trace. A 60 Hz high-cut filter was applied afterwards.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Chapter 3 Reprocessing o f Line ISA 23
was made to have alternating apparent slownesses of 5 and 10 m s/trace as shown
in figure 3.11(a); the filter cut-off used was 3.5 m s/trace. This wavetrain was then
superposed on weaker events with infinite apparent velocity and of similar frequency
content as the ground roll. As can be seen in figure 3.11(b), the filter had limited
success in attenuating the ground roll. Reducing the amplitude of the ground roll
results in improved attenuation.
From this, it can be concluded that the high amplitudes of the ground roll
and the varying apparent velocity (due to the crooked line) contribute to the poor
performance of the f-k filter.
5f Surgical muting
Having rejected the above methods as a means of attenuating the ground roll,
and having decided that some means of removing ground roll energy is necessary,
the only remaining option is to mute this wavetrain prior to AGC (see figure 3.12).
Except where the seismic line hats large bends, the CMP fold should be sufficiently
high that shallow reflection energy could be imaged.
After reprocessing, one can see regions where there is no data coverage due to
bends in the line. One can also see that shallow reflections can be imaged to 0.4 s
travel time (figure 3.13), so the AGC shadow has been significantly reduced. Above
this, the data are contaminated by near offset noise and remnants of the ground
roll energy. Therefore, a better method of attenuating the ground roll would be
required to extend images closer to the surface.
S.S.S Attenuation during Acquisition
Since the ground roll posed such difficulties during processing and is such a
serious source of noise, it will be appropriate to discuss the possibility of attenuating
the ground roll during the acquisition of the data.
The attenuation of coherent noise through careful design of field arrays has
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TRACE 2 5 5 0
W T O W W m W ‘
TRACE 2 5 5 0
(b) ■«?< •<««
0 .0 s
1. 0
Figure 3.11 Synthetic ground roll, (a) before and (b) after t-x domain velocity filter. The ground roll is a 100 ms wavetrain with alternating slownesses of 5 and 10 ms/trace. Slownesses greater than 3.5 m s/trace were rejected. The filter length was 10 time samples by 6 space samples. Note that considerable energy at 5 m s/trace has been passed by the velocity filter.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
TRACE 50 100
V < v±?2!i
o .o s
1.0
2.0
Figure 3.12 Pattern for muting first arrival and ground roll energy. Although an AGC has been applied for display purposes here, the mutes were applied prior to AGC during the reprocessing.
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ission of the
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without
permission.
VP 300 400 500
0.0 s
0.5
1.0
Figure 3.13 Portion of the reprocessed stacked section showing regions of no data coverage where there are bends in the line (a result of surgical muting). Shallow north- dipping reflections can be traced to shallower than 0.5 s in some cases.
Chapter 3 Reprocessing o f Line 12A 24
become a topic of some interest in the geophysical literature in recent years. For
example, a receiver array will attenuate energy in certain spatial wavenumber, k,
ranges. The attenuation characteristics depend on the length of the array, L, as
shown in figure 3.14. Spatial wavelengths greater than twice the array length (for
k < ^ ) will be passed by the array with little reduction in amplitude, while shorter
wavelengths will be attenuated significantly. Thus, the receiver array length can be
chosen so that ground roll wavelengths are attenuated. Note that the spacing of
the array elements (the geophones), I, must be fine enough that there is no spatial
aliasing for the smallest wavelength of the ground roll (i. e., kmax < jj)-
However, one must exercise caution in selecting the length of the array. As is
illustrated in figure 3.15, the angle between the wavefront of the reflected energy
and the surface becomes greater as the depth to the reflector decreases, resulting
in a smaller spatial wavelength. Thus, this energy is attenuated and smeared in
time, reducing the resolution of wide offset, shallow time data. This principle is
illustrated in an experiment conducted in the Albertan prairie in which a survey
was re-shot with smaller array lengths (Taylor [1989]). Indeed, the level of resolution
was increased, but at the cost of a lower S/N (which was partially attributed to less
attenuation of the ground roll).
In some cases, the ground roll energy may not be travelling in the line of the
rec fiver arrays. For example, changes in topography or shallow features can result
in the scattering of surface energy from off the line (Martel et al. [1977]). Another
possibility is that energy can arrive obliquely to the line of the arrays when the
seismic line is very crooked, as is the case with KSZ data. Such broadside ground roll
energy has a greater apparent spatial wavelength and so may be attenuated less by
the array. Moreover, it will have a higher apparent velocity on a shot gather, which
can make attenuating this energy through velocity filtering very difficult (see section
3.3.2). To avoid such difficulties, this energy should in principle be attenuated in the
field using three dimensional arrays; however, this can be impractical as it involves
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 3.14 Receiver array response (from Morse and Hildebrandt [1989]). The horizontal axis is wavenumber, fc, with nodes at multiples of where L is the receiver array length.
F igure 3.15 Wavefront geometry for energy arriving from different depths. On the left, shallow reflection energy has a short spatial wavelength; on the right, deep reflection energy has a long spatial wavelength.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Chapter 3 Reprocessing of Line 12A 25
much greater cost.
Figure 3.8 shows that the ground roll energy for line 12A has wavenumber
between 7.0 X 10“3 and 1.4 x 10-2 m -1 . In order to attenuate this, the receiver
array length, L, would have to be at least 70 m. However, the purpose of this line
was to acquire high resolution shallow information, which prompted the selection
of a 20 m receiver array, which passes wavenumbers below 2.5 x 10-2 m-1 , and so
passes significant ground roll energy.
It is not enough to be concerned only with the length of the receiver arrays;
the shot - receiver configuration (which determines the offset increment, or spatial
sampling, within CMP gathers) is also im portant. The combined response of the
field arrays and the spatial sampling within the CMP gathers is known as the
stackarray response. It is described by Anstey [1986] and Morse and Hildebrandt
[1989], with field examples in the latter and in Prskalo [1988]. A brief summary of
stackarray theory will be given here; if the reader desires more information, then he
is referred to the cited works.
Just as the field array has a response which is defined by its length, L, there is
also a “CMP array” response which is defined by the spatial sampling rate within the
CMP gather, D, i. e., the offset increment (figure 3.16). The stackarray response is
simply the product of the receiver and CMP array responses. If the survey geometry
is designed so that D = L, then the resulting stackarray will pass only energy with
k a 0, as shown in figure 3.16 . In this case, the equivalent receiver array for a
CMP gather (i. e. the stackarray) is in fact continuous and of uniform weight, and
is said to satisfy the stackarray criterion. If a configuration results in a stackarray
which is not continuous and uniformly weighted, then it will pass significant energy
with k 0 (figures 3.17 and 3.18).
In practice, the application of the stackarray criterion has been found to im
prove data quality (Prskalo [1989]). However, crooked Lithoprobe lines result in
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
C M P-array s ta c k a rra y
1 4 4 • • • 4 4 = " I 4 4 • • • 4 . A .“ I 60m r
6 0 m field array re sp o n se CMP a rray re sp o n se s ta c k a rra y re sp o n se
.4 1 1 1 H(m'*) o .02 .033 (m*’) q
Figure 3.16 The top line shown 60 m receiver arrays with a 60 m spatial sampling within a CMP gather, resulting in a continuous, evenly weighted stackarray. The bottom line shows the corresponding wavenumber response. Note that the stackarray only passes energy with k ~ 0. From Morse and Hildebrandt [1989],
15 -e lem en t CMP a rra y s ta c k a rra y
* j ^ 4 . . . ^ ^ ... ^ ^
30m fiald a r ray re sp o n se CMP a rra y re sp o n a e s ta c k a rra y re sp o n se
Figure 3.17 As in figure 3.8, but with a 30 m receiver array, resulting in a discontinuous stackarray. The stackarray passes energy with k ^ 0. From Morse and Hildebrandt [1989],
C M P-array
4 4 4 . . . ' 4 4 =
-1 60m r
00m field a r ra y re sp o n se CMP a rray re sp o n ae s ta c k a rra y re sp o n se
L , _ 4 i . 1 1 H . ______<mM) 0 .02 .033 (m*1) o
F igure 3.18 As in figure 3.8, but with a 90 m receiver array, resulting in a continuous but unevenly weighted stackarray. The stackarray passes energy with k ^ 0. From Morse and Hildebrandt [1989],
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Chapter 3 Reprocessing o f Line 12A 26
uneven spatial sampling within CMP gathers, thus complicating the stackarray
theory. Nevertheless, a brief analysis of line 12A was made.
For line 12A, the offset increment, D, is 40 m whereas the receiver array length
is 20 m. Therefore, the stackarray is discontinuous, having the response shown in
figure 3.19. Note that the danger of this stackarray configuration is that any noise
with k ~ 0.05 m -1 will not be attenuated after stacking. However, the ground roll
energy, with wavenumber between 7.0 x 10~3 and 1.4 X 10~2 m-1 , is attenuated by
40 dB after stacking even if no other attem pt is made to attenuate it. Nevertheless,
reflection energy within the AGC shadow is still of very low amplitude, so only
allowing the stackarray to attenuate the ground roll is not enough. Other methods,
such as those discussed in section 3.3.2 should be attempted.
3 .4 V ib rator N o ise
3.4-1 Attenuation
The near offset coherent noise illustrated in figure 3.20 is termed “vibrator
noise” due to its apparent spatial relation to the vibrators. This noise has linear
moveout equal to that of the ground roll, and is dominated by low frequency en
ergy (20 - 35 Hz) when compared with uncontaminated records (figure 3.4). The
magnitude of this noise is variable from one shot to another, often contaminating
offsets as great as 800 metres.
The muting of the near offset traces did not result in improvement of reflection
quality, which suggests that these noisy traces are attenuated through stacking.
However, reflection energy on these traces has been reduced in amplitude by the
application of an AGC, and so it does not contribute to the stack. Thus, it would
be better if reflection energy were enhanced before AGC.
with permission of the copyright owner. Further reproduction prohibited without permission.
AM
PLIT
UD
E
WAVENUMBER ( l /m )
F igure 3.19 Stackarray response for line 12A. Computed from a receiver array with 12 elements at a sp u in g of 1.67 m, and from a CMP array, 120 fold with an offset increment of 40 m.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
TRACE 50 100
0.0 s
F igure 3.20(a) Near offset, vibrator related noise. TVace spacing is 20 m. The vibrators were located between traces 60 and 61.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
o<s o
siaaiDaa
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figu
re
3.20
(b)
Am
plitu
de
spec
trum
of
the
vibr
ator
no
ise.
Chapter 3 Reprocessing o f Line 12A 27
Velocity filtering was not a viable solution, due both to its limited success and
the large computer effort required. Since the vibrator noise is a “ghost” image of
the ground roll, applying time shifts so that the ground roll is horizontal on a shot
record will also result in horizontal vibrator noise. Thus, the multiple trace median
filtering method discussed in section 3.3.2 would attenuate this noise while passing
reflection energy.
Since the vibrator noise is of predominantly low frequency, the spectral equal
isation applied in section 3.1 results in an improved S/N as is shown in figure 3.21,
although there is still some contamination by the noise. The improved quality of
the section after spectral equalisation was discussed in section 3.1, and shown in
figure 3.2 .
S.4-2 Causes of Vibrator Noise
Since vibrator noise was such a prevalent problem throughout the KSZ data, it
is relevant to examine the possible causes of this noise. It is apparently an echo of the
high amplitude ground roll, so it is likely an artifact of the cross-correlation process.
Seriff and Kim [1970] pointed out that harmonic distortion (i. e. of higher frequency
than the fundamental) early in the sweep correlates well with the fundamental
frequency later in the pilot sweep if a downsweep is used (as was customary in the
1960s). This results in high amplitudes at positive time lags in the cross-correlation
function between the distorted sweep and the theoretical pilot sweep. Thus, the
correlation process will produce ghost energy following high amplitude energy such
as the first arrivals or ground roll.
The use of an upsweep rather than a downsweep results in sidelobes at negative
time lags. Thus, the correlation ghosts of a reflection will be superposed on earlier,
stronger reflection energy, so signal quality does not deteriorate appreciably in the
presence of harmonic distortion. However, now the presence of low frequency rather
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
TRACE 50 100
rs-'li'-
0.0 8
1.0
2.0
F igure 3.21(a) Shot gather contaminated by vibrator noise, before spectral equalisar tion.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
TRACE 50 100
VS*****' >. "-<<v'<.--i»
0.0 8
1.0
2.0
Figure 3.21(b) Shot gather contaminated by vibrator noise, after spectral equalisation.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
C h a p ters Reprocessing of Line 12A 28
than high frequency distortion results in low frequency sidelobes at positive time
lags. This suggests that the KSZ data may have been subject to low frequency
distortion in the emitted vibrator signal.
By what mechanism can low frequency distortion be produced? Martin and
W hite [1989] examine the occurrence of noise with a very similar appearance which
was found to have been caused by the loss of phase control in one of the vibrators.
This problem was caused by hardware failure in the vibrator, and resulted in low
frequency subharmonic or random noise. Vibrators in the KSZ transect did have
phase control problems (sometimes resulting in only three vibrators functioning
during the regional surveys). However, there was no obvious connection between
the amount of phase error and severity of vibrator noise. Furthermore, because this
type of noise is so common in vibroseis data (Billings [1990]), it is unlikely that it
is exclusively caused by hardware failure, but rather is more fundamental.
Uncorrelated vibroseis data acquired by Shell Canada Ltd. in 1984 and analyzed
by the author confirm that prominent subharmonic energy can be produced by the
vibrators which results in correlation noise. Amplitude spectra of progressive time
intervals of an uncorrelated shot record (figure 3.22) show that this energy remains
at half of the fundamental frequency as the latter varies between roughly 60 and
70 hertz. After correlation, this noise has an appearance similar to that of line 12A
data (figure 3.23).
Many physical systems generate harmonic energy, but it is far more unusual
for subharmonic energy to be generated. W hat mechanism might be involved in
the case of vibrators? One possibility is that the vibrator pads decouple from the
ground over a range of frequencies comparable to the natural resonant frequency
of the near surface. This could produce subharmonic energy in two ways: 1) the
pad may bounce every second cycle, or 2) the pad may rock back and forth, being
tilted in one direction during one cycle, then in the other direction during the next
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0 10 20 30 40 SO 60 70 80 90 100Frequency
5
4e■O
I 21
0Frequency----------
20 30 SO 60 Frequency
80 90 100
1.6 1.4-
. 1.2 - % 1.0 -
S 0.8- go.e-
0.4-0 .2 -
0.0100805030 40
Figure 3.22 Amplitude spectra from uncorrelated vibroseis data collected by Shell Canada Ltd. in 1984. Spectra are of progressively later 1 s data windows, with the sweep frequency marked by an arrow. Subharmonic energy is generated when the sweep is between 55 and 70 Hz.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1.4-1.2 -
• I.O- *o2 0 .8 -
’Eo.e-l 0.4-
0 .2 -
0 .0 -20 30 40 100Frequency
2.0
1.5-
3 l.o-
0.5-
0.020 30 50 100
Frequency2.0
1.5-
3 1.0-
0.5-
0.020 30 40 80 90 100
Frequency
1.5-
§ 1 0 -
0.5
0.0 90 1008050 603020
Figure 3.22 (Continued)
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8 - 9 s
9 - 10 s
2.5
2.0 -
1.0 -
0.5-
0.080 00 10040 5020 60
Frequency
100Frequency
3.0
2.5-
0.5-
0.020 30n in 40 SO 60 70 80 90 100
Figure 3.22 (Cc.itinued)
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Reproduced
with perm
ission of the
copyright ow
ner. Further
reproduction prohibited
without
permission.
SHT
J R<P
0 .5
1 .0
1 .5
2. 0
2 .5
TIME
IN
SEC0NDS
SHTx ao to
4 .5
5 .0
5 .5
^SS^SSSSSs&C
mmsSS00^$0 S M p W i i p l ®
Figure 3.23 Vibroseis record acquired by Shell in 1984 showing timilar vibrator noise to that present in the Kapuskasing data.
woz
onm
w
2—
ns
Chapter 3 Reprocessing o f Line 1ZA 29
cycle (Lansley [1988]). Such a possible resonance was visible when the pads were
observed during acquisition of the data in 1987 (West [1990]).
The Shell data were acquired using vibrators which did not use ground force
control, whereas the vibrators in the KSZ transect did. D ata acquired by Shell in
1990 using ground force control show similar vibrator noise. The cause of this noise
has not yet been determined, but is known not to be subharmonic distortion. This
does not rule out the possibility of subharmonic distortion in KSZ data; it only
demonstrates that there are different mechanisms for producing such noise. The
cause of this noise is a topic worthy of future investigation. If it is not possible to
record uncorrelated data in the field so that such distortion can be removed before
further processing, then uncorrelated data should be recorded at least in a region
affected by this noise for later examination to determine possible causes.
3.5 S ta tic C orrection s
3.5.1 Elevation Statics
The elevation static correction, teiev, is given in Yilmaz [198'. j as:
2 Ed — (E s + Er)*eicv —
Vb
where Ed , Es , and Er are the elevations of the datum, shot, and receiver respec
tively, and Vb is the velocity of the bedrock (see figure 3.24). This formula for t eje„
assumes that that the raypaths are vertical near the surface and that the shallow ve
locity structure is uniform along the line. The first assumption is not unreasonable
because the depth of penetration of the seismic rays is greater than the recording
spread length for most of the seismic section. For line 12A, the maximum offset
was 3.6 km; a ray arriving at 45° at this offset will hav ’ been reflected at a depth
of 1.8 km, which corresponds to a travel time of 0.85 s assuming a typical bedrock
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RECEIVER
SHOT
D A TUM
Figure 3.24 Elevation static correction.
3.6 km
SHOT RECEIVER A R R A Y
1.8 km
v = 6 km /sec
t =0.85 sec
Figure 3.25 Depth of penetration for a seismic ray incident at 45° at the maximum offset.
R e p ro d u c e d with perm iss ion of th e copyright ow ner. F u r th e r reproduction prohibited without perm iss ion .
Chapter 3 Reprocessing o f Line 12A 30
velocity of 6 km /s (see figure 3.25). Only for smaller travel times might one ex
pect seismic energy to deviate from the vertical by at least 45 degrees. Moreover,
the weathered, near-surface bedrock has lower velocities them the deep unweathered
bedrock, causing the seismic energy to be refracted towards the vertical. So, a static
correction of 30 ms determined assuming vertical raypaths for a reflector at 0.35 s
will be underestimated by less than 4 ms. Where this assumption is not valid, such
as when extremely large maximum offsets are used, there is no longer one single
time correction for the entire trace. In this case, one can determine time corrections
which Eire dependent on the ray parameter p (related to the angle of incidence), and
apply the corrections in the r-p domain. For example, Wenzel [1988] applied such
a method to DEKORP data collected in the Rhinegraben region in West Germany
with an offset range of 66 to 82 kilometres.
Although the verticeil raypath Eissumption is reasonable, the assumption that
the near-surface velocity function is laterally homogeneous can be significantly in
error. There is a layer of glacial till of variable thickness and of very low velocity
overlying the bedrock (Haeni [1986] found velocities for glacial till ranging from 0.3
to 2.4 km/s , whereas the bedrock velocity was determined from the first arrivals
to be approximately 6 km /s in the Abitibi). Thus, there will be an additional
time delay for the energy to travel through this overburden, with this delay being
proportional to the thickness of the low velocity cover. In order to determine this
additional delay, one must perform a refraction static emalysis.
3.5.2 Refraction Statics
There are a number of ways in which refraction static corrections can be deter
mined (see Russell [1989] for a review of a variety of methods); all of them involve
“picking” the first arrivals on shot gathers, then determining the static corrections
from these travel times. The method available in the DISCO processing package
involves the inversion of the first arrival times to determine a velocity model of the
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Chapter 3 Reprocessing o f Line 12A 31
near-surface, then calculating the corrections from this model.
I t Picking of the first arrivals
The picks made with an automatic picker during the preliminary processing
were found to be of poor quality, so new first break picks were made using the DISCO
automatic picker. This routine computes the envelope of the seismic data over a
time gate centred on a user-supplied estimate. The first break was picked where the
envelope reached 50% of the peak amplitude within this time gate. Although the
vibroseis wavelet is zero phase and therefore should in principle be picked on the
positive peak, the picking routine performed poorly if one picked the maximum value
of the envelope. This difficulty arises because the first arrival energy reverberates
between the surface and bedrock-overburden interface resulting in an extended first
arrival wavetrain. The envelope of this energy has a very flat peak, so that there is
ambiguity in selecting the time with the maximum value. Picking the wrong phase
of the wavelet is not critical — it is the relative pick times which axe important.
The relative static corrections will be accurate provided that the same phase of the
wavelet is picked on all traces.
The process of picking arrivals is computer intensive (picking all shot records
would take 24 hours of CPU time, which corresponds to at least four days of elapsed
time). Thus, it was decided that not all offsets from all shots would be picked since
the additional redundancy did not significantly change the results of the inversion
during testing. For this reason, picks were only made for data in the offset range
500 to 1340 metres (corresponding to traces recorded by only one DFS-V recorder,
which simplified the processing considerably). For offsets of less than 500 metres,
the data are characterized by high amplitude precursors which result in inaccurate
picks. At far offsets, there are two reasons for disregarding the first breaks: 1)
the data have lower S/N, and 2) the refracted energy penetrates more deeply as
the offset is increased, so at the farthest offsets, it may be sampling bedrock of a
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Chapter 3 Reprocessing o f Line 12A 32
greater velocity (Mayrand et al. [1987]). By using the middle offsets, only the most
accurate picks are retained. Picks were made only for every sixth shot, resulting in
an average of twelve picks per station for use in the inversion routine.
In order to quantify the reliability of the picks, the reciprocal time errors were
determined as in Mayrand et al. [1987]. These errors are the differences in pick
times for reciprocal shot-receiver configurations. A histogram of errors for all of
the picks is shown in figure 3.26(a). From the total of 1526 pairs of reversed picks,
the mean reciprocal error is -2.0 ms. and the variance is 50 ms2. Figure 3.26(b)
shows the histogram of errors if one ignores the picks which were rejected by the
inversion routine as being unreliable. The mean is -2.0 ms, and the variance is 19
ms2 for 1460 picks. Thus, the mean error is only one sample, with roughly 80% of
the reciprocal picks being within two samples of each other.
21 Inversion of the first arrivals
The second step in the refraction static method is to determine the static
corrections from the first arrival times. The refraction static inversion routine from
the DISCO seismic processing package utilizes the delay time method as described
by Barry [1967], modified so as to determine a velocity model iteratively. From
this model, the static corrections are then calculated. At each iteration, a bedrock
velocity is fitted to the first arrival times; any picks which fall more than a specified
tolerance away from this curve are rejected, thus ensuring that spurious picks do
not contaminate the results. For a description of the method, see the DISCO User’s
Manual.
R ather than requiring an initial velocity model, the DISCO inversion routine
requires the user to specify the overburden velocity (1.6 km /s) and the upper and
lower bounds on the bedrock velocity (5.3 to 6.8 km/s). Unlike many other inversion
routines, the DISCO routine allows for only one weathering layer.
W hat is the validity of the assumption that there is only one weathering layer?
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Num
ber
of Pa
irs
Num
ber
of Pa
irs
400
200 -
-8 0 -4 0 0 40 80Reciprocal Error (m s)
400
200 -
-8 0 -4 0 0 40 80Reciprocal Error (m s)
Figure 3.26 Histogram of reciprocal pick errors (a ) all picks (b ) not including picks rejected in the inversion routine.
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Chapter 3 Reprocessing o f Line 12A 33
In reality, one would expect to find a layer of very low velocity glacial till (less than
2 km/s) overlying weathered, fractured bedrock (roughly 5 km/s). This weathered
bedrock would grade downwards into unweathered, higher velocity bedrock (roughly
6 km/s). Thus, one would expect that a more reasonable model would include at
least two low velocity layers. However, the first breaks in the range of offsets which
were picked (500 to 1340 metres) generally can be approximated by one straight
line, which suggests that a model of one layer over bedrock is satisfactory (see figure
3.27).
The resulting near-surface velocity model and static corrections are plotted in
figure 3.28 . The best method of testing whether the inversion routine performed
well is to make stacked sections with and without the corrections being applied
(see figure 3.29). The overall improvement shown after the application of refraction
statics is clear.
3.5.3 Surface Consistent Residual Statics
In the days before the digital recording of seismic data, refraction static cor
rections were commonly believed to be the complete solution to the statics problem
(Russell [1989]). In reality, the refraction static method has an inherent limitation in
that the traveltimes of the refracted first arrivals reflect the average velocity struc
ture along the entire raypatfi, thus limiting the i ̂ solution of the statics solution.
Moreover, the earth model used (one constant velocity layer over laterally vary
ing bedrock) is admittedly simplified. Any complexities in the velocity structure
(e. g. varying overburden composition and water table levels resulting in irregular
velocity variation in the overburden, and vertical velocity variation in the bedrock)
will result in complexities in the raypaths, deviations from the vertical raypath
assumption, and inaccurate static corrections.
In the early 1970s, a new method of determining surface consistent residual
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Reproduced
with perm
ission of the
copyright ow
ner. Further
reproduction prohibited
without
permission.
OFFSET a t/1 t/l ( j \ 0 ) 0 ) ( J ) 0 ) O ) « < J < J « > J o J s J 0 0 0 Q Q ) 0 0 C D ( 0 ( 0 ( 0 ( 0 ( 0 t*k t a M t B t , a * M M ^ M M M M M W Wo i ' j C b — Cj i / i ' - j aD — o)C/i * ^ cd — C 5 c n - g i D — o j t / i - j o o s c a s s Q ^ — — — r u r o / u r v j r uu ) C i ) n ) ^ s ^ ^ ^ u ) 9 s s s ) M ^ t u ^ i \ i > u i u ^ ^ s a ) B n j & o ) ( O K O ) ( / i s i ( o ^ u ) u t > j u )
RED-STBTGO CO GO OO QO 03U\ cn o) 5) oi ct)od to s n j u) ^ u i c n N i c o u ) S H i \ j o ) ^ t n o ) s j a ) ( o c a ^ i u u ^ u i o ] s j c o ( o
200
400
cnz
LU
Figure 3.27 First arrival picks are marked as x . The plain, straight line is the initial estimate used in the picking routine. For the offsets used in the inversion routine, one straight line fits the fimt arrivals very well, indicating that the assumption of one overburden layer over a half-space bedrock is reasonable.
Reproduced
with perm
ission of the
copyright ow
ner. Further
reproduction prohibited
without
permission.
QU JCD
200 200OVERBURDEN
t E 100 1 0 0BEDROCK
30f e l 20
3020
6250 6250
6000 6000
5750 5750
550055001 1 0 0 1200 1300
STATION1400 1500
Figure 3.28 Near surface velocity model as determined from the inversion routine. Param eters for the inversion routine are as follows: overburden velocity = 1.6 km/s; allowable bedrock velocity = 5.3 km /s to 6.8 km/s; bedrock velocity model is smoothed over 50 stations; delay times are smoothed over 5 stations; 3 iterations in the inversion routine.
900
1000
11
00
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Figu
re
3.29
(a)
Stac
ked
sect
ion
with
no sta
tics
appl
ied.
900
1000
11
00
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figu
re
3.29
(b)
Stac
ked
secti
on
with
with
elev
atio
n st
atic
s ap
plie
d.
Q.>
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Chapter 3 Reprocessing o f Line 1ZA 34
statics was developed (Taner et al. [1974], Wiggins et al. [1976]). This method,
which is used in the DISCO processing package, is non-deterministic in that the
corrections are determined directly from the data so that reflectors are aligned
within a CMP gather. It has the capability of resolving statics with a wavelength
less than one receiver spread, complementing the refraction static solution. The
following outline of the method is summarised from Taner et al. [1974].
Each trace in an NMO-corrected CMP gather is correlated with a pilot trace
(the stacked CMP trace); the time shift which aligns the two is the time lag of the
peak in the cross-correlation function. The time shift Ty (from shot i and receiver
j ) can be written as follows:
Tij = S i + R j + C k + M kXf j
where Si is the shot static at station i\ R j is the receiver static at j; Ck is the
structural static at CMP k\ Mk is the residual normal moveout (RNMO) correction
at CMP k (the NMO hyperbola is approximated as a parabola); and X , j is the offset.
Si and R j are surface consistent (i. e. the shot correction is the same for all traces
from a particular shot, and likewise for the receiver correction). Ck and M* are
subsurface consistent (the same for the entire CMP gather). A large, overdetermined
set of equations for all of the time shifts is solved to decompose these shifts into the
various time correction terms.
The structural term is arbitrary since all traces in a CMP may be time shifted
by Ck without affecting the alignment of traces within the CMP. Therefore, Ck -my
be constrained to be zero (ibid.). The other parameters were selected empirically
based on two criteria: the quality of reflections on the stacked section, and the
stability of the static values (the solution is considered to be stable if the static cor
rections do not vary wildly between neighbouring stations and between iterations).
Best results were obtained when Mk wer constrained to be zero, and when the
shot and receiver statics were allowed to vary independently of one another. Since
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Chapter 3 Reprocessing o f Line ISA 35
the sources and the receivers were both located on the surface, one might expect
that there would be greater validity in constraining the source and receiver statics
to be equal; however, the vibrator and geophone arrays were not coincident, and
the quality of the stacked section is somewhat better without this constraint. Our
confidence in the solution is bolstered when one examines figure 3.30, which shows
the similarity of restilts when different sets of CMP gathers are used as input.
The maximum allowable static was chosen to be 8 ms. which is one half of a
wavelength for 60 Hz (approximately the highest frequency used in reprocessing).
When the residual statics were allowed to be larger (16 ms), the solution became
unstable for parts of the line. For the parts where the solution remained stable,
static values varied over approximately 8 ms, justifying this choice of a maximum
time shift. The CMP bin size was also found to be crucial for the stability of the
solution, with results becoming unstable when bins were more than 800 m wide.
The improvement in the stacked section after residual statics is remarkable (see
figure 3.31). F.cfiection quality is markedly improved both outside and within the
correlation window which was used to determine the statics. Combined with the
stability of the solution, this is strong evidence that the statics problem has been
attacked with great success.
The reprocessing or the high resolution line 14A by Milkereit of the Geological
Survey of Canada has revealed that there was an 8 ms time delay on that line
between the datasets recorded by the two DFS-V acquisition systems, each of which
recorded half of the recording spread. There was probably a similar delay for line
12A. Not being surface con-:stent, and being equal to the maximum allowable time
correction, this cannot be corrected using the residual static method. Unfortunately,
this face came to light too late to be corrected during processing.
It is clear that the determination of an accurate static correction solution was
crucial to the processing of the data. Recent developments in static methods show
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Stat
ion
720
700
680
660
640
620
600
58015 -1 0 -5 0 5
Static Correction (m s)10
Figure 3.30 Plots of residual receiver static corre' tions determined using separate groups of CMP gathers. Each group contained 35 gathers, with an overlap between the two groups of 5. Note the similarity in the two solutions despite their being computed using different input data.
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900
1000
11
00
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Figu
re
3.31
Slac
ked
secti
on
afte
r ap
plic
atio
n of
surf
ace-
cons
iste
nt
resi
dual
sta
tics.
Chapter 3 Reprocessing o f Line ISA 36
potential for even greater data improvement in the future. A travel time tomo
graphic inversion method was used with success in reprocessing line 14A (Milkereit
[1990b]). Also, methods which use a random searching technique to select the set
of corrections which maximises the power of the stacked traces have been developed
and implemented with great success (Ronen and Claerbout [1985], Rothman [19S6],
and Dahl-Jensen [1989]).
\
3 .6 V elocity A n alysis
Due to the high velocities and long travel times in Lithoprobe data, the normal
moveout (NMO) is small for most of the data. This was illustrated using synthetic
CMP gathers containing a number of reflections, which were stacked using a suite
of velocity functions (figure 3.32). These results show that reflections below 2 s are
insensitive to velocity over a broad range, so only a cursory velocity analysis need
be made for the deep data. The same figure shows the need for a careful velocity
analysis above 2 s. Reprocessing of the data should result in much better velocity
control for the shallow data than during the preliminary processing.
There are two primary methods for the selection of stacking velocity: 1) the
examination of NMO corrected CMP gathers using a range of velocity functions,
with the aid of plots of coherency versus velocity and time (Yilmaz [1987]), and
2 ) the examination of stacked sections which were made using a suite of velocity
functions. Due to the poor pre-stack S/N, the second method is preferred.
Some surprising results were found. In the south half of the line, shallow (< 1 s)
dipping reflections stacked optimally at very high stacking velocities (7 to 7.5 km /s)
— see figure 3.33 . This is more likely a result of dip moveout effects rather than
true high seismic velocity since first arrivals indicate a velocity of approximately 6
km /s at the surface. A comparison of the dip moveout (DMO) and normal moveout
(NMO) equations shows that the increase in stacking velocity is one of the effects
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VE
LO
CIT
Y O o o Oo If) o if)
<0
o<0
N<P
If)to
N<0
1 1 1 1
£ O o o Ow O If) o if)o M If) N
if) If) K) If)
O*0M
OoIf)
0.0 8
1.0
2 .0
3.0
4.0
Figure 3.32 Sensitivity to NMO velocity shown using synthetic CMP gathers. Velocities labelled at the top indicate the velocities at 0 s and a t 4 s respectively, with a linear increase between the two. The correct velocity function is 5750 to 6750 m/s.
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300
400
500
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Figu
re
3.33
(a)
Shall
ow
dipp
ing
refle
ctio
n en
ergy
sta
cked
at
6 k
m/s
(the
bedr
ock
velo
city
at
the
surf
ace)
.
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with perm
ission of the
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ner. Further
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without
permission.
VP 300 400 500
a0 .0 s
0.5
1.0
1.5Figure 3.33(b) Shallow dipping reflections stacked at 7 km /s, much higher than the bedrock velocity of 6 km /s . Note tha t reflections within the box are imaged better than in figures 3.31(a). This indicates tha t DMO effects are present in the data.
Chapter 3 Reprocessing o f Line ISA 37
of dipping reflectors:
.2N M O :
R M S
D M O : t,i2c o r r2 x 2Cos26
* R M S
where t and t corr are the measured and corrected travel times respectively, x is the
offset, Vrm, is the root-mean-square velocity, and the reflector is dipping at an angle
0. Thus, the effective NMO velocity for a reflector of dip 6 is > Vr m s • The
dips in figure 3.33(b), 25° to 30°, imply stacking velocities of 6.6 to 6.9 km /s for
a bedrock velocity of 6 km/s. This is consistent with the stacking velocity used in
the figure (7.0 km/s).
The presence of a dipping reflector also causes the reflection point to move
updip as the offset is increased within a CMP gather, which results in reflection
point smearing (see figure 3.34). In order to remove this effect, one must apply
full pre-stack migration which is prohibitively computer intensive, or pre-stack par
tial migration, otherwise known as DMO removal. Considerable advances in the
efficiency of DMO removal routines have been made during the 1980s, increasing
their practicality. Unfortunately, however, no working DMO program was available
for use in the DISCO software package. The high stacking velocities are a strong
indication that DMO effects are present, suggesting that a DMO routine be made
available for the future processing.
3.7 V ariability o f R eflection E n erg y w ith O ffset
If reflector dips are small and there is little lateral velocity variation, then all
traces within a CMP gather should be sampling the same subsurface geology. Thus,
reflection energy should, be consistent across a CMF gather; however, in reality, there
is great variation (figure 3.35). Although the amplitude of the reflection energy
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Reproduced
with perm
ission of the
copyright ow
ner. Further
reproduction prohibited
without
permission.
COMMONSHOT SHOT SHOT MID RECEIVER RECEIVER RECEIVER
POINT 2 3
F igure 3.34 Reflection point smearing for a clipping reflector. The 3 shot - receiver pairs belong to one CMP gather. As the <. flset is increased (from pair 1 to pair 3), the reflection point moves updip.
OFFSET (m)
500 1000
i0 .0 s
1.0
2.0
3.0
Figure 3.35 A portion of a CMP gather. Note the variation of reflection energy with offset.
R e p ro d u c e d with perm iss ion of th e copyright ow ner. F u r th e r reproduction prohibited without perm iss ion .
Chapter 3 Reprocessing o f Line 12A 38
appears to vary with offset, this could result to some extent when an AGC is applied
if the noise levels vary with time and offset. A controlled gain function should be
applied in order to describe the amplitude variation rigorously. Nevertheless, in
some areas of the figure, there is true amplitude variation (for example, where two
reflections are close in time and the amplitudes of the two vary independently with
offset). Furthermore, the phase of the reflections varies with offset, resulting in
different character and in different time shifts which cannot be described as a static
correction. Although these time shifts are small, they are significant, resulting in
much of the signal being attenuated through stacking. Post-stack reflection energy
is often comprised of energy from only a limited number of traces on CMP gathers.
There are three possible reasons fcr this variability of signal. 1) As a result of
complex interlayering of rock types in the vicinity of the reflection point, reflecting
energy may constructively or destructively interfere and scatter to varying degrees
as the angle of incidence (i.e., offset) is varied. This effect is more likely to play a
significant role for shallow data since the angle of incidence does not vary greatly
for deep data over the range of offsets used. 2) Due to the complexity of the geol
ogy, raypaths may vary considerably, with reflection energy from different reflection
points arriving at similar times. For example, undulations in the reflecting horizons
perpendicular t j the line may result in reflection energy being returned at similar
times lrom very different areas. This may result in time delays and interference be
tween energy travelling along the different paths. It is also possible that reflection
energy from very different geological units could arrive at similar times (i. e. out of
line reflections), which may result in complex interference patterns. 3) Relatively
localized, shallow velocity inhomogeneities could result in significant raypath bend
ing for different surface loc?fions, resulting in reflection point wander within a CMP
gather. Interaction between these three factors in differing degrees could produce
the variability of reflection energy which is seen in the data.
Since reflection energy is stacking coherently only over a limited range of offsets,
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Chapter 3 Reprocessing o f Line 12A 39
it was felt that forming stacked sections for different offset ranges would allow
one to select ranges along the line where most of the coherent signal is present.
However, differences were not significant when offset ranges of 800 metres were used,
indicating that the scale of the variation is smaller than this. An examination of a
number of CMP gathers revealed that the reflection character within each gather
varies rapidly dong the line. Thus, careful, detailed editing would be required
in order to result in an improvement after stacking. Such a procedure would be
practical only if automated.
In recent years, advances in automated trace editing procedures have been
made (e. g. Mayrand and Milkereit [1988]). Such procedures concentrate on iden
tifying traces which are dominated by noise or noise bursts, with this noise being
characterised by anomalous amplitude levels. In the future, it may be worthwhile to
concentrate editing procedures on selecting data which contain consistent coherent
signal rather than simply editing out noise. One possible method would be to apply
a coherency filter to the CMP gathers. This type of filter is generally, applied to
stacked data in order to pass only energy which is coherent over a minimum trace
window and range of dip. This filter could be applied before stacking, passing only
coherent energy with zero dip. Another possibility would be to apply a Ivarhunen -
Loeve filter, which also passes only coherent energy but is very computationally ex
pensive. Advances in the implementation of such methods could be of considerable
benefit to crustal seismic profiling.
3.8 Inclusion o f H igh Frequencies in th e P rocessin g
When the data were resampled to 4 ms (section 3.1), was any useful reflection
energy lost? Having determined a new processing scheme for the data, a portion
of the line was selected to test for the presence of high frequency reflection energy.
This portion of the line included shallow reflections, which would be most likely to
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Chapter 3 Reprocessing of Line 12A 40
have retained high frequency components.
The data were spectrally equalised over the full bandwidth (20 to 120 Hz)
with the 2 ms sampling rate being retained. Only data with offsets up to 1340 m
(recorded by one DFS-V system) were processed, both in order to reduce processing
time and to reduce sensitivity to velocities. Otherwise, the same reprocessing flow
was followed. Filter panels of the resulting stacked section show that there is no
shallow (< 1 s) coherent reflection energy above SO Hz and none above 100 Hz
elsewhere in the section (figure 3.36). However, the application of NMO corrections
to pre-stack data results in a significant downward frequency shift for shallow data
(a phenomenon known as NMO stretch). The shallowest energy on the stacked
section is at 400 ms. At most, this 80 Hz stacked energy would be the result of 94
Hz energy on uncorrected pre-stack data. NMO stretch is not appreciable for the
reflection energy below 1 s.
E ither frequency copmponents above 100 Hz are attenuated and scattered to
such an extent that the reflection energy has a very low S/N, or the processing
method is inadequate to image the high frequencies. For example, high frequency
reflections are extremely sensitive to inaccuracies in static corrections and NMO
velocities, and to presence of dip. The determination of a static correction solution
using more recent advanced methods, the removal of DMO effects, and the resulting
better control on NMO velocities may result in higher frequency reflection informa
tion. W hatever the reason, a substantial amouni of the source effort (20% of the
sweep time) did not contribute significantly to the imaging of reflections. In future
surveys, the lowering of the upper sweep limit to 100 Hz should be considered.
3 .9 S tacked S ection s
The data were initially processed to obtain a preliminary stack (plate 1) using
the flowchart shown in figure 3.37 . The data were reprocessed to give a new
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Reproduced
with perm
ission of the
copyright ow
ner. Further
reproduction prohibited
without
permission.
20 - 35 HZ 35 - 50 HZ
Figure 3.36 Filter panels of a stacked section where frequencies up to 120 Hz have been retained and equalised. Note tha t there is little reflection energy above 80 Hz. Bandwidths used in the spectral equalisation were 20-30 (0.5 weight), 35—45, 50-60, 65-80, 85-100, 105-120 (each 1.0 weight).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figu
re
3.36
(Con
tinue
d)
100
- 12
0 H
Z
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Figu
re
3.36
(Con
tinue
d)
Chapter 3 Reprocessing o f Line ISA 41
stacked section (plate 2) using the flowchart shown in figure 3.38 . A post-stack
coherency enhancement filter was applied to both sections. The section in plate
2 was then migrated using a finite difference method to give the section in plate
3. The improvements after reprocessing are substantial, and will be examined in
chapter 4.
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STACK OF ADJACENT SHOT GATHERS
AUTOMATIC GAIN CONTROL
CROOKED LIN E GEOMETRY
REFRACTION STATIC CORRECTIONS
C M P SORT
VELOCITY ANALYSIS
N M 0 CORRECTION
FIR ST BR]EAK MUTE
NON SURFACE - CONSISTENT RESIDUAL STATIC CORRECTION
Figure 3.37 Preliminary processing flowchart.
STAC
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%'*
*3
RESAMPLE DATA FROM 2 m s TO 4 m s
NEW CROOKED LINE GEOMETRY
SURFACE - CONSISTENT RESIDUAL STATIC CORRECTIONS
STACK
C M P SORT
REAPPLY MUTES
FIRST BRETK PJCKING
SPECTRAL EQUALISATION
AUTOMATIC GAIN CONTROL
REFRACTION STATIC CORRECTIONS
MUTE OF GROUND ROLL AND FIRST BREAKS
VELOCITY ANALYSIS TO COMPENSATE FOR DIP MOVEOUT
F igure 3.38 Reprocessing flowchart.
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Chapter 4
Conclusions
4.1 S u g g estio n s
The quality of the stacked section has been greatly improved after repro
cessing (see plates 1 and 2). The most crucial processes were spectral equalisation
and the determination of a complete static solution (refraction and surface consis
tent residual statics). Spectral equalisation was essential to the imaging of shallow
reflections as was shown in section 3.1 . A comparison of data below 3 s in plates 1
and 2 shows the marked improvement after the new static solution. These processes
must be applied to obtain an adequate signal to noise ratio before further testing
is done, e. g. velocity analysis, or stacks using limited offset ranges. Although the
improvement in plate 2 is notable, further improvement is possible. Throughout
chapter 3, inadequacies in the processing steps and suggestions for improvement
were stated, and are summarised here.
The two m ajor forms of coherent noise — ground roll and vibrator noise —
were not attenuated optimally. 1) The ground roll was surgically muted, thereby
also removing any reflection energy which may be present. 2) The vibrator noise
was partially attenuated through spectral equalisation; however, the S/N was still
too low on many traces to contribute reflection energy to the stack.
Velocity filtering showed some promise in attenuating the ground roll. An
improved routine may aid in extending some of the shallow reflections even closer
to the surface. Other possibilities involve the muting of this noise in the r-p domain,
and removing this noise using a multiple trace median filter (a technique used to
- 4 2 -
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Chapter 4 Conclusions 43
remove coherent energy in vertical seismic profiling data). Being a ghost image of
the ground roll, vibrator noise could also be removed through median filtering.
The problem of vibrator noise is common to much seismic data, both deep
crustal and exploration. Therefore, prevention, not merely attenuation, of this noise
would be of great benefit. However, the causes of this noise must be determined
first; to this end, steps should be taken in future surveys to record uncorrelated
data in an affected area for detailed examination.
The importance of an accurate static correction solution has been emphasized
in section 3.5 . Although data quality was greatly improved, further improvement
may still be possible. First of all, the 8 ms time shift between data recorded by the
two DFS-V systems is not surface consistent, and so should be removed before a
static solution is determined. Furthermore, the use of newer, more advanced static
correction methods, such as travel time tomographic inversion (Milkereit [1990b])
or random search optimisation (Dahl-Jensen [1989]), holds promise for future pro
cessing.
In thu southern half of line 12A, shallow reflections dipping at up to 30° on the
unmigrated section (assuming an average velocity of 6 km /s) were found to stack
optimally at abnormally high velocities, indicating that dip moveout effects are
significant. These shallow reflections are crucial to the interpretation of the section,
linking the surface geology with the subhcrizontal structures at depth. Thus, a
DMO removal routine should be made available for future processing of data from
the AGB.
Reflection energy varies strongly both in phase and amplitude character within
a CMP gather so that few traces contribute reflection energy to the stacked trace.
The development of an efficient method of passing only consistent energy with zero
dip in the NMO and DMO corrected CMP gathers (e. g. coherency enhancement,
Karhunen - Loeve filtering) could benefit the processing of crustal data.
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Chapter 4 Conclusions 44
Finally, no reflection energy above 100 Hz contributed to coherent reflection
images after stacking using the processing methods available for this thesis. How
ever, it is not clear whether this is a physical limitation due to strong attenuation in
the shallow bedrock and overburden, or whether it is simply a result of inadequate
processing methods.
4 .2 E va lu ation and In terp reta tion o f th e R ep ro cessed S ection
The preliminary and reprocessed stacked sections are shown as plates 1 and 2
respectively with the migrated, reprocessed section shown as plate 3. The improve
ment in data quality after reprocessing is striking. The preliminary section has
been interpreted by Green et al. [1990] and Jackson et al. [1990]. In the following
pages, the improvements after reprocessing will be evaluated, with some geological
interpretation being made based on these two citations.
Firstly, the prim ary goal — to image reflections in the upper I s — has been
achieved. In the southern half of the line (VP 100 to 900), much coherent, north
dipping reflection energy is visible (e. g. reflections A, B, and C). This will aid in
extending the known surface geology through the upper several kilometres of the
crust during interpretation. In the north part of the line (VP 900 to 1600), the
improvement in the shallow data is not so great as in the south. Most notably, no
dipping reflections are imaged clearly. The weaker shallow reflectivity in the north
may be due to the complex internal stratigraphy within the Blake River Group
(Dimroth et al. [1983]), which the northern portion of the line traverses. However,
this could also be due to higher noise levels. True am plitude processing must be
done in order to determine if this change in amplitude levels is real.
In addition to the notable improvement in the upper 1 s, data quality in much
of the rest of the seismic section has also been improved. Possible exceptions to
this are D and E, where there is a reduction in reflection energy and coherence
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Chapter 4 Conclusions 45
after reprocessing. The stacking velocity was picked by maximising the coherence
of reflection D, suggesting that the decline in quality is a result of inaccurate static
corrections rather than a poor choice of stacking velocity. The residual static so
lution was determined using a correlation window centred on reflections F and G.
Reflection D is imaged well where data centred on reflection G were used as input to
the residual, static routine; however, where the input data were centred on the more
steeply dipping tail of reflection F, there is a reduction in the quality of reflection
D.
The south half of line I2A is characterised by north dipping reflections in
the upper 5 s. After reprocessing, some of these shallow reflections can be traced
very near to the surface. For example, reflection A, dipping at roughly 25° on the
unmigrated section (assuming a velocity of 6 km /s) can be traced to 0.4 s, and
projects to the vicinity of the Crosby Sill, an ultramafic intrusion. This reflection
may be attributed to this intrusion, a nearby deformation zone, or the contact
between the Kinojevis and Blake River Groups (Jackson et al. [1990]). Reflection
B, dipping at 30°, projects to within the Kinojevis Group at the surface. Reflection
C is more gently dipping (20°), and projects to near the Larder Lake - Cadillac
Deformation Zone at the surface (although it is most likely not an image of the
deformation zone, which is commonly believed to be sub-vertical).
The most prominent mid-crustal reflection, F, has been interpreted as orig
inating from the base of the greenstone volcanic material (Jackson et al. [1990],
Green et al. [1990]). Whereas on the preliminary section, reflection F is laterally
intermittent, after reprocessing it forms a very coherent, more laterally continuous
reflection package. Furthermore, it is considerably thicker, which could either be
the result of extensive interlayering, or of cross line undulations in the reflecting
horizon. After migration, the geometry of F and G is consistent with a synclinal
structure for the base of the greenstone volcanic material.
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Chapter 4 Conclusions 46
The material below F and above 5 s may be comparable to the Wawa Domal
Gneiss Terrain in the KSZ, but Jackson et al. [1990] prefer the interpretation that
it is the Pontiac Metasedimentary Belt and/or its basement, over which the AGB
has been thrust from the north.
Below 5 s, there is a large amount of subhorizontal reflection energy along
with dipping diffraction energy. Reprocessing has resulted in improvement of S/N,
particularly from VPs 100 to 600, and 1200 to 1600. There is a marked increase
in lateral continuity, suggesting that there is more laterally extensive layering in
the lower part of the section than can be surmised from th,* preliminary section.
However, the presence of the diffraction energy suggests that there is still a large
amount of faulting and/or truncation of reflective layers. Based on the crustal model
developed from the studies of the KSZ (Percival et al. [19S9]). this material has been
interpreted to consist of layered mafic/felsic granulites and layered anorthosites
(Green et al. [1990]).
The reprocessing of the data produced very good results. The improved imaging
allows the surface geology to be extended to depth with greater confidence, and
should allow a more detailed interpretation of reflection zones at depth. It is clear
tha t substantial effort should be expended in processing the data which will be
acquired for the Abitibi - Grenville Transect, and the results illustrate the value of
detailed processing of deep crusted seismic data from Archaean cratons in general.
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Appendix
DISCO Job Decks
The job decks used in the reprocessing of the data are shown in the follow
ing pages. For information on the various parameters or on the operation of the
processes, see the DISCO User’s Manual. Where implementation was not straight
forward, a discussion is included.
A .l Surface G eo m etry D efin itio n
♦JOB ABITIBI 12A JAN SURFACE GEOMETRY♦CALL DUMIN♦♦*♦♦♦ D efin e surface lo c a t io n s and e le v a tio n s o f s ta t io n s♦♦♦CALL LINE STATIONSLOCN 101 LXY 59603S 5328926 293LOCN 102 LXY 596038 5328952 295LOCN 103 LXY 596041 5328972 296*♦ e tc .♦♦♦♦ D efine th e recording spreads♦♦♦♦ No gap p attern ( fo r r o l l -o n and r o l l - o f f )♦CALL PATTERN 240 YESPSTAT 11 1240 240♦♦♦♦ 17 s ta t io n gap, a t 181 to 197♦CALL PATTERN 17 YESPSTAT 1891 1180 180
- 4 7 -
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Appendix DISCO Job Decks 48
181 198240 257* *
♦♦ A ssign th e lo c a t io n s of th e shots♦♦♦CALL SOURCE 1 240SHOT 1 101 240 101THRU 66 1 0SHOT 67 168 240 101THRU 70 1 0SHOT 71 177 240 101THRU 183 1 0SHOTTHRU
1841462
2901
17
SHOT 1463 1569 240 1397THRU♦END
1530 1 0
A .2 S u b -su rface G eom etry D efin ition
One trace with an offset of 750 metres from every fourth shot was selected.
The mid point locations for each of these traces were linearly interpolated to define
the CMP line which would be used in sorting the data.
A .3 T race E d its
Traces with low S/N were chosen manually during preliminary processing from
plots of adjacently stacked shot gathers. These same trace edits were used during
the reprocessing.
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Appendix DISCO Job Decks 49
A .4 W rite G eo m etry In fo rm a tio n in to T race H ead ers
** Omit dead sh ots **♦CALL EDIT FFID SEQNOALL OMIT RANGE0 3 29 30 56 58 82110 111 127 129 141 142 168196 197 226 227 257 258 288319 320 343 351 381 382 412443 444 474 476 506 507 537568 569 599 600 615 617 632656 662 690 691 713 714 722753 7F1 784 785 816 817 832849 850 880 887 914 915 942946 947 977 978 1009 1010 10221040 1041 1072 1074 1103 1104 11251159 1160 1189 1196 1226 1227 12571288 1289 1320 1321 1352 1353 13691403 1404 1433 1446 1475 1476 15071539 1540 1571 1572 1603 1604 16191652 1653 1680 1681 1708 1709♦CALL EDIT FFID SEQNOALL OMIT NORANGE133 827 1044**** Write shot number in to tra ce headers**♦CALL HEADPUT SHOT STORE INTEGERINPUT FFID ALL** f i l e shotDATA 4 1DATA 28 25DATA 31 26DATA 55 50DATA 59 51DATA 81 73DATA 84 74DATA 109 99DATA 112 100DATA 126 114
83169289413538633723833943102611321258137715081627
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Appendix DISCO Job Decks 50
DATA 130 115DATA 132 117DATA 134 118DATA 140 ' 124DATA 143 125DATA 167 149DATA 170 150DATA 195 175DATA 198 176DATA 225 203DATA 228 204DATA 256 232DATA 259 233DATA 287 261DATA 290 262DATA 318 290DATA 321 291DATA 342 312DATA 352 313**** For th e north h a lf o f th e spread:DATA 383 344DATA 386 345**** For th e south h a lf o f th e spread:DATA 380 341DATA 383 342****DATA 411 370DATA 414 371DATA 442 399DATA 445 400DATA 473 428DATA 477 429DATA 505 457DATA 508 458DATA 536 486DATA 539 487DATA 567 515DATA 570 516DATA 598 544
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Appendix DISCO Job Decks 51
DATA 601 545DATA 614 558DATA 618 559DATA 631 572DATA 634 573DATA 655 594DATA 663 595DATA 689 621DATA 692 622DATA 712 642DATA 715 643DATA 721 649DATA 724 650DATA 752 678DATA 755 679DATA 783 707DATA 786 708DATA 815 737DATA 818 738DATA 826 746DATA 828 747DATA 831 750DATA 834 751DATA 848 765DATA 851 766DATA 879 794DATA 888 795DATA 913 820DATA 916 821DATA 941 846DATA 944 847DATA 945 848DATA 948 849DATA 976 877DATA 979 878DATA 1008 907DATA 1011 908DATA 1021 918DATA 1027 919DATA 1039 931DATA 1042 932DATA 1043 933
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Appendix DISCO Job Decks 52
DATA 1045 934DATA 1071 960DATA 1075 961DATA 1102 988DATA 1105 989DATA 1124 1008DATA 1133 1009DATA 1158 1034DATA 1161 1035DATA 1188 1062DATA 1197 1063DATA 1225 1091DATA 1228 1092DATA 1256 1120DATA 1259 1121DATA 1287 1149DATA 1290 1150DATA 1319 1179DATA 1322 1180DATA 1351 1209DATA 1354 1210**** For th e north h a lf o f th e spread: DATA 1368 1224**** For th e south h a lf o f th e spread:DATA 1363 1219DATA 1364 1221DATA 1367 1224****DATA 1378 1225DATA 1402 1249DATA 1405 1250DATA 1432 1277DATA 1447 1278DATA 1474 1305DATA 1477 1306DATA 1506 1335DATA 1509 1336DATA 1538 1365DATA 1541 1366
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Appendix DISCO Job Decks__________ 53
♦ ♦♦♦ For the north h a lf o f th e spread:DATA 1544 1369DATA 1546 1370♦ ♦♦♦ For th e south h a lf o f th e spread:DATA 1545 1370DATA 1547 1371♦ ♦* *
DATA 1570 1394DATA 1573 1395DATA 1602 1424DATA 1605 1425DATA 1618 1438DATA 1628 1439DATA 1651 1462DATA 1654 1463DATA 1679 1488DATA 1682 1489DATA 1707 1514DATA 1710 1515DATA 1725 1530♦ ♦
♦♦ For th e north h a lf o f th e spread ONLY, add 120 to th e channel ♦♦ number (to make channels 121 to 240)♦ ♦
’‘CALL HEADPUT CHAN ACCUM INTEGER INPUT FFID ALLDATA 1 120DATA 2000 120
♦♦ W rite geometry in form ation in to tr a c e headers ♦♦♦CALL PROFILE 15 400
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Appendix DISCO Job Decks 54
A .5 F irst A rrival P icks
ABITIBI 12A JAN♦JOB * *
** Read only north h a lf o f th e spread ♦ ♦
240
FTRS7 ARRIVAL PICKING
♦CALL GIN 1000 2DENSITY 6250REEL LF0811 126 161♦♦♦♦ P ick only fo r every s ix th♦♦♦CALL EDIT SHOT SEQN0ALL OMIT RANGE121 125 127 131145 149 151 155
133157
FFID I NCR SEGY
137161
139 143
♦♦ W rite shot numbers, channel numbers, and geometry inform ation in to ♦♦ tr a c e headers (se e s e c t io n A.4)**♦♦ Do not p ick f i r s t a r r iv a ls fo r n earest 500 m etres ♦ ♦
OFFSET OMIT
FFID2000
♦CALL EDITSEL 10 500♦♦♦ ♦
♦CALL FIRSTA SHOTESTIMAT 219♦♦ x l t l x20 0 5000TECH1 40 .5DISK PK.LINE♦END
RANGE
OFFSET 100
t2834
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Appendix DISCO Job Decks 55
A .6 R efraction S tatics
♦JOB♦CALL♦♦♦♦♦CALLSHIFTS♦ ♦
♦♦
VONEVZERO101♦ ♦
FIRSTAPRINT♦END
ABITIBIDUMIN
12A JAN REFRACTION STATICS
STATICG0
v(low )5300
1600
maxit e r3
0 0smooth
v(h igh) p o in ts6800 50
dtsmooth5
max timeerror15 STAT.LN
x(min)500
x(max)1340 PK.LINE
A .7 R esa m p le
♦JOB ABITIBI 12A JAN♦CALL GIN 8000 2DENSITY 6250REEL LF0809 4 79REEL LF0811 80 161REEL LF0813 162 237REEL LF0815 238 314REEL LF0817 315 400REEL LF0819 401 442REEL LF0821 445 520REEL LF0823 521 597REEL LF0825 598 684REEL LF0827 685 764REEL LF0829 765 844
RESAMPLE120 FFID INCR SEGY
♦ ♦
♦♦ Write shot numbers, channel numbers, and geometry in form ation in to ♦♦ tr a c e headers ♦ ♦
♦CALL RESAMP 4♦CALL TAP0UT 6250 RSM1NREEL L00773
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Appendix DISCO Job Decks 56
REEL L00772REEL L00771REEL L00770REEL L00769REEL L00768REEL L00767REEL L00766REEL L00765REEL L00764♦END
Note that the data from the two DFS-V recording systems were resampled
separately, and written to two tape sets (north and south halves). When being
sorted to CMP gathers, these two datasets Eire merged.
A .8 CMP Sort
♦JOB ABITIBI 12A JAN CMP SORT♦CALL TAPIN♦♦ north h a lfSET RSM1N♦♦ south h a lfSET RSM1S♦♦
♦CALL SORT 1000 10000MAJOR CDPMINOR OFFSET♦ ♦
♦CALL TAP0UT 6250 S0RT1REEL L00450REEL L00451REEL L00455REEL L00739REEL L00740REEL L00742REEL L00743REEL L00745REEL L00761REEL L00762REEL L00763
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Appendix DISCO Job Decks 57
REEL L00746REEL L00748REEL L00759REEL L00760♦END
A .9 Spectral Equalisation
♦JOB ABITIBI 12A JAN SPEQ ♦CALL DSKRD CDP.601.630 ♦ ♦
♦♦ Apply r e fr a c tio n s t a t ic s ♦ ♦
♦CALL HEADPUT REC-RFSTATTRI REC-STATSTAT-LN STATICG STATION♦CALL HEADPUT SHT-RFSTATTRI SHT-STATSTAT.LN STATICG STATION♦CALL STATICMULTISHT-RFSTAPPLYREC-RFSTAPPLY♦ ♦
♦♦ F ir s t a r r iv a l mute ♦♦
♦CALL MUTE CDP OFFSET 20 INT INTON 259♦ ♦ X t X t X t X
200 350 800 420 801 210 20002300 470 2900 700 4100 1160♦♦♦♦ Ground r o l l mute♦♦♦CALL MUTE CDP OFFSET 20 INT INTSURG 259♦ ♦ X t ( i ) t ( f )799 0 50800 225 4201000 278 4751400 385 5652000 570 7002200 630 767
t360
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Appendix DISCO Job Decks 58
2900 800 1000♦♦♦CALL SPEQ 150FGATES15 20 30 3530 35 45 5045 50 60 65SPEC0UT25 .5 40 1♦♦♦CALL FILTER CDP
100
NOTCH 60 ♦ ♦
♦♦ Reapply mutes ♦ ♦
♦CALLON2002300♦CALLSURG799800 1000 1400 2000 2200 2900 * *
♦CALL♦END
MUTE259350470MUTE2590
225278385570630800
CDP
8002900CDP
504204755657007671000
OFFSET 20 INT INT
420 801 210 2000 360700 4100 1160OFFSET 20 INT INT
DSKWRT CDP-601.630-SPQ
The implementation, of this routine was found to be extremely problematic.
Being very computationally intensive, the data had to be broken into subsets of
less than 50 CMP gathers for processing. Although using the array processor can
significantly reduce the processing time, the array processor frequently failed while
performing spectral equalisation, making its use virtually impossible. Thus, pro
cessing time had to be reduced by using disk input and output (reading the input
data from tapes and writing output to tapes separately from the spectral equali
sation process). Nevertheless, the total amount of processing time was extremely
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Appendix DISCO Job Decks 59
large — it took two months to complete the processing. Since spectral equalisation
was found to be so crucial in the processing of the shallow data, it is necessary that
the problem with the array processor be solved in the near future.
A .10 Surface - C onsistent R esidual Statics
♦JOB ABITIBI 12A JAN♦CALL TAPIN SET S0RT1♦ ♦
♦♦ Prelim inary V e lo c it ie s ♦ ♦
SURF CONS RES STATICS
♦CALL DEFINE CDPHANDVEL 1♦♦ t V t V
0 5750 4000 6750♦CALL NM0 PRELIM♦♦♦♦ S e le c t and f la t t e n a data♦♦♦CALL PREPARE CDPGATE 0♦♦ cmp t t ( i ) t ( f )41 2200 2000 240070 2200 2000 2400100 2280 2080 2480170 2364 2164 2564226 2500 2300 2700270 2680 2480 2880345 2820 2620 3020390 2940 2740 3140535 3420 3220 3620658 3980 3780 4180760 4640 4440 4840809 4940 4740 5140882 4940 4740 5140914 5060 4860 5260967 5060 4860 5260♦♦♦♦
INV
t8000
311
PRELIM
v7000
345
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Appendix DISCO Job Decks 60
♦♦ nchan maxshft itermax name♦CALL STATICR 110 8 3 SCLN10FSMASH 3 3 3♦♦ cm p(i) cmp(f) s h o t ( i ) s h o t ( f ) r e c ( i ) r e c ( f )GE0M 311 345 455 660 515 710♦END
The residual statics routine was found to have memory limitations, which ne
cessitated that the input data be broken into a large number of small datasets. The
process was applied to groups of 35 CMP gathers each (with an overlap of 5 between
groups) using a correlation window of 400 ms length. Although the process did not
fail if larger input datasets were used, the results were of significantly lower quality;
future processing of other datasets must involve testing to determine the optimal
size of input dataset which can be used.
A .11 NM O Correction and Stack
♦JOB ABITIBI 12A JAN STACK♦ ♦
♦♦ Read s p e c tr a lly balanced CMP gathers ♦♦
♦CALL TAPINSET SPEQ♦ ♦
♦♦ Stack on ly data w ith o f f s e t between 200 and 2000 metres * *
♦CALL EDIT CDP OFFSETSEL 1 967 OMIT RANGE0 199 2001 9999♦ ♦♦♦ R efined v e lo c i t i e s **♦CALL DEFINE CDP INV REFINE♦♦ t V t V t V t V
HANDVEL 10 7000 1000 7250 2000 6750 8000 7000HANDVEL 2200 7000 1000 7250 2000 6750 8000 7000
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Appendix DISCO Job Decks 61
HANDVEL 2400 7800 1000 7500HANDVEL 2800 7800 1000 7500HANDVEL 4700 7000 1000 7250HANDVEL 5200 6250 2000 6750HANDVEL 7600 6250 2000 6250HANDVEL 9670 6250 2000 6250♦CALL NMO REFINE♦ ♦
♦♦ Apply r e s id u a l s t a t i c s ♦ ♦
♦IFCDP 1 43HEADPUT SC.STAT STORESHOT SCLNOF SHOTHEADPUT SC.STAT ACCUMREC-STATSCLNOF REC
RANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI.♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALL
CDP 44 73HEADPUT SC.STAT STORE SHOT SCLN1F SHOT HEADPUT SC.STAT ACCUM REC-STATSCLNIF REC
CDP 74 103HEADPUT SC.STAT STORE SHOT SCLN2F SHOT HEADPUT SC-STAT ACCUM REC-STATSCLN2F REC
CDP 104 133HEADPUT SC.STAT STORE SHOT SCLN3F SHOT HEADPUT SC.STAT ACCUM
2000
2000
2000
8000
4000
4000
6750
6750
6750
7000
6750
6750
8000
8000
8000
8000
8000
SHOT
STATION
SHOT
STATION
SHOT
STATION
SHOT
7000
7000
7000
7000
7000
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Appendix DISCO Job Decks 62
ATTRI♦RESET♦ IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALL
REC-STATSCLN3F REC
CDP 134 163HEADPUT SC.STAT STORE SHOT SCLN4F SHOT HEADPUT SC-STAT ACCUM REC-STATSCLN4F REC
CDP 164 193HEADPUT SC.STAT STORE SHOT SCLN5F SHOT HEADPUT SC-STAT ACCUM REC-STATSCLN5F REC
CDP 194 223HEADPUT SC-STAT STORE SHOT SCLN6F SHOT HEADPUT SC-STAT ACCUM REC-STATSCLN6F REC
CDP 224 253HEADPUT SC-STAT STORE SHOT SCLN7F SHOT HEADPUT SC-STAT ACCUM REC-STATSCLN7F REC
CDP 254 283HEADPUT SC-STAT STORE SHOT SCLN8F SHOT HEADPUT SC-STAT ACCUM REC-STATSCLN8F REC
CDP 284 313HEADPUT SC-STAT STORE SHOT SCLN9F SHOT HEADPUT SC-STAT ACCUM
STATION
SHOT
STATION
SHOT
STATION
SHOT
STATION
SHOT
STATION
SHOT
STATION
SHOT
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Appendix DISCO Job Decks 63
ATTRI REC-STATSCLN9F REC♦RESET♦IFRANGE CDP 314 343♦CALL HEADPUT SC.STAT STOREATTRI SHOT SCLNIOF SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLNIOF REC ♦RESET ♦ IFRANGE CDP 344 373♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN11F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN1IF REC♦RESET ♦IFRANGE CDP 374 403♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN12F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN12F REC ♦RESET ♦IFRANGE CDP 404 433♦CALL HEADPUT SC.STAT STOR-ATTRI SHOT SCLN13F SHOT♦CALL HEADPUT SC.STAT ACCUMATTRI REC-STATSCLN13F REC ♦RESET ♦IFRANGE CDP 434 463♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN14F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN14F REC ♦RESET ♦IFRANGE CDP 464 493♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN15F SHOT♦CALL HEADPUT SC-STAT ACCUM
STATION
SHOT
STATION
SHOT
STATION
SHOT
STATION
SHOT
STATION
SHOT
STATION
SHOT
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Appendix DISCO Job Decks 64
ATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALL
REC-STATSCLN15F REC
CDP 494 523HEADPUT SC.STAT STORE SHOT SCLN16F SHOT HEADPUT SC.STAT ACCUM REC-STATSCLN16F REC
CDP 524 553HEADPUT SC.STAT STORE SHOT SCLN17F SHOT HEADPUT SC.STAT ACCUM REC-STATSCLN17F REC
CDP 554 583HEADPUT SC.STAT STORE SHOT SCLN18F SHOT HEADPUT SC.STAT ACCUM REC-STATSCLN18F REC
CDP 584 613HEADPUT SC.STAT STORE SHOT SCLN19F SHOT HEADPUT SC.STAT ACCUM REC-STATSCLN19F REC
CDP 614 643HEADPUT SC.STAT STORE SHOT SCLN20F SHOT HEADPUT SC.STAT ACCUM REC-STATSCLN20F REC
CDP 644 673HEADPUT SC.STAT STORE SHOT SCLN21F SHOT HEADPUT SC.STAT ACCUM
STATION
SHOT
STATION
SHOT
STATION
SHOT
STATION
SHOT
STATION
SHOT
STATION
SHOT
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Appendix DISCO Job Decks 65
ATTRI REC-STATSCLN21F REC♦RESET♦IFRANGE CDP 674 703♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN22F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN22F REC ♦RESET ♦IFRANGE CDP 704 733♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN23F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN23F REC ♦RESET ♦IFRANGE CDP 734 763♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN24F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN24F REC ♦RESET ♦IFRANGE CDP 764 793♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN25F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN25F REC ♦RESET ♦IFRANGE CDP 794 823♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN26F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN26F REC ♦RESET ♦IFRANGE CDP 824 853♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN27F SHOT♦CALL HEADPUT SC-STAT ACCUM
STATION
SHOT
STATION
SHOT
STATION
SHOT
STATION
SHOT
STATION
SHOT
STATION
SHOT
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Appendix DISCO Job Decks 66
ATTRI REC-STATSCLN27F REC*RESET♦IFRANGE CDP 854 883♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN28F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN28F REC ♦RESET ♦IFRANGE CDP 884 913♦CALL HEADPUT SC.STAT STOREATTRI SHOT SCLN29F SHOT♦CALL HEADPUT SC.STAT ACCUMATTRI REC-STATSCLN29F REC ♦RESET ♦IFRANGE CDP 914 943♦CALL HEADPUT SC .ST AT STOREATTRI SHOT SCLN30F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN30F REC ♦RESET ♦IFRANGE CDP 944 967♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN31F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN31F REC♦RESET♦CALL STATIC SC-STAT♦ ♦
♦ ♦
♦CALL STACK 800NORM♦CALL DSKWRT STACK♦END
STATION
SHOT
STATION
SHOT
STATION
SHOT
STATION
SHOT
STATION
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Appendix DISCO Job Decks 6J_
A .12 C oh eren cy F ilter
♦ JOB ABITIBI 12A JAN♦CALL DSKRD STACK**♦CALL AGC 2000♦CALL FILTER CDPKEYDEFBAND15
1BP20 60 70
♦♦♦♦ W rite su rface (VP) lo c a t io n s♦♦♦CALL HEADPUT VP STOREINPUT CDP ALL SEqNODATA 1 1DATA 967 1♦CALL HEADPUT VP STOREINPUT CDP NONE SEQNODATA 31 150DATA 63 200DATA 93 250DATA 121 300DATA 154 350DATA 188 400DATA 220 450DATA 246 500DATA 278 550DATA 307 600DATA 339 650DATA 367 700DATA 399 750DATA 429 800DATA 459 850DATA 492 900DATA 525 950DATA 558 1000DATA 589 1050DATA 622 1100DATA 656 1150DATA 689 1200DATA 722 1250
COHERENCY FILTER
INTEGER 1
INTEGER 1
967
967
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Appendix DISCO Job Decks 68
DATA 754 1300DATA 787 1350DATA 820 1400DATA 852 1450DATA 883 1500DATA 913 1550DATA 945 1600♦ ♦
♦♦ Coherency f i l t e r ♦ ♦
♦♦ xgate x in c tg a te t in e mindip♦CALL SIGNAL 19 9 100 60 -200♦♦d ip in c th resh f i l t e r15 0 .15 0♦♦ Add back 257. o f th e non co h e r e n c y -filter e d data♦CALL DIGISTK 0.75**♦CALL DSKWRT STACK.C0H♦END
A .13 P lo t
♦JOB ABITIBI 12A JAN PLOT FINAL SECTION♦CALL DSKRD STACK.C0H♦♦♦♦ t p i ip s♦CALL SECPL0T VA 30 3LABEL VP 100GAIN 4SETAMP PEAKPL0T0PT /NAME=P1♦♦ above sc a le width height♦CALL SIDELBL 8 30PL0T0PT /NAME=P2/P0S= (AFTER,PI)SPACE 1 .FREE B0X1SPACE 1 .ARROW NORTHSPACE .5FREE B0X2 1 7.SPACE .5
maxdip200
b ia s- .3
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Appendix DISCO Job Decks_____________________________________________ 69
FREE B0X3 1 7 .SPACE .5FTEXT B0X1 .5 2LITHOPROBEFTEXT B0X1 .3 1(9KSZ TRANSECT LINE 12AFTEXT B0X2 .3 2DATA ACQUISITION FTEXT B0X2 . IS 1<0ACQUIRED BY : VERITAS GEOPHYSICAL ®® <2SOURCE: ®® 2 MERTZ VIBRATORS <0G 20 - 120 HZ SWEEP (9<9 8 S SWEEP LENGTH ®<9 8 SWEEPS PER VP <9(9 20 METRE VP SPACING Q<9 <9RECEIVERS: 0(9 OYO 14 HZ GEOPHONES <9<9 12 PHONES PER GROUP <9<0 20 METRE RECEIVER SPACING <9<2 240 CHANNELS <2<9 SPLIT SPREAD (EXCEPT ROLL-ON/OFF) <2® 180 CHANNELS (SOUTH), 60 CHANNELS (NORTH) ®® 19 STATION GAP FOR VP ®a aRECORDING: ®0 TWO DFS-V SYSTEMS aa 16 S LISTEN TIME ®a 8 S CORRELATED RECORDS aa 2 MS SAMPLE INTERVAL aaFTEXT B0X3 .3 2PROCESSING SEQUENCE FTEXT B0X3 .15 1aPROCESSED AT THE LITHOPROBE SEISMIC PROCESSING FACILITY aUSING THE CYBER/DISCO SYSTEM BY JAN KOZEL (UNIVERSITY OF TORONTO)®
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Appendix DISCO Job Decks 70
« G
1 . CROOKED LINE GEOMETRY cIBIB2.
30 X 800 M CMP BINS «
TRACE EDIT «(B DETERMINED MANUALLY FROM STACKED SHOT GATHERS USED IN «IB/A
PRELIMINARY PROCESSING «
3. FIRST ARRIVAL PICKS IBO/A
DETERMINED USING THE DISCO AUTOMATIC PICK ROUTINE IB<D
4. RESAMPLE TO 4 MS IBIBA
ANTIALIAS FILTER: 100 - 125 HZ ROLL-OFF IB
5. REFRACTION STATIC CORRECTION IBIBA
1 LAYER (1600 M/S) OF VARIABLE THICKNESS OVER BEDROCK IB<86. MUTE OF FIRST ARRIVALS IBIB 0 M 350 MS Q
IB 200 M 350 MS IB0 800 M 420 MS IBIB 801 M 210 MS <BIB 2000 M 360 MS IB(B 2300 M 470 MS IBIB 2900 M 700 MS IBIBA
4100 M 1160 MS IBVS
7. SURGICAL MUTE OF GROUND ROLL IB(B 800 M (225 , 420) MS IBIB 1000 M (278, 475) MS IBIS 1400 M (385, 565) MS IBIB 2000 M (570, 700) MS IBIB 2200 M (630, 767) MS IB<SA
2900 M (800, 1000) MS e<8
8. SPECTRAL EQUALISATION FOR 0 S TO 3 S IB(B (2 0 ,3 0 ) (3 5 ,4 5 ) (50 ,60 ) HZ BANDWIDTHS IBIB 0 .5 1 .0 1 .0 WEIGHTS IB(BA
EQUALISED USING 150 MS AGC IB(89.IB
AGC FOR 3 S TO 8 S (750 MS WINDOW) e
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Appendix DISCO Job Decks 71
10. 60 HZ NOTCH FILTER ISID11. REAPPLICATION OF FIRST ARRIVAL AND GROUND ROLL MUTE ISIS12. COMMON MID POINT SORT IS(013. SURFACE CONSISTENT RESIDUAL STATICS (SID MAXIMUM CORRECTION 8 MS IS10 400 MS CORRELATION WINDOW: IS(0 VP TIMES (MS) ISID 160 2000 2400 ID(0 210 2000 2400 ISIS 260 2080 2480 (S<0 375 2164 2564 ISIS 460 2300 2700 (3(0 540 2480 2880 IS(0 660 2620 3020 ISID 740 2740 3140 (SIS 960 3220 3620 ISIS 1150 3780 4180 IBIS 1310 4440 4840 (S(S 1380 4740 5140 IS<S 1500 4740 5140 isIS 1550 4860 5260 isIS 1636 4860 5260 ISIS14. VELOCITY ANALYSIS AND NMO REMOVAL 0Q15. STACK isIS16. AGC (2000 MS WINDOW) ISIS17. BANDPASS FILTER isIS 15/20 - 60/70 HZ ISIS18. COHERENCY FILTER isIS 19 TRACE WINDOW 9 TRACE INCREMENT ISIS 100 MS TIME WINDOW 60 MS INCREMENT ISIS19. ADD BACK UNFILTERED STACK ISIS 75 IS 25 (S20. PLOT IS
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Appendix DISCO Job Decks 72
<8 VARIABLE AREA (30 */. NEGATIVE BIAS) <89 30 TRACES / INCH <DID 3 .0 INCHES / SECOND <0IS♦CALL TOPLBL 3 30.INPUT VELDEFN VELDEFN 1990DISPLAY 1 967STRIP .5 2 TVBOXTVBOX TV TOP NOANNOTPLOTOPT /NAME=P3/P0S=(ABOVE,PI)♦END
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References
A nderson, R. G ., and McM ehan, G. A . 1989 “A utom atic E d iting of Noisy Seismic D ata”
Geophysical Prospecting 37 (8) pp . 875 - 892
Anstey, N. A. 1986 “W hatever H appened to G round Roll?” The Leading Edge 5(3) pp. 40
- 45
Al-Husseini, M. I., Glover, J . B ., Barley, B. J . 1981 “Dispersion P a tte rn s o f the G round
Roll in E astern Saudi A rab ia” Geophysics 46 (2 ) pp . 121 - 137
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