P O S I V A O Y
O l k i l u o t o
F I -27160 EURAJOKI , F INLAND
Te l +358-2-8372 31
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I lmo Kukkonen , Markku Paananen ,
Seppo E lo , Seppo Pau lamäk i , Jukka La i t i nen ,
H IRE Work ing Group
of the Geo log i ca l Survey o f F i n l and ,
Pekka He ikk inen , Suv i He inonen
August 2010
Work ing Repor t 2010 -57
HIRE Seismic Reflection Surveyin the Olkiluoto Area
August 2010
Base maps: ©National Land Survey, permission 41/MML/10
Working Reports contain information on work in progress
or pending completion.
The conclusions and viewpoints presented in the report
are those of author(s) and do not necessarily
coincide with those of Posiva.
I lmo Kukkonen , Markku Paananen ,
Seppo E lo , Seppo Pau lamäk i , Jukka La i t i nen ,
H IRE Work ing Group
of the Geo log ica l Survey of F in land
Geo log ica l Su rvey o f F in l and
Pekka He ikk inen , Suv i He inonen
I ns t i tu te o f Se i smo logy , Un ive rs i t y o f He l s ink i
Work ing Report 2010 -57
HIRE Seismic Reflection Surveyin the Olkiluoto Area
ABSTRACT
A seismic reflection survey comprising three vibroseismic lines (total length of 31.1
km) was carried out in the Olkiluoto area, western Finland, in July, 2008. The survey is
a part of the project HIRE (High Resolution Reflection Seismics for Ore Exploration
2007-2010) of the Geological Survey of Finland (GTK). The Olkiluoto survey was done
in co-operation with Posiva Oy.
The HIRE seismic reflection survey in the Olkiluoto area revealed numerous previously
unknown structures in the upper crust of the area.
The most prominent structures observed are the subhorizontal strong reflectors which
very probably represent Postjotnian diabase sills intruding both the Svecofennian
gneisses as well as the rapakivi granites. These reflectors can be associated with the
similar seismic structures recorded in marine seismic transects in the Bothnian Sea, and
thus they represent a large-scale structure.
The Mesoproterozoic rapakivi granites can be distinguished as homogeneous,
seismically transparent domains which extend to a depth of at least 4 km. The
interpreted rapakivi structures are in a good agreement with gravity modellings.
On the Olkiluoto Island, reflectors could be correlated with drillhole based data on
lithology and brittle fault zones. The main brittle fault zones detected in drillholes are
represented as reflectors in the seismic sections, and several new structures have been
interpreted. A synthetic seismogram constructed for a 1 km deep hole in the Olkiluoto
Island using down-hole logs of density and P-wave velocity, suggests that the main
brittle fault zones generate strong reflectors. Pegmatitic granite sometimes has a weak
reflection contrast with the surrounding gneisses, and sometimes has no contrast. There
seems to be a simultaneous correlation of reflectivity with fracture zones at the location
of the pegmatitic granite occurrences.
The HIRE 2D seismic reflection data agrees well with the earlier 3D reflection surveys
in the Olkiluoto Island, and most of the reflectors detected in HIRE surveys are present
also in the 3D results. The 2D data can be used to extend the interpretation of the 3D
results in the Olkiluoto Island.
Where the seismic lines cross the lineaments limiting the Olkiluoto bedrock block, the
seismic data can be used to provide estimates of the dip angles of the lineaments. The
results suggest that the lineament separating the Olkiluoto Island from the continent
would be dipping about 38º to SE, and one of the lineaments limiting the NE side of the
block would be dipping 18º to NE, respectively.
Keywords: Seismic reflection surveys, site surveys, spent nuclear fuel, Fennoscandian
Shield, crystalline rocks, Olkiluoto, Finland
HIRE-HANKKEEN SEISMINEN HEIJASTUSLUOTAUS OLKILUODON ALUEELLA LÄNSI-SUOMESSA TIIVISTELMÄ
Olkiluodon alueella Länsi-Suomessa tehtiin heinäkuussa 2008 seisminen heijastus-
luotausmittaus, joka koostui kolmesta vibroseismisellä menetelmällä mitatusta luotaus-
linjasta (yhteispituus 31,1 km). Luotaus kuuluu osana Geologian tutkimuskeskuksen
(GTK) HIRE-hankkeeseen (High Resolution Reflection Seismics for Ore Exploration
2007-2010). Olkiluodon alueen mittaukset tehtiin yhteistyössä Posiva Oy:n kanssa.
HIRE-luotausten perusteella Olkiluodon kallioperästä tuli esiin lukuisia ennen tunte-
mattomia yläkuoren rakenteita. Merkittävimmät havaitut rakenteet ovat vaaka- tai loiva-
asentoisia voimakkaita heijastajia, jotka todennäköisesti edustavat postjotunisia dia-
baasisillejä. Diabaaseiksi tulkitut heijastajat näyttävät leikaavan sekä svekofennialaisia
gneissejä että rapakivigraniitteja. Nämä heijastajat voidaan yhdistää samankaltaisiin
meriseismisissä luotauksissa havaittuihin heijastajiin Selkämerellä, ja siten ne toden-
näköisesti edustavat suuremman mittakaavan kallioperärakenteita.
Mesoproterotsooiset rapakivigraniitit ovat seismisillä profiileilla homogeenisiä, huo-
nosti heijastavia alueita, jotka ulottuvat ainakin 4 km:n syvyyteen. Tulkitut rapakivi-
rakenteet ovat hyvässä sopusoinnussa painovoimamittausten mallitustulosten kanssa.
Olkiluodon saarella heijastajia voitiin korreloida kairausten perusteella selvitettyyn
kivilajijakaumaan ja hauraisiin siirrosvyöhykkeisiin. Tärkeimmät kairauksissa todetut
hauraat siirrosvyöhykkeet havaittiin heijastajina heijastusprofiileilla ja myös useita
ennen tuntemattomia siirrosrakenteita voitiin tulkita. Syvän (n.1 km) kairareiän tiheyden
ja seismisen P-aallon nopeuden reikämittausten perusteella laskettu akustinen impe-
danssi ja synteettinen seismogrammi osoittavat, että hauraat siirrosvyöhykkeet synnyt-
tävät voimakkaita heijastajia. Pegmatiittisella graniitilla on paikoin vain heikko tai ei
lainkaan heijastuseroa ympäröiviin gneisseihin nähden, mutta heijastajat esiintyvät
usein paikoissa, joissa on sekä pegmatiittista graniittia että hauras siirrosvyöhyke.
HIRE-hankkeen 2-dimensionaaliset luotaukset käyvät hyvin yksiin aiemmin Olkiluo-
dossa tehtyjen 3-dimensionaalisten seismisten heijastusluotausten kanssa ja useimmat
HIRE-aineiston heijastajat voidaan havaita myös 3D-aineistosta. 2D-tuloksia voidaan
käyttää laajentamaan 3D-tulosten tulkintaa Olkiluodossa.
Siellä, missä seismiset mittauslinjat leikkaavat Olkiluodon kalliolohkoa rajaavia linea-
menttejä, heijastusluotausten perusteella voidaan tulkita näitä vastaavien kallioraken-
teiden kaadekulmia. Tulkinnan perusteella lineamentti, joka erottaa Olkiluodon saaren
kalliolohkon mantereesta kaatuu noin 38° kaakkoon, kun taas kalliolohkoa koillis-
puolelta rajaava lineamentti kaatuu noin 18° koilliseen.
Avainsanat: Seisminen heijastusluotaus, käytetty ydinpolttoaine, paikkatutkimukset,
kiteiset kivilajit, Fennoskandian kilpi, Olkiluoto, Suomi
1
TABLE OF CONTENTS
ABSTRACT
TIIVISTELMÄ
1 INTRODUCTION................................................................................................... 3
2 SHORT GEOLOGICAL DESCRIPTION OF THE AREA........................................ 5
3 SURVEY METHOD ............................................................................................. 11
4 DATA PROCESSING .......................................................................................... 13
5 RESULTS AND INTERPRETATION ................................................................... 21
5.1 2D and 3D representation of the results .................................................... 21
5.2 Reflection properties of the upper crust in the Olkiluoto area ..................... 27
5.3 Synthetic seismogram and reflection properties of the Olkiluoto Island ...... 29
6 COMPARISON OF REFLECTION DATA WITH LITHOLOGICAL AND FRACTURE ZONE MODELS IN OLKILUOTO .................................................... 33
7 COMPARISON OF HIRE RESULTS WITH PREVIOUS 3D SEISMIC REFLECTION SURVEYS IN OLKILUOTO .......................................................... 41
8 HIRE RESULTS AND GRAVITY SURVEYS ....................................................... 51
9 DISCUSSION ...................................................................................................... 55
10 CONCLUSIONS .................................................................................................. 59
REFERENCES ........................................................................................................... 61
2
3
1 INTRODUCTION
A seismic reflection survey comprising three vibroseismic lines (total length of 31.1
km) was carried out in the Olkiluoto area, western Finland, in July, 2008. The survey is
a part of the project HIRE (High Resolution Reflection Seismics for Ore Exploration
2007-2010) of the Geological Survey of Finland (GTK). The Olkiluoto survey was done
in co-operation with Posiva Oy.
The general aims of the HIRE project are (1) to introduce reflection surveys as an
exploration tool for the Precambrian crystalline bedrock of Finland, (2) to apply 3D
visualization and modelling techniques in interpretation, and (3) to improve the
structural data base on the most important mineral resource provinces in Finland. The
HIRE targets comprise exploration and mining camps in very diverse geological
environments. Targets include Cu, Ni, Fe, Cr, PGE, Zn, and Au deposits, most of them
economic, as well as the Finnish site for nuclear waste disposal (Olkiluoto).
Olkiluoto Island has been chosen for the disposal site of spent nuclear fuel of the power
companies Teollisuuden Voima Oyj and Fortum Power and Heat Oy. The aims of the
HIRE survey in the Olkiluoto area was to reveal the upper crustal structures in an area
much wider than the Olkiluoto Island, and to explore the structures in detail and much
deeper than is possible with other geophysical techniques.
The survey was carried out by the Geological Survey of Finland using SFUE
Vniigeofizika, Moscow, Russia, as the seismic contractor. The HIRE project is partly
funded from the debt conversion agreement between Finland and Russia. The Olkiluoto
survey was agreed between GTK and Posiva Oy based on the offer by GTK dated
February 28, 2008.
The survey comprised three vibroseismic soundings along roads. The field work was
carried out during July 1-16, 2008.
4
5
2 SHORT GEOLOGICAL DESCRIPTION OF THE AREA
The Palaeoproterozoic supracrustal rocks of the HIRE survey area (Fig. 5-1) are
metasedimentary, which is demonstrated by the quartz-feldspar-rich and biotite-rich
layers within the mica gneisses, representing psammitic and pelitic layers, respectively
(Suominen et al. 1997). Almost all the mica gneisses are variably migmatised, and the
type and the intensity of the migmatisation vary considerably. The older component of
the migmatite, or palaeosome, is mainly composed of quartz, plagioclase, biotite and
often also potassium feldspar. It contains macroscopic garnet and cordierite
porphyroblasts, as well as microscopic sillimanite inclusions in the biotite and cordierite
grains. The younger component of the migmatite, or neosome, mainly occurs as coarse-
grained and pegmatitic granite veins parallel to the foliation and banding. Thus, the
migmatites are usually veined gneisses in their migmatite type (Fig. 2-1A). The main
minerals of the neosome are quartz, potassium feldspar and plagioclase. Occasionally it
contains the same porphyroblasts as the palaeosome.
The largest Palaeoproterozoic plutonic intrusions of the Rauma map sheet area are
composed of trondhjemites, tonalites and granodiorites (Suominen et al. 1997), but in
the HIRE survey area they are scarce. The contacts between the different rock types are
gradual. The most felsic of the above rock types, trondhjemites, are medium-grained
and massive or weakly foliated. This group of plutonic rocks also includes gneissose
granodiorites, which are fine- or medium-grained, almost always hornblende-bearing,
and include a few centimetres wide light- and dark-coloured streaks (Suominen et al.
1997). There are indications that they may be paragneisses in origin.
The Palaeoproterozoic granites, porphyritic granites and pegmatites of the HIRE survey
area are very heterogeneous, containing numerous inclusions and granitised restites of
the mica gneiss. They contain garnet and sometimes also cordierite from the assimilated
mica gneisses. The chemical composition of the granites resembles the so-called S-type
granites, which derive from partial melting of the sedimentary rocks (Suominen et al.
1997).
The Palaeoproterozoic rocks are intruded by Mesoproterozoic rapakivi granites. In the
survey area they belong to the Eurajoki rapakivi stock (ca. 70 km2), which is a satellite
massif to much larger (ca. 1400 km2) Laitila rapakivi batholith, located east of the
Eurajoki stock and dated at 1583±3 Ma (Vaasjoki 1996). The Eurajoki stock is
composed of two main rapakivi types: older hornblende-biotite granite, or the Tarkki
granite, dated at 1571±3 Ma, and younger, topaz-bearing microcline-albite granite, or
the Väkkärä granite, dated at 1548±3 Ma (Haapala 1977; Vaasjoki 1996; Suominen et
al. 1997). The contact between the marginal Tarkki granite and the surrounding
Palaeoproterozoic migmatites is sharp. Also the contact between the Tarkki granite and
the central Väkkärä granite is sharp, the Väkkärä granite dipping gently outwards
(Haapala 1977; Elo 2001). The gravity interpretations (Elo 2001) suggest that the
Eurajoki rapakivi granite extends downwards and outwards in every direction, and its
depth exceeds 5 km.
6
The Tarkki granite is homogenous, medium- and even-grained rock, which contains
potassium feldspar ovoids, 3 - 6 cm in diameter, mantled with plagioclase (Haapala
1977) (Fig. X1C). The ovoids are sparsely distributed, the distance between individual
ovoids being often several metres. The Tarkki granite is occasionally cut by topaz-
bearing quartz porphyre dykes, which mineralogically and chemically resemble the
Väkkärä granite, indicating a close genetic connection between the two (Haapala 1977).
The Väkkärä granite is composed of several texturally different types, the contacts of
which are either sharp or gradual (Haapala 1977). The porphyritic (Fig. 2-1c) and
coarse-grained granites are topaz-bearing, the phenocrysts being potassium feldspar and
quartz. Accessory minerals include, in addition to topaz, fluorine-rich siderophyllite,
monazite, ilmenite, Nb- and Ta-rich cassiterite, columbite and bastnaesite. Miarolitic
cavities are common in the porphyritic granites (Haapala 1977). The topaz-bearing
Väkkärä granite resembles typical tin granites in chemical composition. It is enriched in
fluorine, lithium, gallium, rubidium, tin and niobium, and depleted in strontium, barium
and zircon.
The even-grained Väkkärä granite is mainly composed of potassium feldspar, quartz
and biotite, accessory minerals being zircon, ilmenite, anatase, monatzite, as well as
topaz and cassiterite as secondary minerals. The contact type of Väkkärä granite against
the Tarkki granite has potassium feldspar, plagioclase and quartz phenocrysts, 1 - 10
mm in diameter, in a fine-grained matrix. The contact type contains fragments of Tarkki
granite and sends apophyses into it (Haapala 1977).
E-W-trending greisen veins and associated quartz veins occur within the Tarkki granite.
The width of the veins varies from a few centimetres up to two metres, and the drillings
have shown that at least some of the veins and vein swarms extend over 100 m in depth
and over 300 m in a horizontal direction (Haapala 1977). In the Väkkärä granite the
veins are more randomly oriented, and also lensoid, rounded and irregular greisen
bodies occur. The main minerals of the greisen veins are quartz, micas and iron-rich
chlorite. Often they also contain abundant topaz, fluorine, garnet, beryl, genthelvite and
bertnandite (Haapala 1977). The most common ore minerals include sphalerite,
cassiterite, chalcopyrite, wolframite, gahnite, molybdenite, rutile, secondary iron oxide,
pyrite, pyrrhotite, arsenopyrite and galena. The greisen veins were caused by hot,
hydrous fluids, migrating in interstices and fractures of the rapakivi granites (Haapala
1977). The greisen veins both in the Tarkki and Väkkärä granite were formed by fluids
emanated from the Väkkärä granite.
The olivine diabase dykes and sills, dated at 1270 - 1250 Ma (Suominen 1991; Vaasjoki
1996) cut all the other rock types of the survey area. The main minerals of the diabases
are plagioclase, augite, olivine, opaques (ilmenite, magnetite) and rarely biotite
(Hämäläinen 1987: Suominen et al. 1997; Veräjämäki 1998). Usually they also contain
small amounts of quartz and potassium feldspar, which form micrographic intergrowths
(Kahma 1951). The diabases are subophitic in texture indicating a hypabyssal origin.
The diabases are tholeitic in chemical composition, and their geochemical features
suggest that they are feeder channels to continental flood basalts. No such basalts,
however, exists in the area today.
7
In the subhorizontal contact of the Sorkka diabase against the surrounding migmatitic
gneisses, the medium-grained diabase becomes fine-grained or aphanitic towards the
contact (Kahma 1951). Also at the contact between the Sorkka diabase and the Tarkki
rapakivi granite (Fig. 2-1C-D), the diabase is rather aphanitic, but becomes medium-
grained a few metres away from the contact. The diabase, in places, has palingenic
veins, which have resulted from the partial remelting of the country rocks. They are
from a few millimetres up to 10 cm wide and proceed 2 to 3 m, at the most, into the
diabase from the contact. The veins in the diabase-rapakivi granite contact mainly
consist of quartz and alkali feldspars. In addition they contain some biotite, chlorite and
especially near the diabase contact also hornblende (Kahma 1951). One analysed vein
within the Sorkka diabase in Hankkila contains potassium feldspar, plagioclase,
hornblende, muscovite, olivine or pyroxene, with accessory quartz and carbonate
(Suominen et al. 1997). In the contact between the palingenic veins and the olivine
diabase, a narrow reaction zone has been observed. In this zone the olivine and augite of
the diabase have altered into serpentine, amphibole, chlorite and an iron-bearing opaque
mineral, whereas the plagioclase has become corroded (Kahma 1951).
On the basis of outcrop observations during the mapping campaign in 2002 (Paulamäki
2007), the western contact of the Eurajoki stock (Tarkki granite) against the
Palaeoproterozoic migmatitic country rock seems to be gently dipping, the dip of the
contact being about 20° to the west/northwest. The contact is very sharp and intact. No
visible features of contact metamorphism have been observed within the migmatitic
gneiss.
The bedrock of the Olkiluoto Island, where the final disposal of spent nuclear fuel is
planned, is mostly composed of Svecofennian high-grade metamorphic supracrustal
rocks. In terms of their mineral composition, texture and migmatite structure, the rocks
of Olkiluoto can be divided into four major classes: 1) migmatitic gneisses, 2) tonalitic-
granodioritic-granitic gneisses or TGG gneisses, 3) other gneisses including mica
gneisses, quartz gneisses and mafic gneisses, and 4) pegmatitic granites (Kärki &
Paulamäki 2006). The migmatitic rocks can further be subdivided into stromatic
gneisses, veined gneisses and diatexitic gneisses on the basis of their migmatite
structures. All the rock types mentioned above are very sharply cut by ca. 1560 Ma old,
NE-SW striking narrow Sub-Jotnian diabase dykes, which dip steeply to the NW
(Mattila et al. 2008). Post-Jotnian diabases are not known in the Olkiluoto Island.
8
Figure 2-1. A) Veined gneiss, Olkiluoto. B) Homogeneous gneissose tonalite. Ilavainen.
C) Alkali feldspar ovoid with a plagioclase mantle within the medium- and even-
grained Tarkki rapakivi granite, Hankkila. D) Porphyritic Väkkärä rapakivi granite,
Hankkila. E) Subhorizontal contact between the Sorkka diabase (above) and the Tarkki
rapakivi granite (below), Hankkila quarry. F) Close-up photo of the contact in E.
Photos by Seppo Paulamäki, GTK (A-B, E-F) and Aimo Kuivamäki, GTK (C-D).
C D
A B
E F
9
On the basis of the whole-rock chemical compositions, the supracrustal rocks of
Olkiluoto can be divided into four distinct series or groups: a T series, S series, P series
and basic, volcanogenic gneisses (Kärki & Paulamäki 2006), T, S and P standing for
Turbidite, Skarn and Phosphorus, respectively. In addition, pegmatitic granites and
diabases form groups of their own that can be identified both macroscopically and
chemically.
The refolding and crosscutting relationships indicate that the metamorphic supracrustal
rocks have been subject to polyphased ductile deformation, including at least four stages
(D1-D4), the D2 being locally the most intensive phase and producing thrust-related
folding, strong migmatisation and pervasive foliation (Mattila et al. 2008). The age of
the D2 phase is estimated to be approximately at 1860 Ma based on the age of the TGG
gneisses (1863 Ma) deformed in D2. In subsequent thrust-related deformation phase D3,
the earlier deformed migmatites were zonally refolded, rotated or sheared. Zones
dominated by ductile D3 shear structures and tight F3 folds were formed, and often the
D2 structures were rotated parallel to the F3 axial surface (S3) striking to the NE and
dipping to the SE. D3 shear zones are thrust zones parallel to the F3 axial surfaces or E-
W striking oblique-slip shear zones. Simultaneously with D3 deformation new granitic
leucosomes intruded often parallel to the F3 axial surfaces and shear structures. The
approximate age of the D3 deformation at Olkiluoto is 1860-1830 Ma based on the U-Pb
(SIMS) age of one D3 related pegmatitic granite dyke cutting the TGG gneiss.
Subsequently, D3 elements and earlier structures were again redeformed in deformation
phase D4, which produced close to open F4 folds with axial planes striking to the NNE
and dipping to the ESE. In places, a few centimetres to several metres wide thrust-
related ductile D4 shear zones, subparallel to the regional direction of S4, have been
observed. The central and south-eastern parts of the Olkiluoto site, the ONKALO area
and the region ca. 500 m eastward from it, have been affected more strongly by D4. At
least a part of intrusive-like rocks named at Olkiluoto as K-feldspar porphyries seem to
be related with D4 shear zones.
10
11
3 SURVEY METHOD
The Olkiluoto reflection survey consisted of 2D lines located according to the geology,
and road conditions. The applied method is the CMP (Common Mid Point) method with
symmetrical split-spread geometry. At end of lines asymmetric shooting was applied.
The number of active channels was 402, and the channel interval was 12.5 m. The
maximum offset between source and receivers was 2,502 m in case of symmetrical
geometry and at ends of lines in asymmetric geometry up to 5,025 m.
The source point interval was 50 m, and locally, for instance in the proximity of
interesting structures, it was reduced to 25 m. Vibrators were used to generate the
seismic source signal. Three (minimum two) 15.4 ton Geosvip vibrators were used as a
group. The applied force was about 10 ton/vibrator. The sweep was a 16 s linear
upsweep with a frequency band of 30-165 Hz, and the total listening time was 22 s. The
final correlated signal length is 6 s. The number of sweeps/source point was six. The
sweeps were stacked and the stacked data were saved.
Positioning of the line layout and recording station stakes in the field were done
immediately before the acquisition with 25 m steel rope. Horizontal positioning was
done with differential GPS to an accuracy of at least ±2 m, and elevations were
determined with levelling to an accuracy of at least ± 0.5 m.
The survey parameters are shown in Table 3-1 (Zamoshnyaya 2008).
12
Table 3-1. HIRE survey parameters in Olkiluoto.
Recording I/O-4
Number of active channels 402
Sampling interval, ms 1
Record length after correlation, s 6
Preliminary gain, dB 24-36
Notch filter, Hz off
Noise suppression editor
(BURST+DIVERSITY)
on
High-pass filter, Hz off
Tape format SEG-Y
Medium type HARD - DISC
Acquisition geometry Symmetrical
split spread
Stacking fold varying
Receiver group spacing, m 12.5
Spacing of source locations, m 25 or 50
Spread length, m 5012.5
Linear geophone grouping 6 geophones on
12.5 m base
Linear SV-14/150 vibrator grouping 3 on 25 m base
Sweep frequency limits, Hz 30-165
Sweep period, s 16
Number of vibrations at a source point 6
Ground force 65%
Control system and vibrator synchronization control VIB PRO
13
4 DATA PROCESSING
Data processing was done in three main steps. First, on-site processing was done by
Vniigeofizika in the field base. The first results were used mainly for quality control.
Second, basic processing was continued from the field results in the Moscow office of
Vniigeofizika. Third, post stack processing was done by the Institute of Seismology of
the University of Helsinki (HY-Seismo), working as a contractor and research partner
for GTK.
The on-site and basic processing sequence of data processing is shown in Tables 4-1 -
4-2 (Zamoshnyaya 2008).
Post stack processing (Table 4-3) was made by HY-Seismo starting from the NMO
(normal-move-out) stacked sections by Vniigeofizika. The post stack processing
included four processing steps:
1) Whole trace amplitude equalization,
2) Stolt migration with depth dependent velocity function,
3) Spectral balancing,
4) Depth conversion.
The purpose of the first step was to eliminate the amplitude variations along the lines
caused by changes in surface conditions and possible processing artefacts. The second
step improves the migration results as the original migrations were done using the
constant velocity of 5000 m/s. As a part of the basic processing, Vniigeofizika
performed velocity analysis at every 100th CMP. From the measured values the average
(root-mean-square) Vrms-velocity was estimated and this velocity function was used in
Stolt migration, i.e. migration in frequency - wave-number domain. This takes into
account the average increase of velocity as a function of traveltime, i.e. depth. The
stacking velocities as well as the velocity function are shown in Fig. 4-1.
In the spectra of the migrated traces the amplitudes decrease as a function of frequency,
which results correspondingly in decreasing the resolution of the data. This can be
improved by spectral balancing, i.e. by increasing the contribution of higher
frequencies. Spectral balancing was done multiplying the spectra with a linearly
increasing function of the frequency. The applied value of the multiplier was 1.0 at 40
Hz and 2.0 at 160 Hz.
The migrated traces are functions of travel time and before plotting they have to be
converted to functions of depth. The conversion velocity was calculated from the
average interval velocities estimated from the average stacking velocity function. The
values of the depth conversion velocity are listed in Table 6. Between the listed values
linear interpolation was used. The difference between the stacking velocity of Fig. 4-1
and the depth conversion function is very small, less than 200 m/s. As one can see in
Fig. 4-1, the variation of the velocities is of the order of 4 % and similar uncertainties
are possible in depth conversion.
14
Figure 4-1. The stacking velocities (coloured dots) and the velocity function used for
post stack processing (black line). The colour of the dot indicates the line (V1-V2).
15
Table 4-1. On-site processing of vibroseis data.
16
Table 4-2a. Basic processing of vibroseis data.
17
Table 4-2b. Basic processing of vibroseis data (cont.)
18
Table 4-2c. Basic processing of vibroseis data (cont.)
19
Table 4-3. Steps of post-stack processing.
Final NMO stack
Migration (Stolt)
TWT (ms) Vrms (m/s)
0 5152
100 5402
200 5615
300 5751
400 5849
500 5926
600 5980
700 6041
800 6097
900 6144
1000 6184
1500 6307
2000 6369
Spectral balancing
Band pass filter
Frequency Filter amplitude
20.0 0.0 40.0 1.0
160.0 2.0
200.0 2.0 240.0 0.0
Depth conversion
Depth (m) Conversion velocity (m/s)
0 5152
270 5401
561 5611
861 5745
1168 5843
1479 5919
1791 5973
2076 5934
2432 6081
2760 6135
3093 6188
4786 6382
6487 6487
Plotting
20
21
5 RESULTS AND INTERPRETATION
5.1 2D and 3D representation of the results
The location of the survey lines is shown on a geological map in Fig. 5-1. The map
shows two lines for each section, first the receiver station line as it was located in the
field, and second, the common midpoint (CMP) line. The CMP line indicates the
surface projection of average locations of reflection points. For a deep and long section
these may differ noticeably if the survey line is curved or crooked. This issue must be
taken into account, when locating reflectors in the field. Those very close to surface
(less than ca. 200 m) are best located with the shooting line in the terrain, whereas
deeper reflectors are best located with the CMP line.
When interpreting 2D seismic sections the effects of the cross dip of reflecting
structures must be taken into account. The apparent dip of a planar reflector as seen in
the seismic section depends on the true dip and strike of the reflector. If the strike is
perpendicular to the line, the apparent dip is equal to the true dip, but the more the strike
angle deviates from perpendicular, the smaller becomes the apparent dip angle. The
relations between true and apparent dip are shown in a nomogram in Fig. 5-2. As can be
seen in the figure, subvertical structures surveyed at small strike angles are imaged by
apparent dip angles significantly smaller than the true dip.
Datum level of the reflection data is 0 metres above sea level. The uppermost layers
(Quaternary sediments, weathered bedrock) have lower velocities than the intact
crystalline rocks. Velocity and thickness variations of the surface layer generate
spatially dependent delays in the arrival times of reflections, and the data must be
corrected for these „static‟ effects. This is done by shifting the signals to a common
depth level, which is usually the highest level of topography in the survey area. The
datum level is also the level to which the upper boundary of seismic sections (depth 0
m) should be referenced.
Frequency content of the data is very good. Examples of frequency spectra are shown in
Fig. 5-3. The applied frequency band of 30-165 Hz is well covered with received data.
This predicts good resolution in the final images.
The processed sections show a wealth of reflectors. Reflectors as thin as 10 m vertically
and 200-300 m wide horizontally can be distinguished in the sections.
22
Figure 5-1. Survey lines in the Olkiluoto area. Numbers along the lines indicate
receiver station pole numbers (italics) and CMP coordinates (normal text). The
geological map is based on the geological map sheet 1:100 000 Rauma (Suominen et
al., 1997) and the digital geological data base of the Geological Survey of Finland.
23
0
10
20
30
40
50
60
70
80
90
0 10 20 30 40 50 60 70 80 90
True dip (º)
Ap
paren
t d
ip (
º)
10
20
30
40
50
60
70
90
80
Figure 5-2. Relations between true and apparent dip angles of planar reflectors. The
curve parameter is the angle between the survey line and the strike of the reflector at
surface.
The results of the survey are shown in migrated and depth converted NMO (normal-
move-out) sections in Figs. 5-4 – 5-6. The sections were converted from time sections
to depth sections using the velocity function in Table 6. In the figures the reflectors are
displayed as variable area plots of averaged instantaneous amplitude (traces). In
addition, the amplitudes were averaged in moving windows (60 m in vertical and 90 m
in horizontal directions, respectively) and displayed in dB scale in the background as a
colour-coded map. Lithological boundaries are given on the top of the 2D sections
according to the digital database on Precambrian geology (“DigiKP”) of the GTK.
To obtain three-dimensional visualization of the reflector structures, the seismic
sections were input to SURPAC 3D visualization and modelling software. The sections
were imported into SURPAC as images which were draped along vertical „curtains‟
defined by the CMP coordinates of the lines. Relevant units were interpreted first from
the sections in 2D, and the corresponding strings were digitized again in SURPAC on
the drape surfaces of the sections. As a result, the 3D correlation of the strongly
reflective units is presented with the 3D strings.
In addition to seismic reflection images, the available geological map and airborne
magnetic map were also applied to provide a means to correlate reflectors with bedrock
structures. These correlations were used in the interpretations below.
24
Figure. 5-3. Frequency spectra of the vibrator field data from V1 with two offsets from
the shot point. Upper panel: offset 150-200 m, lower panels: offset 1950-2050 m. Shot
point location (SP 1065, CMP 2130) is in Olkiluoto Island. Increasing time windows
correspond to increasing depths (Zamoshnyaya 2008).
Figure. 5-4. Migrated NMO section of line V1. Reflectors with high amplitude are automatically enhanced with red background colour.
Geological boundaries at the surface are taken from the geological map sheet 1:100 000 Rauma (Suominen et al. 1997) and the digital
geological data base of the Geological Survey of Finland. The boundaries of reflectors and their geological interpretations have been
indicated with thick lines. BFZ stands for brittle fault zone. See text for details.
25
Figure. 5-5. Migrated NMO section of line V2. Data representation and geological boundaries as in Fig. 5-4. BFZ stands for brittle fault
zone. See text for details.
26
27
Figure. 5-6. Migrated NMO section of line V2. Data representation and geological
boundaries as in Fig. 5-4. BFZ stands for brittle fault zone. See text for details.
5.2 Reflection properties of the upper crust in the Olkiluoto area
The general observation of the HIRE results in the Olkiluoto area is the very strong
subhorizontal or gently dipping reflectors at the depth of 2-3 km. These reflectors are
typically 500-1000 m thick and very continuous laterally (Figs. 5-4 and 5-5). One of
these reflectors is at the surface on line V2, where it is associated with the Sorkka
diabase sill. Although this reflector is thinner and weaker than those at deeper levels, we
expand this correlation and attribute the strong reflectors to diabase sills. The
interpretation is supported on one hand by the gravity modelling of the Olkiluoto area
(see chapter 8) which is in good agreement with interpreted diabase reflectors at surface.
On the other hand, very extensive subhorizontal reflectors in the uppermost 5-14 km are
known in the Bothnian Sea (BABEL Working Group, 1993). They can be correlated
with Postjotnian on-shore sills in Finland and Sweden, and in one location the reflector
28
reaching the sea bottom is associated with an off-shore diabase outcrop (Korja et al.
2001).
In the Olkiluoto area the subhorizontal strong reflectors consist of numerous reflective
elements at a few tens of metres‟ vertical distances from each other. Due to their
internal reflectivity they cannot represent only homogeneous slabs of diabase. The
internal impedance contrasts may be attributed to shearing or compositional variations,
i.e., more felsic interlayers. Thus, the reflectors may rather represent dyke swarms than
single diabase sills.
Areas of Mesoproterozoic rapakivi granites are characterized by transparent, poorly
reflective domains in the sections (Figs. 5-4 and 5-5). This is attributed to the
homogeneous composition of the granites. There seems to be only weak reflection
differences between the rapakivi granites and the gneisses, which makes it difficult to
exactly locate their contacts at depth. The rapakivi granite is cut by the diabase dykes
(the strong subhorizontal reflectors) in several places. No difference can be found
between the Tarkki and Väkkärä types of rapakivi. Generally the rapakivi granite
extends at least to about 4 km depth in the SE part of line V1 and NE part of line V2.
In areas where the Palaeoprotezoic gneisses dominate the surface geology, such as the
Olkiluoto Island, the reflectors are relatively discontinuous and short, forming weakly
reflective packages. In the Olkiluoto Island, where abundant drilling data exist, the
major shear and fault zones seem to be well observable in the seismic section of V1
(Fig. 5-4). On the seismic section, the reflectors are dipping at an apparent angle of
about 25º, and assuming a strike of about 160º they would have a true dip of about 42º
to SE (Fig. 5-2). The reflectors undulate and they are disrupted at many places but
nevertheless they can be followed for distances of over 1 km. The reflectors are thicker
than the brittle fault zones detected in drillholes, and locally seem to correlate also with
pegmatitic granite (see also discussion in chapter 5.3).
Figure. 5-7. Stacking velocities of line V1. Interpreted geology is taken from Fig. 5-4.
The vertical scale is the two-way-travel time. To obtain an approximate depth estimate
in metres, multiply milliseconds by three.
29
Absolute velocities of the reflectors, if available, are very important information in
interpreting the geological character of reflectors. Seismic reflection data provide only
very coarse velocity information, i.e., the velocities used in compiling the CMP stacks.
These data have very poor resolution but can be applied for general comparison with
reflectors. As an example the stacking velocities of the line V1 are plotted at horizontal
intervals of 100 CMP points (625 m) together with interpreted geology of the reflectors
in Fig. 5-7. The results indicate that high stacking velocities (> 6 km/s) and velocity
gradients are generally associated with the strong subhorizontal reflectors at depths of 1-
4 km. It implies that the strong reflectors are indeed faster than their host rocks, which
means that they represent mafic rocks in felsic surroundings.
5.3 Synthetic seismogram and reflection properties of the Olkiluoto Island
In order to provide geological and petrophysical control to the interpretation of the
reflectors in the Olkiluoto Island, a synthetic seismogram was constructed from logging
data on drillhole OL-KR1 which is one of the deepest holes on the Island. The down-
hole density (gamma-gamma) and P-wave velocity (full wave sonic) logs were provided
for our use by the Client.
Seismic reflectivity is generated by seismic impedance contrasts in the rock. The
impedance, which is the product of velocity and density, was calculated from the depth
matched density and velocity logs. The impedance curve was convolved into a synthetic
seismogram, which can be directly compared with the traces of the measured seismic
section. Results are shown in Fig. 5-8. The projection of the hole on the seismic section
V1 is shown in Fig 5-9.
30
Figure 5-8. Drillhole logs of density and P-wave velocity, together with calculated
seismic impedance and synthetic seismogram of drillhole OL-KR1. Short normal
galvanic log is included as an indicator of fracture zones. Legend for the lithology
column is given in Fig. 5-9.
Lithological boundaries seem to generate only weak if any reflectivity in the OL-KR1
data (Fig. 5-8). An exception is the thin (< 5 m) mafic vein at 221 m, which has a
narrow spike on the impedance curve and relatively strong synthetic reflector at 212-
233 m. In contrast to this, the thick pegmatitic granite at 918-980 m does not generate
reflections, although there are considerable variations in density and velocity in
comparison to the hosting gneiss. Unfortunately, the variations act in opposite directions
and cancel out each other, and the resulting impedance profile and synthetic
seismogram do not show any anomalies. On the other hand, pegmatitic granite layers at
420-490 m associate with weak reflectors on the synthetic seismogram.
31
Figure 5-9. Projection of the drillhole OL-KR1 on the seismic profile V1. View is from
NE.
The synthetic seismogram predicts strong reflectivity to be related to fracture zones at
518-540 m and 609-621 m (depths taken from the galvanic log, Fig. 5-8). The fracture
zones are distinctly represented in the density and velocity variations, but the fracture
zones themselves are thinner than the seismic reflectors. Impedance contrast at the
depth of the fracture zones is of the order 40 %, and the corresponding reflection
coefficients are as high as 0.25. It is much above the detection limit of 0.06 typically
considered representative for seismic reflection data. The widening of the reflections in
comparison to the fracture zones can be attributed also to the applied frequency band of
the seismic signal, which is not sufficient to image accurately structures with
thicknesses in the same order of magnitude or smaller than the wave length of seismic
waves. The wave length is about 33 m assuming a frequency of 165 Hz and a velocity
of 5500 m/s. The Rayleigh criterion for thinnest detectable layer is one quarter of the
wavelength, i.e. about 8 m.
The weak correlation of pegmatitic granite and reflectors in Olkiluoto is probably due to
a small impedance contrast between the gneisses and pegmatite but also to brittle fault
zones that seem to be associated with pegmatite and surround the pegmatite granite
layers (see lithology and galvanic log in Fig. 5-8). In Figs. 5-4 – 5-6 we have interpreted
the uppermost (< 1 km) reflectors mostly as brittle fault zones, but these may include
also pegmatitic granite. A more detailed comparison with drillhole based 3D lithology
models is given in chapter 6.
32
33
6 COMPARISON OF REFLECTION DATA WITH LITHOLOGICAL AND FRACTURE ZONE MODELS IN OLKILUOTO
The comparison of the reflection data with lithological and fracture zone models refers
to Geological Site Model Version 1.1, published in the Site Description Report 2008
(Posiva Oy 2009). The model is an update of previously released model version 1.0
(Mattila et al. 2008). In the model, the geometry and properties of 152 lithological units
and 132 brittle deformation zones has been determined. The depth extent of the model is
1000 m, and it is laterally limited to major lineaments surrounding the Olkiluoto Island
(Fig. 6-1). The model is based on large amounts of data gathered during over 20 years,
including 48 deep and 34 shallow drillholes, over 450 observation points at bedrock
outcrops and 14 investigation trends as well as observations from the ONKALO tunnel.
In addition to geological observations, extensive geophysical surveying has been done
using a wide range of different methods, including airborne, ground and drillhole
measurements. Also a number of seismic surveys with different geometries have been
done, including two campaigns of 3D seismic surveys, VSP surveys from 14 drillholes
and seismic refraction on the ground surface. References to the original survey reports
can be found in the Site Description Report (Posiva Oy 2009).
Figure 6-1 shows that from the three HIRE survey lines, V3 and the NW end of line V1
are located at Olkiluoto Island, enabling the detailed comparison of the HIRE data with
the existing geological model and other (especially seismic) data at these lines.
Figure 6-1. Surface map showing the location of the Olkiluoto site area (white colour)
bound by regional lineaments shown as solid black lines. The locations of HIRE survey
lines V1, V2 and V3 (blue lines), ONKALO facility (red line) and 3D seismic survey
sites are also shown (black rectangles).
V1
V2
V3
34
Figures 6-1 and 6-2 show a plan view and a vertical section of the lithological and
brittle deformation models. The dips of the modelled lithological units are typically
moderate or gentle to SSE – S, following the general orientation of foliation. The only
exception is a number of narrow diabase dykes, dipping steeply to NW. Also most of
the brittle features follow the direction of foliation, but there are also steeply dipping
zones, trending N-S, NNE-SSW and NE-SW.
Figure 6-3 presents the results from the NW end of HIRE line V1 with lithology. It can
be seen, that several reflectors within the depth of 0 – 1000 m correlate rather well with
the lithological features, especially pegmatitic granite veins, giving also a clear picture
of the foliation system of the area. No clear seismic indications are related to diatexitic
or TGG gneisses. The strong reflection at the depth of 1000 – 1700 m seems to be an
extension of two major pegmatitic granite veins, located at the left margin of Fig. 6-4c.
According to drillhole data, P-wave velocity of these granites is relatively high (e.g.
5913 m/s, average for section 815 – 865 m in OL-KR47) compared to the surrounding
gneisses (e.g. 5703 m/s, average for section 740 – 785 m in OL-KR47). The synthetic
seismogram suggests only weak or no reflectivity at the pegmatite-gneiss contacts (Fig.
5-7). Apparently the impedance contrast is variable and may be affected by the gneiss
composition and the directions of schistosity and layering.
35
Figure 6-2. Site Geological Model of Olkiluoto, lithology and brittle deformation. Z = 0
m.a.s.l.
Figure 6-3. Site Geological Model of Olkiluoto, lithology and brittle deformation. N-S
trending vertical section at X = 1526000. BFZ (with numbering) denotes brittle fault
zone. Arrows in Fig. 6-2 show the location of the section.
36
Figure 6-4. NW end of HIRE profile V1 a) with the 3D lithological model, b) as such, c)
with a vertical section of lithology (red = PGR, orange = TGG, light blue = DGN, dark
blue = MGN, green = DB). View to NNE.
a
b
c
37
Figure 6-5 shows the results from the NW end of HIRE line V1 with the modelled
features of the brittle deformation model. Two rather discontinuous lines of reflectors
above the depth of 1500 m nicely correlate with gently dipping fault zones OL-BFZ098,
OL-BFZ080, OL-BFZ099 and OL-BFZ002, forming two separate pairs of fault zones.
The core zone thicknesses of these features are only some meters at maximum, but their
influence zones, characterized by increased fracturing, slickensided fractures, alteration
and elevated hydraulic conductivity, are several tens of meters thick. In some places, the
reflectors are clearly cut, displaced or bent (e.g. locations 1, 2, and 3 in Fig. 6-4a). At
location 1, there is a dense group of steeply dipping fault zones, explaining the
discontinuity of the reflectors. At locations 4, 5 and 6, the reflectors end up or they are
significantly displaced, possibly suggesting the existence of a major steeply dipping
fracture zone related to a lineament, bounding the whole site (it location is shown by the
left arrow in Fig. 6-5a).
Despite of the nice correlation with some features, all the details in the seismic results
can not be explained with the existing brittle deformation model. On the other hand,
there are numerous modelled brittle deformation zones that do not give any seismic
indications, probably due to unfavourable geometry (e.g. steep dips) or limitations in
resolution (small dimensions of the features or insignificant contrasts in acoustic
impedance).
The seismic results indicate that the extents of fault zone pairs OL-BFZ098 & 080 and
OL-BFZ099 & 002 are rather well defined, however, they may be more displaced or
bending in some places. A clear reflection between locations 1 and 3 in Fig. 6-5a is
totally a new indication. On the ground surface, it coincides with the Liikla shear zone,
a major ductile zone, trending ENE – WSW. Its dip is not well known, however.
According to drillhole observations, also brittle deformation is related to this zone. In
GSM V. 1.1, the brittle fault zone OL-BFZ146 has been modelled within the Liikla
shear zone, having an orientation of 155-165/60. The seismic data, however, indicates
that the shear zone and the probable overprinting brittle deformation zone might be
more gently dipping. This is supported by the observations from drillholes OL-KR49
and OL-KR50.
On the short HIRE line V3 (see the location in Fig. 6-1), the same main reflectors can
be observed as on line V1. However, due to the shortness and the sharp turn of the line,
the data are poorer than on line V1 and fewer details can be seen. In Figure 6-6, the
results from line V3 with the main gently dipping brittle fault zones can be seen.
Especially the lowest fault pair (OL-BFZ002 & 099) induces clear seismic reflections,
whereas the indications related to upper OL-BFZ080 & 098 and OL-BFZ056 (&018)
are less clear but still observable.
38
Figure 6-5. NW end of HIRE profile V1 a) as such, b) with some gently dipping main
features c) with a vertical section of the brittle model. View to NNE.
1
2
3
Locations of bounding lineaments
5
4
6
a
b
c
39
Figure 6-6. HIRE profile V3 with the main gently dipping fault zones OL-BFZ056, OL-
BFZ098, OL-BFZ080, OL-BFZ099 and OL-BFZ002. Behind profile V3, also a portion
of profile V1 is shown. View to SW.
V3 V1
BFZ056
BFZ098 & 080
BFZ099& 002
V1
40
41
7 COMPARISON OF HIRE RESULTS WITH PREVIOUS 3D SEISMIC REFLECTION SURVEYS IN OLKILUOTO
Two 3D seismic surveys have been done in Olkiluoto as separate campaigns. The first
survey was done as a pilot study in 2006 (Juhlin & Cosma 2007). The survey covered
an area of about 650 m x 600 m, located slightly NW of ONKALO facility. As a follow-
up to the work carried out in 2006, a new 3D seismic survey was done in 2007, focusing
on the eastern part of the site (Cosma et al. 2008). The survey covered an area of about
1150 m x 1050 m. The locations of the both survey areas with HIRE lines V1 and V3
are shown in Fig. 7-1. HIRE line V1 passes through only the SW margin of 2007 survey
area (with poorer data than in the central area), but crosses the 2006 survey area more
centrally. HIRE line V3 traverses rather nicely the 2007 survey area. However, the
crooked geometry complicates the interpretation, and the SW-NE strike of the brittle
fault zones runs more parallel to the line V3 than V1.
Figure 7-1. Locations of the seismic 3D surveys (purple rectangles) and HIRE profiles
V1 and V3 (green lines).
42
In the processing phase, the data from 2006 have been binned into 12 x 12 m and that
from 2007 into 8.33 m x 12.5 m CDP bins. Fold is highly variable in both surveys
(Figures 7-2 and 7-3). For 3D migration and depth conversion of the migrated volume,
DMO (dip-move-out) velocities between 5300 - 6000 m/s have been used in 2006
survey. In 2007 survey, the applied velocity has been 5750 m/s (Cosma et al. 2008). The
results of the 3D surveys have been available as depth migrated volumes, slices (inline,
crossline and horizontal) (Fig. 7-4) as well as amplitude block models (Fig. 7-5).
In seismic surveying it is not possible to give any exact detectability limit for the
thickness of geological structures, since it depends on so many factors (contrast in
elastic parameters and dimensions of the reflector). The contrasts in Olkiluoto are
typically rather small, and according to wave theory thicknesses down to ¼ of
wavelength are considered adequate for detection, when the reflection coefficient is
above 0.06. The upper limit of frequency range achieved in the 3D surveys of Juhlin
and Cosma (2007) and Cosma et al. (2008) is limited to 200…300 Hz. Using P-wave
velocities 5000 – 6000 m/s this corresponds to thicknesses between 4 and 7.5 m.
43
Figure 7-2. Fold for the 2006 pilot 3D survey. Fold is for 12 m x 12 m square CDP bins
(Juhlin & Cosma 2007).
44
Figure 7-3. Fold for the 2007 3D survey. Fold is for 8.33 m x 12.5 m rectangular CDP
bins (Cosma et al. 2008).
The comparison of HIRE and 3D seismic results (Fig. 7-6) shows that within the
investigation depth of 1000 m for the 2006 and 2000 m for the 2007 survey, there are a
number of common features. Most of the distinct reflectors observed by HIRE have
been detected also with the 3D seismics. Also the depth conversions are well in
concordance. However, there are some clear features in HIRE data at shallow depths
(<300 m) that have not been detected by 3D surveys. The data from the uppermost 300
m in the 3D surveys are incoherent due to the sparse receiver line spacing and
comparably long receiver-transmitter offsets applied in the primary processing.
In Figure 7-7, a part of HIRE line V1 is presented with the data from the two 3D
surveys. The clear reflectors at the depth of c. 500 – 800 m observed in the 2006 survey
(area 1 in Fig. 7-7) are almost identical in the HIRE data. These reflectors are related to
brittle fault zone OL-BFZ099 and 002, forming a pair of gently/moderately dipping pair
45
of thrust fault zones. Furthermore, at location 2 (Fig. 7-7) there are rather weak
reflectors in both datasets, indicating another pair of gently dipping faults, OL-BFZ098
and 098. Due to the low fold, the corresponding data from 2007 survey is poorer at the
location of V1 HIRE line, but there is some correlation deeper down at the depth of c.
1700 m (location 3 in Fig. 7-7) probably related to massive pegmatitic granite veins. In
more central locations, the data from 2007 survey indicates several clear reflectors than
can be correlated to HIRE results and 2006 data.
In Figure 7-8, HIRE profile V3 (with V1 in the background) with 3D data is presented.
The slice of 2007 data at the location of V3 shows only rather weak reflections
compared to the HIRE data (Figure 7-8b). However, the most significant reflectors from
the whole 3D volumes correlate rather nicely with HIRE data (Figure 7-8c) also on V3.
In spite of this, compared to HIRE line V1 the correlation is poorer on line V3 probably
due to the shortness and sharp bending of the line.
Figure 7-4. 3D depth migrated data cubes, 2006 survey (right) and 2007 survey (left)
(Cosma et al. 2008). View from N.
46
Figure 7-5. Seismic 3D data as amplitude block models, 2006 and 2007 surveys. Only
the blocks with highest (> 0.5 for 2006 survey and > 1.8 for 2007 survey) amplitudes
are shown. View from SW.
The most important advantage of HIRE data compared to the 3D surveys is the
considerably greater investigation depth, exceeding 5000 meters. It gives totally new
information on the probable subhorizontal diabase system located at the depth of c.
2500 – 3000 m below Olkiluoto. Due to the high quality of the data, the HIRE survey
supplements the 3D seismic results also at the shallower depths. It confirms the
interpretation of the main gently dipping fault zones and suggests some new features not
observed in the existing 3D seismic data. In addition, the bigger offsets allowed steeper
structures to be observed.
47
Figure 7-6. NW part of HIRE profile V1 a) as such, b) with the most distinct reflectors
from 2006 and 2007 3D surveys (blue colour). View to S.
a
b
48
Figure 7-7. NW part of HIRE profile V1 a) as such, b) with the data from 2006 and
2007 3D surveys.
1
1
2
3
V1
V1 2006 3D data 2007 3D data
a
b
49
Figure 7-8. HIRE profile V3 (with V1 behind) a ) as such, b) with the data from 2007
3D survey, c) with most distinct reflectors from 2006 and 2007 surveys (blue colour).
V3 V1 V1
V1 V1
V3
2007 3D data
a
b
c
V3
50
51
8 HIRE RESULTS AND GRAVITY SURVEYS
Geological Survey of Finland has measured altogether 1994 gravity stations, shown in
Figure 8-1, in the vicinity of the HIRE survey lines. The Common Depth Points (CDP)
of the HIRE survey have been plotted on the corresponding Bouguer anomaly map
shown in Figure 8-2. The major anomaly is the negative gravity anomaly of about -10
mGal due to the Väkkärä rapakivi granite intrusion. There are positive residual gravity
anomalies of +1.0 mGal to +2.4 mGal to the SE and to the NNE from the Väkkärä
intrusion. The gravity anomalies due to the Väkkärä rapakivi granite and the diabase
sills have been interpreted and discussed earlier by Elo (2001). Here we present results
of gravity modelling of the HIRE lines V1 and V2 based on new gravity measurements
along the roads.
Gravity anomalies were modelled using 3D frustum and 2.5D polygonal bodies with the
ModelVision V9.0 program. Some distortion is inevitable due to the measurements
along meandering roads. The models are shown in Figures 8-3 and 8-4. The thickness of
the Väkkärä intrusion is about 3500 m and the thickness of the Sorkka diabase is about
140 m – 170 m. The contacts and depth extents are realistic but the shape of the bottom
Väkkärä intrusion is indeterminate.
Figure 8-1. The location of the GTK gravity stations. New observations along the HIRE
lines are shown in red.
52
Figure 8-2. The location of Common Depth Points (CDP) on a Bouguer anomaly map.
Figure 8-3. The gravity model of the line V1 along the road with projected locations of
Pole locations.
53
Figure 8-4. The gravity model of the line V2 along the road with projected locations of
Pole locations.
54
55
9 DISCUSSION
The HIRE seismic reflection survey resulted in a new deep image of the bedrock
structures in the Olkiluoto area. Numerous previously unknown structures are revealed
in the seismic sections.
The geological correlation of the reflectors indicates that the strong subhorizontal
reflectors are mostly Postjotnian diabase sills, or swarms of sills which form strong 500-
1000 m thick packages of reflectors. The data implies that there is considerable
variation in the internal impedance properties of the reflectors. The outcropping diabase
reflectors in the Olkiluoto area and in western Finland in general do not give a direct
explanation to the implied heterogeneity. The outcropping diabases are dominantly
olivine diabase, but locally also other diabase types are known. In the island of Säppi,
which provides one of the largest surface outcrops of Postjotnian diabase, about 25 km
to NNW of Olkiluoto, the diabase types present are olivine diabase, pyroxene diabase,
megaophitic diabase and diabase pegmatite (Inkinen 1963; Suominen 1991). Magmatic
layering is also observed in Säppi.
The seismic velocities and densities of different diabase varieties in Säppi were
calculated with the code of Hacker and Abers (2004) using the modal composition data
by Inkinen (1963). The results imply that the compositional variations of diabase
generate reflection coefficients in the range of 0.01-0.04 which is too small to generate
the observed internal reflectivity of the major subhorizontal reflectors. Therefore the
internal reflectivity the strong reflectors should probably be attributed to shearing, or
alternating layers of diabase and a more felsic rock type.
Fracture zones are clearly reflective on the Olkiluoto Island. Many of these reflectors
correlate also with pegmatitic granite, which seems to have variable reflection contrasts
with the surrounding gneisses. Therefore, the reflectors interpreted as brittle fault zones
(Fig. 5-4) may also be partly due to pegmatite. A more thorough discussion on this
problem would require a detailed comparison of drillhole data, reflectors as well as
synthetic seismograms calculated for other holes in the Olkiluoto Island. Moreover, we
pay attention to the fact that the research tunnel ONKALO runs across one of the
reflectors interpreted as a brittle fault zone (Fig. 9-1). Therefore the structure could be
directly correlated with geological observations in the tunnel.
56
Figure 9-1. A detailed perspective view of the seismic line V1 from NE together with the
research tunnel “ONKALO” (yellow lines). The tunnel is cutting the brittle fault zone
OL-BFZ098 (pink surface) modelled from earlier drillhole data. BFZ: Brittle fault zone.
The seismic sections provide many reflectors potentially representing the lineaments
constraining the Olkiluoto bedrock block (Fig. 6-1). On line V1, the interpreted brittle
fault zone reaching the surface at CMP 2450 is a candidate for the SW-NE lineament
running along the strait between the island and the continent (Fig. 5-4 and 6-1). The
apparent dip of the reflector on the seismic section of V1 (33º) and the angle between
the lineament and the seismic line (60º) indicate a true dip of about 38º in the azimuth of
246º (Fig. 5-2).
On the other hand the brittle fault zone interpreted on V2 at CMP 2900 (Fig. 5-5) is
located on the extension of one of the lineaments constraining the NE border of the
Olkiluoto block (Fig. 6-1). Here the apparent dip of the structure is 15º NE, and the
angle between the lineament and the seismic line is about 60º. From Fig. 5-2 we obtain
a true dip of about 18º to azimuth 75º (NE). Assuming that the correlation of the
structure on the seismic section with the lineament is correct would suggest quite
different dip direction for the lineaments on the NE side of the Olkiluoto block, than
suggested in earlier studies. The result requires, however, more detailed analysis. An
interesting option would be to combine statistical fracture orientation analysis of
fractures on outcrops in the vicinity of the crossing points of lineaments and seismic
lines. The fracturing is expected to indicate the orientation of the lineaments (strike and
dip) which could then be correlated with the seismic reflection data.
The structures interpreted as rapakivi granite are characterized by low-amplitude
reflectivity, and often seismically transparent areas. It is in contrast with the gneisses
which show commonly internal small-scale reflectivity, which can be attributed to the
migmatitic and other compositional variations. However, the difference between
reflection properties of rapakivi granite and gneisses is small and gradual, and therefore
their contact zones are not obvious in the sections. Here we have applied the gravity
57
models by Elo (2001) as guides to find the most probable structures representing the
rapakivi-gneiss contacts. A comparison with the seismic models and gravity models is
shown in Fig. 9-2.
The HIRE 2D seismic reflection data agrees well with the earlier 3D surveys (Juhlin and
Cosma 2007; Cosma et al. 2008) in the Olkiluoto Island, and most of the reflectors
detected in HIRE surveys are present also in the 3D results. The 2D data can be used to
extend the interpretation of the 3D results in the Olkiluoto Island.
Figure 9-2. Perspective view of seismic section V1 and the gravity profiles 1 and 2 of
Elo (2001). View is from SW. In the gravity models the black and white areas refer to
the modelled boundaries of the Väkkärä rapakivi granite. The Tarkki granite cannot be
modelled with gravity because it has practically no density contrast with the
surrounding gneisses. RKGR: rapakivi granite, DB: diabase.
58
59
10 CONCLUSIONS
The HIRE seismic reflection survey in the Olkiluoto area revealed numerous previously
unknown structures in upper crust of the area.
The most prominent structures observed are the subhorizontal strong reflectors which
very probably represent Postjotnian diabase sills intruding both the Svecofennian
gneisses as well as the rapakivi granites. These reflectors can be associated with the
similar seismic structures recorded in marine seismic transects in the Bothnian Sea, and
thus they are parts of large-scale crustal structures.
The Mesoproterozoic rapakivi granites can be distinguished as homogeneous,
seismically transparent domains which extend to a depth of at least 4 km. The
interpreted rapakivi structures are in a good agreement with earlier gravity modellings.
On the Olkiluoto Island, reflectors can be correlated with drillhole based data on
lithology and brittle fault zones. The main brittle fault zones detected in drillholes are
represented as reflectors in the seismic sections, and several new structures have been
interpreted. A synthetic seismogram constructed for a deep hole in the Olkiluoto Island
using down-hole logs of density and P-wave velocity, suggests that the main brittle fault
zones are able to generate strong reflections. Pegmatitic granite sometimes has a weak
reflection contrast with the surrounding gneisses, but sometimes has no contrast. There
seems to be a common, although not systematic correlation of reflectivity, fracture
zones and locations of the pegmatitic granite occurrences.
The HIRE 2D seismic reflection data agrees well with the earlier 3D surveys in the
Olkiluoto Island, and most of the reflectors detected in HIRE surveys are present also in
the 3D results. The 2D data can be used to extend the interpretation of the 3D results in
the Olkiluoto Island.
Where the seismic lines cross the lineaments limiting the Olkiluoto bedrock block, the
seismic data can be used to provide estimates of the dip angles of the lineaments. The
results suggest that the lineament separating the Olkiluoto Island from the continent
would be dipping about 38º to SE, and one of the lineaments limiting the NE side of the
block would be dipping 18º to NE, respectively.
60
61
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