/

Interpretation of Refraction and Reflection StackData Over the Brevard Fault Zone in South Carolina

by ·

Kenneth J. Laughlin

Thesis submitted to the Faculty of the

Virginia Polytechnic Institute and State Universityl

in partial fuliillment of the requirements for the degree ofMaster of Science

J

Geophysics

APPROVED:(‘ U / r

__ .

’ Cahit Co h, Chairm

< a

John K. Costain dwin S. Robinson

January 27, 1988Blacksburg, Virginia

Interpretation of Refraction and Reflection StackData Ovcr the Brevard Fault Zone in South Carolina

byKenneth J. Laughlin

Cahit Coruh, ChairmanGeophysics

(ABSTRACT)

l Near surface structures across the Brevard fault zone are studied using the refraction and re- .flection arrivals recovered from the Appalachian Ultradeep Core Hole (ADCOH) regional seismic

Line 1. In using refracted arrivals, a new processing approach is introduced that translates refracted

first arrivals from multifold seismic data into a refraction stack of two-way delay time sections.

Reprocessing of reflected arrivals has improved shallow reflectors and allowed better imaging of the

Brevard fault zone. Following processing of refraction a11d reflection arrivals independently, both

data sets are combined into a composite stack section. The composite stack section displays one

bright refractor interpreted as the boundary between the weathered layer and high velocity crystal-

line rocks. This refractor is continuous in the Inner Piedmont with occasional vertical offsets. The

continuity of the refractor diminishes across the Brevard fault zone. In the eastern Blue Ridge, the

refractor is discontinuous with high angle truncations. On the composite stack section, the Brevard

fault zone can be traced from the _surface to 6 km (2 s) where it appears to splay from the Blue

Ridge thrust. Different from previous interpretations, the Brevard fault zone is imaged as having

both an upper and a lower boundary surface as well as a group of reflectors within the zone. This

reflection package initially thickens to 2 km at 3 km depth, then thins as it reaches the Blue Ridge

master decollement. The Blue Ridge thrust is as shallow as 1.5 km (0.5 s) at the northwest end of

the Line l. A deeper decollement is interpreted below the Blue Ridge thrust. The depth of this

deeper thrust is 3 km (1 s) at the northwest end of the line, and also joins to the Blue Ridge thrust

at 6 km depth making the structures below the Brevard fault zone more complex than previously

published.

Acknowledgements

Acknowledgements iii

I Acknowledgements iv

i

I

Table of Contents

Introduction ............................................................ I

Available Seismic Data ................................................,... 8COCORP data ....................................................... 10Seisdata rellection data .................................................. 10

Seismic Data Processing .................................................. I3Refracted Arrivals ..................................................... 13Reflected Arrivals ..................................................... 18Combined Refraction and Reflection Stacks .................................. 18

Discussion and Interpretation ............................................... 24Ability of Refraction Stacks to Image the Subsurface ............................ 24Correlation of available data sets .......................................... 26lnterpretations ..............,......................................... 28

Conclusions ...................................................,....... 37

Bibliography ........................................................... 39

Appendix ............................................................. 4I

Vita ................................................................. 42

Table of Contents v

List of Illustrations

Figure 1. Location map for ADCOH, COCORP, and Seisdata seismic lines ............ 5

Figure 2. ADCOH Linel and HI-RES 8 seismic profiles .......................... 6

Figure 3. COCORP and Seisdata seismic profiles ................................ 7

Figure 4. ADCOH Line 1 noise test ........................................ 12

Figure 5. Concept of the refraction stack processing ............................. 20

Figure 6. Refraction stack processing ........................................ 21

Figure 7. Constant velocity refraction stack (CVRS) panels ....................... 22

Figure 8. Residual statics irnprovements in refraction stack sections ..................22

Figure 9. Portion of reprocessed ADCOH regional ............................. 23

Figure 10. Refraction stack sections with surface elevation plots ..................... 31

Figure ll. Fault signature in shot gathers ..................................... 32

Figure 12. Travel time·offset (t-x) curve of noise test. ............................. 33

Figure 13. Portion of the composite stack section ............................... 34

Figure 14. Portion of the cornposite stack section after migration .................... 35

Figure 15. Final automatic line drawing from composite stack section ................. 36

List of lllustrations vi

Introduction

The purpose of this study is to examine further near surface structures across the Brevard fault

zone (Figure 1) in South Carolina by using refracted and reflected arrivals from the Appalachian

Ultradeep Core Hole (ADCOH) project multi-fold seismic data.

The primary goal of the ADCOH project is to examine the basic tectonic processes and crustal

structure of a composite crystalline thmst sheet formed by continental collision. Because composite

crystalline thrust sheets are among the largest structures in orogenic belts, they are important to the

overall processes involved with mountain building and the evolution of continental crust (Hatcher

and others, in press). The subsurface image of the BFZ has been one of the important targets of

the site selection of the ADCOH project. The 10-12 km ultradeep core hole will sample the

Brevard fault zone (BFZ) permitting comparative structural and thermochemica.1/isotopic studies

of the formation of ductile and brittle fault zone with multiple displacements.

IThe general study area (Figure 1), located in Oconee County South Carolina near the town of ‘

Westminster, encompasses three major tectonic provinces over which a considerable amount of

geological-geophysical investigations have been completed. From southeast to northwest in I

Figure 1 these provinces are: (1) the crystalline Inner Piedmont allochthon and Chauga Belt; (2) I

IIntroduction I

I 1II

II

the Brevard fault zone (BFZ); and (3) the crystalline Blue Ridge allochthon (BRA). Across these

three tectonic provinces a distinct assemblage of rock types have been studied (Hatcher, 1971).\Vithin the Brevard fault zone, rocks are pervasively sheared mylonites, ultramylonites,

blastomylonites, and phyllonites (Hatcher, 1971). The Inner Piedmont rocks in the study area

consist largely of granitic gneiss, arnphibolite, and biotite gneiss. Studies of the Chauga Belt rocks

indicate that they are of lower grade than the high grade terranes of the eastem Blue Ridge and

Inner Piedmont. These are composed of augen gneiss and lineated granitic gneiss. The eastem

Blue Ridge geologic province is predominately granitic gneiss, biotite schist and biotite gneiss,

homblende gneiss, and upper arnphibolite facies of metavolcanic and metasedimentary rocks

(Hadley and Nelson, 1971).

Both the Blue Ridge allochthon as well as the Inner Piedmont and Chauga Belt are proposed

to 11ave been displaced northwest along the Blue Ridge master decollement for tens to hundreds

of kilometers (B1yant and Reed, 1970). Hatcher (1971) further proposed that the BFZ formed from

failure on the northwest limb of tl1e Low Rank Belt Synclinorium due to motion along the Blue

Ridge thrust. The fault initiation, which separated the Irmer Piedmont allochthon and Chauga Belt

from the Blue Ridge allochthon, occurred within incompetent rocks in the northwest limb

(Hatcher, 1971). According to Cook and others (1979), the displacement along the Blue Ridge

master decollement could be as great as 260 kilometers. Hatcher and others (1987a) indicate that

this displacement is significantly greater.

The Brevard fault zone is mapped as a narrow linear belt of intense shearing and faulting from

the Alabama Piedmont to near the North Carolina-Virginia border. Due to motion on the fault,

a variety of different lithologies can be found within a small traverse across the BFZ. Because of

this, the Brevard fault zone has long been considered a significant Appalachian orogenic boundary,A

in addition to a major crustal failure (Jonas, 1932; King, 1955; Reed and Bryant, 1964; Bryant and

Reed, 1970; Hatcher 1971; Hatcher and others 1987a; and Hatcher and others 1987b). The Brevard

as a thrust was first presented by Jonas (1932).

Introduction 2

Imaging of the Brevard fault zone using seismic reflection data was reported with different em-

phasis in the literature. Successful imaging of reflections from the BPZ was first published by Clarkand others in 1978 closely followed by Cook and others (1979) with results from combined

COCORP reflection profiles. Seisdata seismic profiles with line drawings and interpretations of theBrevard zone were published by Behrendt in 1986. Better images of the BPZ were recovered from

the collection of ADCOH seismic data (Coruh and others, 1987; Chowdhury and Phinney, 1987).

The seismic characteristics of the Brevard fault zone as determined by these studies have been

imaged differently in each data set. Using velocity spectra, Clark and others (1978) recovered re- _flections from the top and bottom of the BPZ, near Rosman N.C., giving the BPZ a thickness of

0.9 km with a velocity of 5.9 krn/sec. On the other har1d, interpretations of COCORP data in

Georgia by Cook and others (1979) show the BPZ as a single discontinuous group of reflectorsrooted in the Blue Ridge master decollement (BRMD). Behrendt’s (1986) interpretation of the

- BPZ from Seisdata lines is as a series of southeastward dipping reflections. ADCOH regional Line

1 (Qomh and others, 1987) define the BFZ as a zone of east dipping reflectors between highly re-flective Inner Piedmont and poorly reflective Blue Ridge rocks. The high resolution ADCOH

seismic profile (Chowdhury and Phinney, 1987) again image the BPZ as a zone, with a maximum

vertical thickness of 0.4 s (1.2 km), having distinct top and bottom reflectors.

Despite all these studies, the first 0.6 s (1.8 km) part of the BPZ was in question. While imaged

well at depth, raw continuous reflections were observed in the first 0.6 s on ADCOH regional Line

1 (Figure 2a) and the first full second is not published on ADCOH high resolution Line 8

(Figure 2b). On both COCORP line GAI (Figure 3a) and Seisdata Lines 6 and 8 (Figure 3b)

only deep reflections were observed.

To correlate the imaged part of the BPZ with surface geology, shallow seismic data were con-

sidered to be essential. In this paper, HH attempt is made to image the geometry of the subsurface Kstructure of the BPZ by using both refracted and reflected arrivals from the ADCOH regional

LineIntroduction3

I1 data set. The refraction and reflection arrivals are processed, evaluated, and interpreted. The re-

· sults are three refraction stack sections of refracted first arrivals, reduced to delay time for the entire

ADCOH regional Line 1, reprocessed reflection stack sections (unmigrated and migrated) from

CMP 710 to 1680 to a depth of 4 seconds, ar1d a combined refraction and migrated reflection stack

section. The different refraction stack sections with different offset ranges and fold illustrate differ-

ent degree of smoothing effects. Composite stacks as a combination of specific refraction and re-

flection stack sections are used for final interpretation. This final interpretation also incorporates

results from surface geology, shallow core hole data, and a noise test.

II

Introduction i 4

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Figure 2. ADCOII Lmel and Ill-RES 8 scismic profiles: (a) AD(l()l-I regional Line 1 automatic linedrawing (from Coruh and others, 1987). (b) ADCOII l·lI-RES 8 section (from Cliowdlnury Iand Plnnney, 1987) begins at 1 s depth at which the ßrevard fault zone is approximatclv I(1.4 s tlnck. The BFZ and lower Paleozoic platform sedimcnts (LPPS) are well imaged in ,both sections below 1 s. „

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Figure 3. C()C()Rl’ and Seisdata seismic profiles: (a) COCORP Line GA1 (from Cook and others, I1979) in vicinity of the Brevard fault zone reflections. 'fop arrow indicatcs reflections from Ithe Brevard fault zone. Lower arrow denotes lower Paleozoic platform scdiments. (lm) Line Idrawing intcrpretations of SD-6 and SD·8 (from Behrendt, 1986). Brevard mult zone re- Iflections are indicated as eastward dipping rcflcctions packages. All three of these data sets Iimage few shallow reflcctions or reflections from the Brevard fault zone. I

I

IIntroduction 7

I

Available Seismic Data

A short description of the data sets acquired near the study area with information relative to the

BFZ is given below. The ADCOH regional data set contains 180 km of vibroseis profiles that were

acquired by Westem Geophysical Company in Georgia and South Carolina (Figure 1) for the

ADCOH project.

ADCOH regional Line 1 data acquisition was conducted with 120-channel recording system and

five vibrators using 24 s up-sweeps with 14-56 Hz. The recording time was 32 s to give 8 s of

full-correlation time. Spread configuration was a symmetrical split-spread with near and far offsets

of 200 and 4150 m. Receiver groups consisted of twenty-four 10 Hz digital grade geophones cov-

ering 0.134 km in length and spaced at 0.067 km group intervals. Shot intervals were also 0.067

km giving a maximum subsurface coverage of 60 fold. Because of higher signal-to-noise ratio of

the ADCOH regional lines and their availability, the ADCOH regional Line 1 is used in this study.

Prior to collecting Line 1, a noise test was conducted at the beginning of the line ("NT" in_

Figure 1) within Inner Piedmont rocks. The noise test geometxy was straight line with bunched

receiver groups spaced at 0.003 km intervals. 5.4 km of coverage were obtained in 16 records to

give 0.07 km and 5.4 km near and far offsets. The source was a single vibrator with 8-56 Hz up-Available Seismic Data 8I

sweeps. The resulting shot ordered, as continuous one·fold, data are shown in Figure 4. This data

set is used as a key to interpret the refracted arrivals of the ADCOH regional Line 1.

ADCOH Line 1 was originally processed at the Regional Geophysics Laboratory at Virginia

Tech and published in the forms of conventional sections to a depth of 8 s (Costain and others,

1986) and automatic line drawings to a depth of 14 s (Coruh and others 1987). The imaging quality

of the data is among one of the best over crystalline terrane and possible interpretations and their

implications are also discussed by Hatcher and others (1987b). Interpreted from Line 1 are the Blue

Ridge thrust (BRT) and the Brevard fault zone which appears to splay from the BRT at 6 km

depth. The BFZ is interpreted as a group of eastward dipping rellectors, from a depth of approxi-

mately 0.6 s (1.8 km) to 2 s (6 km) respectively, bounded by the highly reflective Inner Piedmont

and the relatively transparent Blue Ridge (Coruh and others, 1987). Most importantly, the BFZ

was imaged to separate the highly reflective Inner Piedmont rocks from the poorly rellective Blue— Ridge rocks. The BFZ splays into the BRT at a depth of 1.8 s (5.4 km). Additionally, subhori-

zontal reflections interpreted as nearly undisturbed lower Paleozoic platform sediments (LPPS) as

well as Eocambrian-Cambrian('?) rift basins are well represented in the line drawings at approximate

depth 2.6 s (7.8 km).A

ADCOH high resolution Line 8 (Figure 1) was acquired using 120 receiver groups at 0.033 km

intervals leading to a 4 km spread. The source pickets were 0.033 km apart with 16 intermediate

vibrator points with coverage ranging from 30 to 60 fold. A nonlinear sweep of 20-80 Hz was ap-

plied for 27 s giving 5 s of full-correlation time (Chowdhury and Phinney, 1987).

The published results of ADCOH high resolution Line 8 display part of the unmigrated pre-

liminary stack section from 1 s to 3 s two-way traveltime (Chowdhury and Phinney, 1987). Clearly

imaged in this section are two reflectors from 1 s (3 km) to 2 s (6 km), interpreted as the top and

bottom boundary of the Brevard fault zone, separated by a region of seismic transparency of 0.4 s

(1.2 km) maximum thickness. These reflected events are furthest apart at 1 s while closest together

Available Seismic Data 9

I

I

at 2 s depth. This geometric configuration of the BFZ indicates a low angle thrust rooted in the

· BRT at approximately 6 km depth. .

COCORP data

COCORP Georgia Line GA1 (Figure 1) was recorded with a 30 s sweep of 8-32 Hz for 15 s

of full-correlation data. Cook and others (1979) state that the BFZ does not extend beneath 2 s

two-way traveltime (6 km). On GA1, Cook and others (1979) interpret the BFZ to be a single

discontinuous package of reflections that dips l6° east and is rooted in the sole thrust BRT ('?)

above to an apparent offset in the Paleozoic shelf strata beneath beneath the BRT. This relation-

ship supports Hatcher’s hypothesis (1971) that thrust ramping along the west flank of the synformal

Chauga Belt was initiated by a normal fault in underlying sedirnents. Additionally, calculations off

minimum offset on the Brevard fault zone give 8 km of throw and 30 km heave. The same inter-

pretation for the Brevard fault zone was republished by Brown and others (1986).

Seisdata reflection data

Seisdata seismic reflection data (1350 km of 96 channels) along three profiles in South Carolina and

Georgia (Figure 1) were interpreted by Behrendt (1986). The data set has 24 fold maximum cov-

erage and was recorded with a sweep of 48-12 Hz, 24 s long, from four vibrators.

Published line drawing from profiles over the Brevard fault zone display discontinuous dipping

reflectors interpreted as the BFZ to a depth of 2 s (Behrendt, 1986). Using the Seisdata reflection

seismic data set with the line drawing interpretation of Behrendt (1986) and aeromagnetic profiles,

Costain and others (1987) suggested the Brevard fault zone as the "floor" of a large strike·slip duplex

that extends approximately 300 km to its root beneath the Atlantic Coastal Plain ir1 NorthCarolina.AvailableSeismic Data 1 10

Despite all this published data, the Brevard fault zone at shallower depths was not imaged

clearly to give a full correlation with the surface geology. The most shallow image of the BFZ is

ir1 the ADCOH regional rellection seismic Line l and extends up to 0.6 s depth (Coruh and others,

1987). Therefore the ADCOH Line 1 data set is reprocessed in this study to obtain an improved

image of the Brevard fault zone at shallower depths.

l

Available Seismic Data ll

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Available Seismic Data IZ

II

Seismic Data Processing

One of the major objectives of this study is the use of refracted arrivals to image the Brevard fault

zone at shallow depths. To make the refracted arrivals useful for interpretation an unconventional

data processing flow is introduced in which a concept to produce a refraction stack data is followed.

Later, the refraction stack data are combined with the conventional reflection stack data to obtaincomposite stack sections.

Refracted Arrivals

The standard processing sequence for multifold reflection data requires that refracted wave ar-

rivals be discarded by the application of muting because the NMO correction causes a time variant

stretching that effects the early arrivals most. Also, the reflected wavefront follows a hyperbolic

time·shift (moveout) between traces with increasing offset while the refracted headwave undergoes

a linear increase in time with offset. Refracted arrivals are thereby discarded in favor of reflected

events, since both moveout corrections cannot be easily applied to the same data. In this study,

the refracted arrivals are evaluated to image near surface structures. The refracted arrivals were re-

duced to delay times of a common refraction surface. I

l Seismic Data ProcessingI3’

- *

1

Where the refraction interfaces are planar, the interpretation of the refraction method followsstraight forward processing routines with regular traveltime-distance curves (Palmer, 1986). How-ever, most of the time irregular traveltime·distance curves result from refraction surveys because

subsurface geology is represented by non-planar reflectors. Teclmiques for interpreting irregular

traveltime—distance curves are derived from the concept of delay time. For a single seismic interface,

the traveltime t of a headwave arriving at a source-receiver distance x is Z = to +éwhere tl, is the

delay time and V, is the refractor velocity (Figure Sa) (Palmer, 1986; and Barry, 1967). Delay time

to consists of a source term (ZCK) that is the time it takes part of the seismic wavefront to travel to

j the interface at tl1e critical angle of incidence i, and a receiver term (ty,) that is the traveltime for the

wave to travel to the receiver from the seismic interface at the angle i, (Figure Sa). The delay time,

therefore, is the traveltime for the critically refracted wavefront to travel from source to refractor

then to a receiver located at the critical distance x, (Figure Sc). Most refracted data are recorded

at offset greater than x, and the traveltime-distance curve will therefore experience a linear increase

in time with offset called refraction moveout (RMO) (Figure Sb).

To assist in the evaluation of velocities and removal of RMO, the concept of common refraction

(CRS) ensembles is introduced. CRS gathers are composed of traces of source and receiverI

pairs symmetxic about a common mid-point (CMP) which projects to the mid-point of the CRS

(Figure Sa). When CRS ensembles are sorted according to offset, the linear RMO is observed in

adjacent traces (Figure Sb and Figure 6a). Common refracted surface ensembles reduced to delay

time were used to form the input data for refraction stacks. Removal of refracted moveout would

reduce all traces in the CRS ensemble to common delay time. This is displayed in Figure Sd and

Figure 6b.

The motivation for the refraction stack is to exhibit refracted arrivals in the form of a vertical

seismic section in the profile and delay time domain that is ready for interpretation. Taking ad-

vantage of a situation where refracted arrivals are routinely discarded, a scheme was devised to en-

hance near surface imaging by emphasizing refracted events in seismic data. Processing according

i

Seismic Data Processing 14

¤

to the concept of a refraction stack enhances the refracted events and displays the stacked data ina fashion similar to reflection seismic sections. A CRS ensemble before RMO correction (a), and

after RMO correction (b) is shown in Figure 6. Refracted events showing linear moveout by offset

are corrected to represent the same time, t,, in (b), that is equal to delay time. A stacked trace (c)

obtained from stacking the RMO corrected data (b) shows the signal-to-noise ratio enhancement.

The refraction stack process shown in Figure 5 and Figure 6 involves five processing stages unique

to this method. They are 1) selection of time—offset windows for the first axrivals, 2) sorting data

traces into the CRS ensembles, 3) determination of refractor velocity by refraction velocity analysis,

4) application of linear refraction moveout (RMO), and 5) stacking of traces in the CRS ensembles.

This procedure begins by separating the refraction firstbreak wavelet from the rest of the trace

creating refraction·only traces that are sorted to give common refracted surface (CRS) gathers. The

separation is accomplished by applying a mute function that zeroes the entire trace except for a

window that brackets tl1e first arrival wavelet to create refracted data traces. In this study, the

window length of 0.1 s is chosen from the autocorrelation function. For consistency, the mute

window function is determined and and applied to shot gathers.

The multifold reflection data of CMP may be given by:

Y im = 20 + AmA0) = 120+ AE)

A0) = AU + AT.)

where AT„ = [ T,} + — T,} and f,](t) represent the normal moveout time and the zero off-

set trace respectively. T,) id the two—way zero offset reflection time and V, is the reflection velocity.

lf the traces consist of only reflected events, the CMP stacked trace from the above may be given

as:fl!)i

Seismic Data Processing 15

l_ _

>

which represents reflected energy at the zero-offset in n-fold data. Similarly, the multifold refraction

data may be given as: -

8r0) = 80ff+ ötr)i 800) = Soft + öß)

8,00)=i go(f°l' 660)

where 6:,, = : — to = , to is the delay time, and V0 is the refractor velocity. The CRS stacked

trace may be given as:

80)whichcan be considered to represent the refraction event at :0 delay time for n-fold data. These

summation processes can also be used to determine the velocity information needed. In these cases,

V, is the stacking approximation to the root-mean-square velocity to the reflector, and V} is the

stacking averaged apparent velocity of the refractor.

Although there is a strong smoothing effect because of refraction stacking, thegstacked traces

from the CRS ensembles represent two-way traveltimes for critically refracted travel paths (delay

time :0). The difference in two-way traveltime between the refraction delay time to and the two-way

time T0 of zero offset reflected waves is defined by the cosine of the critical angle (Figure 5c). The

refraction delay times can be converted to the vertical two-way times by T0 =ä. This shift is

necessary if velocity contrasts or layer thicknesses are such that two-way delay time through the

subsurface is visibly different from the corresponding reflected two-way arrival time. As addressed

in a later section, the necessary shift for the ADCOH data modelled is less than 4 ms (1 sample

interval). Because of this small difference, no shift is applied to the ADCOH Line 1 refraction stack‘ sections.

The smoothing effect on the data by stacking was a major concem. To observe this effect, three

CRS groupings with different offset ranges were processed. The CRS offset groups are: (1) A11

offsets 0.2-4.15 km (RSTACK2); (2) Near offsets 0.2-2.5 km (RSTACKZB); and (3) Far offsets

Seismic Data Processing 16

[

2.5-4.15 km (RSTACKZA). Refraction stacking velocity analysis is performed separately for each

of these offset ranges after respective sorting into these groups.

The CRS gathers are used to determine the correction (refraction) velocity by constant velocity

analysis. The velocities are chosen using constant velocity refraction stacks (CVRS). CVRS are

generated from panels of adjacent stacked CRS traces that have all been reduced to delay time by

RMO correction with the same velocity. The velocity which results in the panel with the best signal

to noise ratio (S/N) and refracted continuity is the refraction stacking velocity for that group of

adjacent CRS gathers (Figure 7). This refraction stacking velocity is the average refractor velocity

from updip and downdip apparent refraction velocities because the data used were from split spreadn

geometries. From CVRS analysis at CRS intervals along the line, the refraction stacking velocities

at 25 locations for the entire line are determined. CRS gathers summed into one trace after the

most optirnum RMO correction applied enhance the refracted signal and attenuated unwanted

noise components. Therefore, accurate velocity picking and optirnum RMO applications are de-

termined from the evaluation of S/N in CVRS panels.

Stacking of CRS gathers reduced to a common datum of 0.4 km directly followed corrections

for refraction moveout. The ADCOH Line 1 data set resulted in three refraction stack sections

which all exhibit a very continuous refractor at about 0.1 to 0.2 s with a high S/N. Further im-

provements in refractor continuity was obtained by application of one pass of surface consistent

residual statics. Effect of the application of residual statics improved the data quality with some

smoothing effect but the major features are preserved as seen in Figure 8. Three final refraction

stack sections are shown in Figure 10.

Seismic Data Processing 17

1....- -

Reflected Arrivals

The reflected events in the ADCOH regional Line 1 from CMP 710 to 1692 were also reproc-

essed. The main objective of reprocessing the reflection data was two·fold. First, improvement in

shallow (first 0.6 s) data reflection continuity to relate the near surface reflected events with thoserefracted events in refraction stack sections. Second, enhanced recovery of reflected arrivals espe-

cially from the Brevard fault zone to image it from the surface to the depth which it roots into theBRT. FThe final reprocessed reflection stacked section was generated following the standard processing

sequence for the original data (Coruh and others, 1987) with the addition of new mute functions,

irnproved velocity coverage, predictive deconvolution, and multi-pass surface consistent residual

statics with windowing to enhance the BFZ.

g With the application of 35 mute functions, predictive deconvolution, 25 velocity functions, and

residual statics, enhanced reflections, from both shallow and deep events, were obtained

(Figure 9). The most dramatic improvement in reflected data resulted from increased velocity

coverage which irnproved the quality of reflections ir1 the first 0.6 s. Final improvements occurred

from multi-pass multi-iteration residual statics with velocity updating. Special windowing for the

static shift calculations focused on the subhorizontal reflections at the beginning of the line, then

followed the reflections of the BFZ to the surface.

Combined Refraction and Reflection Stacks

As one of the final objectives of this study, specific final refraction and reflection stack sectionswere combined into composite stack sections for detailed composite interpretation. This was ac-

Seismic Data Processing 18

II

I

complished by muting both data sets followed by summing. The mute function of the refraction

and reflection stack data were chosen to taper out the refracted events while beginning to taper in

the reflection events in the stackirig to generate the composite stack (Figure 13). When matching

CMP trace pairs from both refracted and reflection stack sections were not present, a zero trace

was added in place of the missing CMP trace. This procedure is responsible for the gap at the top

of CMP 1145 to CMP 1160 through the absence of refracted stack traces and the gaps at CMP 1410

and 1590 due to the absence of reilection stack traces.

The composite stack section is migrated further increasing S/N and refraction and rellection

continuity. As a result of these improvements, the migrated composite stack sectioni(Figure 14)I

will be used in tl1e final interpretation. The data gap between CMP 790 and 870 in the reflection

data, as a result of acquisition through the town of Westminster, South Carolina, has created edge

effects from migration.

I

Scismic Data Processing 19

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Scismic Data Processing 22

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Discussion and Interpretation

In order to extend the interpretation of ADCOH regional Line l seismic data, two points must

be addressed: 1) the subsurface imaging capabilities of the refraction stack method and its limita-

tions, and 2) determination of the nature of the well defined refractor, in the refraction stacks, in

terms of geologic interfaces. The noise test is examined to identify the refracted events in the re-

fraction stack. Lithologies from ADCOH DHI shallow core hole were used for correlation of the

refraction data with near surface geology.

Abi1—ity of Refraction Stacks to Image the Subsurface

The refraction stack sections after the application of residual statics are shown in Figure 10a,

b, and c for the offset ranges 0.2—4.l5 km, 0.2-2.5 km, and 2.5-4.15 km, respectively. These figures

clearly show that refraction stacks as seismic sections have a high S/N ratio. The smoothing effect

of multi-offset data on the resolution of the data is the most important feature to be addressed.

Since the surface covered by the refracted wave in the CRS ensemble is sampled more by the large

offset range data, the smoothing is directly proportional to offset ranges. Because of larger refractedray paths, large offset traces are also approximated the most by assuming subhorizontal refractors.

Discussion and Interpretation 24

111

As with rellection data, smoothing will also result from the summation process of multifold datacompostion. To observc these results of offset and fold upon refraction resolution, three differentltrace offset ranges were applied to generate refraction sections. The near and far offset rangesections should exhibit more detail than the total offset range section due to lower fold. When themultiplicity of the data is equal, the near offset section has more resolution than the far offset sec-tion because each CMP with near offsets sums traces with shorter sampling paths. The complete

offset range stack will display the least detail of the three stack sections. On the other hand, thissection will have the best SiN and refractor continuity which is desirable in areas of low seismic

reflectivity.

Comparison of all three refraction stack sections (Figure 10) show that RSTACK2

(Figure 10a) has the best refractor continuity. As expected, all three sections display different de-

tailed features while broad features are similar for all three. The sections exhibit breaks in refractors

with vertical offsets. Particularly noticeable examples common in all three sections are the distinct ·breaks in the refraction events near CMP 450 and 730 (Fl and F2 in Figure 10). As a cross check,

shot gathers are examined in detail using a standard refraction prospecting technique to confirm thatthe fault signatures are contained in the reversed refraction time-distance curves. To accomplishthis, shot record 260 at CMP 706 and shot record 290 at CMP 766, one from each side of the ap-

parent fault, are given with respective time·distance curves. These reversed time-distance curves

covering CMP 730 are given in Figure ll. Clearly observed from Figure 11 is a negative time shiftof 0.029 s lI1 the shot gather (260) with source at CMP 706 and a positive time shift of 0.033 s in

the shot gather (290) with source at CMP 766. The negative time shift occurs in all traces recorded,

from shot 260, to the northwest of CMP 750. The positive time shift occurs in all traces, from shot

290, to the southeast of CMP 721. l\/Iodelling from this t-x curve, with V, = 1.67 km/s and

V2 = 6 km/s (ic = l8°), the fault is located beneath CMP 734 with the downthrown side to the

soutl1east (Figure ll). Taking the time shift dt= 0.031 s, the offset on this fault (dz) is about 52

m. In Figure 10a the fault is located at CMP 730 with the offset (dt) of 0.03 s on tl1e downthrown

side to the southeast. This same procedure yields similar results when applied at CMP 4501ocating

Discussion and Interpretation 25

1

the fault at CMP 447 with an offset of 0.025 s. In most cases, detailed features such as refractorundulation and discontinuities are displayed on both RSTACKZA (Figure 10c) and RSTACKZB

(Figure 10b) while not usually observed on RSTACK2 (Figure 10a) because of a high degree of

smoothing. Judging from all sections, the full offset range refraction stack (Figure 10a) exhibit the

common features because uncommon features are smoothed out. Note the refractor displacement e(F3) at CMP 920 in Figure 10b and Figure 10c which is not visible in Figure 10a while the dis-

placernents at CMP 350, 450, and 730 are common in all stacks. This is believed to be due to the ·

higher smoothing in RSTACK2 than in RSTACKZA or RSTACKZB. With respect to these

general features, the different seismic signatures are observed on all three sections for each major

geologic province. With respect to smoothing, the near ar1d far offset range stack sections display

more apparent refractor detail. Tl1e far offset section (Figure 10c) appears to display more refractor

undulation than the near offset section (Figure 10b). After close examination though, the trace-

to-trace detail is discovered to be small with refractor variations emphasized by lower fold. Differ-i

ences between RSTACKZA (Figure 10c) and the other two refraction stack sections (Figure 10a

and 10b) are also possibly due to the depth which the far offset travel paths reach. Since folding

is a major cause of smoothing, the far offset travel paths in RSTACK2 (Figure 10a) are averaged

out by the same number of short travel paths.

Correlation of available data sets

For a correlation of arrivals from refraction stack sections with lithology and geologic structure,

the ADCOH regional Line l noise test, conducted at the beginning of the line, is examined with

the purpose of obtaining a model for the shallow geology in the Inner Piedmont allochthon which

is developed from refracted arrivals. Since the noise test was recorded with 3 m (10 ft) trace intervals

that resulted in a 5.4 km far offset survey, all major refractors that are present in the ADCOH re-

gional Line l data were examined.

Discussion and Interpretation 26

The analysis of the noise test reveals that two principle refracted events are recorded in additionto the direct wave (Figure 12). The velocities of these three arrivals as determined from the shotrecords are: V,:1.1 km/s for the direct wave; V2:2.9 km/s for the first refractor; and I/3:6.05 km/s

for the second refractor. The two refracted waves and the direct wave irnply a three—layer subsurfacemodel with two distinct seisrnic interfaces. Since the wave velocity in each layer has already beendeterrnined, all that is necessary to determine the thickness of the first two layers is the zero·offset

delay times for the two refraction events. Taking the intersection of the air wave and the directwave as the zero·offset source point, the two-way delay times for the two refracted events as pickedfrom the shot records are: (1) tl: 0.05 seconds for Refraction A. and (2) :2: 0.065 seconds for_Refraction B (Figure 12). Using the traveltime equations, the thicknesses of the first layer (h,) and

the second layer (hl) are calculated to be 30 m and 37 m respectively. Of additional interest in thisstudy are the two crossover distances (xa, xa) . Substituting the delay—times and seisrnic velocitiesobserved from the noise test for the first two layers into the appropriate traveltime equations andsolving for offset leaves the distance at which the first refraction anives before the direct wave xa

at 89 m. Sirnilarly, the second refraction becomes the first anival xa at 142 m and remains the firstarrival throughout the 5.4 km of offset for the noise test. These distances are also observed in theactual time·distance curves given in Figure 12. A

i

This result supports our main RMO stage assumption that all traces in a CMP gather from 0.2

to 4.15 km offset can be moved-out at the same apparent refractor velocity. Using the parametersobtained from the noise test, a difference of less than 0.004 s is found between refraction delay timeand two-way vertical reflection time. Since the sample interval is 0.004 s, the correction for this

difference was not applied.

The bright refraction event in the refraction stack is evaluated to be representative of the second

refractor, interpreted from the noise test at a depth of 67 m with an intercept time of 0.065 s. The

time, measured between the surface elevation mark and the refractor, in the refraction stack is also

0.065 s at the same location (CMP 205) as the noise test. The refractor is interpreted to continue

Discussion and InterpretationA

27

I

I

Ifrom the location of the noise test (CMP 202) to CMP 980 (Figure 10) at which it occurs at a

two-way delay time of 0.025 s from the surface elevation mark to the refractor. Located near CMP980 on Line 1, there is shallow ADCOH hole DI·I1 to a depth of 0.3 km. As calculated from the

three-layer model, average velocity to the second refractor is :1.67 km/s. Assuming that this av-

erage velocity is relatively constant between CMP 980 and the beginning of the line, 0.025 s two-way delay time converts to 0.026 s two·way traveltime. This corresponds to a depth of 21 m. Thisdepth correlates within 7 m to the boundary logged in DH1 between the weathered zone and a

granitic gneiss.

Interpretations

The extended interpretation of the ADCOH regional Line 1 seismic data, composite stack using

automatic line drawing, parallels those of Coruh and others (1987) and Hatcher and others (1987b)

with an expansion on the BFZ and the addition of shallow refraction information. In addition to

the seismic data, previous surface geological studies are incorporated to constrain interpretations

of near surface events.

Results of the noise test and shallow core hole data analysis lead to an interpretation of the

refractors as the bottom of the double layer weathered zone (surface of bedrock) with an average

velocity of 1.67 km/s. Additionally, this interpretation is supported by surface geology where some

correlation is found between lithologic units and refractor quality. The refractor is best seen where

amphibolite, granitic gneiss, and Henderson gneiss are exposed at the surface. The refractor is poorbelow Poor mountain amphibolite possibly because of geometrical complexity of the seismic

— interfaces. The Brevard fault zone and Rosman fault to the west are indicated by a no-data window

between CMP 1390 and 1430 to 0.1 s depth. In the eastem Blue Ridge rocks, good refractor seg-

ments tend to correlate with greywacke-schist·amphibo1ite on the surface (Hopson and Hatcher,personal communication). Apparent in the composite stack section is the signature of the three

Discussion and Interpretation 28

major tectonic provinces. The Inner Piedmont allochthon and Chauga Belt (IPA-CB) possess arelatively simple continuous surface below the weathered layer (Figure 14). Near the boundary of

the IPA—CB a decrease in refraction continuity is observed. A strong reflected event can beprojected upward to this approximate location at the surface and is interpreted as the CB-IPAboundary (CB/ IPA) at depth. In other parts of the composite section, reflectors are traced nearsuch tectonic boundaries at the surface and could be evidence of major lithologic boundaries. Thedecrease in refraction continuity near the CB/IPA is slight in comparison with the one exhibitedat the BFZ to Chauga Belt interface. The _surface signature of the Brevard fault zone can best bedescribed by a lack of refracted seismic signal up to 0.1 s (Figure 14). The refracted signal begins

to reappear in the Blue Ridge allochthon as a retractor with relatively high S/N but with manydiscontinuities (Figure 14) that possibly indicates inhomogeneity. The refracted events in the BRA

are completely different from those in the IPA/CB as well as the major tectonic boundary (BFZ)that divides them. Although both have a relatively high S/N ratio, the refracted events are discon-

tinuous at higher angles with truncations occurring.

An automatic line drawing (ALD) of the unmigrated composite stack section (Figure 15) wasgenerated for additional detailed interpretation. In this form, an increase in reflection coherencyover the composite stack was obtained. One of the most improved targets of this study is the BFZ.From the surface, where the BFZ is distinguished in the refractions, to 6 km depth (2 s) the Brevardfault zone can be seen in more detail than previously published. As originally stated by Clark andothers (1978), the upper and lower boundaries of the BFZ are imaged as relatively continuous

eastward-dipping rellectors. Figure 15 shows that the Brevard fault has a larger zone than ismapped on the surface. When the boundaries from surface mapping are considered the upper andlower boundaries are close together near the surface (0.2 s), move apart to a maximum distance of

0.7 s near 1 s depth (3 km) below CMP 1200, then splay together into the BRT at 2.0 s (6 km)

(Figure 15). But the composite stack section in Figure 15 shows that the Brevard fault zone has

a relatively constant thickness, which dips 17°, to a depth of 4.2 km where it flattens to a dip of8°. Between the top and bottom boundaries, the interior of the BFZ is imaged as reflective zones

Discussion and interpretation 29

1with east dipping packages of reflectors. The Brevard fault zone image is more complex than de-

scribed in previous literature. Instead of thinning at the BRT as its root, the BPZ splays from the

Blue Ridge thrust with some considerable thickness (0.6 km). An alternative interpretation(Figure 15) for the Blue Ridge thrust (BRT) places it at a minimum depth of 0.5 s (1.5 km) rightbelow the BRA after paralleling the BPZ as it shallows from 2 s to l s depth although a deeper

thrust (DT) is also clearly visible at depths of 1 s (3 km) and 2.8 s (8.4 km). In the other direction,

the BRT splits as it joins with the BPZ at approximately 2 s (3 km) depth and joins with the deepertrust isolating two distinct tectonic packages between the BRT and DT. The ALD profile displaystwo other major reflection packages. These are reflector packages which are imaged dipping east-

ward (Pigure 15). From published line drawings (Coruh and others, 1987) these reflectors even-tually become westward dipping to the southeast and are therefore interpreted as major lithologicboundaries in the Six Mile Synclinorium.

Discussion and Interpretation 30

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Discussion and Interpretation 32

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Discussion and Interpretation 33

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Discussion amI Interpretation 35

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Discussion and Intcrprctation 36

I.‘I

I

Conclusions

Imaging of the ADCOH Line I data set was enhanced through the introduction of composite ‘

stack processing. Refracted first arrivals from seismic reflection data were processed using the re-

fraction stack method to generate refraction delay time seismic sections for shallow infonnation.

Despite large offsets and high fold, resolution of refracted events is high enough to aid in the in-

terpretation of composite stack sections. These refractors in the refraction stack sections are inter-

preted to be the lower boundary of the low-velocity weathered layer ir1 the Inner Piedmont and

Chauga Belt as well as parts of the Brevard fault zone and Blue Ridge. The average velocity to the

refractor is 1.67 km/s in contrast with stacking velocities for the reflection stack data that range from

4.6 km/s to 6.6 km/s. A velocity of 6 km/s was applied for depth calculations for reflected events.

Distinct seismic refraction signatures are associated with the three major geologic provinces in _

the study area. The Inner Piedmont is recognized as continuous refractions with a high S/N that

give way to the Chauga Belt’s more discontinuous nature. The shallow Brevard fault zone is dis-

tinctive because of its lack of strong coherent imaging by the refraction stack method. Strongly

segmented, high S/N refractors possibly indicating inhomogeneity are typical of the Blue Ridge.

Reprocessing of the reflected data not only increased the reflectivity of the shallow (first 0.6 s) in-

formation but provided improvements in imaging from the Brevard fault zone and shallow

Conclusions 37

lithologic boundaries in the Inner Piedmont as well as the Blue Ridge. An increase ir1 velocitycoverage, in offset and time, as well as residual statics improvements through windowing are re-sponsible for most of these improvements in the deeper events while improved mute functions and

predictive deconvolution recovered the shallow reflectivity. The Chauga Belt and Inner Piedmontexhibit both distinct reflection and refraction boundaries. While the Chauga Belt Inner Piedmontinterface is displayed as a decrease in refractor continuity, a strong reflection event can be traced to

the surface at this same location. The Brevard fault zone and Blue Ridge thrust are seen in greaterdetail than previously published leading to altemative interpretations ofboth the BPZ and the

l BRT. For the BPZ, the highly reflective top and bottom boundaries are well imaged along withreflections from the interior. Reflections from within are recovered as east-dipping packages of re-

i A

llectors. The Brevard fault zone is about l km wide in outcrop as mapped on the surface, reachesa maximum vertical thickness of 2 km at 3 km depth, then thins as it joins into the Blue Ridgethrust at 6 km depth. From the surface to 4.2 km the BPZ clips l7°, between 4.2 km and 6 km the

BFZ flattens to an 8° dip. From this point, the Blue Ridge thrust shallows with the BPZ toward

the northwest end of the line to a depth of 1.5 km. To the southeast, a short thrust segment con-nects the BRT with a deeper thrust (DT) located between the BRT and and the undeformed lower

Paleozoic platform strata (LPPS). "Ihe DT is clearly seen to shallow from approximately 8.4 kmdepth to 3 km at the northwest end of the line. The BRT and DT isolate two wedge packagesobserved below the BPZ and above the LPPS.

Conclusions 38

VW1, Bibliography

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Coruh, C., Costain, J.K., Hatcher, R.D., Jr., Pratt, T.L., Williams, R.T., and Phinney, R.A., 1987,.Results from regional vibroseis profiling: Appalachian Ultra-Deep Core Hole site study,Geophys. J. R. astr. Soc., v. 89, pp. 147-156.

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’ Bibliography 40

Appendix

FIGURE PROGRAM DATASET PROJECT TAPEFigure 4 FIG04.DAT NVSWI DEEP! B0037Figure 6 FIG06.DAT MUTESRT2 DEAPI BOO37

· Figure 7 F!G07.DAT ’I\/IUTESRT2 DEAPI BOO37

Figure 8 FIG08.DAT RSTACK2 DEAPI BOO37

Figure 9 FIG09.DAT FINAL! DEEPBFZ BOO37Figure !0a F!G!0.DAT RSTACK2 DEAPI BOO37

Figure 10b FIGIO.DAT RSTACK2!3 DEAPI B0037

Figure 10c FIGIO.DAT RSTACKZA DEAPI B0037

Figure 13 F!GI3.DAT COMP2 DEEPBFZ BOO37

Figure I4 FIGI4.DAT MIG__COMP DEEPBFZ BOO37

Figure I5 FIG!5.DAT COMP_LD! DEEPBFZ BOO37Tabnc 1. Figures-Iocatcd on backup ß0037—K.lL.ßCK

Appcndix 4l