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107 Chapter 6 Efficient Mapping Of Structure And Azimuthal Anisotropy With Three Dimensional Shallow Seismic Refraction Methods 6.1 - Summary  A three dimension al (3D) se ismic refr action su rvey was carried out across a shear zone. The data were processed with the generalized reciprocal method (GRM) rather than with tomographic inversion because of the relatively small volume of data, the occurrence of large variations in depth to and wavespeeds within the main refractor and the presence of azimuthal anisotropy. The results show that there is an increase in the depth of weathering and a decrease in wavespeed in the sub-weathering associated with the shear zone.  Althoug h the shear zone is general ly consid ered to b e a two dimensiona l (2D) feature, the significant lateral variations in both depths to and wavespeeds within the refractor in the cross-line direction indicate that it is best treated as a 3D target. These variations are not predictable on the basis of a 2D profile recorded earlier.

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Chapter 6

Efficient Mapping Of Structure AndAzimuthal Anisotropy With Three

Dimensional Shallow SeismicRefraction Methods

6.1 - Summary

 A three dimensional (3D) seismic refraction survey was carried out across a

shear zone.

The data were processed with the generalized reciprocal method (GRM) rather 

than with tomographic inversion because of the relatively small volume of data,

the occurrence of large variations in depth to and wavespeeds within the main

refractor and the presence of azimuthal anisotropy.

The results show that there is an increase in the depth of weathering and a

decrease in wavespeed in the sub-weathering associated with the shear zone.

 Although the shear zone is generally considered to be a two dimensional (2D)

feature, the significant lateral variations in both depths to and wavespeeds within

the refractor in the cross-line direction indicate that it is best treated as a 3D

target. These variations are not predictable on the basis of a 2D profile recorded

earlier.

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The amplitudes of the refracted signals are approximately proportional to the

ratio of the specific acoustic impedances between the upper layer and the

refractor and they provide a convenient and detailed measure of apparent

azimuthal anisotropy or rock fabric. The amplitudes also contain additional

useful geological information, although some of the cross-line amplitudes could

not be completely explained.

Qualitative measures of azimuthal anisotropy are obtained from the wavespeeds

and the time-depths computed from the traveltime data with the GRM algorithms

and from the amplitudes. These three methods give similar consistent results,

with the direction of the greater wavespeed being approximately parallel to the

direction of the dominant geological strike. Furthermore, all three methods show

that the direction of the greater wavespeed is approximately orthogonal to the

direction of the dominant geological strike in one region adjacent to the shear 

zone.

The in-line results show that both accurate refractor depths and wavespeeds can

be computed with moderate cross-line offsets, say less than 20 m, of shot points.

These results demonstrate that swath shooting with a number of parallel

recording lines would be adequate for 3D surveys over targets such as highways,

damsites and pipelines. Only a modest increase in shot points over the

requirements for the normal 2D program would be required in the cross-line

direction for measuring azimuthal anisotropy and rock fabric with amplitudes.

6.2 - Introduction

In the last two decades, three dimensional (3D) seismic reflection methods have

revolutionized the exploration for, and production of petroleum resources. The

improved images of the subsurface geology are a result of the recognition that

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most geological targets are in fact three dimensional, and that it is essential to

employ spatial sampling densities and processing methods which recognize and

accommodate this reality. It is now generally accepted that in many cases, two

dimensional (2D) seismic reflection methods give an incorrect  rather than an

incomplete picture of the sub-surface (Nestvold, 1992).

By contrast, 3D refraction methods (Zelt, 1998; Bennett, 1999; Deen et al, 2000)

are not very common. However, there are compelling reasons for the expedient

development of 3D shallow refraction methods for routine use in geotechnical,

environmental and groundwater applications.

Geological structures and the corresponding depths to and wavespeeds within

bedrock, can show as much variation in the cross-line direction as in the in-line

direction. In the vast majority of near-surface studies, such variations are

significant.

There is a need to address azimuthal anisotropy of wavespeeds. Anisotropy can

be caused by lamination, foliation or by the preferred orientation of joints and

cracks within the refractor, and it is another important parameter for assessing

rock strength for rippability and foundation design. However, its most important

near-surface application may be in the determination of fracture porosity in

crystalline rocks for the development of groundwater supplies for domestic and

irrigation purposes, in studies of contaminant transport especially of radioactive

wastes (Barker, 1991), the stability of rock slopes and seepage from dams, the

construction of underground rock cavities for storing water, gas, etc, and the

construction of tunnels.

The relationship between anisotropy and crack parameters has been the subject

of considerable research in the past (Crampin et al, 1980; Thomsen, 1995).

Nevertheless, there are no established approaches for the routine mapping of 

these parameters with shallow geotechnical or environmental targets, although

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radial surveys to measure azimuthal anisotropy (Bamford and Nunn, 1979; Leslie

and Lawton, 1999) represent the first steps in that direction.

6.3 - Data Processing With The GRM

This study describes the results of a 3D shallow seismic refraction survey

recorded some time ago across a shear zone at Mt Bulga in southeastern

 Australia. The data are processed with a traditional approach using the

generalized reciprocal method (GRM) (Palmer, 1980; Palmer1986), rather than

with tomographic inversion for the following reasons.

The wavespeeds in the refractor range from less than 2000 m/s in the shear 

zone to more than 5000 m/s in the adjacent rocks. Recent case histories (Lanz

et al, 1998), demonstrate that current tomographic inversion methods cannot yet

reliably resolve wavespeeds in the main refractor, even though over 90% of the

traveltimes originated from the refractor. In those cases where stable inversion

has been achieved, the variations in wavespeeds are generally less than about

5% (Zelt, 1998).

The volume of data is low in contrast to that generally considered desirable for 

effective tomographic inversion. As a comparison, the approximately 2000

traveltimes for 120 detector positions used in this study are much less than the

more than 50,000 traveltimes for 29 detector positions used in the tomographic

analysis of Zelt and Barton, (1998). For most routine shallow refraction

investigations, the costs of recording at least an order of magnitude of additionalshot points can be prohibitive.

Model studies and case histories (Palmer, 1980; Palmer, 1991) demonstrate that

the GRM can resolve large variations in the depths to and wavespeeds within

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refractors using considerably smaller data volumes than is the case with most

tomography programs.

 Azimuthal anisotropy is rarely accommodated with most tomography programs.

Isotropy is normally assumed in order to employ as many traveltimes from as

many directions as possible in the inversion process. In addition, the traveltime

differences due to anisotropy are quite small, and are often within the accepted

range for the residuals of inversion.

In this study, the amplitudes of the refracted head waves are used to map

anisotropy. Previous studies have shown that the head coefficient, the

parameter which controls the amplitude of the refracted signal, is approximately

proportional to the ratio of the specific acoustic impedances of the overburden

and the refractor (Palmer, 2001b; chapter 5). However, the head wave

amplitudes are generally dominated by the rapid variation due to geometric

spreading. Another study (Palmer, 2001a; chapter 3), demonstrates that the

effects of geometrical spreading and dipping interfaces can be accommodated

with either the multiplication of the amplitudes of the forward and reverse traces,

or by the convolution of those traces. In this study, the ratios of the amplitude

products for pairs of shot points with varying azimuths are used as a qualitative

measure of azimuthal anisotropy.

6.4 - Survey Details

The data used in this study were acquired in approximately the same location asa 2D set of data described previously (Palmer, 2001a; chapter 3). The survey

was carried out shortly after the area had undergone complete clearing of the

native vegetation and subsequent planting of tube stock for a pine plantation. As

a result, the survey pegs which marked out the exploration grid, had been

removed, and so the precise relationship between the two surveys is not known.

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However, the cross-line numbers in this study correlate approximately with the

station numbers on the 2D profile.

Figure 6.1:  Plan of in-line and cross-line geophones and shot points. Shots 1 to

15 are shown as bold symbols and were recorded with in-lines 17 and 21. Shots

16 to 42 are shown as open symbols and were recorded with cross-lines 45 to

69.

The data were recorded with a 48 trace seismic system using a roll switch and

single 40 Hz detectors. Shot holes were drilled to depths of between 1 and 2.5 m

with a small trailer mounted drill rig. Charge sizes were between 1 and 3 kg of a

high velocity seismic explosive.

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Initially, two parallel lines 20 m apart, with each consisting of 24 geophones at a

5 m spacing were set out. These in-lines were located approximately either side

of the earlier 2D profile. Five shot points, nominally 60 m apart, were located

along each line, while another four oblique shot points offset 60 m from the end

of each line of geophones in the in-line direction and offset 60 m in the cross-line

direction were also recorded, making a total of fourteen shots.

 A second series of seven parallel cross-lines which were 20 m apart, and each of 

which consisted of twelve geophones at a 5 m separation were then set out.

There were four shot points on each cross-line and the shots were nominally 60

m apart. These lines were recorded in groups of four by simply rolling through

from one end to the other. A total of twenty seven shots were recorded in the

cross-line directions.

Figure 6.1 is a plan of the two geophone arrangements and shot point locations.

6.5 - Analysis of the In-line Traveltime Data

The traveltimes were hand picked from the field monitors, and standard

corrections for the uphole time and the system delay in the analogue

components were applied. The previous 2D study (chapter 3; Palmer, 2001a),

showed that a three layer model was applicable. It consists of a thin surface

layer of friable soil with a wavespeed of about 400 m/s, a thicker layer of 

weathered material with a wavespeed of approximately 700 m/s, and a main

refractor with an irregular interface with wavespeeds between approximately2000 m/s and 5000 m/s.

The traveltime data for in-line 21 for all fourteen shots are shown in Figure 6.2.

The graphs for the shot points which are offset by 60 m from the geophone

spreads in the in-line direction and which are located along cross-lines 33 and

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81, namely shots 1 to 4 and 8 to 11, all show arrivals which originate from the

main refractor. The graphs in the forward and reverse directions appear to be

essentially parallel, but in fact gradually converge. Also, there is an unresolved

inconsistency in traveltimes between stations 45 and 49, which is related to very

low amplitude arrivals on the shot records.

Figure 6.2:  Traveltime data recorded on in-line 21 with in-line, adjacent and

oblique shot points. In general, the graphs gradually converge in each direction

of recording. The inconsistencies in the reverse traveltimes can be seen

between cross-lines 45 and 49.

Figure 6.3 shows the refractor wavespeed analysis function tV, computed with

equation 6.1, using a 5 m XY value, for four shot pairs. They are shots 2 and 9

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which are collinear with the detectors, shots 3 and 10 which are collinear with the

adjacent parallel line of detectors on in-line 17, shots 1 and 11 which form a

northwest-southeast shooting orientation, and shots 4 and 8 which form a

northeast-southwest shooting orientation.

Figure 6.3:  Refractor wavespeed analysis function computed for the in-line,

adjacent and oblique shot pairs. The wavespeeds for the oblique shot pairs have

been corrected with the cosine of 30 degrees which is the angle between in-line

21 and the line joining the shot points.

tV = (tforward - treverse + treciprocal)/ 2 (6.1)

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where treciprocal is the traveltime from the forward shot point to the reverse shot

point, and it is a constant for a given shot pair and a set of collinear detectors.

The wavespeed in the refractor along in-line 21, is obtained from the reciprocal of 

the gradient of tV for the shot pairs which are collinear with the detectors, namely

shots 2 and 9. Between cross-lines 45 and 50, the wavespeed is not well

determined because of the unresolved inconsistency in the traveltimes

mentioned previously, but it appears to be greater than 4000 m/s. The value of 

5000 m/s shown in Figure 6.3 is taken from the earlier adjacent 2D results

previously referenced.

The wavespeed is 1850 m/s between cross-lines 50 and 60.

Between cross-lines 60 and 69, the wavespeed is 3930 m/s. However, this

region can be further separated into an interval between cross-lines 60 and 64

with a wavespeed of approximately 5000 m/s followed by an interval between

cross-lines 64 and 69 with a wavespeed of approximately 3000 m/s. This

separation is consistent with the results of the earlier 2D survey.

The refractor wavespeeds computed with shots 3 and 10 which are located along

the adjacent in-line 17, are essentially the same as those determined above.

However, the wavespeeds computed with the northeast-southwest and

northwest-southeast oblique shot points are higher mainly because of the angle

of about 30 degrees between the line of the detectors and the line joining the two

shot points. The wavespeeds between cross-lines 50 and 60 of the oblique

shots shown in Figure 6.3 are the product of the measured values and the cosine

of 30 degrees. They show that the corrected wavespeeds are higher in the

northeast-southwest direction than in the northwest-southeast direction.

There is some question about the validity of the wavespeeds derived from the

oblique shot pairs, because no account has been taken of the fact that most of 

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the detectors are not collinear with the two shot points, as is assumed with

equation 6.1. For these shot pairs, the reciprocal time increases as the offset of 

the geophone from the line joining the shot points increases. Nevertheless,

these results have been included, because they are consistent with other results

to be described below.

Figure 6.4:  Time-depths computed for the in-line, adjacent and oblique shot

pairs. The reciprocal times for the oblique shots have been adjusted so that the

time-depths are the same between cross-lines 45 and 49, in order to emphasize

the systematic divergence from the in-line values.

Figure 6.4 shows the time-depths computed with equation 6.2 using a 5 m XY

value, for the four shot pairs used in Figure 6.3. The reciprocal time for the in-

line shots 2 and 9 was computed with equation 33 of Palmer (1980). The

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reciprocal times for the other shot pairs could not be derived as conveniently, and

they have been adjusted until the differences in the time-depths between cross-

lines 45 and 49 were minimized. This facilitates the recognition of the systematic

divergence of the time-depths for the oblique shot pairs from the collinear values.

time-depth = (tforward + treverse - treciprocal)/2. (6.2)

The increase in the time-depths between cross-lines 50 and 60 corresponds to

the region in the refractor with the low wavespeed.

The systematic divergence of the time-depths computed with the oblique shot

pairs from cross-line 49 to cross-line 69, can be employed as a qualitative

measure of azimuthal anisotropy in the following way.

The time-depths tG, are related to the depths zG, with equation 6.3,

zG = tG / DCF (6.3)

where the DCF, the depth conversion factor relating the time-depth and the

depth, is given by:

DCF = V Vn / (Vn2 - V2)½ (6.4)

or 

DCF = V / cos i (6.5)

where V is the average wavespeed above the refractor and

sin i = V / Vn (6.6)

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It is reasonable to assume that the point of critical refraction below each station is

much the same whether the energy propagating in the refractor is traveling in the

northeast-southwest direction or the northwest-southeast direction. This implies

that the depth to the refractor is the same irrespective of the direction of 

measurement. Therefore, any variations in the time-depth at each station will be

related to variations in the DCF through equation 6.3.

Figure 6.5:  The ratio of the time-depths computed with shots 4 and 8 in the

northeast-southwest direction and shots 1 and 11 in the northwest-southeast

direction.

The time-depths for the oblique shots were then re-adjusted in the following

manner. In-line 21 intersects the line joining shots 1 and 11 at cross-line 53 and

the line joining shots 4 and 8 at cross-line 61. The time-depths for the oblique

shots at these points were computed from the in-line depths with equation 3

using the refractor wavespeeds appropriate to each direction as shown in Figure

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6.3. The reciprocal times for the oblique shots were then adjusted until the

depths at the intersections were the same as those just computed. Finally, the

ratios of the time-depths in the two oblique directions were computed and they

are presented in Figure 6.5.

 As with the wavespeed analysis function in Figure 6.3, this method of detecting

azimuthal anisotropy makes no allowances for the variations in reciprocal time for 

the detectors which are not collinear with the shot points. The reciprocal time

subtracted in equation 6.2 should be increased for the detectors offset from the

line joining the shot points, in order to take into account the extra path length in

the refractor. The use of a constant reciprocal time should increase the

computed time-depths at the offset detectors and as a result, the time-depth

profile should appear to be flattened. However, no such flattening is obvious in

Figure 6.4.

Despite these reservations, the results are presented because they are

consistent with those determined with other approaches. In particular, the region

between cross-lines 45 and 49 shows values less than one, while the remainder 

shows values greater than one. These results are qualitatively similar to those

derived from amplitude ratios in Figure 6.8 below and from a comparison of the

in-line and cross-line wavespeeds.

These results also demonstrate the benefits of including an analysis of the

residuals as a function of azimuth with tomographic methods. The differences in

traveltimes between the offset shots in Figure 6.2 show little variation about the

mean and as a result, the time-depths also show the same small variations. For 

example, the variations about a zero mean difference in the time-depths in Figure

6.4 are less than a few milliseconds. Although such variations are within the

acceptable ranges of residuals for most tomographic approaches, nevertheless,

there may still be a systematic correlation with azimuth and therefore an

indication of azimuthal anisotropy.

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Figure 6.6 shows the approximate depths to the refractor obtained with equation

6.3.

Figure 6.6:  Depths to the main refractor computed with an average wavespeed

of 700 m/s in the upper two layers.

6.6 - Analysis of the In-line Amplitude Data

The amplitudes of the first arrivals were hand picked from the trace values with a

utility in Visual_SUNT, a seismic reflection processing software package. A

correction was applied for geometric spreading using a reciprocal of the distance

cubed expression, which previous studies had indicated was appropriate for this

site (Palmer, 2001a; chapter 3). The corrected amplitudes for the four pairs of 

shots described above, were then multiplied.

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Figure 6.7 shows the amplitude products for shots 2 and 9 which are colinear on

in-line 21. They show low values between cross-lines 45 and 49, which

correspond with the wavespeed of 5000 m/s, higher values between cross-lines

50 and 62, which correspond with the wavespeed of 1850 m/s, and lower values

between cross-lines 63 and 69 which correspond with the wavespeed of 3930

m/s. The amplitudes in this last region gradually increase towards cross-line 69,

and correspond with the decrease in wavespeed when the region is further sub-

divided into two regions.

Figure 6.7:  Amplitude products corrected for geometric spreading for shots 2

and 9.

These results are consistent with previous studies which demonstrate that the

amplitude product is approximately proportional to the square of the ratio of the

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specific acoustic impedances of the overburden and the refractor (chapter 5;

Palmer, 2001b). Since the wavespeeds in the layers above the main refractor 

exhibit little lateral variation in the in-line direction, the amplitudes are essentially

a function of the wavespeeds and densities in the refractor.

The amplitudes of the other shot pairs show the same general pattern, as well as

the detailed features such as the higher values at cross-lines 54, 56, 59 and 61

on line 17. These variations can be attributed to changes in the coupling of the

detectors, or near-surface changes in the wavespeeds.

Figure 6.8:  An apparent anisotropy factor obtained from the square root of the

ratio of the corrected amplitudes for the two pairs of oblique shots.

Figure 6.8 shows the square root of the ratio of the amplitudes obtained with

shots 4 and 8 in the northeast-southwest direction to the amplitudes obtained

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with shots 1 and 11 in the northwest-southeast direction. This parameter should

reflect a relative anisotropy factor, since it is not possible to provide an absolute

scale, because as yet, there is no method for compensating for the different

energy levels and coupling of each shot. However, an approximate scaling factor 

was obtained from the ratio of the wavespeeds in the different directions for the

region with the low wavespeeds between cross-lines 50 and 62 in Figure 6.3.

 An examination of the cross-line data described below, shows that in general the

wavespeeds are higher in the cross-line direction, that is along the dominant

geological strike, than in the in-line direction. However, the exception is the

region between cross-lines 45 and 49 where the reverse applies. The fact that

Figure 6.8 is consistent with this model provides confidence in the validity of the

relative anisotropy factor.

6.7 - Analysis of the Cross-line Traveltime Data

The traveltime data recorded in the cross-line direction show that the same three

layer model is applicable in the cross-line direction as for the in-line direction.

However the wavespeeds in the second layer show more variation and range

from 540 m/s on cross-line 57 to more than 1000 m/s on cross-lines 45 and 69.

Figure 6.9 summarizes the traveltime data for the shot points at the ends of each

cross-line.

The refractor wavespeed analysis function for each cross-line is shown in Figure

6.10. In general the pattern is similar to that determined for the in-line directions,namely a zone of low wavespeeds between cross-lines 49 and 61, and zones of 

higher wavespeeds on cross-lines 45, 65 and 69.

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Figure 6.9:  Stacked traveltimes for the shot points at each end of the cross-

lines.

On cross-line 45, the wavespeed is 3380 m/s, while on cross-line 49, there is a

lateral change from that value to 2000 m/s. Furthermore, there is a

corresponding change in the wavespeeds of the second layer shown in Figure

6.9. The 3380 m/s wavespeed correlates with the second layer values of 1020

m/s to 1200 m/s, while the 2000 m/s on cross-line 49 correlates with a value of 

810 m/s for the second layer. This correlation between the wavespeeds in

second layer and main refractor on cross-line 49 together with the change in

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refractor wavespeeds on the two in-line profiles at cross-line 50 are consistent

with a lateral change in wavespeed occurring on cross-line 49. The significance

of this result is that a major change in refractor wavespeed has been resolved

along cross-line 49, even though the contact between the two zones is probably

not orthogonal to cross-line 49. Theoretical studies (Sjogren, 1984, p168-173)

have predicted that there should be errors in the determination of accurate

refractor wavespeeds.

The lateral change in the wavespeed on cross-line 49 also provides an

explanation for the inconsistent traveltimes obtained on the in-line profiles with

the shot points on cross-line 81. The seismic trace consists of a low amplitude

early arrival from the high wavespeed zone, followed by the high amplitude later 

arrival from the adjacent low wavespeed zone. It is possible that one of these

arrivals may be a side swipe.

The wavespeed of 3380 m/s determined in the cross-line direction is significantly

less than the value of about 5000 m/s determined previously with the 2D profile

in the in-line direction. It contrasts with the remainder of the survey area, in

which the wavespeeds are greater in the cross-line direction. However it is

consistent with qualitative measures of azimuthal anisotropy obtained with time-

depth ratios in Figure 6.5 and with amplitude ratios in Figure 6.8.

The wavespeed of 2000 m/s between cross-lines 53 and 61 is 7.5% larger than

the value of 1850 m/s measured in the in-line direction.

On cross-lines 65 and 69, the wavespeed is 6500 m/s between in-lines 19 and

21 and 2000 m/s to 2100 m/s elsewhere. Although the accuracy of the

wavespeed in this center interval is not high because it is measured over a

limited number of points, it is still higher than the in-line value of 3930 m/s

determined between cross-lines 60 and 69.

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Figure 6.10:  Stacked wavespeed analysis function for the offset shots for cross-

lines 45 to 69.

These results are a compelling demonstration that there can be important 3D

effects even with a nominally 2D geological structure. The lateral change in

wavespeeds on cross-line 49 generates inconsistent arrivals on the in-line data

which are more readily explained with the cross-line data. In addition, the rock

fabric in the region between cross-lines 45 and 49, as measured with the

apparent anisotropy factor, is approximately orthogonal to the dominant

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geological strike direction and to the fabric in the remaining regions of the

refractor. One geological interpretation is that this region has undergone rotation

during the formation of the shear zone.

Figure 6.11:  Isometric view of the cross-line time-depths.

Figure 6.11 is an isometric view of the cross-line time-depths and shows that the

variations in the cross-line direction can be considerable even for a nominally 2D

structure with a line orientation which attempted to parallel the dominant strike

direction.

6.8 - The Cross-line Amplitude Data

The cross-lines were recorded in groups of four with the shots being collinear 

with either the second or third line in the group. The amplitudes for the seven

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cross-lines shown in Figure 6.12, were obtained by combining the corrected

amplitude products for each line using the offset shot pairs for that line. As there

was usually some variation in the energy levels from each shot due to shot hole

depth or local geological conditions affecting coupling of the energy, it was

necessary to scale each set of amplitudes to a common level. This was

achieved by determining an average scaling factor between adjacent lines using

the two shot pairs collinear with those two lines.

Figure 6.12:  Isometric view of the cross-line amplitude products corrected for 

geometric spreading.

The amplitude products corrected for geometric spreading are shown in Figure

6.12. In general, the amplitudes reflect the wavespeeds in the refractor. The low

wavespeeds between cross-lines 53 and 61 produce an increase in the

amplitudes, while the higher wavespeeds between cross-lines 45 and 49 and on

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cross-line 65 produce lower amplitudes. However, there are a number of 

departures from this trend where a detailed correlation is made.

On cross-line 45, there is a gradual increase in amplitude from in-line 13 to in-line

25, which is consistent with an increase in the wavespeed in the overlaying layer 

from 1020 m/s to 1200 m/s. On cross-line 49, the decrease in wavespeed in the

refractor from 3380 m/s to 2000 m/s is matched with a decrease in the

wavespeed in the overlaying layer from 1020 m/s to 810 m/s, resulting in only a

minor increase in amplitudes.

There is no obvious explanation for the decrease in amplitudes at each end of 

cross-lines 53 to 61. While the higher amplitudes in the center of each line

correlate with the low refractor wavespeeds, there is little evidence for any

significant variation in wavespeeds in the cross-line direction.

Variations in topography and density might provide an explanation. The

topography along the in-lines 17 and 21 is lower than that of the surrounding

survey area and there is an ephemeral creek located across the southeastern

corner. Accordingly, the edges of the survey area may have lower moisture

levels and therefore lower densities in the second layer. Furthermore, no

account has been taken of shear wavespeeds, which also affect the head

coefficient.

The gradual increase in amplitudes on cross-lines 53 to 61 along in-line 13

correlates with an overall increase of wavespeeds in the second layer from about

600 m/s to about 800 m/s. However, these values are much the same as the in-

line values of about 700 m/s, and so do not provide a complete explanation for 

the decrease in amplitudes at each end of cross-lines 53 to 61.

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The locally high values on cross-lines 55 and 61 correlate with similar peaks on

the in-line results, suggesting a geological source, rather than variations in

geophone coupling.

It is also difficult to fully explain the amplitudes on cross-lines 65 and 69 because

they do not readily correlate with wavespeeds or the in-line results. The high

amplitudes between in-lines 21 and 24 correlate with low refractor wavespeeds

shown in Figure 6.9, but they are not matched with similar amplitudes for the low

wavespeeds between in-lines 13 and 17. Furthermore, the high wavespeed

region between in-lines 17 and 21, exhibits both low but more commonly high

amplitudes.

Figure 6.13:  Summary of wavespeeds and interpreted faults plotted over the

contours of the time-depths in milliseconds. The bold arrows indicate thedirections of the higher wavespeeds.

Despite these apparent inconsistencies, it is possible that the amplitudes are still

providing a viable model of the wavespeeds in the refractor. The low

wavespeeds which are implied by the high amplitudes, correlate with the

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separation of the region between cross-lines 60 and 69 into two intervals with

wavespeed of approximately 5000 m/s and 3000 m/s, and with a wavespeed of 

2600 m/s detected on the earlier 2D data between cross-lines 67 and 73.

Figure 6.13 is a summary of the wavespeeds in the in-line and cross-line

directions plotted over a contour map of the time-depths. The boundaries of the

regions with different wavespeeds are interpreted as faults. An additional fault

along in-line 17 might also be inferred on the basis of the cross-line amplitudes.

 Although this discussion has focused on the variations in amplitudes due to

variations in wavespeeds, it is recognized that other factors, such as inelastic

attenuation, can affect amplitudes. The inelastic attenuation in the refractor at

each detector for a given shot pair, is reduced to a constant amount with the

amplitude product or convolution and therefore, it is not a significant factor. The

inelastic attenuation in the overburden is not compensated with the amplitude

product, and it may be an important factor in lossy media. However, in this case

history, the travel path in the overburden is less than two times the dominant

wavelength of the seismic energy (700 m/s / 35 Hz), and is considered to be a

second order effect.

6.9 - Discussion and Conclusions

The results of this study are a convincing demonstration of the benefits of 3D

shallow refraction methods. Although the shear zone at Mt Bulga is considered

to be a 2D structure, the significant spatial variations in depths, wavespeeds andazimuthal anisotropy demonstrate that it is best viewed as a 3D target.

The depths to the refractor show considerable variation in the cross-line direction

as well as in the in-line direction. There is a general increase in depths which is

associated with the lower wavespeeds of 1850 m/s to 2000 m/s between cross-

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lines 49 and 61. However, the increase in depths between in-lines 20 and 25 on

cross-line 49 could not be confidently predicted on the basis of the results from

either the earlier 2D profile or the two in-line profiles in this study. Similarly, the

lateral variations in wavespeeds on cross-lines 65 and 69 also require the

additional coverage in the cross-line direction to be detected and resolved.

In general, the amplitudes correlate with the ratio of the wavespeeds between the

refractor and the layer above. However, there are a number of anomalies in the

cross-line results for which as yet there are no obvious explanations. The

geology of the survey area is quite complex and it is probable that drilling or 

excavation would be required to obtain a complete explanation of the observed

amplitudes.

The amplitude ratios provide a convenient approach to determining azimuthal

anisotropy. The qualitative correlation between the measures of azimuthal

anisotropy obtained with wavespeeds, time-depths and amplitude ratios provides

confidence in the validity of the results. This is especially the case with the

region between cross-line 45 and 49 where the direction of the maximum

wavespeed is approximately orthogonal to that for the remainder of the survey

area and to the dominant geological strike direction.

 A major benefit of using amplitudes as a measure of the wavespeeds and

therefore anisotropy, is that a value can be determined at each detector, whereas

several collinear detectors are usually required if traveltimes are employed. In

addition, it is probable that the amplitudes may be a more sensitive measure of 

anisotropy than traveltimes.

There were a number of difficulties in combining some of the in-line and cross-

line amplitude results. This suggests that data be recorded in a single pass

using several parallel lines of detectors, rather than with a number separate

recording setups.

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The results of this study also show that shots laterally offset by up to 20 m still

produce results similar to the in-line shots. Therefore, a single line of shot points

together with a number of parallel recording lines, would be efficacious for 

recording 3D refraction data along narrow swaths. A minimum of three parallel

lines is suggested, while five or more would give better cross-line determinations

of wavespeeds with the traveltime data.

Such a recording program would be suitable for many types of geotechnical

investigations as for example with highways and damsites, which require only

relatively limited coverage in the cross-line direction. Accordingly, the benefits of 

the additional sampling in the cross-line direction can be achieved without a

commensurate increase in the number of shot points. Typically, an increase of 

about 100% over an equivalent 2D program may be sufficient.

The refractor mapped in this study has large spatial variations in depths,

wavespeeds and azimuthal anisotropy and therefore it provides a searching test

of any approach seeking to resolve each of these parameters. The results of this

study demonstrate that simple and efficient 3D refraction methods using the

GRM can provide more useful geological interpretations than would be the case

with even detailed 2D approaches.

6.10 - References

Bamford, D., and Nunn, K. R., 1979, In-situ seismic measurements of crackanisotropy in the Carboniferous limestone of North-west England: Geophys.

Prosp., 27, 322-338.

Barker, J. A., 1991, Transport in fractured rock, in Downing, R. A., and Wilkinson,

W. B., eds., Applied groundwater hydrology: Clarendon Press, 199-216.

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Bennett, G., 1999, 3-D seismic refraction for deep exploration targets: The

Leading Edge, 18, 186-191.

Crampin, S., McGonigle, R., and Bamford, D., 1980, Estimating crack

parameters from observations of P-wave velocity anisotropy: Geophysics, 45,

345-360.

Deen, T. J., Gohl, K., Leslie, C., Papp, E., and Wake-Dyster, K., 2000, Seismic

refraction inversion of a palaeochannel system in the Lachlan Fold Belt, Central

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Lanz, E., Maurer, H., and Green, A. G., 1998, Refraction tomography over a

buried waste disposal site: Geophysics, 63, 1414-1433.

Leslie, J. M., and Lawton, D. C., 1999, A refraction-seismic field study to

determine the anisotropic parameters of shales: Geophysics, 64, 1247-1252.

Nestvold, E. O., 1992, 3-D seismic: is the promise fulfilled?: The Leading Edge,

11, 12-19.

Palmer, D., 1980, The generalized reciprocal method of seismic refraction

interpretation: Society of Exploration Geophysicists.

Palmer, D., 1986, Refraction seismics - the lateral resolution of structure and

seismic velocity: Geophysical Press.

Palmer, D., 1991, The resolution of narrow low-velocity zones with the

generalized reciprocal method: Geophys. Prosp., 39, 1031-1060.

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Palmer, D., 2001a, Imaging refractors with convolution: Geophysics 66, 1582-

1589.

Palmer, D., 2001b, Resolving refractor ambiguities with amplitudes: Geophysics

66, 1590-1593.

Sjogren, B., 1984, Shallow refraction seismics: Chapman and Hall.

Thomsen, L., 1995, Elastic anisotropy due to aligned cracks in porous rock:

Geophys. Prosp., 43, 805-829.

Zelt, C. A., and Barton, P. J., 1998, 3D seismic refraction tomography: a

comparison of two methods applied to data from the Faeroe Basin: J. Geophy.

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Zelt, C. A., 1998, Lateral velocity resolution from 3-D seismic refraction data:

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