Directory UMM :Data Elmu:jurnal:J-a:Journal Of Applied Geophysics:Vol44.Issue2-3.2000:

Journal of Applied Geophysics 44 Ž2000. 257–273
www.elsevier.nlrlocaterjappgeo

Multi-component seismic surveying for near surface
investigations: examples from central Wyoming and southern
England
C.R. Bates a,) , D.R. Phillips b
a

Sedimentary Systems Research Group, School of Geography and Geosciences, UniÕersity of St. Andrews, Fife,
KY16 9ST Scotland, UK
b
GeoQuest ReserÕoir Technologies, 3609 S. Wadsworth BlÕd., 5th Floor, DenÕer, CO 80235, USA
Received 25 May 1998; accepted 14 January 1999

Abstract
Within the crust, the weathered layer has been shown to contain some of the greatest values of shear wave velocity
anisotropy. There are two main causes of this shear wave velocity anisotropy. In all rock types, anisotropy results from
structural weaknesses commonly manifest as aligned open fractures. In sedimentary rocks, additional anisotropy can result
from mineral or grain particle alignment and layering or bedding. Unconsolidated sediments also show this preferential
particle alignment which causes ordered heterogeneity or anisotropy that can be described as transverse isotropy. Both forms

of anisotropy can have significant implications not only for the strength of a material but also for the passage of fluids
through it. Two case histories are presented that describe these forms of anisotropy in the near surface and the implications
for environmental investigations. In central Wyoming, a downhole survey was conducted using 28 borehole locations
instrumented with three-component Ž3C. receivers and surface shear wave impact sources. The sandstone bedrock at the site
showed anomalous areas of high shear wave anisotropy that were interpreted to be due to a dominant regional fracture
pattern. In southern England, a downhole survey was conducted in two boreholes using an array of 3C receivers and both
surface and down hole shear wave sources. At this site, the heavily overconsolidated Oxford Clay showed transverse
isotropy due to the strong preferential particle alignment. Both examples illustrate the use of shear wave seismic studies for
mapping near surface geological features that may have important impacts for environmental and hydrogeological
investigations. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Shear wave splitting; Anisotropy; Multi-component seismic; Environmental investigation

1. Introduction
In bedrock, where the hydrology is controlled
by fractures and shear zones, these features
)

Corresponding author. E-mail: crb@st-and.ac.uk

often represent the dominant pathways for

groundwater flow and contaminant migration.
The fractures are typically sub-vertical to vertical and aligned with a dominant azimuth. The
preferential alignment caused by unequal horizontal stress imparts anisotropy which, in its

0926-9851r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 6 - 9 8 5 1 Ž 9 9 . 0 0 0 1 7 - 8

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C.R. Bates, D.R. Phillipsr Journal of Applied Geophysics 44 (2000) 257–273

simplest case, is known as transverse isotropy
with a horizontal axis of symmetry Ž TIH. . Such
a system is described using five elastic constants rather than the two Lame´ constants for
isotropic media. In unconsolidated sedimentary
sequences, an important environmental objective is the mapping of clay and sand strata. In
particular, mapping the continuity of clay layers
as barriers Ž aquitards. to contaminant migration
is a principal objective in characterisation and
remediation investigations. Preferential particle

alignment in clay sequences also imparts anisotropy to the material that can be described as
transverse isotropy but with a vertical axis of
symmetry Ž TIV. . It is possible to measure the
amount and type of anisotropy in rocks and
unconsolidated sediments using seismic waves
and in particular shear waves. The use of shear
waves for this purpose will be a focus of this
paper.
Shear waves are sensitive to different physical properties of earth materials than are compressional waves. Many authors have noted the
advantages in using shear waves in addition to
compressional waves for near surface reflection
seismics ŽClark et al., 1994; Dobecki, 1995. and
for deep seismics Ž Domenico and Danbom,
1987; Corbin et al., 1987. . The advantages of
using the shear waves for near surface investigations include higher seismic impedance contrasts at clay–sand interfaces, insensitivity to
water table and increased resolution for detecting thin strata. However, these benefits are often
not realised for several reasons. Shear waves are
more difficult to generate than compressional
waves and they usually are attenuated more
rapidly in the near surface. Also, shear wave

records are often contaminated by noise from
other wave types such as surface waves. One of
the major benefits of using shear waves is that
they exhibit shear wave birefringence or shear
wave splitting in anisotropic media. This property can be used to characterise rocks and their
anisotropy and has been exploited in the oil and
gas industry for over 15 years Ž Lynn and Thomsen, 1990; Mueller, 1991; Lynn, 1994. . In the

engineering and environmental fields, similar
advantages can be demonstrated but have seen
limited use ŽHasbrouck, 1987; Rai and Hanson,
1988; Bates, 1991. .
1.1. Shear waÕe birefringence
Shear wave birefringence, or shear wave
splitting, describes the action of a shear wave
when it enters an anisotropic medium. The shear
wave splits into two waves that travel at different speeds with different polarisation directions
along the same propagation path. The magnitude of splitting and the polarisation directions
of the split waves contain information about the
anisotropy of the medium. Shear wave splitting

has been comprehensively described in the literature and for a thorough review, the work of
Crampin Ž1985. is recommended.
1.2. Deep shear waÕe inÕestigations
In the hydrocarbon industry, most shear wave
work has concentrated on a particular form of
anisotropy in the ground described as azimuthal
anisotropy or TIH ŽWinterstein, 1990; Crampin
and Lovel, 1991. . Such conditions are commonly observed in naturally fractured hydrocarbon reservoirs with vertical open fractures parallel to the maximum horizontal stress direction
providing the dominant pathways for oil and gas
migration Ž Heffer and Dowokpor, 1990; Teufel
and Farrel, 1992.. Shear wave surveys for oil
and gas targets typically involve acquiring
multi-component seismic data using three-component Ž3C. geophones and one- Žor up to three.
component sources. For surveys with 3C geophones and 3C sources, a complete 9C data set
is recorded. Identification of shear wave splitting and its routine use in the hydrocarbon
industry required a number of important developments: Ž1. digital 3C recording systems, Ž 2.
high energy sources that could produce shear
waves with a broad frequency bandwidth, Ž 3.
computer programs capable of processing the
complex data sets. These developments are now

C.R. Bates, D.R. Phillipsr Journal of Applied Geophysics 44 (2000) 257–273

not only available for near surface investigations but they are cost effective to use as well.
1.3. Near surface shear waÕe inÕestigations
Shear wave studies in the near surface have
recorded some of the greatest values of shear
wave velocity anisotropy ŽCrampin, 1990; Lynn,
1991.. However, few near surface shear wave
surveys conducted over the last 10 years have
been directed at measuring this feature. Rather,
an emphasis has been placed on the fact that
shear waves travel at slower velocities than
compressional waves and, therefore, if they can
be generated with the same frequency then an
increase in seismic resolution is possible Ž Clark
et al., 1994; Dobecki, 1995. . This will hold true
only if waves of the same frequency can be
produced and the absorption per unit wavelength is the same for compressional waves and
shear waves. Sadly, in practice this has rarely

been observed. Before the advent of the minivibrators ŽChristensen, 1992., generating shear
waves with sufficient energy and bandwidth for
surveying to depths greater than 30 m was
difficult as the only engineering shear wave
source was the traction plate and hammer or
some variation of this ŽMooney, 1974.. In addition, because of the high costs associated with
shear wave seismic, most near surface shear
wave surveys have tended to record only a
limited number of components rather than the
full nine components.
One of the other potential uses of near surface multi-component seismic is in hydrocarbon
surveys for naturally fractured reservoirs. In
these types of survey, use is made of the shear
wave splitting to determine directions and degree of fracturing within deep hydrocarbon
reservoirs. One problem with these surveys is
that measurements made at the surface must be
used to infer changes at depth, thus the signatures imparted to the shear wave at depth have
to pass through the surface layer before they are
recorded. If the surface layers contain a high
degree of anisotropy this can overprint the ac-

259

tual deep signal that is of interest. Therefore, a
knowledge of the magnitude of surface anisotropy is critical to interpreting the records for
deeper information.

2. TIH, fracture anisotropy and anisotropic
criticality
The magnitudes of shear wave and compressional wave anisotropy in hydrocarbon investigations range from background levels of 1–4.5%
to anomalous levels of 25–30% recorded near
some fault zones. Crampin Ž1994. has presented
extensive data that suggest a correlation between the magnitude of shear wave anisotropy
and the magnitude of fracturing in a rock mass.
According to Crampin, at background levels of
anisotropy of 1–4.5%, the rock mass can be
considered to be intact and discrete fractures are
surrounded by largely uncracked, competent
rock ŽFig. 1.. It is rare to obtain the quality of

Fig. 1. Schematic realisation of percentage shear wave
anisotropy and crack density in a fracture rock Žafter
Crampin, 1994. together with a schematic realisation of
shear wave anisotropy ŽTIV. in a clay sequence with
preferential particle orientation.

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C.R. Bates, D.R. Phillipsr Journal of Applied Geophysics 44 (2000) 257–273

data necessary to record these background levels of anisotropy and a few percent anisotropy is
often within the experimental errors associated
with a field survey. The range of shear wave
anisotropy between 4.5 and 10% represents a
narrow band of fracture state that can easily be
crossed by only small differences in local in situ
stress inequalities. At values of shear wave anisotropy greater than 10% the rock is in a state
of heavy fracture with a cracking so severe that
a breakdown in shear strength may occur and
increased flow of pore fluids is evident through

the enhanced permeability. This theory of fracture criticality, above which the rock mass loses
structural integrity and thus fluid flow becomes
enhanced, has similarities to fluid percolation
theory. Percolation theory Ž Stauffer, 1985. is
used to describe fluid flow in heterogeneous
media. When occasional fractures are isolated in
a medium of low matrix permeability, flow or
permeability is near zero. As the density of
fracturing increases, the amount of fracture interconnectivity increases. At a critical density of
fracturing, interconnectivity is achieved and the
fluid permeability increases significantly. Zones
of high fracture density have been correlated
with zones of anomalous high hydrocarbon production as is evident in the Austin Chalk
ŽMueller, 1991. . While the work of Crampin
has been based in large part on deeper studies of
crustal rocks where minimum stress directions
are usually horizontal and not vertical there are
implications of his work for the near surface
where the minimum stress is usually vertical.
Some important places in the near surface where

the horizontal stress direction could be at the
minimum are steep hillsides, rock cuttings and
embankments and cliffs where instability is a
sever engineering problem. In addition, vertical
stress release would tend to accentuated horizontal weakness planes such as bedding.
Results from the hydrocarbon industry have
important ramifications for environmental Žnear
surface. investigation. For example, in the
northeast of the US, there is a heavy dependence on groundwater production from frac-

tured bedrock. Similarly, in the southeast of
England, there is a reliance on the naturally
fractured chalk aquifers for approximately 70%
of water supplies. Clean drinking water is produced from fractures in these rocks but fluid
contamination will also migrate along the same
fractures. Typical well head protection programmes use hydrogeological models based on
circular capture zones. In a fractured system,
this assumption is clearly invalid but currently
few techniques are available to adequately describe the fracture systems. Mini-vertical seismic profiles are suggested for mapping fractures
which intersect boreholes, while mapping the
fractures between the boreholes would require
surface reflection and refraction studies. The
following example details an investigation of
naturally fractured sandstone in central
Wyoming.
2.1. Central Wyoming fracture anisotropy study
At a site in Wyoming where unconsolidated
sediment and surface soil horizons are typically
less than 1 m thick, a survey was conducted to
measure the magnitude and variability in magnitude of anisotropy in the near surface. The
survey was conducted as part of a review of
static corrections for a deeper hydrocarbon
multi-component seismic exploitation survey.
However, the area is also one where there is a
heavy reliance on groundwater extraction for
domestic use and thus the example serves to
illustrate the magnitude of variability in near
surface anisotropy that can occur in environmental investigations. Prior to the geophysical
surveys, reconnaissance geological evaluations
were made of the area with particular note taken
to geological structural trends. The bedrock at
the site consists of sandstone interbedded with
fine shale of the Lower Eocene Wind River
Formation. The field site is at the crest of a
large anticline, however, local dip on the beds is
less than 58. Bedrock outcrop fault and fracture
mapping and fold axis trends showed the dominant azimuth to be West–East across the survey

C.R. Bates, D.R. Phillipsr Journal of Applied Geophysics 44 (2000) 257–273

261

Fig. 2. Results of geological outcrop analysis for Ža. structural trend of faults and fractures, Žb. folds, Žc. topographic linear
features from aerial photo-lineament analysis. Žd. shows the results of the down hole seismic survey for fast shear, S1
direction.

area ŽFig. 2a and b.. The results of an aerial
photo-lineament analysis ŽFig. 2c. also showed
an approximate West–East trend in topographic
linear features. The major topographic features
in these arid badlands environment tend to follow the structural weaknesses in the bedrock.
The regional water table is variable and inconsistent at between 20 and 40 m depth.
2.2. Field procedure
As part of a three-dimensional 3C deep hydrocarbon survey using dynamite sources, 28
boreholes were drilled over a 5 km2 area prior
to the main survey and fitted with 3C geo-

phones. The holes were drilled to a depth of 20
m and the 3C assemblies deployed at their
bases. The downhole geophone assembly consisted of three single geophone elements arranged to be mutually orthogonal with two elements in the horizontal plane and one vertical.
The geophones were potted inside empty dynamite explosive sleeves for easy deployment.
The geophones were then used to record surface
to hole shear and compressional wave data sets
to characterise the near surface anisotropy. During deployment no measures were taken to orient the horizontal geophones in the ground.
Orientation was accomplished after recording
by software rotation based on the compressional

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C.R. Bates, D.R. Phillipsr Journal of Applied Geophysics 44 (2000) 257–273

Fig. 3. Acquisition geometry for shear wave to borehole
study in Wyoming.

wave polarisation data. Once the geophones had
been deployed, the holes were back-filled and
left to settle for three weeks before recording to
allow the ground to recover from drilling disturbances.
The acquisition procedure is illustrated in
Fig. 3. For each buried geophone location, a
minimum of three near offset shear and compressional wave source locations separated by
308 to 408 were surveyed. In addition, two far
offset compressional wave source locations were
used per hole for orienting the downhole assembly. The shear wave source consisted of a sledge
hammer and 30 kg steel plate coupled to the
ground using 15 cm long spikes. The plate was
struck on each end to create shear waves of
opposite polarity which aids in the identification
of the shear arrivals. The source was located a
distance of 3 m from the hole in both radial and
transverse orientations. Striking the source vertically created the compressional waves. Seismic noise at the site was minimal as the nearest
source of cultural noise was 2 km from the site.

The following sequence was used in the processing of the shear wave records. The orientations of the horizontal geophone components
were determined using arrivals from the two
offset compressional wave sources to rotate the
data into directions parallel and perpendicular to
the plane of incidence between source and receiver. One set of shear arrivals for both horizontal components at all the 28 stations is shown
in Fig. 4a. The rotated data is shown in Fig. 4b.
A final rotation following the method of Alford
Ž1986. was used to separate the fast and slow
shear waves and to indicate the azimuth of fast
shear wave direction. An example of the final
data for one record after Alford rotation is
shown in Fig. 4c. For each hole, a comparison
of at least three source positions were made in
order to give an estimate of the error in the
recording procedure and the final azimuth and
magnitude of shear wave splitting. The fast and
slow shear wave time series were then crosscorrelated over a 20 ms time window around the
first breaks to obtain the time delay between
fast and slow waves ŽFig. 4c.. The final output
was a measure of the azimuth of fast shear
wave, the time difference between fast and slow
shear waves and the magnitude of anisotropy in
shear wave velocity.
2.3. Results
The results for all 28 geophone locations are
displayed as contour maps. In Fig. 5a, the
VPrVS1 ratio is shown. Only at one place in
the survey did the VPrVS1 ratio exceed 3 and
therefore, following the work of Sipos and Marshall Ž1995. it may be inferred that this is the
only location which truly belongs to a highly
weathered surface layer. For deep multi-component investigations, it is only at this location
that compressional wave static solutions can not

Fig. 4. Ža. Example field record of shear wave data to both unrotated horizontal geophones for all 28 locations, Žb. example
of the same shear wave field records for both horizontal geophones after software rotation based on the compressional wave
arrivals. Žc. Alford rotation for final shear wave data into qS1 and qS2 together with cross-correlogram for time delay
between records.

C.R. Bates, D.R. Phillipsr Journal of Applied Geophysics 44 (2000) 257–273

263

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C.R. Bates, D.R. Phillipsr Journal of Applied Geophysics 44 (2000) 257–273

Fig. 5. Ža. VprVS1 ratio from borehole study. Upper plot shows contour map of ratio with lower projection showing isometric view of the same data, Žb. S1
azimuth shown with arrows and the percentage shear wave anisotropy shown in upper plot as a contour map and lower plot as an isometric view of the same data.

C.R. Bates, D.R. Phillipsr Journal of Applied Geophysics 44 (2000) 257–273

be used to construct shear wave solutions in the
absence of specific shear wave data.
Fig. 5b illustrates the percent shear wave
anisotropy and fast shear wave azimuth. The
fast shear azimuth shows a consistent direction
approximately West–East. This direction was
also plotted on the rose diagram in Fig. 2d and
shows a similar direction to that for the structural geology and linear topographical trends.
Thus, the fast and slow shear directions are
interpreted to indicate a predominant weakness
direction aligned approximately West–East in
the surface rocks. The magnitude of shear wave
anisotropy is a measure of the difference in
velocity between fast and slow shear waves and
is greatest in the northern half of the survey
where values greater than 10% are observed.
This, according to the work of Crampin, would
represent a state of competency in the rock mass
where fracturing is so severe that a breakdown
in shear strength occurs and enhanced permeability or vertical fluid conductivity is inferred.
Thus, vertical hydraulic conductivity is also
greater at this location. This area would be
flagged as anomalous during an environmental
study and caution would be recommended with
regard to well head protection and potential
contamination or leak of hazardous waste in this
area.
It should be noted that the location that contained the problem area for seismic static solutions is not the area of highest anisotropy with
greatest impact for the environmental and engineering surveys. Nevertheless, the technique described here simultaneously addresses both
problem areas. The variation in shear wave
velocity anisotropy across the survey area is
significant and indicates the potential for rapid
variations in shear wave anisotropy in similar
geological settings. It is interesting to note that
the sampling grid for this survey was large and
therefore the question remains whether closer
sampling would have identified even greater
variation in shear wave travel time anisotropy
and if these near surface lateral variations are
also seen at greater depth.

265

3. TIV, clay layering and aquitards
The geological feature that most influences
near surface contaminant migration in unconsolidated sediment sequences is the presence and
continuity of clay layers. Beneath many industrial sites and landfills, contaminants have leaked
into the groundwater and the migration of the
contaminants is directly influenced by the permeability anisotropy of the sediments. Clay layers often inhibit migration and therefore represent an aquitard or barrier to fluid movement.
The degree to which the clay layer is a barrier is
often dependent on the amount of clay particles
in relation to sand particles within the layer and
the distribution of the particles or the anisotropy. The highest permeability anisotropy is usually observed where the clay content is highest
and shows the most distinct preferential clay
particle alignment. This alignment is usually
horizontal to sub-horizontal thus causing a permeability anisotropy where fluid flow is less in
the vertical direction than in the horizontal direction. Holes in clay layers, where either the
clay content is reduced or the clay has been
eroded, often represent sinks where contaminants can leak through to deeper aquifers. Locating these holes is critical to many site investigations. It is postulated that for most clay
deposited in dynamic environments, as the proportion of clay to other minerals, dominantly
sands, decreases so will the degree of layer
anisotropy Žcf. Fracture Criticality in Fig. 1..
Thus, high clay content representing a good
aquitard will also result in high seismic anisotropy. A critical anisotropy level is anticipated
above which the clay layer will form a strong
aquitard, and similarly a base level of anisotropy exists below which the clay will allow the
free flow of groundwater and contamination.
One might therefore expect to observe a correlation between hydraulic conductivity and shear
wave anisotropy in unconsolidated clay sediments with high hydraulic gradients where anisotropy is low and small hydraulic gradients
where seismic anisotropy is high.

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C.R. Bates, D.R. Phillipsr Journal of Applied Geophysics 44 (2000) 257–273

The following example details a site investigation of an overconsolidated clay sequence in
southern England to investigate the degree of
transverse isotropy. The survey was conducted
as an engineering experiment to determine the
degree of anisotropy that could be associated
with unconsolidated clay sequences. Such clays
are often cited as excellent fluid contamination
barriers and are used as such in many landfill
sites.
3.1. Preferential particle orientation study
At a site near Purton, southern England, the
Oxford clay, a heavily overconsolidated clay, is
horizontally bedded to depths of greater than
100 m. Within this sequence, no significant
lithological or structural boundaries are observed and a steep velocity profile has been
recorded for both compressional and shear
waves.
3.2. Field procedure
The field technique, illustrated in Fig. 6,
followed one adapted from White et al. Ž 1983.
and was first used at the site by Al-Azzawi

Fig. 6. Acquisition geometry for surface to downhole study
at Purton, southern England.

Ž1986.. An approximately square array of 3C
geophones was clamped at sequential depths in
two adjacent boreholes. The 3C geophone assemblies were the OYO Borehole Picks with 28
Hz receiver elements. The azimuth of each 3C
geophone assembly was controlled using orientation rods from the surface and the receivers
themselves were clamped to the borehole walls
using inflatable bladders. The surface shear wave
source for horizontally polarised shear wave
energy was a sledge hammer and traction plank
weighed down by a vehicle. The source was
struck on either end in order to correctly identify the shear wave arrivals. In addition, each
record was stacked and normalised in order to
average out any inconsistencies in impact
strength. In Fig. 7a, an example is given of the
stacked shear wave arrival energy from a source
position at the mid point between the two boreholes to an array positioned with geophones at
20, 22, 24 and 26 m depths. Only the horizontal
geophone that is aligned with the source is
shown in this figure. A Bison Instruments Borehole Shear Wave Hammer was used for the
down hole vertically polarised shear waves. This
hammer is deployed in an adjacent borehole and
then clamped to the sides of the borehole by a
mechanical lever action. The hammer is then
activated by rapidly sliding a weight against the
clamped hammer thus inducing a vertically polarised wave. An example from this source at a
depth of 2 m to an array clamped with the top
geophones at 18 m is given in Fig. 7b.
From the arrival times of waves generated by
the sources at the surface and within an adjacent
borehole, the phase and velocity for horizontally
propagating, horizontally polarised waves Ž hSh. ,
horizontally propagating, vertically polarised
waves ŽhSv. and vertically propagating, horizontally polarised waves ŽvSh. were measured
together with the compressional waves. From
the computed waves, the five elastic constants
that characterise a medium with hexagonal symmetry were calculated. A number of array positions were recorded to obtain a vertical profile
of wave velocity and elastic constants.

C.R. Bates, D.R. Phillipsr Journal of Applied Geophysics 44 (2000) 257–273

267

Fig. 7. Ža. Example records for downhole shear wave arrivals from a horizontal shear wave source position between
boreholes to geophones at 20, 22, 24 and 26 m depth on channels 1 and 5, 2 and 6, 3 and 7, 4 and 8, respectively, Žb.
example records from vertical shear wave source at 2 m depth to geophones at 18 m depth on channels 1 and 2 in the near
borehole and 5 and 6 in the far borehole and to geophones at 27 m depth on channels 3 and 4 in the near borehole and 7 and
8 in the far borehole.

In addition to the field investigation, samples
were obtained of the Oxford clay for laboratory
analysis which included velocity logging for
shear and compressional waves and analysis
using an electron microscope to determine the
particle fabric orientation.
3.3. Results
The results of the borehole shear wave survey are plotted in Fig. 8a for arrays from the
surface to 50 m depth together with those from
a previous survey by Al-Azzawi Ž 1986. from 50
to 90 m depth. The computed velocities for hSh
and vSh are plotted together with the results for
a standard cross-hole survey, hSv. Velocities
recorded varied between 175 and 850 msy1.

Plotted in Fig. 8b are the calculated shear wave
velocity transverse isotropy results that varied
between 20 and 50%. These results fall within
the range of transverse isotropy modelled by
Kerner et al. Ž1988. with a strong velocity
gradient in both the compressional wave and
shear wave records. In Fig. 9, a view of the
results from SEM analysis are shown for a
vertical section of the clay obtained from a
depth of 6 m. This image clearly shows the
strong preferential alignment of the clay particles in sub-horizontal orientation.
The results from the Purton site can be explained in terms of a card deck model popular
in structural geology ŽHills, 1972; Thomsen,
1986; Winterstein, 1992. . Three characteristic
wave types have been identified to be of impor-

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C.R. Bates, D.R. Phillipsr Journal of Applied Geophysics 44 (2000) 257–273

Fig. 8. Ža. Downhole results for hSh, vSh and hSv for Purton site and Žb. shear wave velocity anisotropy for the Purton site.
Surface to 50 m depth from this survey together with those from a previous survey by Al-Azzawi Ž1986. from 50 to 90 m
depth.

Fig. 9. Ža. Electron microscope vertical cross section of Oxford Clay together with Žb. sketch highlighting the preferential
particle orientation.

C.R. Bates, D.R. Phillipsr Journal of Applied Geophysics 44 (2000) 257–273

tance to the study of transverse isotropy using
White’s method, namely hSh, hSv and vSh.
The hSh waves propagate in the horizontal
direction with particle motion horizontal and
entirely contained within the transverse planes.

269

This wave type will exhibit no additional shear
wave splitting if the polarisation direction is
perpendicular to the axis of symmetry. Within
the card deck Ž Fig. 10a., this is represented by a
greater strength within each card than bonding

Fig. 10. Card deck model for shear wave propagation in transversely isotropic media showing individual card response and
the impact source orientations. Ža. Horizontal propagation, horizontally polarised waves ŽhSh.. Žb. horizontal propagation,
vertically polarised waves ŽhSv.. Žc. vertical propagation, horizontally polarised waves ŽvSh..

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C.R. Bates, D.R. Phillipsr Journal of Applied Geophysics 44 (2000) 257–273

between cards. Thus, flexure of each card or
buckling within the layers is difficult and analogously, the velocity of a wave is high.
The hSv waves propagate horizontally but
with the polarisation direction vertical and
aligned with the axis of symmetry. On passage
of this wave, particles are flexed and slip with
respect to each other. The card deck analogy
ŽFig. 10b. shows that flexure or bending a card
deck along its length causes the cards to bend or
fold with slip between individual cards and this
requires less force than was required to buckle
the cards during hSh deformation. The velocity
of this wave is therefore predicted to be less
than for hSh.
The vSh waves propagate vertically with horizontal polarisation and thus cause particles to
once again slip over each other. Here, the vertical bonds between particles or the friction between the cards controls the velocity or force
needed for deformation. The card deck model
predicts that vSh waves ŽFig. 10c. and hSv
waves ŽFig. 10b. should have similar velocities
and this is in fact what is found in the field
study ŽFig. 8a.. Furthermore, it is predicted that
these should both be less than hSh waves and
once again this was observed in the field. The
cause of these variations is explained in terms
of the preferential particle alignment of the clay
minerals which was observed in the electron
microscope images.
The directional variations in velocity, and
hence elastic properties at this site, have important implications for engineering, hydrogeology
and environmental investigations. For example,
the likely permeability anisotropy that the preferential particle alignment in the clay causes
will result in very different contamination
breakthrough times horizontally vs. vertically.
The seismic shear wave method provides a
means to determine the amount of anisotropy in
the field over a wide area and thus provides
significant data for characterisation and remediation actions at such a site or where a clay is to
be used as a barrier, either natural or artificial,
to fluid contaminant migration. Further work is

now needed to link the seismic anisotropy to the
engineering properties and hydraulic flow rates
at sites.

4. Discussion
The examples presented above illustrate the
high degree of anisotropy in the near surface
resulting from two very different geological features. In each case, the geological features could
be described with seismic measurements using
five elastic constants. The relatively simple
symmetry system allowed shear wave energy to
be easily identified as the records were uncomplicated by mode conversion or additional splitting. Unfortunately, such simplicity may not
always prevail, and complex systems with up to
the full 21 elastic constants may be necessary to
describe the anisotropy. Such cases are not only
beyond routine surveying today but would be
prohibitively expensive even if the technology
were readily available to make such measurements.
The Wyoming data identified an area of high
Žgreater than 10%. shear wave anisotropy that
would be described using Crampin’s criteria for
fracture criticality Ž Crampin, 1994. as one that
shows a state of heavy fracturing with a cracking so severe that a breakdown in shear strength
is likely. Furthermore, it is speculated that this
zone would also show increased flow of pore
fluids through the enhanced permeability, unfortunately, no hydrogeologic data is available for
corroboration. At the Purton site, anomalously
high anisotropy has been linked to the degree of
preferential clay particle alignment. If this anisotropy were interpreted using the fracture criticality relative to anisotropy argument put forward by Crampin, then it may be possible to
define a level of anisotropy in clay materials
above which the clay acts as an aquitard but
below which, at decreasing levels of anisotropy,
the clay becomes less of a barrier to fluid
migration. At Purton, the subsurface only consisted of clay and thus the whole sequence

C.R. Bates, D.R. Phillipsr Journal of Applied Geophysics 44 (2000) 257–273

would be described as an aquitard with high
permeability anisotropy likely especially below
18 m where the shear wave anisotropy is greatest. Current work seeks to investigate the limits
of thickness of clay layers where seismic shear
wave anisotropy can be measured and the quantitative relation between clay particle alignment,
permeability anisotropy and seismic anisotropy.
What is the future of multi-component seismic in near surface engineering and environmental work? Multi-component surveys are expensive, but there is a subset of the multi-component survey that is applicable to near surface
engineering and environmental problems that
can be used effectively and cheaply. The amount
of information to be gained from a multi-component survey is large and applicable to the near
surface field. Applications can be found in engineering studies where dynamic property evaluation and the assessment of stability of slope
conditions are critical. Also, in earthquake engineering a measure of shear strength, and its
lateral and vertical variation, is important in the
design of structures. In hydrogeologic studies,
many groundwater resources are controlled by
fractured bedrock or anisotropic clay layers. For
example in a hydrogeologic study in the Madison Formation in the Black Hills region of
South Dakota, Cheema and Islam Ž1994. found
using standard engineering tests that the direction of open fractures was closely associated
with that of preferential fluid flow in a groundwater aquifer. Such studies now need to be
accompanied by seismic investigations of the
type outlined in this paper. In environmental
investigations, preferential migration of contaminants is controlled by the heterogeneity in the
subsurface, specifically flow barriers formed by
clays and conduits created by alignment of sand
and gravel bodies. Shear wave surveys are also
recommended at these sites.
The cost of full multi-component surface reflection surveys using three orientations of receivers and three orientations of source will
always be high. However, the risks of acquiring
only a subset of the data Žone-component

271

recording. can be large. Under certain circumstances, such as the relatively simple anisotropic
systems outlined herein and with a good knowledge of the existing geologic conditions, this
risk can be limited and a successful survey will
result. The use of borehole seismic in the form
of fracture logging and shear wave vertical seismic profiles can provide further information on
the possible anisotropic conditions and perhaps
should be a pre-requisite to any surface reflection work especially if one is to limit the number of components that are measured in the
field. To be able to understand and thus eventually fully utilise multi-component seismic will
require additional field data to document the
relationships between geologic and hydrogeologic parameters and the seismic signatures.
Such information should include studies where
the seismic anisotropy has been recorded and
information is available on the current in situ
stress field, fracture orientations, preferential
sediment particle alignment, hydraulic gradients
and permeability anisotropy. Furthermore, additional information on electrical and electro-magnetic anisotropy should be integrated with the
seismic anisotropy as the cause of the anisotropy is likely to be linked. Unlike deep oil and
gas seismic work where over the last 15 years, a
good understanding has been gained of the geological processes that cause seismic anisotropy,
the causes in the near surface field are still
being investigated. A great future therefore exists for near surface multi-component surveys
not only with respect to understanding the causes
of near surface anisotropy, but also in helping to
understand deeper data.

Acknowledgements
The original data recorded at the southern
England site was recorded under NERC award
ŽGT4r86rGSr106.. A. Davis and J. Bennell
are thanked for their invaluable help in acquiring and interpreting this data. The Wyoming
data was acquired as part of the DOE program

272

C.R. Bates, D.R. Phillipsr Journal of Applied Geophysics 44 (2000) 257–273

DE-AC21-94MC31224 and Eugene Lavely is
thanked for software development and insightful
commentary. Support from the oil and gas industry partner is also recognised for access to
the site.

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