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Chapter 9
Stacking Seismic Refraction Data inthe Convolution Section
9.1 - Summary
The refraction convolution section (RCS) is an effective domain to vertically stack
shallow seismic refraction data, in order to improve signal-to-noise ratios (S/N).
The convolution operation essentially compensates for the effects of geometrical
spreading, and generates traces with much the same S/N ratios. Such traces
are optimum for stacking, unlike the traces on the original shot records.
A major benefit of stacking in the RCS domain is that it takes places before the
measurement of times or amplitudes. With other approaches which do not
routinely employ stacking, such as tomography, any variations in data quality are
addressed with the application of statistical methods to the traveltimes
determined on the original field data.
An essential requirement for stacking in the RCS domain are data which have
been acquired with a continuous roll along approach typical of reflection
methods, rather than with the more common static spread. Such operations are
more efficient and produce more data from the critical near-surface layers, but
they would require significant re-capitalization of most shallow seismic field
operations.
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9.2 - Introduction
It is well known that the source energy requirements for seismic refraction
surveys are considerably greater than those for seismic reflection surveys for thesame target. The maximum source-to-detector distance for seismic reflection
surveys is generally less than the target depth, whereas the minimum source-to-
detector distance with refraction surveys is usually greater than four times the
target depth. In addition, the geometric spreading component is the reciprocal of
the distance traveled for reflected signals, while the corresponding function for
refracted signals is the reciprocal of the distance squared. Both the longer path
lengths and the more rapid spreading factors result in low refraction amplitudes
and therefore higher source energy requirements. Commonly, the refraction
source is more than ten times the size of the reflection source for the same
target.
Explosives are the standard energy source in most shallow seismic refraction
surveys, and adequate signal-to-noise (S/N) ratios are readily achieved by
increasing the size of the charge. However, this is not always practical in many
environmentally sensitive or urban areas, and it normally results in either poor
quality data due to insufficient charge sizes, or more commonly, no acquisition of
data at all.
In some cases, it is possible to use vertical signal stacking with repetitive
sources, such as hammers and dropping weights. Nevertheless, this approach
can be of limited usefulness, because many repetitions can be required to obtain
reasonable S/N ratios, especially where urbanization is the major source ofnoise.
In addition, vertical stacking can result in slow rates of progress where there are
many source points. Walker et al, (1991) demonstrate that one of the most
important factors in improving the reliability of shallow seismic refraction
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interpretation is a detailed mapping of the wavespeeds in the layers above the
target refractor using a high density of source points. It is not uncommon to
employ a source point between every other pair of geophones.
This study demonstrates the use of vertical stacking with a CMP-like method
using the refraction convolution section (RCS). Redundant or multi-fold
refraction data are acquired with a continuous roll along approach, which is
standard with reflection acquisition. Multiple overlapping RCS are generated with
pairs of shots with the same shot point to shot point separation. The ensemble of
RCS are then sorted and gathered, in much the same way as reflection shot
records, and the gathers are then stacked.
The RCS is a suitable domain in which to stack refraction data when the shot
size, depth and separation are uniform, because the events have approximately
the same S/N ratios. With effective vertical stacking, the S/N ratio improves as
the square root of the number of traces in the stack, but only when the S/N ratios
of the original traces are much the same. Excessively noisy traces, that is traces
with an anomalously low S/N ratio, can significantly degrade the stack and
reduce the benefits of stacking. This situation occurs with stacking refraction
shot records, because there can be large variations in S/N ratios related to the
effects of geometric spreading. Traces at a given station with nearby source
points will have high S/N ratios while traces at the same station with more distant
source points will have lower S/N ratios. The large range in S/N ratios with
refraction shot records significantly reduces the effectiveness of stacking traces
from various shot records with a surface consistent approach.
Shearer (1991) demonstrates stacking shot records in which the shot-receiver
distance is preserved, but not the individual station locations, in order to improve
S/N ratios with earthquake data (see also Lay and Wallace, 1995, p215-216).
However, this approach is not a viable option with shallow refraction data,
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because it does not accommodate lateral variations in either the depths to or
wavespeeds in the target refractor.
A major advantage of stacking in the RCS domain is that S/N ratios can be
enhanced prior to the measurement of parameters, such as times or amplitudes.
By contrast, tomographic methods measure traveltimes from the shot records
which have varying S/N ratios, and then seek to minimize any errors with a
statistical approach.
9.3 The Cobar Stacked RCS Section
High resolution data were recorded with a 48 trace recorder, and single 40 Hz
geophones with a 10 m spacing, as part of a regional seismic reflection survey
(Drummond et al, 1992) across the Cobar Basin (Glen et al, 1994), in the central
west of NSW, Australia. The aim of these high resolution lines was to image the
near surface layers, which in these areas were dipping predominantly in the
vertical direction. The seismic source was a 10 kg charge of a high velocity
seismic explosives at a depth of 40 m, and the shot point interval was 30 m.
The data were recorded with off-end shots in both the forward and reverse
directions in order the obtain large shot-to-detector distances for the vertically
dipping reflection targets. For example, the first shot was at station 96, and the
geophones were from station 95 to station 48. The next shot was at station 90
and the geophones from station 89 to 42, a shift of 60 m. Subsequent shots
continued through to station 48 with geophones between stations 47 and 0. Thegeophone array then remained static while the shot points at 60 m intervals
within the array at stations 42, 36, etc., were recorded. The recording process
was then reversed. The shot point at station 3 was recorded with geophones
between stations 4 and 51. Subsequent shots were at 60 m intervals (stations 9,
15, etc.), and the geophone array was moved up by the same amount in each
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case (stations 10 to 57, 16 to 63, etc.). The resulting maximum refraction fold is
six, which is comparatively low.
Figure 9.1: Forward shot record number 65. Shot point is at station 33
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Figure 9.2: Reverse shot record number 43. Shot point is at station 90.
The first six to twelve traces near the shot point were arrivals from the surface
layer. In order to generate as many useful convolution traces as possible, pairs
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of shots spaced 63 stations apart were used. This reduced the fold to between
one and four.
Figures 9.1 and 9.2 are two shot records which show the familiar rapid decay of
refracted energy with distance, and in turn, the large variation in S/N ratios with
offset.
Figures 9.3 to 9.8 are a series of RCS over intervals of approximately 30
stations. The structure of the refractor can be readily seen in the RCS. A major
feature of the five RCS is the approximately uniform S/N ratios.
Figure 9.9 is the stacked section, obtained from the five sorted and gathered
sections. While the structure of the refracting interface can be recognized, there
is only a modest improvement in the S/N, due mainly to the low fold of between
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one and three. Nevertheless, it demonstrates that stacking is efficacious.
Clearly, a much higher fold is necessary to obtain the full benefits of stacking.
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Figure 9.9: Stacked convolution section.
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9.4 - The Static Geophone Spread
For the effective use of stacking, it is necessary to use RCS with relatively
uniform S/N ratios, and therefore to employ uniform acquisition parameters. The
relevant parameters are consistent charge size and depth, as well as a uniform
separation between the forward and reverse sources. Although these field
parameters are the norm with the refraction data acquired with routine reflection
surveys for petroleum and coal exploration and for regional reflection surveys in
fold belts, they are uncommon with most shallow seismic refraction surveys
carried out for geotechnical, groundwater and environmental applications.
The majority of shallow seismic refraction surveys carried out for geotechnicaland other shallow applications acquire data in discrete units or spreads. With
these surveys, a static spread of detectors is used with a multiplicity of source
points located at several offset positions on one side, through the spread and to
offset positions on the other side. The number of shot points recorded for each
spread has increased substantially in recent years in order to improve the
determination of the wavespeed stratification above the target refractor, and it is
now common to record more than eleven shots for a spread of 12 detectors
(Walker et al, 1991). For the typical survey length of 400 m for a road cutting,
approximately eight spreads of 12 detectors with a 2 detector overlap are
required (Walker et al, 1991), making a total of 88 shot points.
However, it is questionable whether even this considerable number of shot points
achieves the stated objectives of defining the wavespeed stratification within the
weathered layer. An inspection of published data (Walker et al, 1991, Fig. 6),
shows that the vast majority of the traveltime data (~ 90%) are arrivals refracted
from the base of the weathering. Although it is essential to ensure some
redundancy in the traveltime data in order to resolve the fundamental ambiguity
of determining the number of layers detected (Palmer, 1986, p21-29; Lankston,
1992), the majority of the data from the main refractor are generally not used in
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the subsequent data processing stages. It questions the fundamental
effectiveness of the static spread approach to acquiring shallow seismic
refraction data.
There are also concerns about the efficiency of field operations with static
spreads. There can be a comparatively large number of shot points per unit
distance because of the occupation of many shot point locations on two and
often three occasions, as well as the common practice of using an overlap of
several detectors. The repeated occupation of shot points can be
environmentally damaging as well as time consuming. Furthermore, field
operations do not progress smoothly, because the acquisition of data ceases
while the spread of detectors is retrieved and then re-deployed for the next
adjacent spread.
The static spread approach also results in a wide range of source energy
requirements for the different offsets and the different layers above the target.
Relatively low source energies are required for signals propagating in the shallow
near-surface layers, while considerably greater source energies are required for
the deeper target refractors. Since the majority of traveltimes are from the main
refractor, there can be a large source energy requirement. As mentioned
previously, many of these times are not used in the data processing, which
suggests that more efficient approaches may be possible.
This study proposes the continuous acquisition of shallow seismic refraction data
be employed routinely for geotechnical, groundwater and environmental
applications.
9.5- Continuous Acquisition of Shallow Seismic Refraction Data
Continuous acquisition of redundant or multi-fold data is the norm with seismic
reflection methods. With this technique, the source point maintains a fixed
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position in relation to the detector spread, which for land operations is usually a
split spread with the source in the centre of the detectors. The constant
geometry is obtained by laying out more detectors than there are channels in the
recording instruments and then selecting the required channels with a roll along
switch. Continuous and efficient operations are achieved with a single pass of
the seismic source along the line in conjunction with the continual removal of
detectors from the start of the line after they are no longer required, and their
placement at the other end to which the source is progressing.
Better and more uniform coverage of all refractors commonly occurs.
Roll along acquisition methods can provide better data for either the conventional
or convolution approaches where two or more adjacent static spreads would
normally be employed. Comparisons of field operations show that in fact, there
can be a reduction in the number of shot points per unit distance of coverage.
For example, for a 400 m long survey for a road cutting using a 15 m shot
spacing and a 5 m detector interval, a total of only about 55 shots would be
required. A 12 channel seismic recorder would not be suitable for roll along
operations, because the maximum shot to detector distance of 30 m would
generally be insufficient to record enough arrivals from the base of the
weathering. However, a 24 channel seismic recorder, which is widely used in
shallow refraction surveys, would be suitable, and in many cases might even
permit a reduction in the trace spacing to 3m to further improve the resolution of
the wavespeed stratification in the weathered layer. A 48 channel system would
provide further improvements in data quality through additional reductions in
trace spacing, as well as enhanced capabilities with swath or partial three
dimensional profiling.
The comparatively large number of shot points per unit distance with adjacent
static spreads is a result of the occupation of shot point locations on two and
often three occasions, as well as the common practice of using an overlap of
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several detectors. In contrast, the continuous recording of refraction data with
the roll along method involves the once-only occupation of each source point,
and incorporates a uniform overlap as an integral part of the method.
Accordingly, it represents a more efficient use of equipment and field personnel
with a lower environmental impact.
For surveys in which only limited coverage is required, it is still desirable to
replace the single static spread with a quasi roll along approach. However, the
number of source points may in fact increase with this approach, in order to
obtain sufficient data redundancy for stacking.
9.6 Determination of Fold with RCS Data
The maximum fold obtained with continuous refraction acquisition using a split
spread shooting method is similar to that obtained with reflection data, viz.
Maximum fold = Number of detectors / (Shot spacing x 2) (9.1)
Note that the shot spacing is given as the number of detector intervals.
The validity of equation 9.1 can be demonstrated with a simple example.
Suppose that the recording system has 48 channels, the shot is at station 25 and
that the live geophones are from stations 1 to 24 and 26 to 49. For the same
split spread recording pattern, reversed shots at stations 1 and 49, which
represent a shot spacing of 24 stations, are the minimum necessary to computea time-depth at each detector, provided all arrivals are from the target refractor.
The maximum resulting fold is therefore one.
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At the other extreme, if the shot spacing is reduced to a single detector interval,
then the maximum fold is 24. For the more common shot spacing of two detector
intervals, the maximum fold is 12.
In general, not all detectors will record arrivals from the target refractor. Suppose
that the first six arrivals on either side of the shot point are from layers other than
the target. For a shot spacing of two detector intervals, the fold will be 9. This
represents a substantial improvement in efficiency over the static spread
approach. The fold or redundancy of nine would be adequate to resolve most
ambiguities in layer recognition, as well as providing moderate improvements in
S/N through stacking. In addition, 25% of the arrivals are from the shallow
surface layers, which represents a significant improvement over the 10% for
typical static spreads, while the overall shot density per unit distance has been
decreased by as much as 40%. Further increases in the proportion of arrivals
from the near surface layers could be achieved by reducing the station spacing.
This analysis has used shot points at the stations themselves, rather than
midway between, which is also common. The benefit of shots at the detectors is
that reciprocal times, the times between the forward shot point and the reverse
shot point can be readily measured in both directions and then averaged. Any
traveltime delays caused by disturbed ground caused by previous shots, can be
avoided by offsetting the shot points by a few metres at right angles to the line.
9.7- Discussion and Conclusions
The refraction convolution section (RCS) is an effective domain to vertically stack
shallow seismic refraction data, in order to improve signal-to-noise ratios (S/N).
The convolution operation essentially compensates for the effects of geometrical
spreading, and generates traces with much the same S/N ratios. Such traces
are optimum for stacking, unlike the traces on the original shot records.
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and therefore detailed refraction statics analyses are necessary. Frequently, the
arrivals from the base of the weathering can be poor quality and some form of
signal enhancement prior to the measurement of traveltimes might be beneficial.
9.8- References
Drummond, B, Goleby, B, Wake-Dyster, K, Glen, R and Palmer, D, 1992, New
tectonic model for the Cobar Basin, NSW points to new exploration models for
targets in the Lachlan Fold Belt: BMR Research Newsletter 16, 16-17.
Glen, R.A., Drummond, B.J., Goleby, B.R., Palmer, D. and Wake-Dyster, K.D.,
1994. Structure of the Cobar Basin New South Wales based on seismic
reflection profiling: Australian Journal of Earth Sciences 41, 341-352.
Lankston, R. W., 1990, High-resolution refraction seismic data acquisition and
processing, inWard, S. H., ed. Geotechnical and environmental geophysics, vol.
1, Investigations in geophysics no. 5: Society of Exploration Geophysicists, 45-
74.
Lay, T., and Wallace, T. C., 1995, Modern global seismology: Academic Press.
Palmer, D., 1986, Refraction seismics - the lateral resolution of structure and
seismic velocity: Geophysical Press.
Palmer, D., Goleby, B., and Drummond, B., 2000, The effects of spatial samplingon refraction statics: Explor. Geophys., 31, 270-274.
Shearer, P., 1991, Imaging global body wave phases by stacking long-period
seismograms: J. Geophys. Res., 96, 20,353-20,364.
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Walker, C., Leung, T. M., Win, M. A., and Whiteley, R. J., 1991, Engineering
seismic refraction: an improved field practice and a new interpretation program,
REFRACT: Explor. Geophys., 22, 423-428.