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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.