Near-Surface Seismic Reflection Surveys
Seismic reflection surveys for near-surface engineering, water resource, and mineral exploration are becoming more common for depths as shallow as 20-m to over 300-m.
There are advantages to using seismic surveys in water resource, engineering, and mineral exploration especially in areas where the subsurface structures may be geometrically complex. Large and/or structurally complex areas often benefit from seismic characterizations to fill-in areas between boreholes.
Reflection seismic surveys are not commonly used in engineering projects because drilling has been the traditional technique. To better understand the use of a seismic survey it is important to understand the strengths and weaknesses of drilling. Soils are relatively soft and so costs to drill are not as high as in rock. A single borehole provides samples for a very limited volume and there is a hit-or-miss aspect to drilling that may require many boreholes to properly understand the site. There are also various levels of subsurface drilling/sampling that need to be considered for an exploration or siting question. Solid stem auger techniques are probably the least expensive of the soil drilling techniques but are also the least likely to provide a detailed understanding of the subsurface. Detailed bedding relations are often lost by the time the sample arrives at surface due to the shearing, smearing, and intermixing of the original sample with soils along the wall of the hole. The depth of origin any particular sample from a solid stem auger is also somewhat problematic. Solid stem techniques are probably best applied for reconnaissance drilling in soils where a rough idea of the subsurface layering and/or esitmates of the depth to bedrock are required. The 'refusal' depth encountered by auger drilling is often interpreted as the depth to bedrock but large rocks, or boulder-clay till will also cause refusal. Sampling loose sands and gravels is also difficult with a soild stem auger. Hollow stem auger and sampling techniques provide a superior definition of the subsurface soils and layering. Clays are often sampled by Shelby tube while more granular materials are sampled by split spoon and standard penetration tests. If penetration of rock is required then either percussion drilling with a tri-cone bit to grind through the rock or diamond drilling to core sample the rock are used. The interface between the overburden and bedrock is often a difficult area to sample because augers have difficulty in sampling rock while rock drills have an equal difficulty in sampling soil.
Seismic reflection surveys using P-waves are applied every day for petroleum exploration and these methods have also been adapted to near-surface exploration problems. These surveys are the most expensive of the geophysical techniques because considerable energy, technology and manpower is applied with these methods. Horizontally polarized shear waves show promise to resolve soil structures within 50-m to 80-m of surface. An example of a SH-wave reflection survey to estimate the depth to bedrock within 12 to 16-m of surface is shown in Figure 1 below.
Figure 1. An example of a SH-wave reflection survey to estimate the depth to bedrock within 12 to 16-m of surface.
The upper portion in the above figure shows a vertical cross section of the ground surface elevation along the profile. The lower portion of the figure shows a common midpoint (CMP) stack of the SH-reflection data (346 24-channel records) collected along the same line. The geophone spacing was 0.2-m and the source offset was also 0.2-m. The data has been corrected for spherical divergence, source-receiver elevation statics, removal of dc shift, and bandpass filtering. The intermittent correlating event at about 200-ms is believed to be a reflection from the Precambrian bedrock surface at a depth of about 12 to 16-m.
The use of horizontally polarized shear-waves in shallow reflection studies shows good promise to make a contribution to near-surface structural studies. SH-wave velocities in soil are often a factor of 4 to 6 lower than the P-wave. This means the SH-wave length is 4 to 6 times shorter than the P-wave which results in a much higher resolution for subsurface layering. These SH-reflection surveys are often easier to perform than P-reflection surveys. The figure below shows the results from a subsurface seismic experiment that used a 0.22 cal shell as the energy source detonated in a hole in a typical Manitoba overburden with a 3-component geophone detector at a depth of 9.14-m down a second borehole and an array of geophones also at surface. The P-wave velocity is about 1700 m/s in this overburden but the S-wave velocity is 290 m/s. This remarkable velocity difference provides an important advantage to SH-reflection surveys in characterizing near-surface deposits.
Figure 2. Surface and borehole seismic response to a downhole seismic source (0.22 caliber shell).
The figure above presents the surface and borehole seismic response to a downhole seismic source (0.22 caliber shell). Traces 1 to 9 and 13 to 21 are from 10 Hz vertical mode geophones at the surface centred on borehole BH-21. Traces 10 to 12 are from a downhole 3-component geophone system at a depth of 9.14 m. The source was at a depth of 6.25 m in BH-21a collared 4.96 m NE of BH-21. Please note the large difference in frequency content between the surface and subsurface geophones. The estimate of the dynamic Poisson's ratio is close to the average value determined from surface-to-borehole P- and S-wave studies at this site.
Note the large difference in frequency response between the subsurface geophone package and the geophones at surface. This suggests that a great deal of signal is lost in unsaturated soils.
An array geometry that is frequently used for reflection surveys is shown in figure 3 below:
Figure 3. Common mid-point reflection survey array geometry.
The geophone spacing for shallow reflection surveys may be as small as 0.1 to 0.2-m for very shallow exploration and so the use of rapid sources and large channel count systems is important for productivity. Geophone separations of 2.5 to 5-m may be used for P-wvae reflection surveys to depths of 100-m. P- and SH-wave reflections are often visible on the raw record in the field. This in-field ability to observe shallow structures assists in survey design.