Seismic surveys provide information about stratigraphy, hydrogeology, and bedrock topography. Seismic surveys are also a cost-effective way to extend information derived from borehole logs over a much larger area, minimizing the need for costly multiple borings. Crosshole seismic, MASW, ReMi, and Downhole surveys are used to determine shear wave velocity as a function of depth to help determine geotechnical properties of rock and soil.
Seismic surveys measure the time it takes for reflected/refracted energy waves
traveling through the earth to return to the surface. By analyzing
this data, it is possible to gain some understanding of characteristics
of subsurface layers such as depth, thickness, and attitude.
General Applications
- Municipal and industrial groundwater supply investigations
- Pre-construction surveys
- Environmental audits and site assessments
- Evaluation of active/inactive industrial sites
- Public/private remedial investigations and feasibility studies
- Landfill closures
Specific Applications
- Determine depth of competent bedrock
- Determine depth of water table
- Determine overburden thickness
- Map bedrock topography
- Determine rock and soil properties
- Determine lithology, fractures, and faults
- Determine rock rippability
- Determine Seismic Site Class for IBC requirements
For a seismic survey, geophones are set out along a straight line and connected to a seismograph. A seismic source, usually a weight drop or an explosive charge, generates an energy pulse. The time at which reflected or refracted signals are detected at each geophone is recorded on the seismograph. The time and distance data are then analyzed and interpreted.
Pavement and noise from traffic, pumps, and pipelines in urban areas present challenges to the collection of seismic data. Topographic changes, e.g. steep embankments, may also increase the difficulty of collecting accurate data.
Our extensive experience, state-of-the-art equipment, interpretation methods, and software enable us to overcome the limitations of difficult site conditions and provide our clients with interpretations based on high quality data in places where others can not.
We own two Geometrics Geode 24-channel digital instantaneous floating point seismographs, allowing us to use up to 48 geophones in a single spread. For a seismic source, we often use our Bison EWG-1, an hydraulically operated accelerated weight system mounted in a small utility trailer that we tow behind a 4WD ATV, in order to maximize signal quality.
The EWG has proven to be a particularly valuable source for seismic signals in urban areas, where the use of explosives and firing rods is simply out of the question.
We use the generalized reciprocal method (GRM) to interpret seismic refraction data. This sophisticated seismic refraction interpretation technique has the following benefits:
- Accurate definition of dipping layers
- Minimal influence from surface topography
- Determination of depth of refracting layers under each geophone
- Overcomes "hidden layer" and "blind layer" problems to determine an accurate depth of competent bedrock
We also perform high resolution shallow seismic reflection surveys using the common depth point method (CDP). Seismic reflection data provide detailed information about subsurface stratigraphy, lateral continuity of layers, and structure.
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