GEOVision geophysicists often conduct high resolution seismic refraction and seismic reflection surveys in support of engineering, environmental, and hydrogeologic investigations.
When conducting seismic surveys, acoustic energy is put into the ground at a source location. Sources of acoustic energy include a sledge hammer impacting a metallic plate, weight drops, and a number of different types of explosive sources. The acoustic waves propagate into the ground at a velocity dependent upon the density and elastic properties of the material through which they travel. When the waves reach an interface where the layer velocities change significantly, a portion of the energy is reflected back to the surface, and the remainder is transmitted into the lower layer.
Where the velocity of the lower layer is higher than that of the upper layer, a portion of the energy is critically refracted along the interface. Critically refracted waves travel along the interface at the velocity of the lowerlayer and continually refract energy back to surface. Receivers (geophones), laid out in a line at an even spacing on the surface, record the incoming refracted and reflected waves. The seismic refraction method involves analysis of the travel times of the first energy to arrive at the geophones. These arrivals (first breaks) are from either the direct wave (at geophones close to the source), or critically refracted waves (at geophones further from the source). The seismic reflection method involves the analysis of reflected waves which occur later in the seismic record.
GEOVision geophysicists have experience using several 24/48-channel seismographs including the Geometrics R24/48, Geode, and the OYO DAS-1. Seismic energy sources used on past projects have included a sledge hammer, Betsy Seisgun , EG&G Geometrics Dynasource (a vacuum-assisted weight drop), Bison Elastic Wave Generator (accelerated weight drop), and explosives.
GEOVision geophysicists use the seismic refraction method to
- Map bedrock topography
- Map faults in bedrock
- Estimate depth to groundwater
- Estimate bedrock rippability
- Evaluate rock properties
GEOVision geophysicists use the generalized reciprocal method (GRM) to analyze high resolution seismic refraction data. Several computer programs are used in processing seismic refraction data including (1) the program FIRSTPIX by Interpex, Ltd, which allows manual or automatic picking of first breaks, and (2) the program VIEWSEIS by Viewlog Systems, Ltd., which implements the GRM.
GEOVision geophysicists use the seismic reflection method to
- Map subsurface stratigraphy
- Map lateral continuity of geologic layers
- Map buried paleochannels
- Map faults in sedimentary layers
- Map basement topography
GEOVision geophysicists use the program SEISTRIX 3 by Interpex, Ltd., to process seismic reflection data. Processing steps generally applied to reflection data include format conversion, trace editing, preprocessing (description of field geometry), spectral whitening or deconvolution, velocity analysis, surface consistent statics, velocity analysis, normal moveout corrections, prestack filtering, prestack gains, residual statics, and migration.