The environmental engineering industry has used different types of seismic testing to evaluate subsurface geology since the 1960's. An early form of the technique, called seismic refraction, was used primarily in rippability studies to determine what type of machinery was required to excavate materials at construction sites. The refraction technique is limited in its ability to image complex geologic structure or detailed stratigraphy, but because of the relatively low cost to perform a survey, and the ease with which refraction data can be processed, it is still a popular seismic application. The seismic technique was originally developed by the oil exploration industry. Almost all of the developments of the technique for shallow-depth, environmental problems were adapted from oilfield practices. Seismic reflection surveys have been performed in oil exploration to delineate subsurface structure since the 1930's. The early surveys (2D, single fold, continuous coverage profiling) provided large-scale structural information about the subsurface, but forced oil exploration teams to drill without a completely accurate image of the reservoir (much as is done in environmental engineering today). As the use of seismic surveys became more accepted and as funds were available for research, the technique evolved until it became an effective way to view and interpret large-scale subsurface geologic structural features. The advent of the 2D, multi-fold, common-depth-point surveying techniques, along with advances in instrumentation, computers, and data processing techniques, greatly increased the resolution of seismic data and the accuracy of the subsurface images. However, the technique still yielded little information on the physical properties of the imaged rocks, or the pore fluids within them. It was not until the introduction of 3D reflection surveying in the 1980's that seismic images began to resolve the detailed subsurface structural and stratigraphic conditions that were missing or not discernable from previous types of data. Today potential oil reservoirs are imaged in three dimensions, which allows seismic interpreters to view the data in cross-sections along 360° of azimuth, in depth slices parallel to the ground surface, and along planes that cut arbitrarily through the data volume. Information such as faulting and fracturing, bedding plane direction, the presence of pore fluids, complex geologic structure, and detailed stratigraphy are now commonly interpreted from 3D seismic data sets. In the environmental engineering industry 2D shallow seismic reflection imaging has been performed to map the overburden-bedrock interface at test sites since the 1970's. In recent years seismic reflection profiling has been applied to other geotechnical and environmental problems as well. In 1994, RRI performed the first high-resolution 3D seismic reflection survey at a hazardous waste site (Naval Air Station North Island, California). Since that time, over thirty 3D seismic surveys have been performed by RRI for environmental investigations. 3D seismic reflection surveys provide information that can be essential for characterizing and remediating hazardous waste sites. Seismic surveys are proficient at mapping potential contaminant migration pathways, determining the presence of subsurface fracture systems, and imaging structural and stratigraphic heterogeneities below a site. Seismic imaging technology provides valuable information to evaluate groundwater management alternatives.
Seismic reflection imaging is based on the principle that acoustic energy (sound waves) will bounce, or "reflect" off the interfaces between layers within the earth’s subsurface. This principle is analogous to the process of a human voice echoing off of a building wall. During a seismic reflection survey acoustic energy is imparted into the earth with a seismic source. RRI typically uses noninvasive sources on our...
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