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SEISMIC Surveys:
History:
The environmental engineering Industry has been using 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 became 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 (20, 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 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-sca1e subsurface geologic structural features.
The advent of 2D. multi-fold, common- depth- point surveying techniques, along with advances in instrumentation, computers, and data processing techniques, has greatly increased the resolution of seismic data and the accuracy of the subsurface images. However, the technique stile yields 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 dissemble from previous types of data. Today potentia.1 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 nterpreted from 3D seismic data sets.
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