Use of Computers in Seismic Reflection Prospecting
During the past decade the digital computer has brought about revolutionary improvements in our capability to study geology by the seismic reflection method. New recording and processing technology has brought us much closer than anyone would have predicted ten years ago to our ultimate goal of extracting the same geological information that would be retrievable if there were a borehole to the maximum depth of interest at every shot point.
These improvements in the seismic art have made it possible to map structural features with more accuracy and to present them more correctly; they also have enhanced our ability to identify lithology and deduce strattigraphic relationships from reflection data.
Perhaps the most spectacular development of the past decade in seismic exploration has been the capability under proper conditions of detecting gas deposits on seismic record sections, making use of the fact that reflections from the top of gas-filled sands have a higher amplitude than those from water- or oil-filled sands. Digital recording and processing makes it possible to preserve relative reflection amplitudes on seismic records. This could not be done with analog techniques because of their limited dynamic range.
Geologic structures can be mapped with more precision by automatic techniques for transferring reflections from their apparent positions based on reflection times alone to their true positions in space. Wave-equation and frequency-domain methods are employed for such migration. Three-dimensional recording and processing procedures require computers with high storage capacity but they greatly increase the accuracy with which complex structures, particularly in areas of tectonic disturbances, can be mapped.
New capabilities made possible by digital processing now allow the geophysicist to derive lithological and stratigraphic information of a type that was not obtainable previously from seismic reflection records. Filtering programs are available for extracting from complex source signals simple symmetrical wavelets that enhance resolution of reflections and allow better discrimination of stratigraphic relations. True amplitude registration facilitates determination of reflectivities at lithologie boundaries that permits construction of synthetic velocity logs. Presentation of such logs for all shot points in record-section form allows mapping of velocity, which with proper well ties can be converted to a lithologie cross-section comparable to that which could be obtained from closely spaced boreholes. Such presentations give valuable information on stratigraphy in areas where conditions are favorable. Computer determination of parameters such as instantaneous frequency and instantaneous phase, carried out by complex analysis of waveforms, may provide information on lithology and hydrocarbon content not obtainable from conventional processing.
Important progress has been made in computer modeling of seismic data. This involves ray-path modeling for precise structure interpretation and stratigraphie modeling to relate waveforms and amplitudes to subsurface layering. With the introduction of three-dimensional recording, modeling techniques have been determined particularly useful in the interpretation of the data thus obtained.
KeywordsSeismic Reflection Shot Point Record Section Seismic Trace Apparent Position
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