During the past decade, the oil and gas industry has recorded a large number of vertical seismic profiles (VSPs) in order to acquire improved structural, stratigraphic and seismic calibration information in the immediate vicinity of selected study wells. As is the progressive nature of the technology that the oil/gas industry uses in its exploration and development efforts, those companies who actively use VSP data are continually striving to find techniques whereby an increased volume of VSP data can be recorded in a study well at minimal additional cost. The inverse VSP concept has been developed to satisfy this objective.
Significant progress is currently being made in inverse VSP technology, so it is essential that a description of the equipment, field procedures and potential applications involved in this new type of borehole seismic measurement be available to the industry. This documentation is particularly important for those people who are responsible for evaluating reservoirs for purposes of calculating reserves in place, compiling data for unitization arbitration, or planning secondary recovery processes. Hopefully this paper will be a step in helping to disseminate this information throughout the industry.
In forward VSP, a geophone is positioned at various depths in a well in order to record the transmitted and reflected wavefields created by a seismic energy source located on the surface of the earth. In inverse VSP, the spatial positions of the source and receiver are reversed, with the source being manipulated at various depths in a well and the receiver(s) located on the surface of the earth. In concept, the raypaths between source and receivers, and consequently the travel times, are identical in these two recording geometries, as shown in Figure 1. In addition, the subsurface coordinates of the reflection points created by either measurement method coincide if the radiation patterns created by the surface and borehole sources are reasonably equivalent.