How 3DVSP has become a practical proposition

Fran Doherty, director software development, VSFusion, explains how 3D vertical seismic profile technology has developed into an accessible tool for enhanced subsurface imaging and provides a case history to demonstrate its application. For the past 15 years, 3D surface seismic surveys have been the accepted standard mode of subsurface geophysical illumination. The change from acquiring and processing seismic data in single two-dimensional lines to massive 3D seismic surveys has come about due to large dollar amounts of R&D investments, as well as the dramatic drop in the computation power per dollar ratio. Today, more seismic data can be acquired, and the computer power exists that enables these large data volumes to be efficiently and economically processed. Subsequently, most of the earth’s sedimentary basins have been imaged by 3D seismic data. VSP (Vertical Seismic Profile) technology has likewise evolved significantly over the last decade. Perhaps the most important technological advance in the borehole seismic sector has been the development of the multi-level downhole array tool. Before this development, each desired VSP receiver position required that the tool be transported up or down the hole, locked to the borehole wall at some discrete depth, and after recording the data, unlocked and transported to the next desired location. This is a very time-consuming and hence expensive operation. The VSP multi-level array tool generally contains between five and 20 downhole receiver satellite assemblies. The spacing of these receiver satellites is variable, but in general they are spaced at approximately 15 m intervals. Each receiver satellite contains three orthogonally positioned geophones which are capable of recording the full downhole seismic vector waveform response. Compressional (P) and Shear (S) wave data can be recorded, and the waveforms identified and separated with careful VSP processing. Being able to deploy and record waveform data at multiple subsurface depths from a single source position has dramatically reduced VSP data acquisition cost, while dramatically increasing the amount of borehole seismic data that can be acquired.