VSP fundamentals that improve CDP data interpretation

Abstract

This paper emphasizes the fundamentals of the vertical seismic profile (VSP) that improve the interpreter's confidence and understanding in its use as an exploration tool. The areas covered are VSP acquisition, critical points of the processing sequence, and applications to borehole information in depth and CDP data in time. Correlating reflection character to the stratigraphic section is done typically with synthetic seismograms computed from sonic logs and check shot traveltimes. A VSP extends check shot surveys from first break traveltime analysis to reflection analysis by recording the complete seismogram at a fine depth interval in the borehole (at least two depths per seismic wavelength). The unique feature of a VSP is the simultaneous measurement of downgoing and upgoing seismic waves as they propagate at depth. A repeatable source such as Vibroseis and the fine depth sampling make it possible to separate the VSP downgoing waves from the upgoing reflections by f-k filtering. The downgoing wave field can be used as signatures to deconvolve the upgoing wave field as the best estimate of the primary reflections in the vicinity of the boreholes. VSP reflection character indicates the significant features in the sonic log velocities that produce the reflections observed in themore » surface seismic data. The sonic log only penetrates a short distance into the formation but provides detailed velocity information vertically. A VSP, however, has poor vertical resolution (50 to 100 ft (15 to 30 m) intervals) but samples a large area around the borehole similar to CDP data. Therefore, the VSP correlates well with CDP data because they have the same resolution.« less