Development of a vibratory source for shallow reverse VSP applications
Joe Wong, Robert R. Stewart
Shallow VSP surveys are useful for near-surface geotechnical applications and for obtaining data for statics corrections. When dense surface coverage is desired, VSP acquisition is much more efficient if it is done in the reverse configuration, i.e., with surface geophones and a downhole source. Considerations of safety and acquisition efficiency make it highly desirable to use a non-destructive, non-explosive downhole source for RVSP surveys. A suitable candidate for such a source is a vibrator controlled by frequency sweeps or pseudonoise sequences. In cooperation with JODEX Applied Geoscience Limited, the CREWES Project plans to adapt the CORRSEIS piezoelectric vibrator so that it can be used to conduct shallow RVSP surveys. The existing CORRSEIS vibrator produces seismic waves with dominant frequencies in the 2000 to 5000 Hz range. Such high frequencies are suitable for crosshole applications in high-Q rocks, but to be usable for RVSP where geophones are set on unconsolidated materials in the overburden, the vibrator must be redesigned to have dominant operating frequencies of 200 to 500 Hz. Other requirements are higher output power, and easy interfacing with recording instrumentation.