Experimental studies on downhole seismic sources

Abstract

Reversed vertical seismic profiling (RVSP) and crosshole seismology can produce detailed images around a well since they record higher frequency data than surface seismic. The key issue, however, is whether or not a source can be constructed that can deliver enough energy downhole and yet do no significant damage to the borehole. We have investigated various candidate downhole sources for reversed vertical seismic profiling and crosshole seismology. The potential sources studied included explosive charges, a perforating gun, an air gun, and a water gun. The studies were conducted in both open and cased holes in various lithologies. We found that an explosive charge or an air gun can be a viable direct source in both open and cased wells (cemented or free casing). In an open borehole or in a cased well (with new casing), neither explosive charges nor air gun produced damage that was detectable by the borehole televiewer. However, the long‐spaced sonic logging tool indicated that both sources may cause a deterioration of the cement bond between the formation and the casing where the cement is new and its integrity is questionable. The direct P-wave signal is found to be approximately linearly proportional to the amount of explosives used. The signal amplitude decreases as the transmitting distance increases approximately as the power law (−2.14).