SEISMIC DECOUPLING FOR EXPLOSIONS IN SPHERICAL UNDERGROUND CAVITIES

A series of paired explosions in a salt mine near Winnfield, Louisiana, has been conducted to test a theory by A. L. Latter concerning seismic decoupling by underground cavities. The theory predicted a decoupling of 130. Free‐field and surface measurements from an explosion in either a 6‐ft‐ or a 15‐ft‐radius spherical cavity were compared with similar measurements from a completely tamped explosion of equal size. Shot sizes were from 20 pounds to a ton. Surface measurements were made out to 100 km and covered the frequency range from 0.05 to 100 cps. The experiment confirmed that decoupling does occur. For explosions that produce an average cavity pressure up to one‐fifth and possibly more of the lithostatic overburden pressure, seismic waves were decoupled by more than 100, i.e., two orders of magnitude. Even for explosions producing an average cavity pressure of six times the lithostatic overburden pressure, the seismic waves were decoupled by 20—more than a full order of magnitude. Minimum decoupling factors as a function of frequency are presented.