Unexpected Waves Observed in Fluid-Filled Boreholes

Abstract

During a continuing program involving pressure and particle velocity measurements in fluid-filled boreholes, several unexpected results have been observed which can be explained by very simple computations. One is an acoustic square wave, a flat topped pulse which is generated when a slug of water, broken off from the main column by cavitatlon, rejoins the column. In another instance, the geometry is such that a wave along a steel casing is the first signal to arrive at a pressure detector, and the initial pressure is negative, although the explosive pressure generating the transient is initially positive. The manner in which a steel casing causes a reversal of pressure in the “casing-break” is described. A damped low-frequency signal observed in a fluid-filled hole after a small explosion indicates that the gaseous products of the explosive constitute a pressure-release surface and hence that the fluid above the shot is “hanging in space,” acoustically free on both ends. An expression relating the negative pressure observed at the bottom of an uncase hole to the elastic constants of the fluid and solid is also presented.