Secondary Shear Waves From Source Boreholes

Abstract

The purpose of this paper is to synthesize the most important results of the thesis work of Meredith (1990) concerning radiation from seismic sources in boreholes. Previous studies of radiation from sources in boreholes have been far-field studies and have neglected the explicit contribution of the borehole. In general, this is fine for P-wave radiation and for S-wave radiation into high velocity rocks. However, tube waves "leak" shear conical waves (Mach waves) which propagate when the tube wave velocity is greater than the shear wave velocity of the surrounding medium. These Mach waves are of high amplitude because of the dominance of the tube wave and radiate away from the borehole in a fixed conical shape. The shape of the cone is dependent on the shear wave velocity of the medium and the tube wave velocity. This paper defines the conditions under which these Mach waves exist and thoroughly describes them in a physical sense and less so in a mathematical sense. Finally, the relationship of Mach waves to data sets is examined and how Mach waves may be confused with receiver borehole tube waves. To keep the presentation simple, radiation from axial or torsional sources or radiation from empty boreholes is omitted in this paper but fully addressed in Meredith (1990).