Asymmetric Modes of Elastic Wave Propagation in a Fluid-Filled Borehole

Abstract

A numerical investigation of asymmetric modes of elastic wave propagation in a fluid-filled borehole is treated in this paper. The dispersion curves of guided waves (called flexural waves) and synthetic microseismograms are presented. The guided waves are found to be highly dispersive, with the phase velocities decreasing from the formation shear wave velocities in both hard and soft formations. They have much lower cut-off frequencies than the pseudo-Rayleigh waves in axisymmetric modes of propagation. We found that under conditions of low frequency (2-3 kHz) and long source-receiver separation (3-4 m), the flexural wave is the prominent arrival in a microseismogram generated with an asymmetric source, traveling with a phase velocity equal to the formation shear wave velocity, while the compressional wave arrival before that is generally suppressed. The results of this study may be meaningful for designing a shear wave logging tool.