Acoustic modes of propagation in the borehole and their relationship to rock properties.

Abstract

Residue theory is used to predict the relative excitation of various modes contained in experimental waveforms. A plane- geometry model involving a layer of fluid between two elastic half-spaces is shown to provide velocity dispersion curves for propagating modes that are very similar to those for the fluid-filled borehole and is used to illustrate the effects of the confined borehole fluid on surface and body waves traveling along the borehole in the elastic solid. Excitation functions are computed for some of the lowest-order symmetric modes, time-domain response of the trapped modes following the shear head waves calculated and compared to waveforms recorded in boreholes through several homogeneous formations. The insight into the mode composition of the experimental wavelengths obtained in these formations is used to construct amplitude logs that should be especially sensitive to variations in the presence of fluid-filled fractures in the borehole wall.-from Authors