Contrasting stress dependence of compressional and shear velocities: Implications for laboratory, logging, and seismic measurements

Abstract

Stresses are a dominant factor in controlling velocity in any particular rock. Unperturbed in situ stresses are usually different from those found around a borehole or typically applied in a laboratory. Compressional (Vp) and shear (Vs) velocities were measured on five sandstone samples as a function of triaxial stress. Vp is primarily controlled by the stress applied axially or parallel to the direction of propagation. Both axial and lateral or normal stresses have a strong influence on Vs. Isovelocity contour plots over axial and lateral stress space show a complex pattern significantly differing among the different sandstones tested. In spite of this, Vp and Vs are described well by a simple dependence on stress or pressure to the one-third power. Lateral stresses are significantly altered around a borehole much more than axial stresses. As a result, compressional sonic logs should give correct values but shear velocities will be shifted. Also, complex Vp-Vs relationships can be expected if in situ conditions change from the more typical unequal lithostatic stress state to the equal hydrostatic state as zones of high geopressure are approached.