Influence of Stress State and Stress History on Acoustic Wave Propagation in Sedimentary Rocks

Abstract

Effects of currently and previously applied stresses on P- and S-wave velocities in sedimentary rocks (sandstones, chalks, shales) have been studied. Acoustic velocities have been measured on core samples in two orthogonal directions simultaneously under varying (isotropic, anisotropie) conditions of applied stress. Also, a series of model experiments has been performed, where synthetic sandstones were formed under stressed conditions, and where acoustic velocities were measured during and after unloading. The experiments show that the sensitivity to stress generally is largest at low stress levels, in particular below the maximum stress experienced previously. The symmetry of the stress field is to a large extent reflected in the wave propagation. Anisotropic unloading leads to an acoustic anisotropy which remains after the unloading is completed. The results underline that laboratory acoustic measurements should be performed under proper stress conditions. The unloading effect may help to quantify thein situstress field.