INTRODUCTION
We present and analyse the results from tests performed on sandstone cubes in which the bulk acoustic emission (AE) and the ultrasonic P and S wave velocity changes in the three principal directions were monitored and measured simultaneously with the stress and strain as samples were deformed in three orthogonal directions independently. This combination of measurements has enabled us to directly link AE to the formation of new crack damage. The AE data confirmed the existence of the Kaiser stress-memory effect during uniaxial cyclic stressing, and the effect was also observed in the variation of P and S wave velocities in directions normal to that stressing. The data suggests that the crack damage formed during stressing is highly anisotropic, with the new microcracks formed during each stress cycle having minor axes parallel to the minor principal compressive stress direction. The observed effects have been modelled in terms of the elastic closure of pre-existing cracks and the formation of new strongly-oriented dilatant cracks. The dilatant cracks produce AE. By modelling these two separate processes we have demonstrated a clear quantitative correlation between the AE output and the new dilatant crack damage formed during
deviatoric stressing.
OVERVIEW OF PREVIOUS WORK
It has been well-known for many years that microcracks change the velocities of acoustic P and S waves in rocks, and that hydrostatic compression increases these velocities as cracks are closed [1]. Where deviatoric stresses are applied crack closure becomes anisotropic and the changes in acoustic wave velocity caused by stress, especially compressive stress, are also anisotropic. However, little attention was previously paid to the fact that both pre-existing cracks which close under compression and new dilatant cracks which open under deviatoric or tensile stress will contribute to changes in the acoustic wave velocities in rocks under stress until a comprehensive experimental investigation was carried at University College London by Jones [2]. In the course of Jones's investigation, for the first time we believe, AE was also measured simultaneously with the acoustic wave velocities during triaxial deformation experiments. Jones used the classical triaxial testing method with a cylindrical specimen geometry in which two principal stresses are equal. This method has a limitation in the investigation of the anisotropy of the crack damage. The latter limitation was removed in the present study in which cubic samples were loaded, using servocontrol, along the axes normal to the three pairs of orthogonal cube faces independently [3].
The Kaiser effect in the AE of rocks has been investigated for some years with a view to its possible use in determining crustal stress. However, current techniques yield no information about the quantity of damage, whereas acoustic wave velocity measurements can in principle provide such information. Holcomb and Costin [4] demonstrated the anisotropy of crack damage by observing the Kaiser effect in samples which were re-loaded at different orientations, but the method did not provide quantitative information on this anisotropy, whereas acoustic wave velocity measurements made in orthogonal directions do provide such information.
We conclude therefore that the combination of AE with acoustic wave velocity measurements is a good method for quantitatively studying crack damage in rocks as a function of stress-state.
