Elastic wave velocity and attenuation as used to define phases of loading and failure in coal

Abstract

This paper discusses a procedure used to investigate the behaviour of elastic wave velocity and attenuation as an indicator for changes in load and structural integrity of coal samples. By measuring the changes in compressional(P)- and shear(S)-wave attenuation and velocity under uniaxial and triaxial testing conditions, their effectiveness for distinguishing changes in applied load and structural failure of the sample can be observed. The velocity and attenuation values were used in further calculations such as ratios of P-wave to S-wave attenuation and velocity values, dynamic elastic constants, normalized velocities, and attenuation coefficients to reveal any trends that may exist upon loading and failure. The behaviour of both P-wave and S-wave attenuation and velocity considered together define distinct and consistent phases of load change and failure under uniaxial and triaxial testing conditions. The S-wave attenuation and velocity are more sensitive than those of the P-wave to changes in axial load and the initial development of microfractures within the sample preceding structural failure. The attenuation and velocity ratio curves and the dynamic elastic constant curves are not very sensitive to changes in load, but are responsive to structural failure of the coal sample. The attenuation coefficient and normalized velocity curves revealed the same trends observable in the original velocity and attenuation curves.