Abstract
A method of velocity anisotropy analysis based on conventional ultrasonic transmission technique is proposed. Multiple transducers are used both to monitor acoustic emission and to measure velocities in different propagation directions.
A sparse network of transducers located on the rock core surface allows approximation of elastic waves velocity distribution as an ellipsoidal surface. The shape and orientation of velocity ellipsoid is a measure of velocity anisotropy.
A study of changes in velocity anisotropy and its orientation was performed on uniaxially loaded migmatite rock samples with distinct foliation. The velocity measurement was carried out during applying a uniaxial constant strain-rate loading.
Orientation of the velocity ellipsoid corresponded to the anisotropy of rock structure up to activation of a failure plane. Prior to brittle failure, the axis of minimum velocity rotated from its initial direction normal to the foliation to a direction normal to the failure surface.
The practical importance of the ellipsoid model lies in its simplicity and in its general applicability, which allows the use of measurement in a sparse sensor network. The-use of time-variable ellipsoidal velocity model led to a better localization of acoustic emission events during the sample loading when compared to the use of time-variable isotropic velocity model.
The position of the failure plane determined by clustering of acoustic events foci coincided closely with observed sample failure.