Determination of the anisotropy of elastic waves monitored by a sparse sensor network

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.

► Triaxial ellipsoid as a model of velocity anisotropy.
► Changes of velocity anisotropy with respect to the rock stress–strain state.
► Minimum axis of velocity ellipsoid turns perpendicular to the future failure plane.
► Maximum axis of velocity ellipsoid turns into the future failure plane.
► Velocity ellipsoid as an anisotropic model for acoustic emission events localization.