Seismic visibility of fractures

Description/Abstract

Laboratory measurements of seismic travel times of compressional waves, propagated through both intact and naturally fractured specimens of quartz monzonite, were used to determine velocities of propagation. Velocities calculated from a quasi-static model were found to be too low compared to the measured velocities. A theoretical velocity is derived, based on the displacement discontinunity model of wave propagation across a fracture, which depends on the dynamic stiffness of the fracture and on frequency. Comparisons of the measured velocities and those computed from the theory agreed, and confirm the appropriatness of the displacement discontinuity model to simulate wave propagation across the fracture. From this model, changes in group time delay and signal amplitude occur at the fracture and are not distributed throughout the rock. These effects are a function of frequency and the stiffness of the fracture. This suggests that the use of seismic tomographic techniques would yield both the location and mechanical properties of discrete fractures.