Estimating crack parameters from observations of P‐wave velocity anisotropy

Abstract

Cracks are a very common feature of crustal rocks, and their behavior and properties under various conditions are of growing importance to a variety of applications, including investigations of oil and water resources in cracked reservoirs, extraction of geothermal heat, and earthquake prediction by the seismic effects of dilatancy. This paper discusses mapping cracks by measurements of velocity‐anisotropy in underground refractors. The technique is demonstrated by interpreting the velocity measurements of Bamford and Nunn (1979) in shallow fractured limestone. The estimated crack distributions are found to be consistent with rose‐diagrams at neighboring outcrops, and the degree of saturation of the cracks has a major effect on velocity variations. Elastic constants can be derived for an equivalent purely elastic solid which has the same velocity variations as the cracked solid. This allows synthetic seismograms to be calculated for propagation through cracked structures, which demonstrates that use of polarization anomalies in three‐component recordings is a sensitive technique for investigating cracked solids.