Seismic velocities of fractured carbonate rocks (southern Apennines, Italy)

Abstract

SUMMARY A study of the effect of fractures on seismic velocities has been carried out through a comparative study in the field and the laboratory. P- and S-wave velocities were obtained from surface and borehole seismic measurements and, in the laboratory, ultrasonic velocities were measured on intact and artificially fractured specimens of carbonate rocks from the Miranda area (southern Apennine chain, Italy). Calcareous and calcareous-chert sequences are present in the area, with a discontinuity spacing from very close to very wide. Essentially dry rocks have been tested. A correspondence was found between visible rock fracturing and compressional wave velocities from surface seismic measurements. The downhole velocities were lower than the corresponding ultrasonic velocities on core specimens and increased with depth. Specimens were then cut to simulate field discontinuities, and loaded to study the influence of vertical pressure on the seismic velocities. Laboratory and field velocities were finally compared as functions of vertical pressure. It turns out that fractures reduce and vertical pressure increases velocities. Since the rock quality designation (RQD) indices were generally very low, the increasing seismic velocities with depths in poor quality rocks can be explained in terms of better contact between fracture surfaces.