Behaviour of shear waves in rocks with two sets of parallel cracks

Abstract

SUMMARY The behaviour of shear wave polarizations and shear wave splitting observed at the surface suggesting propagation through parallel vertical cracks has been the stimulus for many recent investigations, both in earthquake and exploration seismology. Cracks in surface outcrops, however, frequently display multiple sets of parallel vertical intersecting cracks. This paper examines seismic shear wave propagation in media with two sets of parallel vertical cracks (biplanar cracks) to determine whether the behaviour of shear waves can distinguish between the effects of multiple crack sets and the effects of single sets of parallel cracks (monoplanar cracks). This study shows that the difference between the overall patterns of polarizations of biplanar and monoplanar systems of vertical cracks within the shear wave window in many circumstances is marginal, and unlikely to be easily recognized in the field. We conclude that it is frequently not possible, from analysis of surface observations of shear wave polarizations alone, to distinguish between the effects of biplanar sets of parallel vertical cracks and those of a single parallel set. The difference can usually be recognized if an accurate estimate of both polarizations and time delays between the split shear waves is available over a wide range of azimuths and angles of incidence within the shear wave window. However, in areas with complex fracture and stress systems, time delays may be much harder to estimate than the polarization angles of the leading split shear waves, and it may not be easy to distinguish, from seismic data alone, the difference betwekn parallel and multiplanar sets of vertical fractures.