The effect of a changing aspect ratio of aligned cracks on shear wave vertical seismic profiles: A theoretical study

Abstract

Rocks containing parallel cracks or preferentially oriented pores behave at low frequencies as transversely isotropic solids. A change in the aspect ratio of these inclusions affects the resultant anisotropy. Curves showing the velocity dependence of the two shear waves on the direction of wave propagation in such rocks intersect in a direction of propagation which depends on the aspect ratio. The angle this direction makes with the symmetry axis of the anisotropic rock decreases for increasing aspect ratio. Characteristic of wave propagation around this angle is a change in the polarization of the leading shear wave. To demonstrate how this affects vertical seismic profile (VSP) measurements, multioffset VSPs at different azimuths are modeled with synthetic seismograms for rocks containing inclusions with a range of aspect ratios. For inclusions with small aspect ratios, the polarization of the initial shear wave may change abruptly as the direction of propagation changes, whereas for large aspect ratios there is no change or the change may be more gradual and spread out over a wider range of directions. This feature may be of interest in assessing the physical character of fluid reservoirs. The observations of changes of polarization and variation of delays between the split shear waves may also be used to monitor changes in stresses in the crust and in monitoring changes during hydrocarbon extraction.