Wave propagation through fluid-filled inclusions of various shapes: interpretation of extensive-dilatancy anisotropy

Previous examinations of the effects of extensive-dilatancy anisotropy (EDA) on seismic waves have been largely restricted to discussions of parallel fluid-filled cracks of small aspect ratio. It is recognized, however, that EDA may be caused by stress-aligned fluid-filled cracks, microcracks, and preferentially oriented porespace, in a variety of shapes, dimensions, and distributions, which may not be adequately modelled by the effects of uniform distributions of thin parallel cracks. The effects on seismic waves are examined for distributions of inclusions ranging from spherical pores and oblate spheroids (bubbles), to penny-shaped cracks. Unlike thin cracks, distributions of aligned oblate spheroids induce significant P-wave velocity anisotropy. In contrast, the parallel polarizations of the leading split shear waves within the shear wave window, which is one of the most distinctive features of shear waves in the crust, are preserved for all aspect ratios except spherical bubbles. The 3-D effects show minor variations that are most distinctive at small aspect ratios. Shear waves are very sensitive to changes in the geometry of such thin inclusions, and there is some observational evidence for temporal variations in splitting as the stress acting on the rockmass changes. If this sensitivity is confirmed it would suggest that the detailed geometry of a reservoir during production or enhanced oil recovery (EOR) might be monitored by repeated shearwave VSPs.