Inverting shear-wave polarizations for anisotropy using three-component offset VSPs: synthetic seismograms

Abstract

Summary A numerical algorithm is presented for interpreting shear-wave polarization directions, from offset VSP data, as subsurface anisotropy. The technique compares observed (synthetic) horizontal polarization angles with expected values for a range of physically plausible anisotropic models with different orientations. The comparison is implemented in a database inversion scheme. The anisotropic models are formed by combining the effective anisotropy of parallel, aligned, vertical cracks with matrix anisotropy due to thin-layering or lithology. Different models are defined by varying the crack and matrix properties. The technique is applied to synthetic offset VSP data for a multilayered anisotropic structure, generated by multi-azimuth sources. Analysis of the results indicates that polarization measurements from non-zero offsets with at least two and preferably three different azimuths are required in order to reasonably constrain the final inversion solutions. The parameters which are resolved depend upon the depth of the subsurface zone of interest. Crack strike is well resolved in the deeper layers for which directions of propagation are sub-vertical, whereas other model parameters such as aspect ratio and crack content can only be resolved for wider ranges of incidence angle. The key to a successful inversion appears to lie in the selection of propagation paths through the relatively unique patterns of disruption created by singularities.