ABSTRACT

Azimuthal vertical seismic profile (AzVSP) surveys have commonly been used for fracture characterization by analyzing the P-to-S converted wave response across fractured zones. Fractured media produce characteristic two-cycle patterns on AzVSP gathers that are disrupted in the presence of complex structures. Aiming to characterize the complexity of the AzVSP response in the presence of geologic structures, we have derived anisotropy parameters for effective horizontal transverse isotropic media, generated AzVSP signatures for the flat-interface case as the baseline, and compared the baseline signatures with AzVSP signatures for the dipping-interface cases. The fracture fill and fracture intensity are varied to capture the effect of the intrinsic fracture parameters on the AzVSP signatures. We find that dry fractures produce a stronger response compared with fluid-filled fractures for the same fracture density. The structural response on AzVSP signatures is isolated by generating synthetic seismograms across structurally equivalent isotropic models. Transverse energy, resulting from structures, could be misinterpreted as evidence for fracturing. The AzVSP signatures for a fractured dipping-interface two-layer model show significant distortion of the fracture response for the 10° dip case compared with the flat-interface signatures. As a possible solution to the structural problem, we use the arrival azimuths of the transmitted P-to-S event in synthetic signatures generated with the isotropic structural model, to orient the radial and transverse components parallel and perpendicular to the isotropic P-to-S event, respectively. Such structurally consistent orientation negates the effect of structure on AzVSP gathers and uncovers the underlying fracture response. This methodology can be extended to complex overburdens that are structurally well constrained.

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