Home – Home – Jornals – Russian Geology and Geophysics 2006 number 2

We report the method of azimuthal QVO (QVOA) analysis (seismic quality factor Q versus offset and azimuth) of P -wave attenuation variations as a function of wave-propagation direction in an azimuthally anisotropic (HTI) medium, which is an effective model of vertically fractured rocks. The method is intended to determine crack orientation in fractured reservoirs. The derived approximation of attenuation as a function of the wave-normal direction appears to have the same structure as Rüger's approximation for the PP reflection coefficient widely applied in the azimuthal AVO (AVOA) analysis. We introduced two new parameters: QVO gradient, an azimuth-dependent seismic attribute, and the QVO intercept corresponding to attenuation in the isotropy plane. The use of the QVO gradient, suggested by analogy with the AVO gradient, approaches the new QVOA method to the known AVOA analysis. The QVO gradient is maximum when the source-receiver line is directed along the symmetry axis (or normal to crack planes) and minimum when the two are orthogonal. The relative difference in P -wave attenuation in crack-parallel and crack-normal directions was found out to depend on crack parameters and on V_S/V_P ratio in the background rock, the impact of V_S/V_P being greater than that of the crack density and aspect ratios of cracks and their filling. Azimuthal variations of the QVO gradient can be used to determine crack orientation, and the maximum QVO gradient divided by the intercept provides estimates of V_S/V_P ratio.