AMBIGUITIES IN AVO INVERSION OF REFLECTIONS FROM A GAS-SAND

We examine the reflections from a thick sand layer embedded in shales deposited in an open marine environment of Miocene age. Borehole data indicate that the sand bed is gas saturated. Making the assumptions of single interface reflections, plane-wave propagation in elastic and i sotropic media, and correct amplitude recovery of the actual seismic d ata, we try to invert the amplitude variation with offset (AVO) respon se for the compressional velocity (alpha(2)), shear velocity (beta(2)) , and density (rho(2)) of the gas-sand layer, knowing the parameters o f the upper layer and the calibration constant. The actual reflections reach incidence angles up to 54 degrees at the farthest offset. Notwi thstanding the large range of incidence angles, the outcomes of the in version are ambiguous for we find many solutions that fit equally well , in a least-squares sense, the observed AVO response. We present the locus of the solutions as curves in compressional velocity (alpha(2)), shear velocity (beta(2)), and density (rho(2)) space. To gain a bette r understanding of the results, we also perform the same inversion exp eriment on synthetic AVO data derived from the borehole information. W e find that when inverting the AVO response in the same range of incid ence angles as in the real data case, the exact solution is found whic hever starting point we choose; that is, we have no ambiguity. However , if we limit the incidence angle range, e.g., to 15 degrees, the inve rsion is no longer able to find a unique solution and the set of admis sible solutions defines regular curves in alpha(2), beta(2), rho(2) sp ace. We infer that residual noise in the recorded data is responsible for the ambiguities of the solutions, and that because of numerical no ise, a wide range of incidence angle is required to obtain a unique so lution even in noise-free synthetic data.