ROBUST AVO ANALYSIS

Amplitude variation with offset (AVO) analysis is sensitive to errors in the moveout correction. To reduce moveout errors, AVO-effects are e stimated using time windows. For each time window, block-NMO is used t o avoid NMO-stretch, and correction for residual NMO is performed by a n iterative crosscorrelation technique. The data in a moveout-correcte d time window are modeled as a constant pulse multiplied by an amplitu de function that is approximated by a polynomial in the offset coordin ate. Conversion of data from offset to angle, or slowness, is therefor e avoided. The seismic pulse and the polynomial coefficients are found by the least-squares method. This is a separable least-squares proble m and the polynomial coefficients and the pulse can be estimated separ ately. The reduced nonlinear optimization problem for the coefficients is expressed as a Rayleigh quotient, providing the solution in one st ep. Once the coefficients have been found, the estimate of the pulse i s computed by an explicit formula. This gives an efficient computation al scheme. The method is demonstrated on a shallow seismic anomaly in the Barents Sea, offshore northern Norway.