MAKING AVO SECTIONS MORE ROBUST 1

A bstract When large quantities of seismic data are involved it is impossible to examine all gathers by eye for AVO anomalies. The standard approach is to compute, for each amplitude profile (at a specific time) on each gather, the intercept and gradient of a straight‐line fit to seismic amplitudes. These intercepts and gradients are each plotted as a sort of seismic section ‐ an intercept section, and a gradient section. Estimation of the intercept and gradient for a straight‐line fit to each amplitude profile proceeds traditionally via least‐squares. Two undesirable features can be hidden from the user by the fitting procedure, namely (i) the effect of outlying or uncharacteristic amplitudes on the intercept and gradient estimates, and (ii) complete breakdown of the straight‐line model for the amplitudes, thus rendering meaningless the intercept and gradient estimates. It should be remembered that least‐squares can always fit any sequence of numbers to any other sequence of numbers; checks are needed to show that the result is meaningful. It is shown that statistically robust estimation methods can greatly limit the damage done by outlying amplitudes, and that a simple test on the model, the runs‐statistic, is capable of detecting breakdown of the straight‐line assumption. It is demonstrated using two seismic data sets that these two techniques, used in tandem, facilitate much better quality control of AVO intercept and gradient calculations.