Coherency calculations in the presence of structural dip

We have used crosscorrelation, semblance, and eigenstructure algorithms to estimate coherency. The first two algorithms calculate coherency over a mul tiplicity of trial time lags or dips, with the dip having the highest coher ency corresponding to the local dip of the reflector. Partially because of its greater computational cost, our original eigenstructure algorithm calcu lated coherency along an implicitly flat horizon. Although generalizing the eigenstructure algorithm to search over a range of test dips allowed us to image coherency in the presence of steeply dipping structures, we were som ewhat surprised that this generalization concomitant ly degenerated the qua lity of the fault images in flatter dip areas. Because it is a local estimation of reflector dip (including as few as five traces), the multidip coherency estimate provides an algorithmically corre ct, but interpretationally undesirable, estimate of the best apparent dip t hat explained the offset reflectors across a fault, We ameliorate this prob lem using two methods, both of which require the smoothing of a locally ina ccurate estimate of regional dip. We then calculate our eigenstructure esti mate of coherency only along the dip of the reflector, thereby providing ma ximum lateral resolution of reflector discontinuities. We are thus both bet ter able to explain the superior results obtained by our earliest eigenstru cture analysis along interpreted horizon slices, yet able to extend this re solution to steeply dipping reflectors on uninterpreted cubes of seismic da ta.