Time-Varying Fold In High-Resolution Seismic Reflection Data: A Recipe For Optimized Acquisition And Quality Control Processing And Interpretation

A simple technique is presented to optimize seismic reflection acquisition and processing<br>parameters using the results of walkaway tests, time-varying trace fold, and geologic target<br>weighting. Fold varies significantly with travel-time on shallow seismic reflection surveys due to<br>mute operations and normal moveout (NMO) corrections. In the absence of well control,<br>increased fold may provide the only confidence measure that primary reflection energy is present,<br>based on stacking velocities. The recipe to optimize a seismic survey involves acquiring walkaway<br>tests (oversampling in space and time) at suspected end-member geologic targets. All mutes<br>and stacking NMO velocities are then applied to the walkaway gathers to calculate the optimum<br>window for maximum coherence for primary reflection energy. With the optimum window<br>derived from each walkaway gather, the operator can then calculate the acquisition parameters<br>needed to best image each target of interest, based on the number of seismograph channels available.<br>If the number of channels in shot space for a given target (in travel-time) falls below a<br>threshold (we recommend 6-channel coverage), then the operator must decide either to sacrifice a<br>depth range to image (define a no confidence or low confidence zone), employ more seismograph<br>channels to image all desired targets, or modify the acquisition experiment. If the above recipe is<br>followed, a contour map of the fold (which is one measure of reflector confidence) can also be<br>generated and displayed with the final stacked section as an overlay. To obtain this map, the operator<br>sorts a copy of the shot data into common midpoint (CMP) gathers, sets all amplitude values<br>to unity, applies mutes (zeroing those samples excluded from the processed seismic data), corrects<br>for NMO, and sums the data (as opposed to stacking or averaging the data). The result for both<br>acquisition and processing phases is a record of true fold as a function of time for all locations.<br>True fold displays are useful for quantifying the confidence of primary reflectors for geologic<br>interpretation or subsequent attribute analysis, and also for measuring the uncertainty of seismic<br>velocity estimates which, in turn, can affect seismic inversion-results and petrophysical parameter<br>estimates.