Many geological features associated with oil accumulation show up on seismic maps as interruptions of regional trends rather than as true structural closures. Among such features are reefs, which are often best detected by draping of overlying formations; erosional escarpments which truncate porous limestone beds on their updip sides; and buried ridges which cause productive stratigraphic build-ups in overlying beds. In the presence of regional tilting, seismic indications from such features can be so obscured that special data processing techniques are required to make them readily recognizable on seismic maps.

The problem here is very similar to that of separating gravity and magnetic effects of features having economic interest from regional background. The techniques developed for accomplishing this type of separation, such as residual and second derivative determinations and orthogonal polynomials, can be applied advantageously to seismic data where regional structure obscures significant anomalies. Both contour-smoothing or grid methods can be used depending on the nature of the problem and the preference of the interpreter. As with gravity or magnetics, the grid methods are particularly adaptable for high-speed electronic computation.

Some examples are shown where regional effects are removed from seismic maps over known reefs and productive erosional escarpments by derivative and residual techniques using electronic computation.

A somewhat different approach is necessary when it is desired to remove the effect of velocity variation from time maps by treating the velocity function as a regional effect. Here the regional is multiplicative rather than additive and cross-product terms must be taken into account. By relating the time maps and velocity maps using this approach, the principal hazards of using time maps for interpretation can be avoided.

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