SEISMIC STRIPPING HELPS UNRAVEL DEEP REFLECTIONS

Due to multiple reflections and low signal‐to‐noise ratio, very deep primary reflections often can be recognized only by using a high multiplicity of common‐reflection‐point (CRP) stacking, encompassing a large range of shot‐geophone distances. But in some areas, the structures of the near‐surface layers, say down to a depth of 1 km, may change substantially within the shot‐geophone distance range. In such cases it becomes necessary to include all of this upper geologic section in the time corrections normally used in CRP stacking. This is achieved by seismic stripping. In many cases, slant raypaths can be replaced by vertical raypaths and the inhomogeneities within the upper layers may be compensated for by normal surface consistent static time corrections. This method resulted in substantial improvements in definition of deep horizons in various field surveys. If the depths of the inhomogeneous layers become a large percentage of depths of the desired reflection horizons, say if they amount to 30 percent or more of the depth of interest, the corresponding stripping time corrections are no longer static in a strict sense, but may still be handled by simple approximate formulas, provided the deep reflectors of interest are nearly horizontal and formations in the overburden are moderate. In this case the stripping corrections are slightly varying with reflection time. Stripping corrections may also be included in the normal dynamic time corrections by dividing the total range of shot‐geophone distances into several subranges and determining the optimum stacking velocity for each subrange. Large stacking velocity variations occur in this case.