GRADING SYSTEM FOR SEISMIC REFLECTIONS AND CORRELATIONS

Abstract

The modern reflection seismograph places progressively more emphasis on continuous profiling. The use of the continuous profile method automatically makes possible the computation of dip and the plotting of dip cross sections. Fundamentally the accuracy of a dip computed on such a continuous‐profile symmetrical record is of the same order as the accuracy of a “correlation” between adjacent records because the dip consists of a continuous‐profile correlation over the same length of subsurface base line as that obtained by “correlation” of adjacent, continuous profiles. Dip computation is the fundamental procedure in areas of discontinuous correlation. Further, the use of dip and of correlation provides two essentially independent approaches to the determination of subsurface structure. The combination of the two is used to provide more uniformly accurate results than can be had through the use of either one alone. We require a practical method for sorting and grading the accuracy of dip indications and of correlation, which system should be based on substantially the same fundamentals in each case and should be as free as possible from the vagaries of personal judgment. A system is outlined which is adapted to both purposes. In evaluating dip attitudes, the grade indicates: (a) The certainty with which the event may be identified as a true reflection, and (b) The accuracy of the indicated dip. Similarly, in evaluating correlation, the grade indicates: (a) The certainty with which the events being correlated are known to be correlatives, and (b) The accuracy of the indicated correlation drop. These grades, placed on the cross section, serve as a guide in sorting out any conflicting information at the time the interpretation (and map) is being made. The use of such grades allows the concentration of all pertinent information in one place where any conflicting evidence can be weighed in terms of relative merit.