GB1489908A - Determination of interval velocity in the presence of dip - Google Patents

Abstract

1489908 Seismic data processing MOBIL OIL CORP 11 Dec 1974 53556/74 Heading G1G Seismic interval velocities for a subsurface region having dipping interfaces are computed automatically from common depth point (CDP) traces obtained using at least three seismic profile lines crossing in a common surface point. In the embodiment described several profile lines intersect in a grid of surface points with three lines passing through each point. The main data processing steps are as follows and relate to one such point, the processing being repeated for the other points:- (a) From the CDP data for profile lines A, B, C through the point the travel time T, apparent velocity V, and amplitude A of each trace event (reflection) are determined and stored, (b) for each profile line the average time #T, average dip and average apparent velocity V of any interface are taken over the CDP's defining that interface, (c) data not satisfying validity criteria for amplitude and continuity (i.e. presence on all three lines A, B, C) are rejected, (d) for those data satisfying the criteria the average time TCR for any given interface is taken over the three lines, (e) values of the true dip # CR and true strike CR for each interface are computed, knowing # and the actual azimuth of the profile lines with respect to magnetic north, (f) for each interface and each profile line the average apparent velocity V is corrected in dependence on the true dip and strike relative to the line to give an RMS velocity value VRMS, (g) for each interface the consistency of the three values of V RMS is checked, and if found consistent those values are averaged, (h) the averages of VRMS are plotted against their corresponding TCR values to form a curve which is then subjected to a smoothing process, (i) true interval velocity V i between interfaces is computed from the smoothed curve by a known iteration process see Specification 1317765 in Division H3-5, which involves fitting an arbitrarily dipping Snell's Law layered model to the curve data. By integrating V i with respect to time, average velocity may be computed. A plot of dip, strike, V, V i and VRMS against a common travel time scale may be produced, with strike represented by the orientation of vectors and dip by the displacement of the vectors from the time axis. Details of a Fortran routine for the data processing are given.