GB1399904A - Seismic data event velocity analysis - Google Patents

Abstract

1399904 Seismic signal processing CONTINENTAL OIL CO 16 Oct 1972 47591/72 Heading H4D Common depth point seismic traces are processed to determine event velocities by a procedure which analyses individual events rather than fixed time windows of the traces and so avoids distortions inherent in the use of such windows. The resulting data, after editing, are displayed in an arrow head symbol format which enables event time, velocity, amplitude and polarity to be depicted. The following main steps are included in an embodiment for which a flow diagram is given (Figs. 6, 7, 10, 11, not shown): (a) The traces of one common depth point (CDP) set are given individual mutual time shifts which are each a function of the associated source-detector offset and a reference time shift (corresponding to an assumed velocity). (b) The shifted traces are composited to give a sum trace termed a 'scan". (c) Steps a and b are iterated for other CDP sets and other reference shifts. (d) The resulting scans for one CDP are reduced to "scan logs" by storing digital amplitude values relating only to the main lobes 152, 156 and their immediate side lobes (Fig. 8). (e) The tiein of events from one scan log to another is checked by comparing the scan logs in pairs. (f) The events are edited to reject those which are of low amplitude, are redundant, or do not tiein. (g) The edited scan logs are compared to determine the scan log of greatest amplitude for any given event and hence to compute the velocity of the event. (h) The scan logs are reedited to reject events for which, on the scan log of greatest amplitude, the side lobes outweigh what appears as the main lobe on other scan logs. (i) Steps d to h are iterated for other CDP's. (j) The residual events of several CDP's are tied together into horizons, using time search windows and scan search windows. (k) Average values of event time, velocity and amplitude are computed for each horizon and are assigned weights, the weighting depending on the degree of tie-in of the individual CDP data. (1) The horizons are edited to include those which lie within a given time and velocity difference from the highest-weighted horizon, or to reject certain horizons which share more than a given percentage of events with the highest-weighted horizon. (m) The residual horizons are plotted on a seismic section, in the Fig. 3A, arrow format where the vertical co-ordinate of line 32 represents event time, the horizontal co-ordinate of line 34 presents event velocity (indicated also numerically), the arrowhead size represents amplitude and the arrowhead direction indicates polarity.