GB1290152A - - Google Patents

Abstract

1290152 Seismic signal processing CHEVRON RESEARCH CO 15 Oct 1969 [16 Oct 1968] 50779/69 Heading H4D [Also in Divisions G4-G6] In a method for the automatic conversion by machine of a seismic time section to a depth section whereby events on the time section are migrated both vertically and horizontally to depth section positions representing the actual subsurface reflector positions, time section traces are processed a group at a time to produce from each group a respective "sonogram" trace identified with a particular moveout and hence with a particular direction of arrival of the seismic waves at the seismic detector stations, individual time segments of the sonogram traces are transferred to zones identified by particular depth and horizontal co-ordinates and representing portions of the depth section, and the final depth section is formed by producing a plurality of further sonogram traces from the said segments within the zones. Fig. 4 illustrates four stages in the conversion. Stage 1 represents an event RA on three adjacent traces in a conventional time section: it is argued that the moveout on the traces indicates that the seismic waves were received from the left, so the event must be migrated leftwards. Stage 2 shows a sonogram segment formed by compositing the three traces of stage 1. Stage 3 shows that segment migrated to A using an assumed seismic velocity/depth function and assumed dip angle corresponding to the moveout. Stage 4 shows the final traces formed by "sonogramming" the traces of stage 3. The conversion is preferably made by means of a digital computer and plotter, but can be effected by the Fig. 5 analogue system. A group of side-by-side traces on time section 12 are reproduced by heads 15a, 15b, 15c on mount 18 which is set at an angle corresponding to a selected assumed dip angle #, and are summed at 19 to form a sonogram trace which is recorded on a zone of intermediate record 22. Circumferential increments represent time t on a drum 11 carrying time section 12 and represent subsurface reflector depth z on a drum 21 carrying record 22; variable ratio gearing comprising cam 61a and friction wheel 67 between drums 11 and 21 ensures that the relation: is preserved, where v(z) is an assumed velocity function and z i is a reflector depth. Horizontal increments along drum 21 represent different selected dip angles #, and along drum 11 represent horizontal distance x in the earth; to ensure that the traces selected by heads 15a, 15b, 15c are correct for a given sonogram zone on record 22 the heads are moved horizontally (corresponding to a horizontal migration distance #x under the control of cam 81a which preserves the relation: The correct relationship between the angle to which head mount 18 is set and the parameters v(z), #, is maintained by means of belt-driven cam 95a, reel 98, and servo-mechanism 99, 101. Since cams 61a, 81a, 95a correspond to one value of # and one value of v(z), further cams in sets are provided for other values of # and v(z). The final depth section is formed by reproducing sets of sonogram traces from record 22 with offset heads 35a, 35b, 35c, summing the reproduced traces at 39 and recording the resulting sonograms at 42.