GB1285769A - Method of collecting seismic data of strata underlying bodies of water - Google Patents

Abstract

1285769 Seismic exploration CHEVRON RESEARCH CO 30 Dec 1969 [30 Dec 1968 1 Dec 1969] 63387/69 Heading H4D In a marine seismic exploration method for collecting seismic data suitable for processing to enhance the signal-to-noise ratio, a linear hydrophone array with interhydrophone spacing X is towed by a recording boat at a steady speed V along a survey line, a seismic source is moved at a steady speed V<SP>1</SP> by a shooting boat along a course making an angle α with the survey line and is fired at time intervals T, the resulting hydrophone outputs are individually recorded as traces, and the speeds and courses of the boats are controlled such that the centre points, each located halfway between the source point and hydrophone position (at the recording instant) of a respective source point-hydrophone pair, form a two-dimensional grid including sets of centre points aligned perpendicularly to the survey line. As shown in Figs. 1, 2, 3, a recording boat 12 tows hydrophone array 13 along survey line 11 and is flanked by shooting boats 22, 23 carrying seismic sources 26, 27 and moving along in-phase zig-zag courses 24, 25 respectively. To maintain the flanking position, V<SP>1</SP> cos α = V. A source is fired at points A, B, E, F, G, H in turn on outward legs of the courses to define centre points C1-C24 for source-point A, centre points C25-C48 for source-point B and so on. In this example the between-firing movement components of the source in the direction of and perpendicular to the survey line are about equal (α=26¢ degrees) and equal to 2X, since T is made 2X/V. Since the different centre-points forming a set aligned perpendicular to the survey line are associated with common depth points but different source/ hydrophone offsets, their associated traces can be processed by applying normal moveout and static corrections and composited to suppress random noise and multiple reflections. In addition the traces may be "beam-steered" by applying differential time shifts to them to produce displayed sets of directional traces indicative of seismic energy received from particular directions. This enables both in-line and cross dip components of subsurface formations to be determined. The headings and velocities of the boats may be programmed and controlled automatically by a system including Doppler sonar, radar transponder and gyrocompass navigational aids on the boats (details given). In modifications the two shooting boats may zigzag behind the recording boat, the two zig-zag courses may be in anti-phase, or a single shooting boat zig-zagging behind the recording boat may be used.