GB1098360A - Method and apparatus for geophysical seismic exploration - Google Patents

Abstract

1,098,360. Seismic prospecting. ESSO PRODUCTION RESEARCH CO. Jan. 14, 1966 [Jan. 18, 1965 (2)], No. 1932/66. Heading H4D. [Also in Division G2] In a system for providing a time-varying two-dimensional display the visual effect of which is the same as if one were viewing seismic waves appearing on the earth's surface as the result of a single induced seismic pulse, light intensity-modulated in accordance with the amplitude of seismic signals which would be detected at a plurality of spaced geophone locations is transmitted to the display area by an optical system including an array of elongated light-wave guides. In a first embodiment, seismic impulses are generated by a dynamite, dropping weight or vibrator source positioned sequentially at 2500 points arranged on a 50 by 50 grid and are detected by a geophone or geophone cluster located at the centre of the grid (Fig. 2, not shown). Separate seismogram traces for each impulse point are recorded photographically or magnetically and converted to variable-density or variable-area traces on a film negative 125 (Fig. 3) by processor 123. The traces have a common time origin and are arranged in side-by-side groups on film 125, each group corresponding to a different one of the fifty lines of impulse points. Film 125 is moved past a linear light source 129 to modulate the intensity of light incident on one end of a fibre-optics unit 131 comprising fifty ribbons, each made up from a plurality of fibre-optics strands. Those ends of the ribbons adjacent to film 125 are linearly arranged edge-to-edge so that each ribbon scans a different trace group, while the opposite ends of the ribbons are stacked to form a raster end 135. A magnified image of end 135 is viewed on a screen 139, areal points of which correspond to points on the impulse grid. Seismic " events " appear on the display as rings centred on the screen centre and gradually increasing in diameter (Fig. 14, not shown). Details of the fibre-optics unit 131 (Figs. 4 to 9, none shown), and mechanical system for moving film 125 past unit 131 (Figs. 10, 11 and 15, none shown) are given. A method for extending the subsurface coverage by using three detection locations on a line through the impulse grid centre is described (Fig. 13, not shown), and details of other prospecting methods including a method for providing continuous subsurface coverage (by generating a seismic impulse at each of several locations spaced along a first line, detecting reflected signals at each of several locations on a second line substantially orthogonal to the first and then transferring all impulse locations and detecting locations in a given direction by a distance which is one-half the length of the detecting geophone array) are given. In an alternative embodiment in which continuous waves, rather than pulses, are induced in the earth, the traces to be processed are " correlograms" derived by correlating a detected seismic signal with a counterpart of the seismic signal transmitted into the earth and recording the correlation result.