Patent GB-1124181-A (GB)
Title: Common tangent stacking method and system for locating sonic energy velocity discontinuities
IPC: G01V1/30
Filed: 1965-09-14
Abstract: 1,124,181. Processing seismic records. TEXAS INSTRUMENTS Inc. 14 Sept., 1965 [30 Sept., 1964], No. 39219/65. Heading H4D. [Also in Division G5] In a seismic signal processing system, seismic response data recorded at a plurality of geophone positions spaced along the earth surface is corrected for static and dynamic errors, converted to data representative of scalar wave-front patterns, and the wave-front patterns are combined to give a composite pattern in which wave-front reinforcement patterns represent the scalar positions of subsurface interfaces. The static and dynamic error corrections make the seismic response data representative of that which would have been received by geophones at spaced points (a to i) on a datum plane (36), (Fig. 4) for seismic energy induced at those points. For a uniform velocity of propagation of seismic energy in the subsurface material, the conversion of the data recorded at any one of the geophone positions would give a series of concentric arcs centred at the corresponding point on plane (36), but due to variations in propagation velocity the wave-front pattern for the point will in general comprise a series of arcs of radius and apparent centre a distance below datum plane (36), where Vd is the propagation velocity at plane (36), a is the rate of change of the propagation velocity, and t is elapsed time. In a first embodiment (Fig. 6) for plotting the wave-front patterns, the output of a 2000 c/s. sine-wave oscillator 66 is phaseshifted to provide X and Y circular time-base waveforms for a cathode ray tube 52 whose electron beam is intensity-modulated by the played-back, corrected, seismic response data. - The amplitude of oscillator 66 output, and hence the diameter of the tube trace, is varied in accordance with R t , the centre of the trace is shifted in the Y-direction in accordance with H t  by a bias circuit 78, and a blanking signal from a circuit 82 is applied to the tube 52 to give a sector-shaped wave-front trace which is recorded by photographing on to a film 56. Wave-front patterns for the several geophone positions are recorded serially in side-by-side and overlapping relationship on film 56, the film being continuously or intermittently indexed in the X-direction by distances representative of the spacing (#X), (Fig. 4), between the points (a to i). The time scale of the variation in accordance with R t  may be expanded or compressed, relative to the time scale of the geophone response data, by varying the data play-back speed. The decrease in trace intensity with increase in diameter of the trace on tube 52 is compensated for by a circuit 80. In a second embodiment, (Fig. 7), overlapping sector-shaped wave-front patterns are recorded on a magnetic record sheet 102 by means of a recording-head 116 carried on an arm 118 which rotates or oscillates through a predetermined arc. Head 116 is moved along arm 118 in accordance with R t , and a mechanism 114 moves sheet 102 towards the pivot point of arm 118 in accordance with H t . Sheet 102 is moved in the X-direction, by rollers 104 and 106, between the recording of successive wavefront patterns, and the composite pattern is scanned simultaneously by a pick-up head 136 moving in the Y-direction. The output of head 136 is fed to a recorder 142, which may have a recording transducer moved in synchronism with head 136 relative to a record sheet moved in synchronism with sheet 102, and to a readout device 140 such as an oscilloscope. The air gap of recording head 116 may be maintained in a fixed direction, relative to the X or Y- direction, by means of a parallelogram movement (Fig. 8, not shown), or gyroscope to facilitate compositing of the wave-front patterns. Various pick-up head arrangements are illustrated (Figs. 13, 14, 15, none shown), including a multiple-head assembly, for alignment with anticipated wave-front reinforcements. In a third embodiment (Figs. 9, 10, 11, none shown) an alternative magnetic recording head comprises a plurality of concentric arcuate recording head elements (182) which are sequentially energized by means of a radially sliding contact (188) which is moved in accordance with R t . In a fourth embodiment (Fig. 12) head 116 and arm 118 of the Fig. 7 embodiment are replaced by a rotating disc 202, a rotating arm 212, and a recording head 208, the rotational speed of disc 202 being slightly greater than that of arm 212 so that head 208 moves along an expanding spiral path with R t  and H t  motion determined by the profile of a groove 206. In a fifth embodiment (Fig. 16, not shown) successive wavefront patterns are recorded on separate storage surfaces (T 1 -T 6 ), and the wave-front intensity values at corresponding co-ordinate values on the several surfaces are scanned and summed by respective pick-ups (S 1 -S 6 ) feeding a summation circuit. The summation signal is recorded as a visible record (230) by a recorder (228) synchronized with the scanning of pick-ups (S 1 -S 6 ). The storage surfaces (T 1 -T 6 ) may be constituted by recording storage tubes, a magnetic recording system, a photographic recording system, or a xerographic recording system. In a sixth embodiment, the seismic response data from the several geophones may be processed by digital computer to give wavefront pattern data. In the various embodiments, the seismic energy propagation velocity involved in the variables H t  and R t  may be determined by well logging.