Patent GB-998867-A (GB)
Abstract: 998,867. Processing seismic records. SOCONY MOBIL OIL CO. Inc. Sept. 27, 1962 [Oct. 26, 1961], No. 36725/62. Heading H4D. In an arrangement for producing, from an extant seismogram containing multiple reflections, a seismic record which is free from such reflections, signals derived from the seismogram are time domain filtered to produce signals which, when combined in suitable amplitude and polarity with the original seismogram signals, cause cancellation of the multiple signals. The multiple reflections may consist of (I) " First order surface multiples " such as shown at 23 and 23A to 23C (Fig. 1), the various sections of which are to be considered as superimposed, each having one reflection from the surface 13 and two reflections from subsurface interfaces 11, 12: (2) " Second order surface multiples," such as shown in Fig. 8, comprising two surface reflections and three reflections from subsurface interfaces 50, 11, 12 and (3): " Internal multiples " comprising three reflections from subsurface interfaces such as 11, 12, 13 (Fig. 14). The series of reflections shown in Fig. 1, resulting from a seismic energy source 20, as detected by a geophone 24 and recorded at 8 is shown in Fig. 2, as trace S(t) and comprises the " first break " b(t), primary reflections, P(, P 2  (e.g. due to reflections 21, 22 and 21a, 22a, " first order surface multiples " MP 1 , MP 2  (i.e. of the primaries P 1 , P 2  and due to reflection paths 23, 23a) together with a further multiple MP 12  + MP 21 , termed a summation multiple, and due to (equal path length) reflections 23b and 23c which arrive at detector 24 simultaneously and consequently produce a signal of enhanced amplitude. Fig. 3 shows the arrangement for elimination of the " first order surface multiples." As shown, signals representative of the seismogram S(t(, Fig. 2, and derived from a magnetic recorder 8 are supplied via amplifier 32, to a summing amplifier 35 and via a switched potential dividing circuit 39 to a time domain filter 58 comprising a recording head associated with a rotating magnetic drum recorder 41. The signals thus laid down are then reproduced by two reading heads 42, 43 which are positioned with respect to head 40 such that (1) at the instant the primary P 1  is detected by head 42 its corresponding multiple MP 1  is being supplied to summing amplifier 35; (2) when head 43 detects P 1  the primary P 2  is being detected by head 42 and (3) when head 43 detects P 2 its corresponding multiple is being supplied to summing amplifier 35. The detected signals are then supplied via reversing switches 44, 45 (which are operated to ensure that the signals are of opposite polarity to the multiples to be eliminated in amplifier 35) to amplifiers 46, 47 the gains of which are adjusted in accordance with the reflectivity coefficient #r(t) of the reflecting interfaces and then via an adder 50 and amplifier 53 (which introduces a factor corresponding to the surface reflection coefficient R o  and also compensates for circuit losses) to a filter 54 comprising a low-pass filter circuit the output signals of which are fed to the summing amplifier 35. In this manner signals are fed to the latter amplifier with such polarity and amplitude as to eliminate the undesired multiples the operation being such that signals sequentially derived from the primaries P 1 , P 2  are utilized to eliminate their respective multiples whilst the summation multiples MP 12  + MP 21 , which is of enhanced amplitude, is eliminated by signals derived simultaneously from P 1 , P2 by heads 43, 42, respectively. When, however, the reflectivity of the subsurface interfaces is such that summation multiples of significant amplitude (e.g. F, Fig. 4) are produced from primary rereflections which, themselves, have such low values that they may be neglected (e.g. A, Fig. 4), it is necessary either to provide an additional reading head on drum 41 for such primary or, as shown in Fig. 3, to increase the level of the signals supplied; to the recording head 40 by means of a switch Sl which is operated at the instant of occurrence of such primary under the control of a counter 72. The latter, together with a counter 59 (which earths head 40 except when signals are to be recorded) is driven by a motor 30 which also drives the seismogram reading device 8, the recording drum 41 and a trigger arrangement 62 for an oscilloscope 65. On the latter is displayed the uncorrected signals S(t), via line 66, the cancellation signals CS 1  via line 67 and the corrected signals DM, via line 68. A seismic record free from the first-order multiples may then be produced by means of a suitable recording device supplied from terminals 37, 38. In a similar manner, second and higher order multiples and internal multiples may be eliminated by employing a separate time-domain filter recording drum such as 41, or separate sets of recording and reading heads associated with separate tracks for each such multiple as described with reference to Fig. 9 (not shown). In each case the completeness of elimination of the multiples is dependent on the setting of the reading head with respect to the recording heads and the Specification includes a discussion on methods involving the mathematical process of convolution and correlation by means of which the correct settings for the heads may be derived with a high degree of accuracy.