Process and apparatus for the measurement or treatment of seismic signals

Abstract

Abstract 1335101 Seismic exploration SOC NATIONALE DES PETROLES D'AQUITAINE 24 Nov 1970 [28 Nov 1969] 55886/70 Heading H4D [Also in Division G4] In a seismic exploration system in which the transmitted signal is a train of impulses and the received reflection signal is to be recorded in digital form, data-reduction of the digitized received signal is effected in a series of memory elements by compositing digital samples while the impulse train is being emitted, so that minimum recording capacity is used. In the simplified example of Figs. 1 and 2, the train comprises six impulses (in practice there may be from 10 to 10,000 impulses over a period of 10-200 seconds) occurring at clock times( i.e. elapsed time) a, d, o, p, w, ah and giving rise to received reflection signals R<SP>1</SP>1, R<SP>1</SP>2, R<SP>11</SP>1 &c. To synthesize a received signal record similar to that which would have been obtained if the transmitted impulses had been coincident in time, the compositing is such that the received signal due to the impulse 2 is effectively retarded by a time d - a and added to the received signal due to impulse 1, the received signal due to impulse 3 is effectively retarded by o - a and added to the impulse 1 received signal, and so on. For a maximum two-way travel time, T max., of interest for the seismic energy the addition step need include only those received signals for which the associated transmission impulse precedes the elapsed time by a period less than T max. The Fig. 5 embodiment provides digital control of the impulse transmission and received signal sampling, storage, compositing and recording. A succession of sampling intervals is defined by clock pulse generator 26. The pulses are fed to counter 28, which thus registers elapsed time, and to sampler 21 which samples the output of seismic sensor 20. The samples are digitized at 22 and fed to arithmetic unit 25. The elapsed time is compared at 30 with numbers stored in a register 34 and defining required impulse transmission instants. When coincidence is detected by comparator 30, transmitter 24 is energized and the number is transferred to register 41; this sequence is then repeated. The contents of register 41 are updated and its capacity is such that it stores the transmission instants occurring within a period of duration T max. preceding the current value of elapsed time. A register 48 has storage elements allocated respectively to elapsed times a, b, c &c., and a digital received signal sample arriving at arithmetic unit 25 is addressed to several such elements, where it is added to any contents already in the elements. Each of these addresses is computed at 25 from the difference (i.e. the required retardation time), between the current elapsed time (less T max.) and a respective transmission instant stored in register 41. When the complete impulse train has been transmitted, as determined at 56 by comparing the elapsed time with a preset value, the sums accumulated in the elements of register 48 are read out and recorded on magnetic tape unit 60 which is started up by a control signal from comparator 56. In a more complex embodiment wherein the received signal is obtained from groups of sensors, the outputs of the groups are cyclically connected to the sampling and digitizing devices and register 48 capacity is expanded accordingly. The invention is applicable to land or marine seismic exploration.

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