Patent GB-979719-A (GB)
Title: Method and apparatus for seismic prospecting
IPC: G06G7/19
Filed: 1961-11-23
Abstract: 979,719. Seismic surveying. JERSEY PRODUCTION RESEARCH CO. Nov. 23, 1961 [Dec. 2, 1960 (3)], No. 41922/61. Heading H4D. In an arrangement for seismic surveying a seismic vibration is produced in the earth in the form of a number of discrete-frequency narrow band-width vibrations the reflections of each of which is independently detected and recorded. In one embodiment, Fig. 8 (not shown, a vibrator produces a succession of constant single discrete - frequency vibrations (e.g. between 20 and 70 c.h.s.) which are picked-up by a geophone adjacent the vibrator and by a remotely located geophone or group of geophones. The relative phase and amplitude ratio between the adjacent and remotely received signals are derived for each transmitted discrete - frequency vibration and separately recorded on, e.g. punch cards or separate magnetic tracks &c. The latter is then played-back and fed to a computing device comprising a convolution means in which the signals are operated on by a predetermined function (i.e. a running integrated product is produced) to correct the signals for decrease in amplitude with respect to travel time from deep strata. The output from the convolution means comprising the frequency spectrum of such signals is then fed to a recorder provided with means to translate such frequency spectrum into a time-function signal which is the desired composite seismic signal. Fig. 9 shows a preferred arrangement for carrying out the latter embodiment. A s shown, signals received by geophone 16 adjacent the seismic vibrator 10 and by the remotely positioned group of geophones 22 are fed via identical amplifiers 18, 24 and identical narrow, band-pass filters 20, 26, the centre of the pass bands of which are adjusted to coincide with each discrete vibration frequency produced by 10, to respective gates 28, 32 which are opened by means 11 (controlling vibrator 10) when the " steady-state " condition of each discrete frequency has been attained. The outputs of the gates are then supplied to a phase meter 30 and via respective peak selectors 40, 46 and sample and hold circuits 42, 48 to a divider 44. The output of the latter consisting of amplitude ratio signals and of the phase difference signals from meter 30 are then supplied via averaging circuits 50, 34 and gates 52, 36 (opened between 10 and 60 seconds after the opening of gates 28, 32 open) to recording means 38 on which the " state " of vibrator 10 is also recorded via gate 54 and means 11. The recorded signals are then " played-back " and supplied to the convolution means 70 (comprising known computing means) from which signals representative of the frequency spectrum of the corrected seismic signal are derived and recorded by means 72. A corrected time-function seismic signal may then be synthesized from the recorded signals by means (Fig. 10, not shown) comprising, for each discrete vibration frequency, a generator of such frequency, an adjustable phase shifter and a variable gain amplifier, the outputs of the amplifiers being added and supplied to a recorder. Fig. 11 shows an arrangement, generally similar to that of Fig. 9, in which the seismic vibrations are produced by counter-rotating weights 144, 154 and in which time domain filtering of the signals received by the adjacent and remote geophones 176 and 178 respectively is effected by recording such signals on respective rotating magnetic tracks 164, 164 from which they are read off via a plurality of suitably spaced heads and combined in respective adders 184, 190. The outputs of the latter are then supplied via band-pass filters 186, 192 to apparatus generally similar to that described in connection with Fig. 9 to produce on recorder 200 a record of the relative phase and amplitude ratio between the transmitted and received signals for each discrete vibration frequency produced by the vibrator. An embodiment in which the single discrete frequencies vibrations are generated simultaneously is described with reference to Fig. 12 (not shown). In another embodiment the total frequency band-width necessary to obtain proper resolution for the area being surveyed is selected and a seismic vibration of such bandwidth is then produced by imparting into the earth, either sequentially or in sequential and simultaneous groups, a finite number of discrete frequency vibrations which together " add-up " to the required band-width (Figs. 13 to 19, not shown).