Patent GB-1046771-A (GB)
Title: Improvements in methods of and apparatus for echo-ranging
Inventor: ANSTEY NIGEL ALLISTER
IPC: G01V1/02
Filed: 1962-01-19
Abstract: 1,046,771. Seismic exploration. SEISMOGRAPH SERVICE (ENGLAND) Ltd. Jan. 18, 1963 [Jan. 19, 1962], No. 2027/62. Heading H4D. In a system of seismic exploration in which the artificially produced seismic disturbance covers a broad frequency band, e.g. 20 to 80 c.p.s. and has a duration comparable to the travel times of interest, the masking of early (reflected) echoes by direct or refracted arrivals is minimized by producing the seismic disturbance as a plurality of separate narrow band signals, together embracing the broad band, and arranging the effective spacings of the geophones in the receiving spread to be such that, for each narrow-band signal, the wavelength of the direct or refracted signals corresponds to a null in the pattern response of the spread. Fig. 3 shows a preferred embodiment in which the spread for each recording channel comprises a plurality of geophone stations 3 each consisting of two strings 4 each of about five geophones so disposed that there is an even distribution of geophones along the spread. The " high " side of each string is connected to individual conductors 5 and these together with a " low " side conductor common to all strings are formed as a cable 8 leading to the recording station. At the latter the outputs from the strings are combined in various proportions (as discussed with reference to Fig. 4, not shown) by means of the resistor network to provide a sensitivity pattern which falls away in uniform steps from the centre (of the spread) so that the effective output corresponding to a vertically-arriving reflection is constant. In operation, the total frequency band, e.g. 20 to 80 c.p.s. of the seismic disturbance is divided into four half-octave ranges 20-28, 28-40, 40-57 and 57-80 c.p.s. and during respective ones of these sweeps switch 6 is positioned successively at contacts D, C, B, A, the effective output signal of terminal 7 being applied to a known form of recording amplifier. The Specification discusses the theory of operation of the invention including the effect of ground mixing and shows that for the particular embodiment of Fig. 3 the rejection of the (horizontally-arriving) interference (direct or refracted) signals is better than 50: 1 for all frequencies at the edge of each sub-range and virtually complete in the central portion. Greater rejection may be achieved by employing a larger number of sub-ranges. When the required spread involves the use of a plurality of recording channels of the Fig. 3 kind a reduction in the total number of geophones and associated cables required may be realized by distributing the output of each geophone so that it contributes to several channels as discussed with reference to Fig. 5 (not shown). The division of the total frequency into parts may be effected when the seismic signals are produced instead of when they are received by employing an array of transmitting transducers arranged in a manner analogous to the arrangement described for the receiving transducers (geophones). The receiving transducers may have a directional sensitivity which is a function of time and means (e.g. the addition of supplementary mass or compliance) may be employed to resonate the ground coupling of the transducers within each sub-range in turns. The artificially produced seismic disturbance may be of the pulsecompressive, frequency-modulated continuous wave or pulsed-continuous wave type.