Patent GB-930669-A (GB)
Title: Improvements in or relating to seismic surveying methods and apparatus
Inventor: PICOU CLAUDE HENRI
IPC: G01V1/00
Filed: 1959-10-14
Abstract: 930,669. Seismic surveying. COMPAGNIE GENERALE DE GEOPHYSIQUE. Oct. 14, 1959, No. 34886/59. Class 40 (7). In a seismic surveying system including means for producing a seismic recording, signals derived from a plurality of geophones in response to a single shot and due to seismic waves which arrive at the surface being surveyed at an angle of incidence greater than a predetermined angle are suppressed so that only signals due to waves arriving at angles of incidence less than the predetermined angle and therefore due to simple reflections are recorded. The predetermined angle # depends on the nature of the terrain being surveyed and is defined as SIN #o =Co/Cn(#O#‹), where Co is the velocity   of seismic waves in the surface terrain and Cn the velocity in the deepest terrain reached by the wave. It is stated that signals due to seismic waves having angles of incidence greater than this predetermined angle #m (caused, e.g. by parasitic reflections &c.) may be eliminated by modifying the signals from each geophone by a function f(n,w) and then summing the modified signals. One form which the function   may assume is f(n,w) =SIN nk/nk where k =     WL/c SIN # m, k1 = W1 L /c SIN # m, c = the   (estimated) velocity in the surface stratum, L = the (constant) distance between adjacent geophones, n = the order of the geophone with respect to the centre of the line of geophones, W = the pulsatance of the particular signal considered and W 1 = the lowest value of pulsatance it is desirable to record. This form of the function produces a curve of the signals admitted as a function of the angle of incidence which has a rectangular form. It is stated that, in practice, a finite number of modifying operations applied to (different) narrow frequency bands of each geophone signal (assuming a constant frequency throughout each band) gives satisfactory results if the frequency bands are sufficiently narrow for the summation process to be validly applied. In one embodiment for carrying out the invention eight geophones 11<SP>1</SP>, 11<SP>8</SP> (Fig. 3), are employed, the output signals from each being supplied to respective groups of twelve band-pass filters 13<SP>1</SP> 1 ... 13<SP>1</SP> 12 , 13<SP>8</SP> 1 ... 13<SP>8</SP> 12  which provide the narrow band signals referred to above. The latter signals are then supplied to individual voltage dividers, each comprising resistors 15 of equal value and resistors 14<SP>1</SP> 1  ... 14<SP>1</SP> 12 , 14<SP>8</SP> 1 ... 14<SP>8</SP> 12  the value of each of which is determined by the function f(n,w) and is such that the voltage at the junction of the resistors 14 and 15 is equal to the product of the output voltage of the filter considered and the function where n is the order of the particular geophone considered and w the pulsatance of the central frequency of the pass-band of this filter. The junction points of all the pairs of resistors 14, 15 are then connected together through isolating resistances 16 and the signals thus summed are fed via an amplifier 12 to a recorder 17 in which a composite recording containing only signals due to wave incident at angles less than #m is produced. The arrangement just described however, has two disadvantages, namely: the large number of filters required, i.e. 288, and the inaccuracies produced by phase-shifts inherent in the filters and to overcome these disadvantages the modified arrangement shown in Figs. 4 and 5 may be utilized. In this embodiment the output signals from each geophone are modified by means of voltage divider networks comprising resistors 19 of equal value and resistor 18 the values of which are determined as described for the resistors 14 of the embodiment of Fig. 3. Twelve divider networks are employed for the output of each geophone and corresponding networks associated with different geophones are connected together and feed respective band-pass filters 20 1  ... 20 12  the " W " of the central frequencies of which is utilized in calculating the values of the resistors 18. The output signals of each filter, the number of which has been reduced to twelve by the arrangement employed, are still subject to the inherent phase-shifts of the filters and to remove the latter the signals are supplied via amplifiers 21 1 , 21<SP>1</SP> 12  to magnetic tape recorders 22 1  ... 22 12  and, after recording, the tapes are fed through reading heads 30 1  30 12 , in the reverse direction to that in which they were recorded, and the reproduced signal again supplied via the filters and amplifiers 20, 21 to a summing resistor 24 and a common amplifier 25 and recording means 26, which produces the desired final recording. The reversal of direction of travel of the tape causes the phaseshifts introduced by the second passage of the signals through each filter to be of equal value and opposite sense to those produced by the first passage so that the inherent phase-shifts are eliminated and an additional advantage resides in the improved filtering caused by the double passage through the filters. For best results conventional static and dynamic corrections should be applied. Instead of having the form referred to above the function f(n,w) may take the form where # is a constant and results in a (Gauss) curve of signals admitted as a function of the angle of incidence which has a continuous transition. It is stated that although the application of the above function theoretically requires the line of geophones to be of infinite length a finite length of several hundred metres is sufficient in practice and also that the effect of " secondary lobes " due to the finite spacing L of the geophones is not objectionable if such spacing is below a limit of from 10 to 20 metres depending on the terrain. In addition to its use to eliminate unwanted parasitic reflections the invention may be applied to differentiate between signals produced by seismic refraction. Fig. 6, shows one example of such use in which regularly spaced and aligned geophones 1 . . . 8 are disposed at a distance from a shot point 60. If it is desired to study the signals due to waves travelling over the path 72 ... 77 such signals are recognized by their apparent horizontal speed along the ground (equal to the speed of propagation of the seismic waves in the terrain between the interfaces 64, 65) and the signals due to propagation over other paths are eliminated. The latter is effected by retarding the signals from successive geophones by amounts related to the propagation speed in this terrain to produce eight theoretically synchronous signals. These signals are then processed as described above.