Patent GB-806359-A (GB)
Abstract: 806,359. Seismic prospecting. SOUTHWESTERN INDUSTRIAL ELECTRONICS CO. Aug. 1, 1956 [April 13, 1956], No. 23737/56. Drawings to Specification. Class 118 (2). Apparatus for compositing seismic exploration results obtained from generation of a plurality of seismic waves in the earth and translating vibration waves received therefrom at one position into a plurality of geophone signals in order to emphasize reflections and minimize the effect of noise, comprises a magnetic recorder having at least two recording- and-reproducing heads, a mixing circuit having two effective inputs and an output, first switch means for alternately connecting said output to the first and the second head, means for supplying a geophone signal to one of said inputs, and second switch means for alternately connecting said other input to the second and the first head, said mixing circuit being operable to add voltage signals supplied to its inputs and produce a composite voltage at its output. Shots may be fired using a plurality of boreholes (Fig. 2, not shown) each containing a shot, or a single borehole may be used with shots vertically spaced therein (Fig. 1, not shown)'. The signals from a number of geophones at different locations are dealt with concurrently but each geophone has an individual amplifier, two heads on the recorder and individual associated equipment. A common recorder drum is used to carry a multi-channel magnetic tape having the necessary tracks (two per geophone) and the compositing switch means for alternating the heads of each pair between the appropriate mixed circuit input and output comprises several separate but ganged sections. The effect of noise in the recording system is reduced by frequency modulating carrier waves with the geophone signals before recording and de modulating upon reproduction. The compositing switch has as many positions as there are shots to be fired. In its position for the first shot there will be no recorded signal for reproduction from the second head and the geophone signal will be the only input to the mixer circuit and will be recorded by the first head without modification. In its second position the reproduction from the first head is fed to the mixer input to be combined with the second shot signal from the geophone and the composite output is recorded by the second head. In its third position the composite signal is reproduced by the second head and fed to the mixer input to be further composited with the third shot signal from the geophone and this output is recorded by the first head, the previous recording being automatically erased as the subsequent signal is recorded. This alternation of heads for reproduction of the prior composite recording and recording of the said reproduced signal mixed with a further geophone signal continues until all the shots have been fired. In a described embodiment a four-position swicth is used in conjunction with five geophone channels, each of which is arranged as in the block diagram of Fig. 6 (not shown). The final record on the tape is thus a composite of all the signals supplied from the several shots, and for this to be satisfactorily representative of all signals the mixer circuit must add its inputs in varying proportions as the composite result builds up. This is done by further sections of the compositing switch, ganged to the operation described above, which connect suitable resistors into circuit for each shot stage. Fig. 7 (not shown) illustrates this for three separate channels. The geophone amplifiers are provided with automatic gain control to give a constant average output which would modulate the carrier 100 per cent if applied directly, and the mixing circuit may be arranged to provide a constant 33 1/3 per cent modulation of the carrier at each mixing operation (Fig. 8, not shown) or a stepped increase such that the final result provides 100 per cent modulation. (Fig. 9, not shown). If it is desired to retain permanent records from the individual shots the geophone signals are first recorded on separate tapes and then these are reproduced in turn to take the place of the geophone inputs. The arrangement of Fig. 11 (not shown) comprises the arrangement of Fig. 6 with an additional recorder for the making an play-back of permanent record tapes. The final record may be reproduced, demodulated and fed, for example, to an oscillographic recorder to produce a composite record of the type illustrated in Fig. 4 (not shown) for three geophone channels. In order that the seismic reflections from the various shot locations may be aligned for useful summation in the mixing circuit the recorder drum is provided with an adjustable cam-operated switch for firing the shots. The position of this switch with respect to the recording-reproducing heads is adjusted in accordance with an educated guess as to the differences in travel times of waves from the various shots to the geophones or these may be predicted from a previously made borehole velocity log. This arrangement of the firing circuit is schematically illustrated in Fig. 10 (not shown) which also includes circuits for impressing a " time break " signal and " uphole " signal on the trace from the first geophone and relay means for disconnecting these circuits as soon as substantial seismic energy reaches the said first geophone.