Patent GB-868067-A (GB)
Title: Improvements in or relating to methods of and apparatus for use in interpreting seismographic records
Inventor: ANSTEY NIGEL ALLISTER
IPC: G01V1/28
Filed: 1956-02-20
Abstract: 868,067. Geophysical prospecting. SEISMOGRAPH SERVICE (ENGLAND) Ltd. May 20, 1957 [Feb. 20, 1956], No. 5203/56. Class 40(7) [Also in Group XXXV] Apparatus for use in the interpretation of seismic records comprises means for generating an electric signal with an adjustable waveshape and amplitude and the production of a visual representation of it which is compared with a visual representation of a geophone signal received from a transmitted seismic impulse. In the first embodiment a sawtooth generator 1 provides for time-displaced pulses 6a, 6b, 6c at the outputs of adjustably biased comparator circuits 5a, 5b, 5c, and for timebase sweep via amplitude control 4 of the long persistence cathode-ray tube (C.R.T.) 3 over the face of which the seismic record on translucent paper to be interpreted is placed. The amplitudes and polarities of pulses 6 are adjustable in units 7a, 7b, 7c whose outputs are fed together to pulse shaper 9 comprising resistance-capacitance, Figs. 4A-4C (not shown), and differentiating networks. The pulse shaper output is amplified in unit 11 whose electrical characteristics are the same as those of the amplifier used for producing the seismic record to be interpreted and includes corresponding filters; unit 11 output is fed to a C.R.T. Y deflection plate which receives additionally delayed pulses from unit 13 whose times correspond to the centres of the respective shaped pulses. For the identification and timing of the first signal received by a geophone in refraction seismology the width and amplitude of pulse 10a are varied until a visual match is obtained with the first half-cycle of the recorded signal; the second half-cycle will usually match also. The amplitude of pulse 10b is then increased from zero and adjusted until a match is obtained on the third and fourth half cycles. With the aid of the timing markers derived from unit 13 the arrival times of the first and second received signal group centres are determined. For a reflection record pulse 10a waveshape is adjusted for a match between display and record, pulse 8<SP>a</SP> being inverted if necessary thus indicating that the reflection arose from an interface across which the acoustic impedance decreased; for a "long" reflection, matching to the record of second and third pulses 10b, 10c is carried out, arrival times being ascertained to the different signal components by the timing markers. The apparatus may be used for determining the most suitable filter for the recording process; and a record on which the outputs of adjacent geophone channels have been combined to improve signal/noise ratio may be interpreted. For determining geological dip a display unit showing artificially generated signals on several traces, each trace of the seismic record of spaced geophone outputs being simultaneously matched, is used. Pulses of variable width, separation and polarity may alternatively be produced from coils on soft-iron pole pieces in the magnetic path of a rotating horse-shoe magnet (see Group XXXV). In another embodiment a known function generator tube is used for the comparison waveform generation and additional networks are included to allow for the effects of (1) long initial pulses produced by the explosion, (ii) non-linear geophone characteristics, and (iii) electrical noise, Figs. 1A, 2A, 3A-K (not shown).