GB1261836A - Improvements in and relating to seismic prospecting - Google Patents

Abstract

1,261,836. Seismic prospecting; automatic gain control. VSESOJUZNY NAUOHNOISSLEDOVATELSKY INSTITUT GEOFIZICHESKIKH METODOV RAZVEDKI. 6 Feb., 1969 [8 Feb., 1968], No. 6353/69. Headings H4D and H4R. [Also in Division G4] Multi-channel seismic signals are fed through a commutator to a single stepped-gain amplifying channel which provides dynamic range compression, the amplified signals being recorded in digital form. In the embodiment shown, seismographs 1 are connected through pre-amplifiers 2 to commutator 3. Commutator 3 couples the seismic channels in turn to amplifying channel 4, each for a 30 Ásec. period. Amplifying channel 4 comprises n cascaded stages 6, 7, 8, each with a gain which is some multiple of two (e.g. eight), coupled to the signal inputs of n + 1 gates 9, 10, 11, 12. The gate outputs are coupled to sample- and-hold circuit 24 and level comparator 13. Timer 18 provides pulses at 5 Ásec. intervals to one input of a coincidence gate 14 coupled to a counter 15 (e.g. comprising bi-stables 16, 17) whose logical "1" and "0" outputs are the inputs for n+ 1 coincidence gates 20, 21, 22, 23. At the start of a 30 Ásec. period of commutator 3 the counter outputs give coincidence in gate 20, thus opening gate 12 to pass signals with an amplification of 8<SP>3</SP> to comparator 13. If the amplified signal level exceeds a reference level in the comparator, a comparator output signal is given and will result in a coincidence signal from gate 14 after 5 Ásec. This coincidence signal advances counter 15, thereby closing gate 12 and opening gate 11 to pass signals with an amplification of 8<SP>2</SP> to comparator 13. If these amplified signals still exceed the reference level, counter 15 is again advanced after a further 5 Ásec. interval. This procedure repeats until the reference level is not exceeded, the amplification then being held constant. 5 Ásec. after the nth pulse from timer 18, circuit 24 is allowed to sample the amplified signal and the sample is converted to digital form by A-to-D converter 25 and recorded magnetically at 26. After the sampling, timer 18 resets counter 15. During this reset a code signal representing the prevailing amplification factor is derived from the counter 15 outputs and stored in memory 19. The stored signal is then recorded at 26 and the memory is erased. -The digitally recorded seismic signals and amplification code signals may be played back, amplified and shaped at 27 for application at 32 to a digital computer, and the seismic signals may be reconstituted and recorded in their analogue form with their original dynamic range by applying the output of amplifier/shaper 27 to D-to-A converter 28, dynamic range restorer 29, de-commutator 30 and analogue recorder 31.