GB1295332A - - Google Patents

Abstract

1295332 Selective signalling COMPAGNIE GENERALE DE GEOPHYSIQUE 27 Feb 1970 [5 March 1969 9 Feb 1970] 9712/70 Heading G4H [Also in Division H4] The outputs of a geophone array are timemultiplexed and applied to a single transmission line for recording at a remote point, the multiplex sampling being controlled by pulses sent along the line. In the embodiment shown, geophones C are coupled to transmission line 1 by respective assemblages A<SP>1</SP> detailed in Fig. 7. Line 1 is terminated by a pulse generator G and remote digital recording unit E. Each geophone output is fed to line 1 via a noise-reduction filter 31, FET gate 32 and impedance matcher 33 With the apparatus in a recording mode, all geophone outputs are scanned within a 2 msec. period by sequential opening of the gates 32, each particular gate being opened in response to recognition by assemblage A<SP>1</SP> of an address code identifying its position along line 1. In this example, generator G produces a pulse train and coincidence circuit 42 opens gate 32 when the number of pulses in the train reaches a number stored in counter 35. Prior to the recording mode, the apparatus operates in an addressing mode during which the counters 35 store different numbers, constituting the address codes, as follows. Recording unit E sends along line 1 a pulse code signifying "start addressing"; this code is recognized by units 68 which open gates 41 to pass clock pulses from high frequency sources 40 to counters 35; the code is also received by generator G which transmits in response an "end addressing" code; the "end addressing" code is recognized by units 39 which then close gates 41; thus gates 41 are open for periods respectively proportional to the line transmission times between generator G and the different assemblages A<SP>1</SP> and hence t he numbers stored at 35 identify the assemblage positions along line 1. In an alternative embodiment (Fig. 3, not shown) which does not have the prior addressing mode, units 35, 38, 39, 40, 41, 42 of assemblage A<SP>1</SP> are replaced by a counter with a pre-wired output decoder arranged to open gate 32 when a pre-determined count of the pulses in the train from generator G is reached. This counter is reset to zero (or to a present count if it is not desired to sample all the geophone outputs) at the beginning of each 2 msec. sampling period in response to recognition by the assemblage of a synch. signal. In a modification of the Fig. 3 embodiment generator G produces 8-bit address codes instead of single pulses, and the counter is replaced by an address decoder. Details of the synch. signal recognition circuit are given (Fig. 4, not shown).