Seismic group recorder control system

Claim 1

In a cableless seismic digital recording system using a plurality of portable seismic group recorders to record at selected time intervals the output of geophones located near said recorders, each such recorder being actuated by a limited energy source and possessing a receiver adapted for receiving high-frequency signals modulated by low-frequency signals from a control point remote from at least part of said recorders through a communication link, said receiver having a plurality of frequency-selective low-frequency circuits for channeling demodulated signals to control automatically the operation of said recorder, including selectively turning on only certain predetermined units of said recorders, actuation of recording medium motion by said source, recording digitally the output of geophones of said medium following recording of identifying header data, and turning off said recorder, the improvement comprising a. modulating a high-frequency signal radiated from said control point only by blocks of substantially constant amplitude low monofrequency signals, each block existing for a time T where T is constant and chosen such that each block contains an integral number of cycles of sine wave signals, plus one essentially continuous monofrequency timing wave, the frequency of each said block monofrequency being different from all others and related to the frequency of said timing wave by one being an integral multiple of the other, and all blocks being coherent, that is, each starting sinusoidally from zero in the same polarity using frequency-shift keying for said modulating; b. receiving and demodulating said high-frequency signal at said recorders and filtering the demodulated low-frequency signals in parallel through a plurality of filter channels, each tuned to one only of the monofrequencies of said blocks; c. detecting relative amplitudes of the electric output in said channels periodically and substantially at a time T in a plurality of predetermined combinations, each being sent automatically in accordance with each separate combination and the number of uninterrupted repetitions of said combination to a separate control circuit of said recorder, and d. quenching all of said filter channels tuned to the frequencies of said block signals briefly, substantially simultaneously, and periodically at a time T, such time of quenching occurring just subsequent to the detecting step (c).