Patent GB-1440073-A (GB)
Title: Time synchronisation particularly for seismic work
IPC: G04G7/02
Filed: 1972-07-11
Abstract: 1440073 Clock systems SEISMOGRAPH SERVICE (ENGLAND) Ltd 11 July 1973 [11 July 1972 24 Nov 1972] 32451/72 and 54435/72 Heading G3T [Also in Division H4] In an arrangement for obtaining correct time synchronization at remote points without direct communication of time between those points and particularly for seismic survey operations, separate timing units are provided one for use at each point and each having a constant frequency oscillator from which are derived timing pulses and the units are brought together and inter-connected for synchronizing the pulses and then located at their respective operative points. TIMING UNITS Fig. 1 illustrates a typical unit which may be used either as a master or as a slave unit. It comprises a 1 MHz quartz oscillator which is maintained at a constant temperature (75‹C). The oscillator output is shaped by a Schmitt trigger 12 the I MHz output of which is divided down by a decade counter 14 to give a pulse every 10 secs at outputs 16, 18. Each pulse initiates drive current of adjustable duration in a monostable multivibrator 20, which operates contacts of a relay 26. Closure of the relay contacts in a remote slave unit is used to trigger a seismic signal by actuating a blaster or other seismic wave source such as a vibrator, as described with reference to Fig. 3 (not shown). Their simultaneous closure in a master unit causes a time datum to be recorded in a seismic recorder. To start the recorder in the master unit ahead of a seismic 'shot', the last four decade counter outputs D4 to D7 are decoded by a decoder-timer 38 from which an ouput pulse can be provided at any time in steps of 1 m.sec. relative to one of the 10 sec pulses. This output pulse initiates drive current of adjustable duration from a monostable MV 40 which operates a relay 42 to start the recorder. SYNCHRONIZING. Fig. 2 To synchronize the seismic signal trigger pulse in the or each slave unit so as to coincide with the time datum pulse recorded in the master unit, each unit is provided with a monostable 56 and a Schmitt trigger 50. Closure of a switch 52 in the slave unit permits a pulse from monostable 56 in the master to reset all counters in the decade counter 14 of the slave, the monostable 56 pulse being initiated by the decoder timer 38 output which initiates the time reference pulse from monostable 20. Square waveforms ( e.g. at 200 KHz 40% duty cycle for synchronization are produced by an AND gate 70 which receives the Dl and D6 outputs from decade counters 14. The waveforms A and B, Fig. 4 (not shown), from the master and slave respectively are applied to a gate 70 in the master, the output of gate 70 being a square waveform C (Fig. 4) indicating the degree of coincidence. The output of the gate 70 is connected via a diode (not shown) to a meter 47 which measures a D.C. level substantially proportional to the pulse width of waveform C, i.e. to the degree of coincidence. From a count of the meter deflections from zero to a maximum and back to zero again, the difference between the oscillator frequencies in the master and slave units can be calculated. The frequency of the slave unit oscillator is adjusted to decrease the frequency difference. In operation, an operator at the shot point closes a firing switch and the seismic signal is triggered by the next pulse from monostable 20 in the slave unit. The pulse from monostable 20  in the master is recorded simultaneously. At the same instant a return check pulse is sent from the blaster 80, Fig. 3 (not shown), to set a monostable 40 and allow pulses to pass from timer 38 to system start-output 44 for transmission via radio link to the master. The recorder is also connected to a geophone array (not shown).