GB1153975A - Digital System For Controlling A Seismic Transducer - Google Patents

Abstract

1,153,975. Data storage arrangements. CONTINENTAL OIL CO. 10 Oct., 1967 [16 Nov. 1966], No. 46223/67. Heading G4C. [Also in Divisions G3 and H4] To eliminate equipment-induced timing errors in a waveform control signal for a seismic vibrator the signal, comprising a sequence of digital " words ", is placed in a buffer store and successive words are then read out from the store at a precisely controlled rate for application to a digital-to-analogue (D/A) converter and thence to the vibrator. In the Fig. 3 embodiment the control signal is computer-generated or derived from an analogue waveform by an analogue-to-digital converter and comprises digital samples (of a frequency sweep signal) in the form of sequentially recorded words, each word being two sequential " characters " of seven " bits " each, on magnetic tape 112. The seven bits are read in parallel by head 114 and applied to OR gate 120 to provide one clock pulse per character for driving shift register 124. These clock pulses, taken from one output of the shift register, are fed also to phase comparator 106 to provide an error signal for phase-locking the playback speed of tape 112 to a precision oscillator 90. Sequential outputs on lines 150-168 of register 124 open respective gates 130-148 to steer successive characters from playback amplifier 126 to an N-word buffer store 190-200 (e.g. comprising bi-stable circuits). The words are read out from the store by opening gates 212- 220 sequentially with pulses provided by shift register 222 at a precise rate controlled by oscillator 90. The readout rate is thus unaffected by wow and flutter in the tape playback process. The words from gates 212-220 are reconstituted as an analogue signal by D/A converter 98 and applied directly, or via a radio link 254, to the vibrator 18. This analogue signal may also be used in subsequent signal processing (e.g. correlation) of the output of a geophone array. As soon as a particular word has been read out from store the associated storage element is reset by an output from reset shift register 238 so that the storage element is freed for storing a further word from tape 112. If tape or other speed errors are large or have a long term drift a large buffer store is required and the store may then comprise magnetic cores or may be a random-access store of sufficient capacity to store the total digital sweep signal (in which case the tape record 112 is unnecessary). Details of the application of the individual data bits to the gates 130-148 and store 190- 200 are given (Fig. 4, not shown).