GB1500958A - Automatic computer-performed methods for analysing the operation of an earth vibrator used in geophysical prospecting - Google Patents

Abstract

1500958 Monitoring seismic vibrator operation STANDARD OIL CO 23 April 1975 [24 April 1974] 16871/75 Heading G1G To determine whether a seismic vibrator injects into the earth a faithful replica of its control (pilot) signal or not, a representation of the injected signal is automatically analyzed by a computer controlling an output display on which measures of the pilot and injected signals can be compared. Faulty vibrator operation which would give spurious seismic data or indicate incipient failure of a component of the vibrator or of its control elements can thus be detected. In the Fig. 1 system a CW frequency-sweep pilot signal is transmitted over a radio link to electronics 2 controlling the electro-hydraulic vibrator 3. Corrective phase shifts are applied at 3 to the received pilot signal to give a reference signal. Vibrator operation is synchronized with the reference signal by a phase-lock loop including an accelerometer 4 on the vibrator. The injected seismic signal is sensed by a pick-up 5 on the vibrator base-plate. The various signals are processed by a minicomputer at the vibrator site, or are recorded for later processing at a remote computer centre. The processing provides several analyses of the data. One analysis divides the pilot, reference and base-plate signals into time windows at 8 which are then Fourier analyzed at 9, 11, 13. From the results there is plotted at 15 a respective contour map for each of the three signals, with contours joining points of equal amplitude plotted in frequency versus window-centretime co-ordinates Figs. 3, 4, 5 (not shown). A second analysis e.g. at 16-19 determines the autocorrelation and cross-correlation functions of all or some of the pilot, reference, accelerometer and base-plate signals and these functions are displayed at 15, Fig. 6, (not shown). The autocorrelation function of the pilot signal represents the ideal seismic input to the ground. The pilot/reference cross-correlation function is a measure of the functioning of the electronics 2. The pilot/baseplate cross-correlation function and reference/ accelerometer cross-correlation function likewise enable monitoring of the vibrator's overall operation and phase-lock system respectively. A third analysis, at 20-23, obtains by rectification and integration the smoothed envelopes of the correlation functions. These envelopes are normalized at 24-27 to bring their central peaks to a common amplitude, then displayed at 15 Fig. 6, (not shown). A fourth analysis removes correlation components due to harmonics and noise, by truncating the correlation functions at 28-31. The remaining central portions Fig. 7, (not shown) are Fourier analyzed at 32-35 to give amplitude/frequency and phase/frequency spectra Fig. 8, (not shown) for display at 15. Details of, and a Fortran statement for, the computer processing are given.