Patent GB-768845-A (GB)
Title: Improvements in or relating to method of and apparatus for determining travel time of signals
Inventor: CRAWFORD JOHN MARION
IPC: G06G7/19
Filed: 1954-07-17
Abstract: 768,845. Geophysical prospecting. CONTINENTAL OIL CO. July 17, 1954, No. 20906/54. Class 118 (2). [Also in Group XL (c)] Apparatus for determining the travel time of wave energy through a medium from records bearing traces which are respectively parameters of a signal as transmitted and as received over a path through said medium, comprises a playback device for playing such records, means for progressively shifting the timephase relationship of the respective played-back signals, means for multiplying together the outputs derived from such traces, means for integrating the products of said multiplication, and means for recording the continuing integration sum so produced to indicate the value of said sum for each value of time-phase relationship. Thus signals (which must be non-repetitive over a period of time as long as the greatest travel time to be measured) are effectively auto-correlated to provide a correlation function having a distinct peak at the point where the shift in time-phase relationship corresponds to the delay due to the travel time, which is thereby determined. As applied to seismic exploration the record traces may be obtained as in Fig. 1, wherein a vibrator 1, comprising an accelerating and decelerating off-centre rotating weight 2, sends signals of varying frequency into the earth. A detector 5 picks up a signal which is substantially identical with that transmitted, while a plurality of detectors 6, 7 and 8 pick up signals travelling via the earth after reflections or refractions at the subterranean surfaces 10, 11 and 12. These signals are amplified and recorded in the units 20 and 21 respectively. The recorder 21 may be photographic, electric, acoustic or magnetic, but the records must be capable of being stored for a substantial period as after being made in the field the correlation procedure may be carried out at a later date in a central laboratory. An electro-mechanical transducer may be used instead of the vibrator with off-centre weights, or electro-magnetic radiation may be used. In the case of elastic vibrations the frequency range 20 c/s to 80 c/s is stated to be critical for best results and may be produced by alternately accelerating and braking the engine driving the vibrator 1 over four-second intervals, four seconds being the greatest practical travel time yielding useful information. The absolute rates of acceleration and deceleration are not critical nor need they be duplicated exactly during successive runs. The recorded signals may be of variable area such as those shown simplified in Fig. 3, wherein the trace S is that representing the transmitted signal picked up by detector 5, while the traces 1, 2, 3 and 4 represent signals picked up by four detectors such as 6, 7 and 8 in Fig. 1. The lowest trace T is a timing trace which cannot in practice by used to determine the phase relationship of traces but is useful in subsequent procedure when play-back is speeded up to reduce analysis time. In practice noise and interference renders the signals much less clearly defined (Fig. 3A, not shown), but the following multiplication and integration procedure enables their correlation to be ascertained and their travel times deduced. Even so, improved results may be obtained by using multiple detectors and/or multiple vibrators and combining the records in to a composite record. The effect of multiple vibrators may be obtained by operating one vibrator in a plurality of positions (Fig. 2, not shown), the composite record being the same as if separate vibrators were operated simultaneously. Interchange of vibrator and detector positions to give a composite record of energy travelling in both directions may further improve record quality. A further technique which may be utilized in obtaining records for subsequent correlation by this invention comprises spacing the vibrator positions so far apart that reflected signals arriving at any one detector are so shifted that they cannot be directly added in timephase. In this case approximate corrections have to be applied when recording the transmitted signals to enable the detected signals to be satisfactorily aligned in relative timephase. Such corrections will be applicable to a limited range of reflection time and detector spacing, but a mean correction may be chosen and the geometry of vibrator shift and detector spread made compatible with the depths to be investigated. Field records made as above are reproduced and correlated by apparatus shown schematically in Fig. 4, wherein the reproducer 35 provides signals S 1  ... Sn representative of those received at. the detectors while signal S represents the signal transmitted. The signal S is fed through the earth simulator 36 to compensate for the phase and amplitude distortion introduced in the various frequency components of signals S 1  ... Sn by their passage through the earth. After passing through a variable delay device 37 it is simultaneously multiplied by each signal S 1 . . . Sn in separate multipliers M and the products integrated at I and then indicated or recorded on a mirror-galvanometer type of photographic recorder adapted to record, for each of the respective integrators, the final sum at the end of an integration period. The fixed delay introduced at 37 is maintained for a predetermined integration time and then advanced an incremental amount till a total time range of about one second (varying in the neighbourhood of the actual time delay of the signals in passing through the earth) has been processed. The integration time is chosen such that the consistent phase relationship of the signals S 1  . . . Sn with the delayed signal S builds up a correlation value that is sufficiently predominant over that of the random-phase noise and interference. The correlation function will be a maximum when the delay introduced at 37 is equal to that due to the earth travel time of the particular signal, the precise travel time being thereby determined. A magnetic drum having a plurality of pick-up heads (Fig. 4A, not shown) may be used to delay the signal S by different amounts simultaneously, in which case individual sets of multiplying and integrating circuits must be provided for each pick-up head. In the preferred embodiment of Fig. 7 a magnetic drum variable delay device 38 has a fixed recording head (with erasing head E located just before it) and a movable reproducing head having its adjustment calibrated so that the exact time elapsing between recording' and reproducing can be read accurately and directly. Only one multiplier 39 and integrator 40 are shown for simplicity. A voltmeter 42 indicates the sum accrued at the end of an integration period and switch 43 serves to discharge condenser 41 to provide a zero reference base at the beginning of each integration period. Alternatively a recorder (as in Fig. 8, not shown), may be used. Mechanical or electrical multipliers and integrators may be used but electronic types are ‹ preferred. The reproducer 35 may be provided with movable pick-ups at the generation points S 1  to Sn for relative adjustment of the several traces. Typical delay values used in practice are given and correlation curves are plotted in Figs. 6 and 10 (not shown). With successive delay times at 0.002 second intervals and an integration time of 30 seconds for each, over four hours is required to process a time range of 1 a second. This may be much shortened by speeding up the play-back by, for example, recording the signals on film at 75 mm. per second for a record length of 30 seconds and, after processing, forming the film into a continuous loop for reproduction at 300 mm. per second or faster. The correlation apparatus must then handle higher frequencies and the variable delay device must be calibrated to allow for the speed-up ratio. Figs. 11 and 12 (not shown), illustrate auto-correlation functions theoretically obtainable from a stipulated square wave form and using frequencies of 20 to 80 c/s and of 40 to 80 c/s respectively. Fig. 13 (not shown), gives the result of using an infinite frequency range.