Patent GB-992406-A (GB)
Title: Method and apparatus for producing a seismic record
IPC: G06G7/19
Filed: 1961-10-24
Abstract: 992,406. Seismic prospecting. ATLANTIC REFINING CO. Oct. 24, 1961 [Nov. 1, 1960 (4)], No. 38017/61. Heading H4D. In an arrangement for producing a seismic record involving the comparison of seismic waves received at a point adjacent to and at a point remote from a source which generates in the earth continuous elastic waves of cyclically varying frequency, the two received waves together with timing signals are simultaneously recorded on separate tracks of a single record which is subsequently "played-back" under control of the timing signals to reproduce the two seismic signals one of which is variable delayed in time and compared with the other to produce a resultant signal from which the desired record is produced when the two signals compared are in phase and exhibit similar frequency variations. As shown in Fig. 3, the elastic waves, having a characteristic as shown at 1, Fig. 1, which are produced by a generator 13 comprising shaker weights controlled by an hydraulic system and received by an adjacent geophone 15 and a remote geophone 23 are supplied via filters 65, 73 and amplifiers 67, 75 to respective recording heads 71 and recorded on a magnetic tape 69 together with timing signals from a source 77 which also controls the tape transport synchronous motor 81. After a number of cyles of generator 13, e.g. 5 to 15, depending on the depth of interest, type of terrain &c., the generator and geophones are moved to other positions until the survey has been completed. The recording tape is then removed and utilized in the apparatus shown in Fig. 4, to produce the final desired record 107. The tape 69, which is now preferably connected to form an endless band, is driven by a synchronous motor 83 controlled by the timing signals derived via reproducing head 87. The seismic signals due to geophone 15 are reproduced by head 91, the position of which is varied by means of a motor 99 which also drives the transport mechanism 101 for record 107, such reproduced signals being fed to the receiver 19 'described with reference to Fig. 2, not shown) comprising relays operated by the reproduced signals such that when these signals have the same phase and exhibit the same frequency variations as the signals due to geophone 23 determined by the " time-delay " position of lead 91 the latter signals reproduced by head 89 are converted by 19 into a useful low-frequency signal which is recorded per 105 on record 107. During this stage switches S1, S2 and S3 are open and switch S4 closed. An advantage of the arrangement thus far described over known arrangements which do not utilize the internediate step of recording the signals (on manetic tape) in the field is that the recorded signals (which may be analysed at leisure e.g. in ,he laboratory) may be played back continuously until all the reflections of interest i.e. those over paths 61, 61<SP>1</SP> (Fig. 3) result in the production of signals on record 107 (Fig. 4). This also results in improved resolution and a better signal-to-noise ratio since by continuously playing-back the tape 69 the pass-band 51 filters forming part of receiver 19 may be decreased. In a modification of Fig. 4, the tape 69 nay be split into two parts and the time delay between the two recorded (seismic) signals produced by variation of the relative tape playback speeds. The invention, additionally, provides means for eliminating undesired signals due e.g. to the early arriving refracted signal over path 57 (Fig. 3) and the horizontally travelling signal over path 59. For such elimination the tape 69 (Fig. 4) is first " played-back " with switches S1, S2, S3 open and switch S4 closed and the travel time of the undesired signals deduced from recording 107. Delays appropriate to such times are then set-up between recording and reproducing heads associated with magnetic recording devices in a variable delay unit 25 (comprising one set of heads and recording member for each undesired signal, Fig. 6 (not shown)) from which cancelling signals are derived when unit 25 is supplied with the signals due to geophone 15 from head 90 so that when tape 69 is again " playedback" (S2, S3 open and S1, S4 closed) the cancelling signals are effective to oppose the undesired signals in receiver 19. A further undesired signal which arises from the recording on record 107 of signals passed by receiver 19 when momentary synchronism occurs between the transmitted signal 1 (Fig. 1), due to geophone 15 and the reflected delayed signal 3, due to geophone 23, at the " cross-over " points 7, 9, still remains to be eliminated. In one arrangement such elimination may be effected by filter 112 (Fig. 4) the pass-band of which is as shown by bracket 163 (Fig. 1). For determination of the actual " cross-over " frequencies manual plotting may be employed or such frequencies may be derived from apparatus 94 (Fig. 4), described in detail in connection with Fig. 7 (not shown), and comprising a double-beam long-persistance oscilloscope (producing a display similar to Fig. 1) on which the frequencies may be read directly or by noting the adjustment of a filter in the signal 1 channel required to remove frequencies outside the " crossover " points from the display. Instead of employing a filter such as 112, frequencysensitive switches or a manually operated switch may be employed to restrict the band to that shown by bracket 163 (Fig. 1). The facilities provided by the present invention for eliminating undesired signals such as those over paths 57, 59 and for eliminating signals due to the " cross-over " frequency points 7 and 9 may be employed in conjunction with known arrangements for seismic prospecting utilizing frequency-modulated cyclically-varying con tinuous elastic waves (Figs. 2 and 6, not shown) The arrangement for producing the required elastic waves (i.e. generator 13) involving an hydraulic motor driving eccentric shaker weights is described with reference to Fig. 5 (not shown) The invention may also be employed in well velocity operations. Specification 992,407 is referred to.