Patent GB-1047357-A (GB)
IPC: G01V1/26
Abstract: 1,047,357. Geophysical prospecting. MOBIL OIL CORPORATION. Aug. 24, 1964 [Aug. 29, 1963], No. 34487/64. Heading H4D. [Also in Division G5] In a seismic exploration system in which a spaced source and detector are moved together along a traverse, the source being actuated periodically to generate a family of seismic signals which are recorded serially on one track of a magnetic tape, and a constant-frequency timing signal being recorded on a second track of the tape as described in Specification 945,272, a second constant-frequency signal is superimposed on the timing signal to provide a geographical marker which is subsequently used for the automatic control of a tape playback apparatus and seismic-section writing apparatus. Field recording (Fig. 1).-Seismic signals are generated by a ship-borne gaseous-detonation device 10, detected by a towed string of transducers 12, passed through an amplifier 20, and serially recorded as an F.M. signal on a twintrack magnetic tape 50 and as an intensitymodulated signal on an electro-chemical recorder 24. The seismic signals may be composited on a compositor of the " delay drum " type (not shown) before being applied to the recorder 24. A stabilized 1000 c/s. oscillator 31 drives the tape 50 and recorder 24 in synchronism through a frequency divider 32, amplifier 33 and synchronous motors 30, 71, and also provides the timing signal. A 240 c/s. tone burst from an oscillator 60 may be superimposed on the timing signal to mark a known geographical location. A cam-operated switch 42, driven by the recorder 24, actuates the detonation device periodically, a " time-break " pulse 67 (Fig. 3, not shown) being simultaneously recorded to mark the beginning of a seismic record. A cam-operated switch 69 is arranged to blank off the recording amplifier 20 to give a short duration " null " signal 65 between the end of each seismic record and the commencement of the next record. Signal processing (Fig. 2, not shown).-A seismic section writer 82 comprises a motordriven drum 86 carrying a magnetic recording medium 83 and photo-sensitive paper 91. The field tape 50 is played back on a machine 51a and the recorded timing signals are fed through a frequency-divider 11 to provide a synchronized drive for the drum 86. The recorded seismic signals are fed to a magnetic recording head 81 and subsequently picked off the recording medium 83 by a movable playback head 87. Up to fourteen recording heads 81 (not shown) may be used to permit compositing of the seismic signals. Signals from the playback head 87 are demodulated, amplified, and fed to a galvanometer 90 which directs a beam of light on to the paper 91 to produce a photographic trace. The galvanometer 90 is mounted on a carriage 92 which is moved a given distance in a direction parallel to the axis of the drum 86 by a stepping mechanism 93 actuated by a valve circuit (Fig. 6, not shown) which detects the presence of the " null " signal 65 between two seismic records on the tape 50. A filter and amplifier circuit (Fig. 4B, not shown) detects the presence of any 240 c/s. tone burst on tape 50 and causes the mechanism 93 to make a double step, thereby leaving a blank space on the photographic trace to indicate a geographical feature. The circuit of Fig. 4B also gives audible and visual indications of the geographical marker. Automatic operation of the section-writer.- A series of microswitches in a housing 94 on the drum 86 switch sequentially according to the angular position of drum 86 and operate relays 121-181 in a control circuit 101, described in detail with reference to Figs. 4A and 4B (neither shown). By means of relay logic switching, the drives to the drum 86 and playback machine 51a are stopped and started automatically such that the " time-break " signal of each seismic signal is placed approximately at a predetermined position relative to a splice 91a on the photographic paper 91, and such that on receipt of a geographical marker signal the doublestepping of the galvanometer 90 occurs just before the arrival of a time-break signal. Exact alignment of the " time-break " signals on the photographic paper 91 is achieved by manual adjustment of the position of playback head 87, the operation being monitored on a C.R.O. 100 fed with the " time-break " signals and with synchronizing signals generated electromagnetically by a toothed tone-wheel 105a on the drum 86. Automatic stop, when the galvanometer carriage 92 or tape 50 reaches the limit of its travel, is provided.