Patent GB-849455-A (GB)
Abstract: 849,455. Geophysical prospecting. SOCONY MOBIL OIL CO. Inc. March 19, 1957 [April 2, 1956], No. 9017/57. Class 40(7). Relates to a system for determining the velocity of propagation of acoustic waves in the ground of the type disclosed in Specification 774,098 in which an elongated probe carrying an acoustic pulse transmitter T, Fig. 1, and two spaced piezo-electric receivers R1 and R2 is located in a borehole 10, means being provided at the surface of the ground for measuring the time interval #t between the reception at R 1  and R 2  of a pulse from T propagated through the strata surrounding the borehole. According to the present invention the two receivers are connected to the surface equipment by a single transmission line, an electronic switch S being mounted on the probe for sequentially connecting the receivers to the lower end of the transmission line. As described the transmission line comprises two pairs of balanced conductors 12, 12; 20, 20. The output from the receiver R 1  is normally connected by the switch S to the conductors 12 which are coupled to an amplifier 23 whose output is rectified at 24 (output E, Fig. 2) and applied in parallel to first and second receiver gating units 18 and 19 which are normally blocked. Each transmitted pulse produces wave pulses D 2  and D 3 , Fig. 2, at the receivers R 1  and R 2  and a sync pulse A which is applied to the conductors 20; a spurious direct pulse Dl is also induced in the conductors 12. The sync pulse A is amplified at 32 and triggers a phantastron pulse generator 33 whose integrated output B is shaped at 34 giving a delayed negative pulse C which opens the first receiver gating unit 18 after the spurious pulse Dl has died out. The rectified pulse E 2  from the first receiver R 1  which occurs at a time to is therefore passed by the gate 18 through an amplifier 27 to trigger a monostable multivibrator 16 producing a positive output pulse G from which is derived (1) a delayed negative pulse I which opens the second receiver gate 19 and (2) a delayed positive pulse H which initiates operation of the switch S to connect the receiver R 2  to the conductors 12 and to disconnect R 1  therefrom, the delays being sufficient to ensure that the pulse E1 has ended before the changeover is effected. A negative output from the pulse generator 16 gates on a bootstrap generator 17 producing a sawtooth waveform F which is coupled to a storage capacitor 14 via a normally open electronic switch 37. The rectified pulse E 3  from the second receiver R 2  which occurs at time ti is passed by the second receiver gate 19 through an amplifier 35 to trigger a blocking oscillator 36 whose output pulse J momentarily closes the switch 37 thereby charging the capacitor 14 to a voltage of magnitude equal to the instantaneous amplitude of the sawtooth F at the time ti which is proportional to #t, said magnitude being recorded at 13 on a record sheet which is moved synchronously with the probe by means of a linkage 15. Alternatively the time interval #t may be measured by counting the number of pulses generated by an oscillator during the period #t. The switch S on the probe comprises two pentode gates, Fig. 4 (not shown), controlled respectively by push-pull pulses derived from a phantastron pulse generator triggered by the waveform H which is applied thereto over a phantom circuit utilizing the conductor 12; the A.C. supply for the probe unit is applied via a phantom circuit utilizing the conductors 20, Figs. 3A and 4 (not shown). Figs 3A and 3B (not shown) give the circuits of the surface equipment. The pulse recurrence frequency of the transmitter T is preferably adjustable within the range 15-50 c/s. In a modification, Fig. 5, the control pulse I for the second receiver gate 19 is derived from a monostable multivibrator 163 which is triggered via a pentode gate 184 by the trailing edge of a variable duration control pulse from a monostable multivibrator 169 which is triggered by a pulse corresponding to the pulse I in the embodiment described above. The control pulse is also applied to open an additional noise gate 160 which is also fed from the rectifier 24, so that if any noise signals occur within the duration of the control pulse, the resultant output from the noise gate 160 changes a capacitor negatively thereby cutting off the pentode gate 184 and preventing generation of the pulse I. As shown in Fig. 5 the noise gate 160 comprises a triode 179 in series with a diode-connected triode 181, the input signal being applied to the junction of the two valves such that when the valves are conductive the input signal is shorted to earth but when a negative gating pulse is applied to the grid of the triode 179 the valves are cut off; the first and second receiver gates 18 and 19 are also of this type. The rectifier 24 is shown in Fig. 5 as full wave but it may be switched to half wave, Fig. 3A (not shown).