Patent GB-1020565-A (GB)
Title: Well logging system
IPC: G01N29/07
Filed: 1962-11-26
Abstract: 1,020,565. Seismic surveying. SCHLUMBERGER WELL SURVEYING CORPORATION. Nov. 26, 1962 [Nov. 29, 1961], No. 44674/62. Heading H4D. In an acoustic velocity well logging system the borehole instrument 10, Fig. 1, comprises two transmitters T1, T2 and two receivers R1, R2 operated in a sequence which measures the time of travel of acoustic waves in the media surrounding the well bore between the transmitters and the receivers, in the order T1R2, T1R1, T2R1, T2R2. Expressing such times as ta tb, tc and td respectively the average time of travel between the twc receivers is then given by ta - tb - tc - td and since 2 such average time is based on transmissions from above and below the receivers errors due to tilt of the apparatus in the well bore or due to caves in the latter are minimized. The borehole instrument, which includes the devices of Fig. 3 (not shown), is connected to the surface equipment (Fig. 2, not shown), by means of a special armoured sevenconductor cable 12 having six conductors spirally wound about a central conductor, equalizing means being provided for signal transmission purposes so that signals sent up the central conductor accurately represent the instant of occurrence of such signals. This conductor is indicated at 61 in Figs. 2 and 3. In operation a periodic pulsing source 20 (Fig. 2, not shown) of about 20 C.P.S. or, depending on switch 22, a pulse generator 21 controlled by the cable sheave 13a, controls a flip-flop 24 the oppositely phased outputs of which (25a, 26a, Fig. 4, not shown), control further flip-flops 27, 28 to produce switching waveforms 27a, 28a (Fig. 4) for transmission to the borehole equipment over conductors B and C. The control pulses (considering source 20) are also fed via a 9 m.sec. delay 29 to trigger a " fire-pulse " generator 30 the output of which (30a, Fig. 4) is also sent down hole over conductor A. At the down hole equipment the waveforms on conductors B and C control the sequences of selections of the transmitters and receivers indicated above by means of electromagnetically operated switches 33, 37 whilst the waveforms on conductor A is employed, to control the transmitting pulse generator 40 and, via a flip-flop 45, to close a noise gate 55 in the receiving channel 34 for a period embracing the actuation of a transmitter. To ensure the latter condition the " fire-pulse " on conductor A is delayed in 39 for 100-200 m.sec. before application to pulse generator 40. The output of the latter (40a, Fig. 4), in addition to actuating the transmitters, is also fed to a pulse shaper 46 and sent up the cable over conductor 61. Thus, for each actuation of a transmitter, a pulse (T1, T2, Fig. 4) indicative of such instant is received at the surface equipment each such pulse being followed by signals (R1, R2, Fig. 3) indicative of the instants of reception of acoustic waves produced by actuation of the transmitters. At the surface equipment (Fig. 2) the received signals are fed via amplifier 70 to normally closed gates 71, 72, the former being opened by the " fire-pulse " 30a (see also 30a, Fig. 5) after delay in 73 (waveform 73a, Fig. 5), which is less than the delay of circuit 39 (Fig. 3), to enable the pulses indicative of the instants of actuations of the transmitters to be received. The latter pulses passed by gate 71 then operate a " timeof-occurrences" detector 74 (e.g. a trigger circuit which produces an output when the amplitude of the input signal exceeds a predetermined level) the output signal of which (74a, Fig. 5, not shown), in addition to being fed back to close the gate 71, controls a flip-flop 76 which opens a timing gate 77 to pass the output of an oscillator 80 (e.g. a 5 Mc/s. crystal controlled oscillator) to a binary counter 81. The latter continues to count the cycles of oscillator 80 until the reception of a receiver signal which is fed via gate 72 (already opened by waveform 78, Fig. 5, derived by delaying the output of detector 74 in delay device 78) to a detector 82 similar to 74) the output 82a (Fig. 5) of which, in addition to closing gate 72, also closes gate 77 via flip-flop 76. The count in counter 81 is then indicative of the interval between the transmission of an acoustic signal and its reception by one of the receivers. For each cycle of operation, i.e. T1R2, T1R1, T2R1, T2R2, the corresponding times are combined as ta - tb + tc - td (as described above) and to effect this, " add " and " subtract " control circuits 84, 85, respectively, operate on the counter 81 under the control of the outputs of flip-flop 24. The net count is then supplied in to a transfer circuit 86 under the control of waveform 87a (Fig. 5), which is derived by delaying the waveform on conductor C in device 87. The transfer circuit divides the input count by two to provide the required average time and this is fed to a binary-to-analogue converter 89 which produces a voltage output for application to a recorder 90. Counter 81 is then reset by a pulse 87e (Fig. 5) from a circuit 87b which delays the output of 87 for an interval 87c (Fig. 5). Details of the counter 81 and the transfer circuit 86 are given with reference to Figs. 6A, 6B and 10 (not shown).