Patent GB-916053-A (GB)
Abstract: 916,053. Geophysical prospecting; valve gating and pulse circuits. SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ N.V. Aug. 24, 1960 [Aug. 26, 1959], No. 29244/60. Classes 40 (6) and 40(7). [Also in Groups XIX, XXXVI and XXXVIII] In a system for logging wells wherein a device measures a physical property of the earth formations surrounding the well bore as a function of depth and converts the measurement to a time interval, surface equipment includes a gate which is controlled by the beginning and end of the time interval and which controls the transmission of pulses from a fixed frequency source to a digital counter. The invention is described as applied to a velocity logging system, i.e. in which the physical property is the velocity of acoustic waves in the earth formations; the invention may also be applied to well logging of substrata electrical conductivity. General arrangements, Figs. 1, 7, a, b, c, d, 2a. The well probe housing 10 is lowered by a cable 11 which passes over a calibrated sheave the coupled selsyn 12 of which is connected to circuits 13 provided to initiate operation of sweep circuits, a camera and its film transport circuits, depth-indicating circuits and cathoderay (C.R.) oscilloscope beam-exciting circuits. Probe 10 comprises upper and lower cases 21, 30 which house a pulse generating circuit 32 for exciting transmitting transducer 26 and switching amplifiers 33, 34, 35, a flexible link member 23 provided to afford operation in crooked well bores and to reduce transmission of acoustic energy to the probe housing, receivers 22, 24, 25, and a closure head 31 consisting of a lead sinker. The surface recording equipment comprises a photo-oscillographic recorder 40 and a curve-plotting recorder 41. Each transducer comprises a cylindrical magnetostrictive core carrying a toroidal winding; the core may be a nickel cylinder of stated dimensions. A sound pulse of 100 microsecond duration and frequency 15 kc/s. is transmitted three times a second and after traversing the surrounding earth formations impinges on transducers 25, 24, 22 connected to corresponding switching amplifiers 35, 34, 33. Due to reverberation effects, the output of a receiving transducer, e.g. 25, is as illustrated by Fig. 7a, and amplifier 35 modifies the signal to the form depicted in Fig. 7b in which part 44 represents distortionless amplification of the first half-cycle of the transducer output the remainder of which is suppressed immediately followed by a short pulse 45, of amplitude greater than that of possible interfering noise signals; such " spike " pulses are therefore used for measurement in preference to pulses 44. Amplifiers 33, 34 are arranged to provide outputs of opposite polarity to that of amplifier 35 and the ensuing combined pulse train transmitted to the surface is of the form illustrated by Fig. 7c. The combined pulse train, after amplification in unit 50, is transmitted via delay line 51 to one beam of a double-beam C.R. oscilloscope 52 to produce vertical C.R. beam deflexions proportional to signal amplitudes, and is transmitted to the oscilloscope horizontal sweep circuit 53 which is triggered by pulse 45. Circuit 53 also produces a C.R. beam brightening pulse of width equal to that of the horizontal sweep. The C.R. tube display is photographed by camera 54. Synchronizing circuits 13 (1) periodically couple a timing wave signal generator to the C.R. oscilloscope, (2) control the film transport in the camera at each repetition of the horizontal sweep, and (3) control a lamp so as to illiminate a counter connected to indicate the depth of the well probe, the counter being contained in the camera and being mechanically coupled to the receiver of the selsyn device driven by the probe cable sheave. Amplifier 50 output is transmitted also to a gating pulse generator 60 which produces a pulse, Fig. 7d, and a negative-going counterpart, the former being fed to gate circuits 61 and latter to a counter reset pulse generator 62. Circuits 61 are supplied with a continuous train of pulses derived from a crystal-controlled oscillator and this train is gated by pulse, Fig. 7d, to scale-of-two digital counter 64. Converter 70 produces a voltage, proportional to the number of pulses counted in unit 64, for proportional indicator 73, and produces signals for digital indicator 71 and digital recorder 72. The digital indicator is a lamp display and the digital recorder paper tape is fed in synchronism with the sheave selsyn and is punched by solenoid-operated tape punching devices in binary code, Fig. 5 (not shown). Proportional indicator 73 may be a moving-coil voltmeter with a scale calibrated in microseconds of travel time. A proportional recorder 74 is also provided and contains a motordriven slide wire pen recorder, Fig. 2b (not shown). The photo-oscillographic record produced, Fig. 2a, consists of successive traces corresponding to waveform, Fig. 7c, and to the positive-going output, Fig. 7d, of gating pulse generator 60, and occasional timing pulse traces 85. The mechanical counter depthindicating numbers, e.g. 1090, 1089 are recorded as illustrated. Transmitter pulse generator, Fig. 4. The pulses periodically energizing winding 113 of magnetostrictive transducer 26 are generated by circuit 32 in which double-base diode 100, resistor 101, capacitor 102 and semi-conductor diode 103 cause voltage pulsations across the secondary of transformer 104 which are fed to the bases of transistors 105, 106 comprising a grounded-emitter amplifier. Ensuing high A.C. voltages across transformer 107 secondary are rectified and charge capacitor 112 exponentially until cold-cathode thyratron 114 conducts and discharges the capacitor into transducer winding 113. Semi-conductor diodes 116 conduct when the oscillatory current in the winding reverses in order to quench ringing. Zener diodes 108, 109 maintain constant the voltages to diode 100 and the transistors in order to ensure a constant transmitter pulse repetition frequency. Switching amplifiers, Figs. 4, 7. Resistor 121 in conjunction with the input capacitance of triode 120 attenuates V.H.F. voltages coupled into the receiver circuits by stray magnetic fields from transmitting transducer 26. Immediately following distortionless amplification, by valves 120, 132, 133, of the first half-cycle of the burst of sine waves, Fig. 7a, to switching amplifier 35 from receiving transducer 25, the rising positive voltage at valve 120 grid is fed back via a positive feedback circuit including valves 130, 124 and 125, and capacitor 122. Valves 132, 133 are cut-off shortly thereafter by bias voltage developed across circuit 134, 135 as a result of cathode follower 123, conduction ensuing from the capacitative coupling of its grid to valve 125 anode. The circuit output, from transformer 131 secondary winding, thereby consists of an undistorted half-cycle 44, Fig. 7b, followed by a " spike " 45. It is arranged that the outputs from switching amplifiers 33, 34 are of reverse polarity thereto, Fig. 7c. Photo-oscillographic recording, Figs. 1, 2a, 7. The outputs, Fig. 7c, from the " downhole " apparatus, after amplification, is passed to deflect one beam of a double-beam C.R. oscilloscope 52 via a delay line 51; the C.R. tube incorporates a beam-splitting electrode operated at cathode potential and includes a separation-enhancing magnet. The other beam is deflection-modulated by a pulse, Fig. 7d, generated by a bi-stable circuit in unit 60 and corresponding in time to " spike " pulses 45, 47, Fig. 7c. Unit 13 periodically switches calibration markers 85 to deflection-modulate the upper C.R. beam. A selsyn 12 mechanically coupled to the sheave over which cable sheath 11 passes controls a counter registering the probe depth, the indication on which, e.g. 1090, is illuminated by a lamp energized from circuit 13 and is photographed simultaneously with the C.R. oscilloscope display. Sweep circuit 13 is triggered by spike 45, Fig. 7c, and produces a monotonic horizontal C.R. beam deflection and a beam-brightening pulse. Counting, indicating and recording, Figs. 1, 3a, 3b, 7. Square wave, Fig. 7d, opens a gate 61 and permits pulses at a known and constant repetition rate from generator 63 to pass to a known type of binary cascade counter 64 which utilizes bi-stable triggered pairs 190... 199 which are reset before each transmitting transducer emission by relay 255 disconnection of the counter H.T. supply. Each stage of the counter is connected to a corresponding stage in a digital-to-analogue converter 70; the " 1 " binary condition of a counter stage causes (1) the removal of a short-circuit across a corresponding resistor, e.g. 201, (2) the energization of a neon display lamp, e.g. 234, in digital indicator 71, and (3) the operation of a tape-punching solenoid, e.g. 235, in digital recorder 72 (see also Fig. 5, not shown). Converter 70 is connected as shown to a highimpedance voltmeter 73 having a scale indicating travel time between receiving transducers 24, 25. Analogue recorder 74 utilizes a slide wire 230 the sliding contact of which is mechanically coupled to a recording pen and to a two-phase servomotor 232 controlled by the difference in the voltages on the slide contact and at terminal 210. For recording of acoustic velocity the resistance at a point on the slide wire from earthed terminal 211 is made inversely proportional to the distance from the other end of the slide wire to that point; the chart-recording paper is moved in a direction representing probe depth by mechanical connection to a selsyn connected to sheave selsyn 12. Converter 70 is reset as follows: relay 206 is energized for a brief period after a predetermined delay introduced by mono-stable circuit 242, 243 in unit 75 the input to which is a positive voltage pulse produced in pulse generator 60, Fig. 3c (not shown), at the termination of gating pulse, Fig