Patent GB-904012-A (GB)
Title: Improvements in or relating to bore hole logging apparatus
IPC: E21B47/00
Filed: 1958-08-21
Abstract: 904,012. Recording-apparatus. SUN OIL CO. Aug. 21, 1958 [Sept. 10, 1957], No. 26979/58. Class 106 (4). [Also in Groups XXXVI and XL (c)] In borehole lagging equipment (see Group XXXVI) the value of a physical property is first recorded on magnetic tape, the first record then being transferred to photographic film. Measuring head construction.-Fig. 1 shows a hollow borehole drill 2 containing a casing 11 carrying a flexible probe 13. The casing 11 is mounted so that when the drill 2 is raised the casing 11 can slide within the drill 2, the probe 13 projecting some 20 ft. or more through a tube 6. The flexible probe 13 is terminated in a rubber ball 15 and may carry a number of electrode surfaces (as in Fig. 2, not shown) or capacitive or inductive couplings. Resistance and voltage measurement.-Fig. 3 shows a circuit of the apparatus contained within casing 11. The output of a 100 c/s. oscillator 70 drives a constant current through the earth via an electrode 78 and the drill 2 (shown in Fig. 3 as an earth symbol). The oscillator output is also fed to a potentiometer 84 driven at a constant speed by a motor 54. The voltage at electrode 90 is proportional to the earth resistance and is of opposite polarity to that at the slider of potentiometer 84. The voltage at point 96 will thus reach a null at one point in each rotation of potentiometer 84 and will have opposite polarity on either side of the null. The voltage is converted into a square wave by means of the limiting amplifier 100 using silicon transistors. A square wave of similar amplitude is provided from the oscillator 70 output by means of transistor 106. When the two square waves are added the waveform at point 118 will consist of a large amplitude square wave on one side of the null point followed by one of much smaller or zero amplitude. Since the null point relative to the rotation of potentiometer 84 is dependent on the amplitude at electrode 90, the number of large amplitude square pulses in each train must depend on this amplitude. This waveform is recorded on magnetic tape 62. Voltages at other electrodes such as 122 are similarly recorded. Direct voltages present in the earth may be determined by electrode 122. The component of oscillator 70 is removed by a filter 142, after which the direct potential is compared with the output of a similar motor-driven potentiometer 148 but fed from batteries 154. The sum of voltages at electrode 122 and on the slider of potentiometer 148 is converted to alternating current by the chopper 164 driven by the oscillator 70 and converted to pulse train width modulation by means of amplifier 172 as before. Borehole logging.-The time switch 52 is set to start the apparatus at some predetermined time. While the drill is lifted clear of the foot of the borehole, the apparatus in the casing 11 is lowered or pumped down the drill shaft with the probe extending beyond the bit. The drill is then raised to the top of the borehole so that the earth characteristics are recorded. Time signals from the time switch 52 may be recorded on the magnetic tape 62. By recording the rate at which the drill is raised the data on the tape may be related to depth. Making photographic film records from magnetic tape (Fig. 4).-The data on the magnetic tape 62 is in the form of trains of pulses the length of each train being indicative of the physical state of the earth. Each pulse train is taken from the reproducing head 196, rectified at 197 and differentiated at 198 to provide a trigger pulse for a saw-tooth generator 202 which provides a horizontal sweep on the cathode-ray tube 192, each sweep starting a predetermined time after the beginning of each pulse train. The pulse train output is also fed via an intensity circuit 208 which brightens the cathode-ray tube trace for the duration of each train. The cathode-ray tube display is recorded on a photographic film 190 providing the graph shown. Time signals are picked up by the reproducing head 222 and cause a glow lamp 228 to flash so providing time markers 236 on the film. Specification 904,013 is referred to.