Patent GB-602335-A (GB)
Title: Improvements in method and apparatus for removing random fluctuations from intensity measurements
IPC: G01V5/04
Filed: 1945-03-20
Abstract: 602,335. Voltage measurements ; determining physical qualities of materials. TEXACO DEVELOPMENT CORPORATION. March 20, 1945, No. 7006. Convention date, March 4, 1944. [Class 37] [Also in Group XIX]. A 'voltage having an average value that varies and in addition is subjected to random fluctuations is impressed upon a resistance and capacitance connected in series so that the random fluctuations are smoothed out, the product of the resistance and capacitance determining the time constant of the smoothing circuit ; on the occurrence of a change in the average value of the voltage this time constant is adj usted so that the voltage changes are indicated without being affected by the smoothing circuit. Fig. 1 shows apparatus for determining the nature and location of underground strata 12, 14, 15 by passing through a bore-hole 10 traversing the strata a radiation detector 24 for measuring the natural radioactivity of the strata. The gamma radiation detector 24 may be a Geiger-Muller counter or a proportional counter and is contained in a casing 18 together with a pre-amplifier 26 arranged to amplify its output. The casing 18 is suspended in the hole by means of a cable 20 passing over a pulley or drum of a device 22 for indicating the depth of the instrument in the hole. The output of the pre-amplifier 26 is passed through the cable 20 to an amplifier 28 at the surface connected through an integrator 30 to a recording galvanometer 32. The record obtained as the instrument 18 is lowered through the bore-hole indicates changes in radioactivity due to the different nature of the various strata and also includes irregularities due to the statistical laws of radioactive emission. These irregularities are smoothed out in the integrator 30 comprising resistances and capacitances, which may include three resistance-capacitance circuits 48, 50, 52 in series as shown in Fig. 5. The product of the resistance 54 and capacitance 56 of the circuit 48 is such as to provide a time constant of onehalf second, the time constants of the circuits 50, 52 being 2 seconds and 20 seconds respectively. The voltage from the first circuit 48 is amplified by a D.C. amplifier 66 before passing to the second smoothing or integrating circuit, 50. A double-diode rectifier 68 is connected across the resistance 58, batteries 70 being connected in series with the cathodes so that the rectifier does not become conducting until the voltage across the resistance 58 becomes greater than the voltage of the batteries. Similarly, a double-diode 72 with batteries 74 is connected across the resistance 62. Whilst the instrument 18 is traversing the homogeneous formation 12, the output of the detector 24 is substantially constant except for statistical fluctuations. These are eliminated by the high time-constant circuit 52 and a straight trace is obtained. When the detector 24 j ust reaches the boundary of the formation 14 in which the radioactivity is slightly higher than in the formation 12, the output voltage of the circuit 52 increases and causes the diode 72 to conduct and short-circuit the resistance 62, thereby rendering the time constant of the circuit 52 ineffective so that the instrument 32 records a pronounced hump. When the detector 24 reaches the upper boundary of the formation 15, in which the radioactivity is considerably higher, the diode 68 conducts and short-circuits the resistance 58 so that the time constant of two seconds is no longer in effect. A 'sharp rise of voltage occurs across the resistance 62 causing operation of the diode 72 which short-circuits the resistance 62. The time constant of one-half second of the circuit 48 then comes into effect and a pronounced peak is recorded. The detector 24 may alternatively include an ionization chamber, the D.C. current through which modulates the amplitude of a carrier wave. The modulated wave is sent to the surface and the D.C. component is separated in the amplifier 28. This component is passed through the circuits 48, 50, 52 to the recording instrument 32. The Specification as open to inspection under Sect. 91. comprises also a mechanical device shown in Fig. 7 (Cancelled), for preventing random fluctuations from appearing in a record. The indicating pointer 86 of an instrument such as a recording galvanometer is pivoted at 88 and supports by means of springs 94 a hollow block 90 to which is fixed a pen or mirror 92. Slight vibrations of the pointer 86 are damped out by the springs 94 owing'to the inertia of the block 90. The amplitude of the fluctuations to be suppressed may be controlled by changing the width of the slot 95 in the block 90 or by moving the block along the pointer 86. This subject-matter does not appear in the Specification as accepted.