GB921100A - Improvements in or relating to well logging by nuclear radiations - Google Patents

Abstract

921,100. Neutron well-logging. CALIFORNIA RESEARCH CORPORATION. March 6, 1959, No. 7890/59. Class 40 (3). [Also in Group XL (c)] Apparatus for determining the nature of the constituent elements in an earth formation traversed by a well bore comprises a neutron source adapted to irradiate the earth formation, a gamma-ray detector above the neutron source, a first shielding means having a thermalneutron capture cross-section less than 0.05 barns and a mass number exceeding 200, and arranged to shield the detector from gammarays emitted directly by the neutron source, second shielding means having a thermalneutron capture cross-section exceeding 200 barns, arranged to shield the detector from thermal neutrons diffused within the earth formation, and disposed between the first shielding means and the detector, means for converting the dissipated energy of neutroncapture gamma-rays detected to electrical pulses which are proportional in magnitude to that energy, and recording means for determining in relation to the depth of the neutron source and gamma-ray detector in a well bore, the frequency of repetition in a given time of one or more of electrical pulses having a characteristic magnitude. The first shielding means is preferably of bismuth and the second of boron. In the embodiments the gamma-ray detector is a thallium-activated sodium iodide crystal, preferably not more than one cubic inch in volume, the scintillations of which are detected by a photo-multiplier (p.m.) tube; the crystal and tube are insulated from high well bore temperatures, which would cause " dark current " in the tube, by an ice-loaded Dewar flask. The p.m. tube pulse output may be transmitted to the surface, after heightanalyzing in the well log housing, as pulsefrequency-modulated sub-carriers which are demodulated at the surface before recording takes place, Figs. 1 and 2 (neither shown). Or, in order to permit the use of a narrowbandwidth cable, pulses are lengthened in unit 60, Fig. 21, after amplitude discrimination in unit 58 to eradicate low-amplitude pulses, interference by incoming pulses during the pulse-lengthening process being avoided by unit 59 which acts to remove the H.T. voltage from discriminator 58 after a short delay introduced by a monostable multivibrator which is triggered by the discriminator output. The cable output is applied to the vertical deflection plates of cathode-ray tube 62 to the horizontal plates of which are applied timebases triggered by successive cable output pulses. Circuit arrangement for pulse discrimination and stretching. Figs. 22, 23. Input pulses are applied to discriminator 58, Fig. 22, via an R.C. networkin corporating variable resistor 66 which permits adjustment of the pulse amplitude below which no pulses are transmitted on line 85. The output of cathode follower 67 is applied to the pulse-stretching circuit, Fig. 23 and to a short-delay multivibrator 70, 71 via amplifier 69. The multivibrator output negative pulse is differentiated and the positivegoing pulse so formed triggers a " dead-time " multivibrator 76, 77; capacitor 78 and resistors 79, 80 may be varied to control the duration of the " dead-time " and, utilizing valve 81, H.T. is removed from valve 67 for this period. The incoming pulse on line 85 of the pulse-stretching circuit, Fig. 23, charges capacitor 88 to a potential equal to the pulse amplitude; concurrently, the input pulse is applied to valve 86 to cause it to conduct and the ensuing negative pulse at its anode is applied to cut-off valve 90 which remains cut-off for a period adjustable by means of variable resistor 91. At the end of this period capacitor 88 discharges. Valves 87 and 92 form the output stage, the latter acting as the cathode resistor of the former. The amplitude of the output pulse is substantially equal to the amplitude of the input pulse on line 85. In one embodiment of the sonde, Fig. 3, the detector crystal 10 and p.m. tube 11 are shielded from pollonium-beryllium neutron source 36 by bismuth block 35, bismuth shield 37 and boron or boron carbide shield 38; the Dewar flask is provided at 13A. Aluminium shell 39 is internally coated with a layer of magnesium oxide for reflecting all the crystal-developed light. In another embodiment, a monitor cathode-ray tube and associated camera are disposed in the sonde, Fig. 4 (not shown). In a further embodiment, Fig. 15, a neutronabsorbing material 52, e.g. comminuted boron or boron carbide bonded to a latex, is adhered to housing 34; the effects of thermal neutroncapture gamma-rays from iron or steel housings is thereby reduced. Or borax may be added to the well fluid. An inner coating 54 of a material containing boron may be added. A discussion of energy spectra resulting from the use of the sondes is given, Figs. 5-14 and 16-18 (none shown). The neutron source may be a radium-beryllium mixture. Reference has been directed by the Comptroller to Specification 743,449.