Improvements in or relating to well logging by nuclear radiation

摘要

1,002,540. Gyromagnetic resonance apparatus. CALIFORNIA RESEARCH CORPORATION. Aug. 24,1961, No. 30547/61. Heading G1N, In a free precession nuclear magnetic resonance well logging arrangement, remanent fields arising from metallic deposits or metal in the drilling fluid on removal of the polarising field are compensated or eliminated by the application of reverse or diminishing alternating fields respectively, subsequent to the switch off of the polarising field. In the arrangement of Fig. 1 application of the polarising field by coil 12, which serves also to pick-up the resonance signal is controlled by push-button 44 through a relay 16, contacts 18 of which connect the polarising battery 39 in one position and compensating battery 36 in the other, the precession signal being applied to oscilloscope 51 through blocking condensers 48, 49 and an amplifier 47. Experiments to determine the strength of the compensating signal by applying over-compensating signals are described. In an alternative arrangement an additional threeposition double-pole switch may select for connection to the coil, the compensating voltage, a relaxation voltage, or neither. In a further alternative arrangement described with reference to Fig. 7 (not shown) a capacitor is connected across the polarising coil so that an oscillating current is set up at switch off, the frequency of oscillation being substantially higher than the expected former frequency and the decay time being sufficiently short that precession may be detected. A thyrite resistor or a plurality of cold cathode tubes may be shunted across the coil to control the decay time. A multiple switching arrangement for periodically or cyclically reversing the polarising current flowing in the field coil after initial interruption of the main current source is described with reference to Fig. 12 (not shown).