GB745873A - Analysis of substances by measurement of nuclear magnetic relaxation times - Google Patents

Abstract

745,873. Automatic frequency control systems. SCHLUMBERGER WELL SURVEYING CORPORATION. Jan. 13, 1954 [Jan. 13, 1953], No. 1019/54. Class 38 (4). [Also in Group XXXVI] Apparatus for analysing substances having atomic nuclei embodying magnetic properties comprises means for subjecting the substance to a constant magnetic field in the presence of a second field alternating at the frequency of precession of the nuclei, and means for determining the relaxation time characteristic of the nuclei, which is indicative of the composition of the substance under test. As shown (Fig. 1) for analysing a sample contained in a non- magnetic capsule 10, the constant magnetic field is produced by a permanent magnet 13, and the alternating field is produced by a winding 11 fed by an oscillator 12. The constant field is modulated at 500 c.p.s. by windings 14, 15 fed from a 500 c.p.s. oscillator 16, causing a corresponding modulation in the voltage across coil 11, which is amplified at 17, rectified at 18 and indicated at 19. To maintain the frequency of oscillator 12 at the precessional frequency of the test material for a given constant field, the voltage across coil 11, which contains only 1000 c.p.s. modulation at the correct frequency and a 500 c.p.s. component at any other frequency, is fed via amplifier 20 to a phasesensitive detector 21 obtaining a reference signal direct from the 500 c.p.s. oscillator 16, so that a reactance tube 22 is controlled to bring the oscillator frequency to the required value. The amplitude of the output voltage of oscillator 12 is also maintained at the optimum value by feeding the 1000 c.p.s. signal in amplifier 20 to a phase-sensitive detector 27 fed with a reference signal from 500 c.p.s. oscillator 16 through a frequency doubler 28, the output from detector 27 being a voltage proportional to the 1000 c.p.s. signal across coil 11, and having a peak value at the optimum voltage of oscillator 12. Thus, when modulated with a 50 c.p.s. voltage from an oscillator 30, a 100 c.p.s. frequency signal is obtained at the optimum value, and a 50 c.p.s. signal at other values, the latter being fed to a phase-sensitive detector 33 which thus supplies a control voltage to the oscillator 12. With the output of oscillator 12 controlled as above for frequency and amplitude, the voltage indicated at 19 is shown to be inversely proportional to the square root of the relaxation time of the nuclei, and therefore indicative of the composition of the test sample in the capsule 10. For analysing the substances located in the borehole of a well the capsule 10, coil 11, magnet 13 and modulating field coils 14, 15 are mounted in a cushion member 50 (Fig. 5) urged against the sides of the bore-hole by springs 52, 55. The diameter of coil 11 is made large to allow the associated magnetic field to penetrate an appreciable distance into the walls, and the magnet 13 comprises an electromagnet with a plurality of poles P<SP>1</SP>-P<SP>8</SP> to make the field distribution at the capsule 10 similar to that in the walls penetrated by the alternating field from coil 11. In another construction (Fig. 5A) for deeper penetration into the walls of the bore-hole, the constant magnetic field is produced by D.C. fed coils 92, 93, and the modulation by A.C. fed coils 14<SP>1</SP>, 15<SP>1</SP>, all the coils being wound on magnetic cores 88, 89 joined by a hollow magnetic casing 87. When the apparatus is lowered into the bore-hole the indicator 19 shows the optimum voltage V 1 (Fig. 6) for water until the oil level is reached, when a new optimum voltage V 1 <SP>11</SP> is shown, corresponding to the curve R which is the algebraic sum of the response curve 29a for water and 34a for oil. The output of phase-detector 27 may be photographically recoided on a strip moving at the same rate as the apparatus is being lowered into the well, or it may be indicated on a cathode-ray oscilloscope. The apparatus may be combined with means for measuring the depth in the bore-hole.