Patent GB-746114-A (GB)
Title: Methods and apparatus for the nondestructive testing of materials
IPC: G01V11/00
Filed: 1954-02-05
Abstract: 746,114. Magnetic and electric tests. SCHLUMBERGER WELL SURVEYING CORPORATION. Feb. 5, 1954 [March 9, 1953], No. 3524/54. Class 37. [Also in Group XXXVIII] A method of measuring the strength of a magnetic field particularly adapted for investigating the constituents of the walls of a borehole, comprises detecting the magnetic resonance condition between a free radical and a constant magnetic field in the presence of an alternating magnetic field. For measuring the strength Ho of a magnetic field, e.g. the earth's field, a non-magnetic container 10 (Fig. 1) containing the free radical tris-para-nitrophenyl methyl is placed in the magnetic field and supplied with a highfrequency alternating field by a coil 11 surrounding the container 10 and fed from an oscillator 12. At the magnetic resonance frequency fo, indicated by a drop in voltage across the coil 11, the relationship fo=2.8 Ho exists, and hence Ho can be determined. To measure variations in the field strength, as produced by moving to different locations, a measured auxiliary field is supplied by coils 13, 14 to maintain the resonance frequency at a fixed value, the additional magnetizing current being indicative of the change in field strength. In an alternative arrangement (Fig. 3) the constant field Ho is modulated by a superimposed L.F. alternating field Ha produced by coils 13, 14 when fed from an. oscillator 15. The L.F. component in coil 11 is fed to a phase-sensitive detector 18 together with a reference voltage from the oscillator 15 to give a D.C. voltage indicative of the magnitude and direction of the change in field strength Ho. In a further arrangement (Fig. 5) the output of the phase-sensitive detector 18 is used to control the fiequency of oscillator 41 feeding coil 11 at the resonance frequency via a reactance valve 44, so that a change in field strength Ho produces a change in the resonant frequency of the free radical and hence a change in the output frequency of oscillator 41, and the change is determined by mixing this new frequency output with that of a fixed frequency oscillator 12 and indicating the beat frequency on a meter 19. To investigate the paramagnetic constituents in the walls of a bore-hole by measuring the variations in the vertical component of the earth's magnetic field, the equipment is contained in a non-magnetic housing 50 (Fig. 2) adapted to be lowered in the bore-hole 51 by a cable 52. By rotating the coils 13, 14 through 90 degrees the horizontal component of the earth's field may be investigated. To perform the investigation by measuring the magnetic retentivity of the earth formations traversed by the bore-hole, the apparatus consists of a strong magnet 70 (Fig. 6) with its poles disposed horizontally, attached by an inextensible, flexible rod 75 to the magnetometer apparatus with coils 13, 14 disposed horizontally on either side of coil 11, the whole apparatus being urged against the side wall 73 of the bore-hole by a bowed spring arm 79. Either apparatus may be associated with a simultaneous depth recorder. The apparatus may also be used to examine the walls of a bore-hole which has been cased, or to locate a lost section of drill pipe, or to examine samples of material, e.g. well cuttings, placed in juxtaposition to the container 10. Determining physical qualities and dimensions.-The above magnetometer principle is used to detect variations in the dimensions &c. of a foil or ribbon 93 (Fig. 9) by passing the ribbon through a pre-adjusted gap 91 in a magnet 90 producing the constant magnetic field for the free radical in container 10, changes in the magnetic resonance frequency due to variations in the ribbon being determined with the aid of field modulating coils 13, 14. The ribbon may be magnetic, or conductive or of insulating material when the presence of foreign magnetic matter is indicated The same principles are also employed in the construction of a seismometer (see Group XXXVIII).