Patent GB-528568-A (GB)
Title: Improvements in or relating to electromagnetic apparatus for determining location of concealed bodies
IPC: G01V3/10
Filed: 1939-02-22
Abstract: 528,568. Magnetic tests and measurements. BRITISH WESTERN UNION, Ltd. Feb. 22, 1939, No. 5910. Convention date, Feb. 26, 1938. [Class 37] The depth to which a submarine or other cable or body capable of distorting a magnetic field is buried in the ground or otherwise concealed is determined by means of a plurality of search coils or a plurality of sets of coils, at least some of which have their axes spaced apart and so arranged that the curve of voltage response generated in one of the.coils or sets of coils does not coincide with the curve of voltage response generated in another coil or set of coils as relative movement is effected relatively to the cable, so that the ratio of the resultant of the generated voltage responses varies in accordance with the distance and direction of the body. Coils 48, 49, 51, 52, Fig. 5, are contained in a sled 41 which is dragged along the sea bed in which is buried a cable 43 having a sheathing of magnetic material which causes distortion of the earth's field. The coils are connected in pairs with opposite polarities so that equal and opposite E.M.F.s are generated in each pair by motion through the earth's field. When passing over the cable 43, the distorted field produces a greater E.M..F. in the nearer coil 49 than in the coil 48 so that a nett E.M.F. is produced depending upon the depth d at which the cable is buried. The second pair of coils 51, 52 is so disposed or constructed that the relation between response and depth d is different than for the first pair of coils. This may be effected by making one pair of coils of flat pancake shape and the other pair with windings concentrated at the outer diameter. The difference between the E.M.F.s generated by the two pairs of coils is a function of the depth d and is unaffected by other factors, such as speed of towing. Instead of making the two pairs of coils geometrically different, similar coils may be differently arranged, e.g. one set may be more widely separated than the other, or one set may be above and one below the axis of the sled. Two coils either similar or geometrically different may be arranged in the axial plane of the sled and separated axiallv. In a modification three coils are arranged axially, the front and rear coils being similar and with similar polarities, while the centre coil is of opposite polarity and equal to the sum of the other coils in inductive effect The rear pair of coils 51, 52, Fig. 5, may be replaced by two pairs of coils or by three coils of which the centre coil is equal and opposite in inductive effect to the other two. An additional coil or balanced set of coils may be provided in the sled and energized with direct current for establishing a magnetic field. The coils are rigidly mounted in separate pots 81, 82, Fig. 17, of non-magnetic alloy, which are completely filled with castor oil to reduce the pressure difference between the inside and outside of the pots. A sylphon bellows 83 compensates for different temperature coefficients of expansion between alloy the and the liquid in the pots. Signals generated in the sled 41 are transmitted to the ship over a messenger cable 42, Fig. 19, from which they pass through a balancing circuit 86, an interrupter and input transformer 87, an amplifier 88, a current limiting device 89, and a recorder R. As shown in Fig. 20, five coils 68, 69, 71, 72, 73 are arranged in the sled 41 and are connected in series with each other and the messenger cable 42 by which connection is made to the balancing circuit 86 in which any D.C. arising from thermo-electric, contact potential, and galvanic phenomena is balanced out by introducing a counterbalancing D.C. potential. In the balancing circuit a condenser 93 acts as a reservoir whilst the interrupter contacts are open. Two condensers 94, 96 byepass various forms of interference, and a potentiometer 97 allows an earth connection to be adjusted to give minimum noise. A potentiometer 98, low-voltage battery 99, and resistance 101 are connected across a resistance 102 to provide the balancing voltage. The interrupter and transformer circuit 87 contains cam-operated contacts 107, 108 by which the signal is interrupted at a constant frequency and applied to the two primary windings 104, 106 of a transformer 103 so as to produce a sharppeaked voltage in the secondary winding 109 which is connected to the amplifier 88 by conductors 111, 112, 113. In a modification, Fig. 21, a single contact 114 is employed, and in another arrangement, Fig. 22, a tongue 117 vibrates between two contacts 118, 119. In these arrangements the effect of contact potentials is minimized. Parallelism between a pair of coils 124, 126, Fig. 23, is secured by arranging the coil 126 so that it may be rocked on a ball 128 and tightened by nuts 129. Where more than two coils are employed in a set, only one is adjustable. In a modification, Fig. 26, two coils 133, 134 are rigidly fastened substantially parallel to one another to a frame 136 on which is also mounted a small balancing coil 138 by means of a ball joint. Alternatively two small balancing coils may be rigidly fixed at right angles to one another and to the main coils. The balancing coils are tapped so that an adjustable portion may be short-circuited. In another modification, two small balancing coils at right angles to one another are each angularly adjustable in one plane. In order to make the flux linkages of a pair of coils 159, 161, Fig. 32, equal, the coil 159 has an open conducting ring 162 connected at one end to the outgoing lead 166. An arm 163 is connected to the coil 159 and may be moved over the ring 162 so as to add or subtract any portion of a complete turn to the coil 159. In order to increase the difference between the flux cut by the two coils 169, 171, Fig. 33, of a pair, an iron or steel plate 168 may be placed between them. Specifications 452,141 and 603,326 are referred to.