848,428. Electric tests. CANADIAN AIRBORNE GEOPHYSICS Ltd. Nov. 12, 1957 [Dec. 4, 1956], No. 35239/57. Class 37 Apparatus for geophysical cryoloration comprises a pair of transmitting coils mounted at right angles to one another, a pair of receiving coils similarly mounted, means feeding the coils alternately with A.C. and means for indicating the difference between the signals in the receiving coils at corresponding intervals. The transmitting coils may be carried on one aircraft while the receiving coils are in a case towed behind or in a different aircraft or a case towed behind it. In the arrangement shown in Fig. 2 the A.C. source is connected by switch 4 alternately to transmitting coils 2, 3. Receiving coils 2SP1/SP 3SP1/SP detect the fields of the corresponding coils in the absence of disturbing bodies and their outputs are amplified by amplifiers 19, 18 respectively. Imperfect alignment of the receiving with the transmitting coils may mean that receiving coil 2 1 will receive a signal from transmiitter 3, and 3SP1/SP from 2. This is avoided by switches 17, 17SP1/SP coupled to switch 4 to disable the appropriate amplifier channel when the other coil is receiving a pick-up signal. In addition coils 2SP11/SP, 3SP11/SP, near the transmitting coils combine their signals with the outputs of the receiving coils so as to eliminate unwanted pick-ups. The outpots E 1, Ex may be rectified and fed with appropriate signs to a long time constant indicator which indicates their difference. Alternatively, as shown, the difference between the two signals is split in to in-phase and out-of-phase components at 7, 8 using the A.C. source 1 as a reference. In the arrangement of Fig. 1 (not shown) the outputs of the receiving coils are fed through non-linear amplifiers which eliminate the small signals produced by unwanted coupling when the other coil of the pair is energized.
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