GB899607A - Electromagnetic prospecting apparatus - Google Patents

Abstract

899,607. Electric tests. INTERNATIONAL NICKEL CO. OF CANADA Ltd. Oct. 21, 1958 [Oct. 24, 1957], No. 33633/58. Class 37. Apparatus for detecting and measuring conductive and magnetic bodies comprises means for transmitting at least two primary electromagnetic fields at different frequencies, means for transmitting a reference field at a third frequency, means for receiving and detecting the signals due to these fields, including the responses caused by the presence of conductive or magnetic bodies, and means for comparing the detected signal at the reference frequency with the detected signal at each of the primary frequencies. As shown, in airborne apparatus for detecting and measuring ore deposits, three A.C. sources 10, 11, 12 in an aircraft 13 are connected to transmitter coils 14, 15, 16, respectively, primary coils 14, 15 being disposed in parallel planes and the reference coil 16 being set in a plane perpendicular thereto. Primary receiving coils 18, 20 and a reference receiving coil 22 are located in a bird 17 towed by the aircraft, each primary receiving coil being coplanar with one primary transmitting coil, and the reference transmitting and receiving coils being coaxial. Source 10 operates at a relatively low frequency F1, preferably in the range 100-1000 c.p.s.; source 11 operates at a higher frequency F2, preferably between 500 and 5000 c.p.s.; the reference frequency F3 may be near one of F1, F2, or may lie between them. Receiver coils 18, 20, 22 are tuned to frequencies F1, F2, F3, respectively, and the received signals are passed to filter and amplifying circuits 24, 26, 28, the outputs of which are combined in an amplifier and modulator 30. The modulated signal is fed through a conductor 36 in the tow rope between bird and aircraft to an amplifier and detector 38, the output of which is passed to filters f1, f2, f3, which pass the primary signals and the reference signal, respectively. The reference signal from f3 is compared separately with the signals from f1, f2 in comparing circuits 46, 48, the outputs of which operate pen or magnetic tape recorders 50, 52; alternatively, the outputs of circuits 46, 48 may themselves be compared to produce a single record. One transmitting coil radiating both primary fields may replace the two primary coils; similarly, a single primary receiver coil may be used instead of two. Misalignment between transmitting and receiving coils produces an error signal in a detector coil 23 which is orthogonal to the transmitting coils when they are correctly aligned; this error signal is amplified at 29, modulated at 31 and passed to a detector 39, the output of which is fed to means 53 for recording the extent of misalignment or for re-aligning the coils. Additionally or alternatively, the electro-optical system described in Specification 899,608 may be employed. The apparatus of the invention may be mounted in a helicopter, or in a land or water vehicle. Specification 899,606 also is referred to.