Patent GB-813577-A (GB)
Title: Apparatus for qualitative electromagnetic surveying
IPC: G01V3/165
Filed: 1956-02-10
Abstract: 813,577. Geophysical surveying. CANADIAN AIRBORNE GEOPHYSICS Ltd. Feb. 10, 1956 [Jan. 12, 1956], No. 4241/56. Class 37. Apparatus for the electromagnetic detection and analysis of conductive bodies comprises two electric generators of different frequencies, a transmitting coil in a doubly resonant circuit with the generators, or alternatively two separate transmitting coils each in circuit with one of the generators, the coil or coils being capable of producing two separate primary magnetic fields at the different frequencies, two separate receiving coils mounted with their mutual axes parallel and in circuit with separate detecting and recording means, one of said means being adapted to measure and record a characteristic of the secondary field, induced in said conductive bodies by one of the primary fields and the other one of said means being adapted to measure and record a characteristic of the secondary field induced in said conductive bodies by the other of the primary fields. The generators and the transmitting coils or coil are mounted in an aeroplane and the magnetic axes of the or each coil is vertical. The receiving coils and the detecting and recording means are mounted in an enclosure which is towed by the aircraft. In one embodiment as shown in Fig. 3 each coil is connected to a respective phase meter through a filter and each phase meter is connected to a reference coil which is placed in the vicinity of'the transmitting coil or coils. The primary fields generated by the transmitting coil or coils are distorted by secondary fields produced by eddy currents in conductive material and the resultant fields are picked up by the receiving coils and the phase meter connected to each coil indicates the change in phase caused by such a conductive body. The output of each phase meter is amplified and applied to a recorder. In the alternative embodiment shown in Fig. 4 the amplitude of the quadrature component is recorded for each transmitted frequency, Fig. 4 shows the apparatus associated with one receiving coil only and the apparatus associated with the other receiving coil is identical. The coil is connected through an attenuator Z with a phase detector 1 and the phase detector is also supplied with an inphase voltage from a reference coil positioned near the transmitting coil. The output of the phase detector 1 is amplified and used to position an attenuator Z2 which controls the magnitude of an inverted reference voltage which is applied to the phase detector 1 through the attenuator Z2. The inverted reference voltage is obtained from the reference coil through a phase-shifting circuit and the effect of this inverted input to the phase detector 1 is to maintain the in-phase component at junction A at a minimum. Hence the resultant signal at the junction A is almost entirely the quadrature component of the record signal and it is applied to a phase detector 2 which is also supplied with a quadrature reference phase through a 90 degree phase shifter from the reference coil. Phase detector 2 acts as a second stage of selection for the quadrature component and the output is amplified and recorded. The Specification explains that a graph showing the amplitude of the quadrature component of the resultant field against the conductivity of the body causing the secondary field shows that there is a peak in the quadrature component response and the use of two frequencies is necessary to resolve the ambiguity caused by this peak, by observing the ratio of the two responses.