1887

Abstract

Early in the nineties, the effect decreasing sharply efficiency of most popular and widely used TEM (Transient Electromagnetic Method) method of electrical prospecting was described in a number of publications. Especially intensively this effect manifested itself in cases when close located or coincide transmitter and receiver loops were used. Detected process of induced current attenuation contained the component with small speed of decay which partially or fully masked the useful signal. The study of rock samples producing this effect permitted to connect the measured process with the phenomena of magnetic viscosity also known as superparamagnetic effect - SPM. SPM effect is caused by the peculiarities of magnetization dynamics of ferromagnetic particles having size of order of the first hundreds of angstrom. In rocks, SPM effect is mainly connected with availability of thin-dispersive grains of magnetite, maghemite, limonite and other minerals. After switching off external magnetic field, magnetic moments of these particles oriented in exciting field relax in course of time to chaotic state according to the logarithmic law. In TEM technology the detectors register time derivative of magnetic field and SPM effect has the decay time like ~ 1/t . The results of investigations presented in the present paper were received when studying the SPM-effects produced by subsurface samples of terrigene sediment rocks and soils bedded above gold and nickel containing ore deposits. The main objective of the investigations was to study correlation between SPM anomalies and spatial disposition of ore objects. TEM-FAST ProSystem device manufactured by AEMR Ltd. and specialized induction sensor was used. In the hollow of the sensor as a core, friable samples of investigated rocks were placed. These samples were mixture of sandy-clay deposits and coils selected from subsurficial layer at the depth smaller than 15 cm. Magnetic field with ~200 A/m magnitude was created in a sample by a pulse current flowing through the sensor’s winding. The time decay voltage induced in this winding after switching off the primary current was measured during 1000 μs. The following parameters were determined using H’(t) transient characteristic: a) superparamagnetic susceptibility χSPM characterizing the “energy” of SPM effect; b) anomaly part of H’(t) process - research parameter (d); c) integrated "energy" of IP effect (Induced Polarization) characterizing electrochemical properties of rocks. Case of histories has two examples. First example presents the data received along profile crossing gold placer deposit (Chelyabinsk region, Russia). Close correlation between χSPM and χstat distribution testifies to the fact that relation of ferromagnetic grains concentration which is under SPM state, as well as larger particles, change feebly along profile being in the limits of 1:3 - 1:4. However, these parameters don’t correlate in space with deposit zones and their nature, probably, is not connected directly with gold placers. IP effect’s distribution does not correlate with ore zones and depends on water saturation of rocks: as soon as the samples get dry, IP-effect disappears.

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/content/papers/10.3997/2214-4609.201406479
1999-09-06
2020-07-15
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