THE TWO-FREQUENCY RADIO-HOLOGRAPHIC METHOD TO VISUALIZE IN THE EARTH’S CRUST THE LOCATION OF GEOELECTRICAL HETEROGENEITIES ASSOCIATED WITH LOCAL ORE BODIES (ON THE EXAMPLE OF THE MONCHEGORSK ORE REGION, THE KOLA PENINSULA)

The radio-holographic method is an advanced tool for solving problems of ore geophysics. Holographic reconstruction of geoelectric heterogeneities in the earth’s crust allows visualizing effectively anomalous zones with high electrical conductivity associated with local ore bodies, by using areal surface observations of magnetic components of harmonic electromagnetic field from the controlled source. The visualization of the distribution of heterogeneities in the earth’s crust by using the radio-holographic method can be considered as a special case of the inverse scattering problem. When measuring electromagnetic fields at monitoring points the superposition of two fields is observed. One is the primary field from the controlled source, which, in holographic terms can be treated as a reference wave. The other ones are secondary anomalous fields caused by geoelectric heterogeneities in the earth’s crust, which are analogs of object waves. The integral equation relates the holographic reconstruction of the magnetic field H_H with unknown distribution of the density of fictitious magnetic currents j_m, which are sources of anomalous fields. The system of linear equations with respect to unknown values j_m at grid nodes is obtained after grid approximation of the integral equation. The definition of these values is the solution of the inverse scattering problem. However, the analysis of previous experimental and model results demonstrated the possibility of “false” anomalies in the holographic reconstruction of the distribution of heterogeneities. It was suggested to conduct observations at several frequencies as one of the ways for rejecting such “false” anomalies. Therefore, in 2017 extensive field studies were carried out on the use of a two-frequency radio-holographic method at the Loipishnjun site of the Monchegorsk ore region. As a result of field experiments it was shown that the two-frequency modification of the radio-holographic method significantly improved reliability of the holographic reconstruction of distribution of heterogeneities in the earth’s crust. The comparison of results of the holographic reconstruction at different frequencies allows rejecting “false” anomalies, and identifying anomalous zones that can be associated with ore bodies.

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