This paper presents a novel numerical method for simulation controlled-source audio-magnetotellurics (CSAMT) and radio-magnetotellurics (CSRMT) data. These methods are widely used in mineral exploration. Interpretation of the CSAMT and CSRMT data collected over an area with the complex geology requires application of effective methods of numerical modeling capable to represent the geoelectrical model of a deposit well. In this paper, we considered an approach to 3D electromagnetic (EM) modeling based on new types of preconditioned iterative solvers for finite-difference (FD) EM simulation. The first preconditioner used fast direct inversion of the layered Earth FD matrix (Green’s function preconditioner). The other combined the first with a contraction operator transformation. To illustrate the effectiveness of the developed numerical modeling methods, a 3D resistivity model of Aleksandrovka study area in Kaluga Region, Russia, was prepared based on drilling data, AMT, and a detailed CSRMT survey. We conducted parallel EM simulation of the full CSRMT survey. Our results indicated that the developed methods can be effectively used for modeling EM responses over a realistic complex geoelectrical model for a controlled source EM survey with hundreds of receiver stations. The contraction-operator preconditioner outperformed the Green’s function preconditioner by factor of 7–10, both with respect to run-time and iteration count, and even more at higher frequencies.
The classical radio-magnetotelluric (RMT) method is nowadays routinely applied to various environmental, engineering, and exploration problems. The technique uses radio transmitters broadcasting in the frequency range of 10 kHz to 1 MHz, and the measurements are carried out in the far field. The well-known disadvantages of RMT are a lack of robust radio transmitters in remote areas; the absence of transmitters broadcasting below 10 kHz, which limits the penetration depth; and a possible low signal-to-noise ratio. To overcome these difficulties, controlled sources can be used (controlled-source RMTs [CSRMTs]). We extend the CSRMT method to perform measurements not only in the far field but also in the transition zone. In CSRMT practice, it often is challenging to maintain far-field conditions for logistical reasons. Therefore, part of the measured data contains signatures of the source field, which cannot be interpreted with magnetotelluric software. In addition, the source placed directly in the survey area allows us to increase the signal-to-noise ratio and resolution. Such CSRMT in the transition zone is, in fact, a controlled-source electromagnetic method but with full impedance tensor and tipper vector transfer functions. We develop new procedures for the 3D modeling and inversion of the tensor radio-frequency data measured in the transition zone of two perpendicular horizontal electric dipole sources. In this case, the geometry of the source must be considered in the forward modeling. The developed modeling and inversion software is tested on a synthetic 3D model. The 3D resistivity models derived from the real data confirm the geologic settings and are consistent with the available borehole information. Therefore, we conclude that the CSRMT approach extended to include the source field is feasible and that the developed procedures are reliable.
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