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Magnetovariational sounding; new possibilities

Magnetovariational sounding; new possibilities

Izvestiya - Russian Academy of Sciences Physics of the Solid Earth

Geoelectric research provides unique information on the stratification of the Earth and its deep structures. A basic geoelectric method is believed to be magnetotelluric sounding (MTS), underlain by the inversion of frequency responses of the impedance tensor, which are determined from the electric and magnetic fields. The MTS data interpretation is complicated due to near-surface inhomogeneities distorting the electric field. Modern geoelectric methods allow more or less reliable identification of these distortions and their elimination. The MTS method is complemented by magnetovariational profiling (MVP) using the tipper (the Wiese-Parkinson vector), which is determined from the magnetic field alone. The MVP role reduces to the recognition and localization of geoelectric structures disturbing the horizontal homogeneity of the medium studied. However, the theorem of uniqueness proved in this paper for the 2-D magnetovariational inversion problem indicates that the tipper not only reflects the geoelectric asymmetry of the medium but also provides constraints on its stratification, i.e., enables the construction of layered inhomogeneous sections. Thus, local magnetovariational sounding (MVS) based on the straightforward inversion of frequency responses of the tipper can be applied together with the MTS. The main distinctive MVS feature is the fact that distortions of the magnetic field caused by near-surface inhomogeneities attenuate with decreasing frequency and do not spoil the information on deep structures. Evidently, local MVS allows one to avoid the difficulties inherent in the MTS due to the near-surface distortion of apparent resistivity curves, which are constructed from frequency responses of the impedance tensor. We arrive at the conclusion that MVS can substantially improve the reliability of geoelectric reconstructions. In this respect, several problems related to the combination of the MVS and MTS methods arise. In this paper, we analyze an MVS-MTS complex in which the MVS plays the role of a basic method and the MTS is employed for checking and refining MVS data. An algorithm based on the principle of successive inversions is proposed for the integrated interpretation of MVS and MTS characteristics. The paper consists of theoretical and experimental parts. The uniqueness theorem is proven in the theoretical part for the 2-D MVS inversion problem. The experimental part of the paper describes model experiments on the interpretation of MVS and MTS synthetic data and presents a geoelectric model of the Cascadian subduction zone constrained primarily by MVS data.

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