The Interpretation Model
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The Interpretation Model
Interpretation of the magnetotelluric and magnetovariational data starts with constructing an interpretation model. It is composed on the basis of a priory geological and geophysical information and plausible hypotheses. And great consideration is given to the analysis of magnetotelluric and magnetovariational response functions and their rough tentative inversions. Summing up different evidences (lithologic, petrophysical, tectonic, geodynamic, seismic, geothermic, geoelectric), we create reasonable qualitative image of the medium under consideration and constrain a model class, within which the solution of the inverse problem is sought for. The interpretation model should take into account the aim and the area of the search, that is, designate the target objects, which we are going to find, and characterize (at least presumably) the host medium, in which these objects are to be found. Just at this stage of interpretation we form the correctness set (a set of geophysically meaningful solutions) and transform the ill-posed unstable inverse problem into a conditionally well-posed stable one. Just at this stage of interpretation we compose an optimal grid for the inversion of field data, choose the normal background and starting values of resistivities (conductivities), suggest the strategy of solution of the inverse problem. To get the trustworthy and sufficiently complete magnetotelluric indications necessary for constructing the adequate interpretation model we examine and remove the static distortions of the apparent resistivities caused by near-surface inhomogeneities.
11.1 Analyzing the Static Distortions The static distortions caused by near-surface inhomogeneities extend over the whole low-frequencies range and severely plague the interpretation of the apparentresistiviy curves, generating false geoelectric structures. In Part II of our book we considered a few models illustrating two kinds of the static distortions: the S−effect caused by variations in the conductance S of the upper layer underlaid with resistive bedding and the −effect caused by small M. Berdichevsky, V.I. Dmitriev, Models and Methods of Magnetotellurics, C Springer-Verlag Berlin Heidelberg 2008 DOI 10.1007/978-3-540-77814-1 11,
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11 The Interpretation Model
surface inhomogeneities of higher or lower resistivity . Now we are going to review the current state of the art in this field and consider some techniques for correcting the apparent-resistivity curves distorted by static effects. Static effects are observed in the period range T >Ts , where the skin-depth is much larger than the dimensions of the inhomogeneity. At these periods the local induction dies out and galvanic anomalous field caused by charges becomes quasi-static. Anomalies of this kind manifest themselves in the vertical shift of the bilogarithmic apparent-resistivity curves. The shape of the shifted curves and corresponding section of the phase curves remain unchanged. The initial period Ts depends on the dimensions and position of
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