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In-Depth Information
Chapter 11
The Interpretation Model
Interpretation of the magnetotelluric and magnetovariational data starts with con-
structing 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 nec-
essary for constructing the adequate interpretation model we examine and remove
the static distortions of the apparent resistivities caused by near-surface inhomo-
geneities.
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 apparent-
resistiviy curves, generating false geoelectric structures.
In Part II of our topic 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
