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2. Classifying the anomalies by their scale, we discern the continental, regional
and local anomalies. The effects caused by electromagnetic interaction between
oceans and continents are considered as
continental anomalies.
The effects con-
nected with influence of the first- and second-order tectonic structures (mountain
ridges and valleys, shields, crystalline massifs, platforms, vast depressions and
elevations) are designated as
regional anomalies.
These anomalies are observed
at distances numbered in the hundreds or even thousands of kilometers. Third-
order structures (minor folds, salt domes, traprocks, permafrost lenses, small-scale
inclusions) produce the
local anomalies
. These effects create mosaic pattern with
characteristic dimensions from tens of meters to kilometers. If their dimension is
much less than the measurement spacing, the local anomalies are considered as
noninterpretable
geoelectric noise
.
3. Anomalies generated by elongated inhomogeneities with great aspect ratio
(their length is much larger than their width) are considered as
quasi-two-
dimensional anomalies
(“two-dimensional” anomalies). Mathematical modelling
enables one to establish criteria for quasi-two-dimensionality that are valid in the
central part of the inhomogeneity. Anomalies that do not satisfy the quasi-two-
dimensionality criteria are taken as
three-dimensional anomalies
.
4. The most difficult is to classify the anomalies by their physical nature. We
observe complicated phenomena, in which the sources and vortices of the anoma-
lous field interact with each other. For simplicity we separate the vortex-free (poten-
tial) and vortex (solenoidal) mechanisms of the excitation of the anomalous field.
Using the terminology suggested by Kaufman (1961, 1974), we divide the anoma-
lous electromagnetic field into two parts: the galvanic (Coulomb's) part which is
generated by the excess charges and the induction (Faraday's) part which is gener-
ated inductively by the closed excess currents. The galvanic and induction parts of
the anomalous field are responsible for
galvanic distortions
and
induction distor-
tions
of the magnetotelluric and magnetovariational response functions
.
The galvanic distortions are most pronounced at low frequencies when the pen-
etration depth of the normal field far exceeds the depth and dimensions of the
inhomogeneities. Here the inductive influence of the excess currents is too weak
to reveal itself, while the galvanic part of the anomalous field obeys the direct cur-
rent laws. Inasmuch as the anomalous field is proportional to the normal field, both
the fields have the same phases and in the same ways depend on frequency. The
galvanic anomalies manifest themselves in
static shift
of the low-frequency parts
of the apparent-resistivity curves and do not affect the corresponding parts of the
impedance-phases curves. These effects extend over entire spectrum of sufficiently
low frequencies and do not attenuate even as
0. The structure of the galvanic
anomalies is vividly imaged in the current pattern: electric currents flow around
resistive zones and gather within conductive zones.
Quite different are the regularities of the induction distortions caused by the
excess currents. These effects manifest themselves at high frequencies when the
Faraday induction in the inhomogeneous medium is sufficiently intensive. Here
anomalous and normal fields have different phases and in different ways depend on
frequency. With lowering frequency the induction effects vanish. The characteristic
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