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Chapter 7
Models of the Near-Surface Distortions
We will consider three kinds of near-surface effects distorting the magnetotelluric
field: (1) effects of small-scale inclusions located at shallow depth or even out-
cropped, (2) effects, caused by variations in the conductance of sediments underlaid
with resistive basement, and (3) effects caused by structures in the basement topog-
raphy. Our concern is to gain a more penetrating insight into physical mechanisms
of distorting effects.
7.1 Distortions Caused by Small-Scale Near-Surface Inclusions
This kind of distortions will be referred to as
-
effect
. Let us examine two-
dimensional and three-dimensional
-effects.
7.1.1 The Two-Dimensional
-Effect of the Semicylinder
and Prism
A model of the two-dimensional
-effect that provides simple analytic solution is
presented in Fig. 7.1. The model consists of three layers: the sediments (
1
,
h
1
),
1
,
the resistive lithosphere (
3
). The
sediments contain an outcropped inclusion in the form of an infinitely long horizon-
tal semicylinder (
2
>>
h
2
>>
h
1
), and the conductive mantle (
1
,
a
<<
h
1
). Assume that at the periods under consideration the
effective penetration depth
h
eff
is many times larger than the semicylinder radius
a
.
So, considering the anomalous field, caused by the inclusion, we can neglect the
induction effects and reduce the problem to the direct-current statement demon-
strating the galvanic effects in the TM-mode.
Introduce a cylindrical coordinate system
r
x
with
x
being the axis of the
semicylinder. Ignoring influence of lithosphere and mantle, we determine the scalar
potential of the electric field in the Earth as
,
,
