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1.6 On the Magnetotelluric Anomalies
We consider models, in which the horizontally homogeneous Earth of normal
electric conductivity
N
(
z
) contains two-dimensional or three-dimensional inhomo-
geneities
z
), causing
magnetotelluric anomalies
(distortions).
Magnetotelluric anomalies carry information on geoelectric structures.
The
normal field
E
N
,
H
N
is a field observed within the host Earth in the absence
of the lateral inhomogeneities. In such a one-dimensional model the currents flow
along layers, charges do not arise, the magnetic field has no vertical component,
leading mechanism is the electromagnetic induction.
Considering the normal field, we determine the
normal impedance
,Z
N
=
(
y
,
z
)or
(
x
,
y
,
E
x
/
H
y
E
y
/
H
x
, and the
normal apparent resistivity
,
2
/
o
, that are
independent of orientation of coordinate axes and reflect vertical variations in
conductivity.
In the presence of the inhomogeneities the
anomalous field
E
A
,
H
A
appears.
Magnetotelluric anomalies caused by lateral geoelectric inhomogeneities distort
the normal impedance and normal apparent resistivity. Now we determine the
impedance tensor and oriented apparent resistivities that reflect not only vertical,
but also horizontal variations in conductivity.
Take the real Earth with three-dimensional conductivity distribution
=−
= |
Z
N
|
N
,
,
(
x
y
z
).
A
(
x
o
,
(
x
o
,
Let the apparent esistivity,
y
o
), and impedance phase,
y
o
), be obtained
at a point M
o
(
x
o
,
y
o
) on the Earth's surface. We will juxtapose these distorted
values with the
locally normal apparent resistivity
n
(
x
o
,
y
o
) and
locally normal
phase
n
(
x
o
,
y
o
) ,calculated from the
locally normal impedanceZ
n
(
x
o
,
y
o
) corre-
sponding to the one-dimensional model
n
(
z
)
=
(
x
o
,
y
o
,
z
), where
n
(
z
) is a local
conductivity-depth profile at the point M
o
.Thevalue
log
A
(
x
o
,
y
o
)
A
(
x
o
,
y
o
)
=
(1
.
101)
n
(
x
o
,
y
o
)
is a measure of distortion of the apparent resistivity. Similarly
(
x
o
,
=
(
x
o
,
−
n
(
x
o
,
.
y
o
)
y
o
)
y
o
)
(1
102)
is a measure of distortion of the impedance phase.
With strong anomalies, that is, with large
the apparent-resistivity and
impedance-phase curves depend heavily on orientation of the measurement axes.
For each observation site we can plot a lot of conflicting curves. It is evident that the
formal one-dimensional interpretation of such magnetotelluric curves is senseless.
We classify the magnetotelluric anomalies by their depth, scale, dimensionality
and physical nature.
1. The geoelectric medium can be divided into near-surface and deep parts.
According to this specification, we differentiate the
near-surface inhomogeneities
and
deep inhomogeneities
,
which cause the
near-surface anomalies
and
deep
anomalies
.
Traditionally, we associate the near-surface anomalies with sediments
(partially or in full) and the deep anomalies with consolidated crust and mantle.
,
