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Chapter 8
Models of Deep Geoelectric Structures
In the preceding chapter, we studied the near-surface magnetotelluric anomalies
caused by geoelectric inhomogeneities in the sediments. Now we turn our attention
to the deep geoelectric inhomogeneities located in the consolidated Earth's crust and
the upper mantle.
Figure 8.1 shows generalized resistivity-depth profiles for stable (SR) and active
(AR) regions, compiled from geothermal and geoelectric data as well as from lab-
oratory measurements (Vanyan, 1997). In stable regions, the resistivity decreases
monotonically from 10
4
10
5
mat
depths of the order of 400 km. This global resistivity decay is conditioned by the
gradual warm-up of the Earth's interior and its phase changes. The total resistance
of the lithosphere is about 10
9
÷
Ohm
·
m near the Earth's surface to 10 Ohm
·
m
2
in
active regions (Kuvshinov, 2004). Two local minima of resistivity can be recognized
against this background in active regions, one in the crust (C), and the other in the
upper mantle (A). The resistivity minimum in the Earth's crust is explained by the
fluidization (
m
2
10
8
Ohm
·
in stable regions and 3
×
Ohm
·
m) of
crystalline rocks. The resistivity minimum in the upper mantle is caused by astheno-
spheric partial melting (
=
10
÷
250 Ohm
·
m) or the graphitization (
=
0
.
1
÷
100 Ohm
·
m) and hydrogen diffusity. Studying the
deep conductive anomalies, C and A, we can obtain an unique information on
the fluid regime, petrophysics, reology, thermodynamics and geodynamics of the
Earth's interior.
Keeping this in mind, we examine models of three kinds: (1) models of crustal
conductive zones, (2) models of asthenosphere conductive zones, (3) models of deep
conductive faults.
=
10
÷
50 Ohm
·
8.1 Models of Crustal Conductive Zones
Figure 8.2 demonstrates the one-dimensional apparent-resistivity curves calcu-
lated for crustal conductive layers located at a depth of 20 km, their conductance
being two-four times greater than the sediments conductance. Under these condi-
tions, the crustal conductors display themselves in vast minima or bendings of the
