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In-Depth Information
Fig. 7.38
Model of the
two-dimensional horst
1 Ohm.m
2
km
20
km
10000 Ohm.m
98
km
0.1 Ohm.m
conductance model (Figures 7.18, 7.19). Their ascending branches are close to the
locally normal curves ¨
n
, while their descending branches are drastically shifted
, they are little
different from those in the three-segment conductance model and they have a false
minimum caused by the inductive influence of excess currents concentrated at both
sides of the horst (the effect of false conductive layer).
Figure 7.40 displays the tipper curves reflecting pictorially the horst structure.
They are much like those in the three-segment model (Fig. 7.20).
Figure 7.41 demonstrates the field profiles, which pass across the horst in the
upwards (the
S
−
effect). As for the longitudinal curves,
and
y
−
direction. The electric and magnetic fields are normalized to the normal fields
E
N
E
N
H
N
y
. In many respects the field profiles replicate
those in the three-segment model (Fig. 7.24), but it is notable that in the horst model
we get the box-like
E
y
−
x
,
y
,
observed at
|
y
| →∞
profiles without side minima, so that the transverse electric
field more adequately reflects the structure of the medium than in the three-segment
model.
Now we come to the two-dimensional approximation of the three-dimensional
horst. What is the condition of quasi-two-dimensionality, which allows for the
two-dimensional interpretation of the response functions obtained over a three-
dimensional horst and in its vicinity? Reasoning from the Berdichevsky-Dmitriev
model (Sect. 7.3.3), we can suppose that the quasi-two-dimensionality condition
in the middle part of the elongated horst depends on its elongation (aspect ratio)
and conductivity contrast. It seems that the same is valid for the horst vicinity
if a distance to the horst is far less than its half-length. It can be also presumed
that with increasing frequency, the two-dimensional approximation becomes more
accurate (due to skin effect, which extinguishes the influence of the far ends of
a horst).
Consider a three-dimensional horst shown in Fig. 7.42. Let us examine a set of
models with fixed parameters
1
=
·
,
h
1
=
,
=
.
,
2
=
10 Ohm
m
1km
h
0
7km
10
3
·
,
h
2
=
,
3
=
·
v
=
.
,
Ohm
m
99 km
10 Ohm
m
and variable parameters
7
5km
