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surface inhomogeneities of higher or lower resistivity
. Now we are going to review
the current state of the art in this field and consider some techniques for correcting
the apparent-resistivity curves distorted by static effects.
Static effects are observed in the period range T > T s , where the skin-depth
is much larger than the dimensions of the inhomogeneity. At these periods the
local induction dies out and galvanic anomalous field caused by charges becomes
quasi-static. Anomalies of this kind manifest themselves in the vertical shift
of the bilogarithmic apparent-resistivity curves. The shape of the shifted curves
and corresponding section of the phase curves remain unchanged. The initial
period T s depends on the dimensions and position of the causative inhomogene-
ity (Berdichevsky and Dmitriev, 1976; Jones, 1988; Vozoff, 1991; Singer, 1992;
Weaver, 1994; Zhdanov and Keller, 1994; Berdichevsky and Dmitriev, 2002).
11.1.1 Recognising the Static Distortions
To recognize and evaluate the static shift of the apparent-resistivity curves
A ,we
need some references. It would be natural to measure the static shift of
A -curve
from the locally normal
n -curve calculated for the observation site. Unfortunately
such estimation is possible only in theory. In practice, we can correlate adjacent
A -curves or use some references derived from frequency, transient or magnetovari-
atonal soundings.
Let us start with the S -effect. Figure 11.1 shows a two-dimensional model
composed of an inhomogeneous upper layer (sediments) underlaid with a hori-
zontally homogeneous layered substratum (the crust and upper mantle). The sed-
iments contain several 20 km wide sections with stepwise twofold increase in the
resistivity. The sediments conductance varies from 100 to 2.5 S. The substratum
consists of a thick resistive strata (the lithosphere) and a highly conductive base-
ment (the asthenosphere). The resistive lithosphere includes a crustal conductive
layer.
The transverse apparent-resistivity curves obtained in this model are presented
in Fig. 11.2. They have two maxima which are separated by a distinct minimum
reflecting the crustal conductive layer. As the upper-layer conductance decreases
from 100 S at site 1 to 2.5 S at site 6, the
-curves with their ascending and
descending branches move conformally upwards, holding the same two-humped
shape. But note that the ascending branches which carry information on the upper-
layer conductance merge with the locally normal
n -curves. These branches are
slightly distorted. Their one-dimensional inversion gives adequate values of S .
At the same time the descending branches carrying information on the conduc-
tive basement are dramatically distorted. They are shifted from the locally nor-
mal
n -curves. The vertical shift amounts up to 2.5 decades. It is easy to imagine
what a crazy substratum would be obtained by one-dimensional inversion of these
branches.
Let us estimate the intensity of t he S -effect. Figure 11.3 s ho ws the correlation
between apparent resistivities
( T
( T
=
100 s 1 / 2 ) and
=
1s 1 / 2 ) related
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