Information Technology Reference
In-Depth Information
low frequencies. On the Siberian Platform, zone I, we have bell-shaped
A
-curves.
⊥
-curves with
deep minimum in a period range of 100-500 s. This minimum is most pronounced
in the western Trans-Baikal (zone IV). In the southeastern Trans-Baikal it degen-
erates into the flattened descending branch (zone V). The rough estimates allow
us to associate the minima of apparent resistivities with an inhomogeneous crustal
conductive layer at a depth of 15-30 km.
- and
Approaching the Baikal graben (zones II, III), we get the
12.6.3 Interpretation Model
Generalizing the reliable geological and geophysical data, we distinguish the follow-
ing properties of the Baikal rift zone that control the structure of the interpretation
model: (1) The Baikal rift is a linear structure trending northeast, (2) The Baikal rift
zone is dissected by deep faults striking mostly northeast, (3) The crust of the Baikal
rift zone includes an inhomogeneous conductive layer, which may correlate with
the seismic waveguide, and (4) The upper mantle of the Baikal rift zone is laterally
heterogeneous and may contain such structures as a mantle diapir or asthenosphere
upwarp.
Constructing the interpretation model, we assume that the Baikal rift zone admits
the inversion of apparent-resistivity curves in the class of two-dimensional structures
with a northeast strike. Evidently, a two-dimensional interpretation model should
incorporate the following elements: (1) a heterogeneous surface layer (sediments)
whose conductance varies in accordance with the known electric-prospecting data,
(2) a high-resistivity upper crust with a conductive layer, (3) vertical conductive
channels simulating deep faults, and (4) a heterogeneous upper mantle with con-
ductive inclusions.
12.6.4 Bimodal Inversion in the Hypotheses Test Mode
The strategy of bimodal interpretation depends on the sensitivity of the TM- and
TE- modes to the target structures.
The TE-mode is the main source of information about deep structure in areas with
a monolithic upper lithosphere, where the TM-mode is hardly informative due to the
screening effect of the high-resistivity lithospheric layers. The situation is different
in areas with fault-block tectonics. Conductive (fluid-saturated, graphitized) faults
favor the vertical redistribution of telluric currents induced in various layers of the
crust and upper mantle and thus increase the sensitivity of the TM-mode to deep
structures. Under these conditions, the TM-mode can play an important (perhaps,
even leading) role in the interpretation of MT soundings aimed to studies of the
Earth's crust and upper mantle.
One of the most objectionable problems in the bimodal interpretation of MT
soundings is the correction of longitudinal
-curves distorted by the regional
S
-effect. Some crude estimates can be performed by ignoring the lateral variations in
