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In our topic we will present some approaches to the two-dimensional MT and
MV interpretation that are characteristic of the Russian magnetotelluric school.
Discussing the strategy of two-dimensional interpretation of MT- and MV-data,
we have to address Part II of our topic and answer three questions. In decreasing
order of importance, these questions are: (1) what field mode is more sensitive to the
near-surface and deep structures which are the targets of magnetotellurics? (2) what
field mode is more robust to the 3D-effects caused by real geological bodies? (3)
what field mode is more susceptible to the static distortions caused by near-surface
inhomogeneities?
12.4.1 Sensitivity of the TM- and TE-Modes
to the Target Structures
This point is crucial in determining the magnetotelluric informativeness. We know
that the TM-mode may be more sensitive to near-surface structures and fluid-
saturated faults as well as the lithosphere resistance (porosity, fissuring), whereas the
TE-mode may be more sensitive to deep conductive (fluidized, graphitized, partially
melted) zones.
Consider the basic lithosphere model consisting of the upper conductive layer
(the sediments), the intermediate resistive layers (the consolidated crust and upper
mantle), and the conductive basement (the asthenosphere).
We will start with examining the sensitivity of TM- and TE-modes to near-
surface structures. In Fig. 12.3, the sediments with thickness of 1 km contain a
two-dimensional horst-like resistive elevation with amplitude of 0.7 km and width
of 1 km. The horst is clearly marked by the transverse apparent resistivities
for
periods 0.1-10000 s covering the S 1
and h
intervals and by the transfer phases
interval. But it is
dramatically smoothed in corresponding graphs of the longitudinal apparent resis-
tivity
for periods 0.1-1 s relating to the very beginning of the S 1
, while the real and imaginary tippers, Re W and Im W ,
are close to the detection threshold. The narrow horst is readily detected using the
TM-mode, but it may be missed when using the TE-mode.
Next we examine the sensitivity of TM- and TE-modes to deep structures. Con-
sider a model that contains a two-dimensional prominent asthenosphere elevation
with amplitude of 75 km and width of 150 km.
Figure 12.4 displaces a case typical for stable regions. Here the lithosphere resis-
tance is about 3
and the phase
m 2 . The asthenosphere elevation is indiscernible in the
transverse apparent resistivities
10 9
·
Ohm
·
and phases
for periods 100-10000 s (the
h
interval), but it reveals itself markedly in the corresponding graphs of the longi-
tudinal apparent resistivities
and generates the real and imaginary
tippers, Re W and Im W , that considerably exceed the detection threshold. Clearly
the asthenosphere elevation can be indicated with confidence using the TE-mode,
but it may be missed when using the TM-mode (the screening effect).
Figure 12.5 displays a case typical for active regions. Here the lithosphere
resistance is reduced to 2.5
and phases
·
10 8
·
m 2 . This visibly affects the TM-mode: the
Ohm
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