Information Technology Reference
In-Depth Information
crust, (7) tz - transition zone, (8) cf - crustal fluidized layer, (9) ca - continental
asthenosphere, (10) mantle. Varying the resistivities of blocks sp, cc, tz, cfl, and
ca, we construct a set of models representing different geodynamical situations and
enabling the estimation of the sensitivity of the TE- and TM-modes to the subduct-
ing plate, continental crustal conductors, and continental asthenosphere. The initial
model has a continental section, in which subducting plate, crustal conductors and
asthenosphere are absent. It is successively complicated by introducing the follow-
ing components into the continental section: (1) a crustal conductive layer (either of
infinite extent or 300 km wide), (2) a downsinking conductive plate either connected
or unconnected with the crustal conductive layer, and (3) a conductive asthenosphere
(either of infinite extent or 300 km wide) connected or unconnected with the oceanic
asthenoshere. Analysis of these models leads to the following evident conclusions
that are valid for the continental profile crossing 200 km wide alongshore zone:
1) the resistance of the continental upper crust can be estimated by the TM-mode,
2) the crustal conductive layer of infinite and finite extent is best expressed by the
TE-mode,
3) the conductive junction of the plate with the crustal conductive layer is best
resolved by the TM-mode,
4) the conductive continental asthenosphere is best resolved y the TE-mode; even a
well-defined asthenosphere 300 km wide may be overlooked by the TM-mode,
5) the conductive junction of the continental asthenosphere with the oceanic
asthtenosphe is poorly reflected in both modes,
6) the difference between continental conductors of infinite and finite extent is bet-
ter resolved by the TM-mode.
These conclusions agree with the results of trial inversions of synthetic data
obtained for the examined models. Interpretation has been performed with the pro-
grams Inv2D-FG (Golubev and Varentsov, 1994) and IGF-MT2D (Novozhynski
and Pushkarev, 2001), which provide a regularized solution of the magnetotelluric
and magnetovariational inverse problems in the class of piecewise-uniform (blocky)
media. The inversion of the TE-mode (longitudinal apparent resistivities, longitu-
dinal impedance phases, tipper components) successfully reconstructed the conti-
nental cross-sections with crustal and astenospheric conductors. The inversion of
the TM-mode (transverse apparent resistivities and transverse impedance phases)
was less efficient (crustal and astenospheric conductors are defined with gaps and
distortions). However, the TM-mode determined more reliably the resistivity of the
continental upper crust and fixed the junction of the subducting plate with the crustal
conductive layer.
12.7.4 On the Regional Near-surface Distortions
Now we have to give due consideration to regional near-surface heterogeneities
crossed by the Lincoln line and elucidate their influence on magnetotelluric and
magnetovariational response functions.
