Inversion Strategy - Models and Methods of Magnetotellurics - page 517

The main drawback of the EMSLAB-II model is its schematism caused by
the limitations of the INV2D-FG program. Nowadays we have at our disposal
more effective programs for two-dimensional automated inversion of MT and MV
data. They include the smoothing program REBOCC, implementing Occam's razor
(Siripunvaraporn and Egbert, 2000), and the programs IGF-MT2D (Novozhynski
and Pushkarev, 2001) and II2DC (Varentsov, 1999), which provide the optimiza-
tion of models containing 512 and more blocks of a fixed geometry. Advances
in computing magnetotellurics open up new avenues for the interpretation of
EMSLAB data.

12.7.6 Analysis of Observations on the Lincoln Line

Figure 12.54 shows the transverse apparent-resistivity curves obtained on the con-
tinental part of the Lincoln line. The curves consist of two ascending branches
separated by an inflection or a minimum. The low-frequency ascending branches
of these curves have identical slopes and occupy nearly two decades. To get a better
insight into the behavior of apparent resistivities, we normalize the ⊥ -curves by

shifting them vertically so that their left-hand ascending branches fit best the line
of the average conductance S = 50 S of the upper layer. The normalized ⊥ -curves

demonstrate a simple regular relation: the greater the distance from the coast, the
deeper the central minimum of these curves and the lower their right-hand branch.
Comparing Fig. 12.54 with Fig. 12.45, we find a striking similarity between the
normalized

⊥ -curves
calculated for the models A and S. It seems evident that the continental trap effect is
observed on the Lincoln line and that precisely this effect rather than the influence
of lithospheric and asthenospheric structures governs the transverse

⊥ -curves obtained on the Lincoln line and the theoretical

⊥ -curves at

various distances from the coast.
The longitudinal apparent resistivity curves obtained in the same period range
are shown in Fig. 12.55. With distance from the coast, the -curves change in

shape, showing bell- and bowl-type branches. In many cases the -curves have

gently ascending or descending low-frequency branches lying at various levels. One
might assume that the longitudinal -curves reflect variations in the geoelectric

structure of the lithosphere and asthenosphere but are distorted by static shifts and
occasionally by 3D effects.
Note that the transverse curves of

⊥ satisfy the dispersion relations at all
sites of the Lincoln line. However, the longitudinal curves of

⊥ and

episodically
violate the dispersion relations (sites 3, 4, 10, and 13), as is seen in Fig. 12.56.
Now we turn to the analysis of the inhomogeneity parameter N (in Swift-Eggers
determination) and asymmetry parameters ske

and

B , which help to identify
geoelectric structures and determine their dimensionality. The pseudo-sections of
these parameters are presented in Fig. 12.57. At high frequencies ( T << 1s),the
inhomogeneity parameter N fluctuates at the level of 0.1, indicating the acceptability
of one-dimensional estimates of the resistivity for near-surface rocks. With lower-
ing frequency, we notice an influence of deeper inhomogeneities. At T

w S and ske w

=

1s, the