Inversion Strategy - Models and Methods of Magnetotellurics - page 507

conductance of the sedimentary cover and R 2 = h 2 2 is the average resistance of the

high-resistivity crust that separates the sedimentary cover from the deep conductive
zone.
Another part of the oceanic electric current bypasses the continental trap. The
current leaks into the ocean floor and is distributed among continental deep conduc-
tive zones.
The proportion of the currents flowing into the sedimentary cover to the current
penetrating into deep conductors defines the degree of low-frequency distortions
of the transverse MT-curves and their sensitivity to crustal and mantle conductivity
anomalies.
The inductive anomalies arise as the electric current flows parallel to the coast
(the TE-mode). They are caused by the inductive interaction between oceanic
and continental longitudinal electric currents. At high frequencies the longitudi-
nal excessive currents concentrate in the near-coast zone. Inductive distortions of
longitudinal MT-curves are observed near the shore and they decay at distances of
the same order as a depth to the well-conducting mantle. In the near-coast zone, the
asymmetry of longitudinal electric currents generates the vertical component of the
magnetic field, which can exceed its horizontal component.
By way of illustration, we turn to the work of Berdichevsky et al. (1992) and
consider two-dimensional models A and S (Fig. 12.45). Geologically, these models
are substantially different. Model A imitates an active tectonic zone. Its continental
part involves a thick conductor, encompassing the lower crust and upper mantle. In
the same depth range, model S contains only a thin crustal conductor, characteristic
of stable tectonic zones.
Let us examine the behavior of the transverse and longitudinal apparent-
resistivity curves of A , S calculated for models A and S. Relation-

ships observed on the oceanic profile are rather simple. In the near-coast zone, the
⊥ -curves are thrown down reflecting the current leakage from the ocean to the
continental crustal and mantle conductors. The leakage effects slowly attenuate with
distance from the continent. Even at distances about 1000 km, the low-frequency
resistivities

A , S

and

n .The
induction effects distorting the longitudinal apparent-resistivity curves are far more
local. They are visible in the near-coast zone, but they vanish at distances of
about 50 km and the resistivities

A , S differ dramatically from the normal apparent-resistivity

A , S virtually merge with the normal apparent-

n . Somewhat different relationships are observed on the continental
profile. Near the coast, the ascending branches of the transverse

resistivity

⊥ -curves coin-

n . However, with decreasing fre-

cide with the normal apparent-resistivity curve of

n -curve: their ascending branches lengthen,
and the descending branches shift upward by 2.5 decades. No evidence of a crustal
or crust-mantle conductive layer is available there. With distance from the ocean,
the shape of the

⊥ -curves depart from the quency, the

⊥ -curves slowly changes: gentle inflections and minima reflect-
ing a deep conductive layer appear and the descending branches shift downward.
Finally, at a distance of about 700 km (six adjustment distances), the

⊥ -curves

merge with the normal curve of

n . The behavior of the transverse apparent resis-

⊥ tivity curves can be accounted for by the continental trap effect. The situation