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reflecting the geoelectric inhomogeneity and asymmetry (Swift, 1967; Bahr, 1988;
Caldwell et al., 2004; Berdichevsky and Dmitriev, 2002).
Using the Berdichevsky-Dmitriev magnetotelluric test on sufficiently low fre-
quencies, we ignore the magnetic anomalies caused by near-surface inhomo-
geneities. So, following Bahr (1988) and Caldwell et al. (2004), we proceed from
the truncated decomposition (1.75), [
Z
]
[
e
][
Z
R
], where [
e
] is the real-valued
tensor of local electric distortion and [
Z
R
]is the regional impedance tensor.
The initial invariant parameters in the Berdichevsky-Dmitriev test are:
1. The inhomogeneity parameter determined by (2.46) and (2.53):
=
1
N
mt
=
ζ
1
−
ζ
2
4
Z
xx
Z
yy
−
Z
xy
Z
yx
ζ
1
+
ζ
2
=
−
,
(
Z
xy
−
Z
yx
)
2
where
ζ
1
,ζ
2
are principal impedances derived from the Swift-Eggers decomposi-
tion.
2. The Swift asymmetry parameter determined by (1.60):
.
Z
xx
+
Z
xy
ske
w
S
=
Z
xy
−
Z
yx
Its analog is the angle asymmetry parameter determined by (2.54):
ske
w
ang
=
A
= ||
α
1
−
α
2
| −
π/
2
|
,
where angles
α
1
,α
2
define the principal directions derived from the Swift-Eggers
decomposition.
3. The Bahr phase-sensitive asymmetry parameter determined by (1.61):
Im(
Z
xy
Z
yy
+
Z
xx
Z
yx
)
Z
xy
−
Z
yx
ske
w
B
=
,
where the bars denote the complex conjugation. Its analog is the Caldwell-Bibby-
Brown asymmetry parameter
ske
w
CBB
, determined by (3.80) from the phase tensor:
2
arctan
=
1
xy
−
yx
xx
+
yy
1
2
arctan(
Mske
B
)
ske
w
CBB
=
w
,
where scale factor
M
is
Z
xy
−
Z
yx
2
(
Re
Z
xy
Re
Z
yx
)
.
M
=
xx
+
yy
)(Re
Z
xx
Re
Z
yy
−
that charac-
terize the level of measurements errors. In the magnetotelluric test, we usually take
All the parameters are estimated with respect to threshold values
