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In-Depth Information
Chapter 3
Separation of the Local and Regional
Magnetotelluric Effects
3.1 Using the Local-Regional Decomposition
Applying the eigenstate techniques in a region with homogeneous superficial for-
mations, we can identify the buried regional structures, define their dimensionality
and strike, split the observed magnetotelluric field into the TM- and TE-modes,
discern and analyze galvanic and induction responces. However the local superficial
inhomogeneities may dramatically distort the eigenstate interpretation. A superim-
position model exemplifying distortions of this kind was shown in Figs. 2.3, 2.4,
2.5, 2.6.
Separation of local and regional effects is a critical question of magnetotellurics.
This question agitated scientists for a long time. A possibility to identify and remove
the local near-surface distortions was examined in pioneering works of Bahr (1985)
and Zhang, et al. (1987). The problem has been advanced in basic papers by Bahr
(1988, 1991), Groom and Bailey (1989), Singer (1992), Smith (1995), Chave and
Smith (1994), Chave and Jones (1997), McNeice and Jones (2001) and Caldwell
et al. (2004). An excellent (though a bit outdated) review of developments in this
field is available in Smith (1995).
Relation between local and regional effects can be described functionally with
the local-regional decomposition (1.74), [
Z
S
]
[
e
][
Z
R
][
h
]
−
1
, involving the
regional impedance tensor
[
Z
R
], the
electric distortion tensor
[
e
] and
magnetic dis-
tortion tensor
[
h
]
=
[
h
][
Z
R
]. However, the number of unknowns in this matrix
decomposition is too large to get a resolvable system of equations (12 unknown
complex values in [
e
]
=
[
I
]
+
[
h
] against 4 known complex values in [
Z
S
]). The
local-regional decomposition needs some restrictions. There are three levels of
restrictions.
[
Z
R
]
,
,
1. First, we neglect magnetic anomalies caused by local superficial inhomo-
geneities and use the truncated low-frequency decomposition (1.75).
2. Second, we limit ourselves to the two-dimensional (or axisymmetric) regional
structures.
