Artefacts of isotropic inversion applied to magnetotelluric data from an anisotropic Earth
Miensopust, M., and A.G. Jones
Two-dimensional (2D), isotropic approaches are standard for magnetotelluric (MT) data modelling and inversion.
Unfortunately, the real subsurface structure is not isotropic everywhere
and one should be aware of the possible consequences of applying an isotropic inversion to
data from an anisotropic Earth.
The work presented was motivated by discovering an unusual
mid- to lower-crust conductor that appeared to be bent downward into the lithospheric mantle,
compared to the neighbouring terranes, when applying routine 2D isotropic inversion.
One major difference between the terranes is the presence of the Okavango giant mafic dyke swarm
(northeastern Botswana), beneath which the conductor is imaged to be in the lithospheric mantle rather than the lower crust.
The limited width of the dykes makes them more an anisotropic
feature than a normal 2D structure at MT scale.
To examine the possible effects of the dykes,
synthetic data were generated from a 1D model, accounting for the dyke swarm by using an
anisotropic block, and then inverted isotropically.
The synthetic tests showed that the normal
decomposition and strike analysis techniques are not removing these large scale anisotropic effects, and that an isotropic inversion result obtained in the presence of an anisotropic structure
has to be treated with caution.
The comparison of the synthetic data with the presented case
history strongly suggests that the conductor imaged at lithospheric depths is an artefact, and
is most likely located in the lower-crust, as everywhere else in that area.
inversion of the dyke swarm area supports the assumption of a crustal anisotropic structure
related to the Okavango Dyke Swarm that affects the isotropic inversion results.
Geophysical Journal International, 187, 677-689, doi: 10.1111/j.1365-246X.2011.05157.x. [PDF]
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Alan G Jones / 18 October 2011 /