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| Titel |
Inverting MT 3D data using local coordinates and taking distortion parameters into account: progress and primlimary results |
| VerfasserIn |
M. P. Miensopust, C. G. Farquharson, A. G. Jones |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250024171
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| Zusammenfassung |
| In recent years different 3D inversion codes for magnetotelluric data, using finite differences,
finite elements or integral equations, have been developed, and some are beginning to become
available for general use. All of these codes are designed to accommodate different aspects
(e.g., topography, marine applications, including vertical transfer functions, -¦). Our inversion
program presented here uses a finite-element forward engine, and differs from previous ones
by solving simultaneously for galvanic distortion parameters as well as Earth structure, and
by using individual, local coordinate systems for each MT site instead of one global
coordinate system. The code is called MCMT3DID (Marion and Colin’s MT 3D Inversion
code including Distortion).
The reason for using local coordinate systems is obvious: Using a global coordinate
system for modelling means that measured data need to be rotated from the coordinate system
in which the data were recorded to the common, global modelling system. Therefore a
rotation will be applied to the complex 2x2 impedance matrix which results in
transferring noise of one channel into the others – a huge disadvantage that causes
degradation in information, especially if only one channel is strongly affected by
noise but the others are not. Hence our philosophy is to leave measured data in their
acquisition coordinates and not to apply any rotations to them. Of course there
will be a global coordinate system used for the inversion itself, but for comparison
between measured data and synthetic data, the noise-free, synthetic data will be rotated
to the local coordinate system of each site to avoid the noise propagation into all
components.
Another common problem of magnetotelluric field data is galvanic distortion due to
charges on small-scale, near-surface conductivity anomalies. Since the magnetic effects of the
galvanic distortion decay rapidly with increasing periods they can be neglected in the MT
case and only the electric effects, which persist to all periods, will be taken into account.
Therefore the relation between the measured field data (Zmeasured), the impedance
tensor of the true structure (Ztrue) and the distortion matrix (C) can be written
as:
Zmeasured = C Ztrue
Inverting for conductivities only would result in an image of a structure related to Zmeasured
rather than being an accurate representation of the subsurface. To gain a result being more
alike the true structure, we invert for conductivities and distortion parameters simultaneously
to accommodate these small-scale, near-surface anomalies causing the galvanic
distortion.
We will present the structure of the 3D finite-element code with the focus on some of the
more interesting aspects (e.g., local coordinate system and distortion) including the different
options and features (e.g., different approaches to include the distortion in the inversion
process and to define the starting values for the distortion parameters) as well as the status of
progress at the time of the conference. We will also show some preliminary results obtained
from the first test data sets using the options in the code, which are fully implemented at that
stage. |
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