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Rapid approximate 3D inversion of transient electromagnetic data

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Schaa, R (2010) Rapid approximate 3D inversion of transient electromagnetic data. PhD thesis, University of Tasmania.

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Abstract

Three-dimensional geophysical inversion of EM data aims to give an idealised
estimate of the 3D geoelectrical structure of the subsurface which can meaningfully
describe a set of observations. Rigorous 3D inversion of time-domain
electromagnetic data is an onerous computational challenge, often taking several
hours or even days. Motivated by the demand for a rapid and reliable 3D inversion
method for TEM data, a fast, approximate 3D invert:>iun t:>dierne uf Lnt11t:>ie11L
electromagnetic data has been developed. The new scheme combines the TEM
moments concept and geologically constrained 3D inversion methodology. The
TEM moment inversion scheme builds on the pre-existing VPmg potential field
modelling and inversion framework which has been extended and modified so as
to accommodate approximate 3D TEM inversion. The method has potential for
fast, constrained 3D inversion of large airborne TEM data sets.
The moment transform of TEM data is a time-weighted integral of the impulse
response which accentuates late-time features. Due to the time integration, a
TEM decay is effectively reduced to a single value. Depth resolution is lost due
to the time integration, which, in effect, converts the nonlinear 3D TEM inversion
problem into a linear 3D magnetic inversion problem.
Using TEM moments, EM interaction may tolerably be ignored, therefore
justifying linear superposition of TEM responses. 3D TEM forward modelling
is realised as a linear combmation of a discretised 3D target response and a
continuous background response. Superposition of the TEM moment responses of
magnetic dipoles, distributed on a cubic mesh, gives the target response whereas
the TEM moment response of a half space serves as the background response.
Computations are based on analytical formulae. For the approximate scheme,
a reduction in accuracy is accepted as a trade-off for much improved speed of
calculation.
For the 3D inversion, a starting model (or a set of weights) generated from
conductivity-depth imaging ( cor) is used to inject depth resolution. The CDis are
based on total magnetic field data which facilitates derivation of unambiguous
apparent conductivities for fixed-loop TEM. Depth resolution is also recovered by
means of geological constraints and depth weighting. The underlying model is
both geological and petrophysical, so that the inversion can be focused on selected
units. A fast steepest descent method is employed, so that computationally
slow matrix inversion is not required Typically, for ground-TEM, the fast
approximate 3D inversion completes in minutes, thus facilitating exploration
of non-uniqueness. The inversion scheme was successfully tested on synthetic
fixed-loop TEM examples and on fixed-loop TEM field data from South Africa.

Item Type: Thesis (PhD)
Keywords: Imaging systems in geophysics, Geological mapping, Geophysics
Copyright Holders: The Author
Copyright Information:

Copyright 2010 the Author

Additional Information:

Available for use in the Library but NOT for copying until 17/02/2012. After that date, available for use in the Library and copying in accordance with the Copyright Act 1968, as amended. Thesis (PhD)--University of Tasmania, 2010. Includes bibliographical references. 1. Introduction -- 2. Methodology of the TEM moments approach -- 3. Approximate 3D modelling of TEM moments -- 4. 1D imaging of TEM data -- 5. 3D inversion of TEM moments -- 6. Application of approximate 3D TEM inversion to field data -- 7. Conclusions

Date Deposited: 03 Feb 2015 03:20
Last Modified: 11 Mar 2016 05:53
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