Electromagnetic &amp; Seismoelectric sensitivity analysis using resolution functions

In the field of exploration geophysics various methods are applied to determine the physical properties of the subsurface of the Earth. Some of the methods most widely used are seismic and electromagnetic surveys, which are each used according to the type of information that is being sought and their ability to provide that information. The method of seismoelectrics is in that regard a promising technique because theoretically it should be sensitive to a wide range of subsurface parameters, spanning both the acoustic and the electromagnetic methods. We aim to perform a parameter sensitivity analysis for the seismoelectric problem, investigating how perturbations in different parameters affect the data and how well these perturbations can be inverted for. We will first study some elements of inverse theory with a special focus on how to construct resolution functions from the basic integral equation of scattering theory. This integral equation can be derived using both superposition and reciprocity principles. This will be shown for both the electromagnetic and the acoustic case, before moving on to the seismoelectric case; it will become clear that the latter poses additional challenges. In addition, the results for the electromagnetic case can be compared directly with the results obtained by [Slob and Mulder, 2011] who performed an electromagnetic parameter sensitivity analysis. The results of this comparison could serve as a validation of the seismoelectric forward modelling code ESSEMOD [Grobbe and Slob, 2013], which will be used to generate synthetic data for this thesis. It is concluded that the superposition principle and the reciprocity theorem provide identical expressions for a scattered field. This conclusion was the basis for deriving the complete reciprocity theorem for the seismoelectric system, which was in turn used to derive resolution functions for a perturbation in bulk density and a perturbation in the seismoelectric coupling coefficient. For the electromagnetic case, we show that the resolution functions computed, using explicit analytical Green's function solutions are identical to resolution functions computed with data produced by ESSEMOD and that this indeed serves as a validation of ESSEMOD. Using the same approach, we have successfully computed a resolution function for inversion for the coupled seismoelectric system for a perturbation in bulk density. It is recommended that this innovative result is used as the basis for the analysis of the seismoelectric sensitivity to more complex parameter-contrasts, which could serve as an assessment of the true potential of the seismoelectric method.

Subject

seismoelectric
electromagnetic
resolution-function
sensitivity

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Student theses

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master thesis

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