Modelling of Controlled Source Electromagnetic Data

Abstract

This work treats modelling of electromagnetic fields from controlled sources in geophysical applications. The focus is on modelling the marine CSEM (controlled source electromagnetic) method in planarly layered media. The recent introduction of SeaBed Logging (SBL) as an application of the marine CSEM method for direct hydrocarbon identification has resulted in increased survey activity, and expanded as well as renewed the interest for investigating electromagnetic field propagation in the subsurface of the earth.

The material within this document consists of a short introduction to the CSEM and SBL methods and four self-contained papers:

• Low-frequency electromagnetic fields in applied geophysics: Waves or diffusion? treats propagation of low-frequency fields in conductive media, and compares their behaviour to nondistorted field propagation in lossless media.

• The first test of the SeaBed Logging method describes the first laboratory test of this method. The scaled experiment was performed in order to validate if the detection of thin resistive layers within conductive surrounding media is possible.

• Asymptotic evaluations of the marine CSEM field integrals elaborates on how electromagnetic signals propagate in an idealized stratified earth model. To this end, the method of steepest descents is applied in order to separate the various wavemodes.

• Electromagnetic fields in planarly layered anisotropic media formulates a mathematical description of the field propagation in stratified media with arbitrary anisotropy. The field equations are solved by using the matrix propagator technique.

Even if electromagnetic field propagation in layered media is a rather mature research subject, the current development of the CSEM and SBL methods demands reinvestigations and new theoretical insights. Optimal survey planning and solid interpretation rely on a thorough understanding of the signal propagation in the subsurface. The main motivation in this thesis is to contribute to increased knowledge of how electromagnetic fields travel in the earth.