Electromagnetic IP response of a conducting dike in a conducting earth

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dc.contributor.authorCharters, Robert Anthonyen_US
dc.date.accessioned2024-08-13T17:47:25Z
dc.date.available2024-08-13T17:47:25Z
dc.date.copyright1984en_US
dc.date.issued1984
dc.degree.departmentDepartment of Physics
dc.degree.departmentDepartment of Physics and Astronomy
dc.degree.levelMaster of Science M.Sc.en
dc.description.abstractIn this thesis, the electromagnetic coupling component of the spectral IP signal is studied for the case of aver­tical conducting dike embedded in a uniform conducting earth. A scale laboratory analogue model - was used to obtain spectral IP measurements that were comprised mainly of an electromag­netic coupling component. A graphite plate simulated the dike while a brine solution in a plywood tank simulated the uniform earth. The IP effect for the materials and frequen­cies used in this model was negligible. Normalized spectral IP measurements were obtained along traverses over the model. These measurements displayed a "gull-wing" anomaly that was symmetrical about the dike. This "gull-wing" feature is typi­cal of the electromagnetic response of a naturally occurring dike. The normalized spectral IP measurements were converted to apparent resistivities that were characteristic of both the dike and the uniform host earth. Inversions were made of the analogue model apparent re­sistivities using the Marquardt inversion algorithm and vari­ous empirical models of EM coupling based on the Cole-Cole dispersion. The Cole-Cole dispersion is not a suitable model for EM coupling since it yields only apparent resistivities with small, negative phases. The phase of the analogue model apparent resistivity in the region over the dike is positive. Inversions of analogue model apparent resistivities were made using the generalized Cole-Cole dispersion and the product of two generalized Cole-Cole dispersions. The inversion calcu­lations broke down in the region of positive phase over the dike, but successful inversions were made in regions far from the dike. It was concluded, therefore, that the Cole-Cole dispersion and its derivatives were better suited to modeling EM coupling for a uniform earth than for a conducting body em­bedded in a uniform host earth. An analytical model of EM coupling for the embedded dike was developed, and is referred to as the Dipole-Dipole-Loop (DDL) model. In the DDL model, the dike embedded in a uniform host earth was replaced by a conducting loop embedded in a uniform half-space. The resulting EM coupling was derived using the principles of electromagnetic induction. All linear current elements in the DDL model were treated as strings of point electric dipoles. The expressions for the EM fields of a point electric dipole embedded in, or on the surface of, a uniform conducting half-space are well known. The analogue model apparent resistivities were inverted using the DDL model. Inversions were made of apparent resistivities along individual traverses for different frequencies and dipole separations with the dike at a constant depth. Ex­cellent agreement was obtained between the DDL model and ana­logue model apparent resistivities along these traverses. The parameters of the DDL model were constant with array position, but varied with frequency and dipole separation, indicating that an equivalent current loop existed for each electromag­netic configuration of the conducting dike. Thus, the DDL model was capable of describing EM coupling for the embedded dike for the whole frequency range of measurements obtained. Further work is required before the frequency dependence of the DDL model parameters for the embedded dike can be predicted. When this is achieved, the DDL model will provide more accurate descriptions of EM coupling for buried ore bodies and will fa­cilitate removal of the EM coupling component from the spectral IP signal obtained from such bodies.
dc.format.extent134 pages
dc.identifier.urihttps://hdl.handle.net/1828/17463
dc.rightsAvailable to the World Wide Weben_US
dc.titleElectromagnetic IP response of a conducting dike in a conducting earthen_US
dc.typeThesisen_US

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