Performance Analysis and Characterisation of a new Magneto-Electrical Measurement System for Electrical Conductivity Imaging

Abstract

An often used proxy in hydrogeology is the electrical condutctivity distributioni to study flow and transport of groundwater in soil. In order to map the electrical conductivity distribution of soil columns (length 0.5 m and 0.1 m radius), traditionally ERT (Electrical Resistivity Tomography) is used. In this thesis the ERT measurements will be complimented by magnetometric resistivity (MMR) mearurements. Due to a low frequency current injection (25 Hz) a current distribution is generated in the soil column of which the electrical potential is measured at several electrodes. This current has an associated magnetic field, also depending on the internal electrical conductivity distribution, this resulting magnetic field is measured in the MMR method.<br /> The magneto-electrical resistivity imaging technique (MERIT) combines the measurement of both electric and magnetic parameters. In this thesis the development of a small laboratory scale system will be described. The electrical potentials are measured by small stainless steel screws inserted into the plexiglass mantle of the soil column, the magnetic fields are measured with especially designed sensor modules. The three component magnetic field modules are composed of AMR sensors, and are located on a vertical moving scanning torus. The magnetic field measurement system is designed to operate under ’typical’ laboratory noise conditions. <br /> The development of this new measurement system also includes optimization of the sensor positions and current injection geometries. The technique of singular value decomposition is applied to the Jacobian (or sensitivity matrix) of a combined ERT and MMR dataset in order to obtain information on the behaviour of the system in the so-called model space as well as its data space. This information can be used to obtain a quantitative measure of the optimum measurement configuration and the optimum resolution of the MERIT system, depending on the target of the survey. <br /> Finally a 2D quasi-Newton inversion algorithm with Broyden type Jacobian updating is used in order to image the electrical conductivity distribution of the soil column.