LAUSR

The injection of a low frequency electrical current in the ground between two electrodes A and B generates a magnetic field that can be measured at the ground surface with sensitive magnetic sensors. The map of the magnetic field, measured at the frequency of the injected current, can be used to determine the paths of the current through the ground. When the current is channelled along preferential conductive paths, the MagnetoMetric Resistivity (MMR) method can be used to detect these paths. Conductive current paths can be associated with preferential flow paths of groundwater when the two electrodes A and B are in the direction of the flow and when the flow path is highly electrically conductive with respect to the background. We first review the background equations for the magnetic field in MMR. Then, we provide the kernel of the problem using Biot and Savart law to connect the components of the observed magnetic field to the current density distribution. We also develop a simple approach to invert the magnetic field in terms of electrical current paths. To illustrate how the method works, we develop five synthetic models to test the sensitivity of the method to the properties of the conductive targets channelling the electrical current. The targets are characterized by different shapes, sizes, depths, and conductivity contrasts with the background. Then, we proceed with a case study for which the MMR method is used to identify and map preferential groundwater flow paths bypassing a mine waste rock dump drainage collection trench into the tailings pond. In this case, the conductivity of the flow paths is much stronger than the background conductivity due to the high mineralization of the ground water along these paths. The method underlines the 3-D architecture of these flow paths.