Magnetometric Resistivity (MMR) Imaging of Subsurface Solute Flow: Inversion Framework and Laboratory Tests

Abstract

In support of the development of the magneto-electrical resistivity imaging technique (MERIT), a framework is presented for forward and inverse modeling of near-surface magnetometric resistivity (MMR) data, and results are reported from initial laboratory tests. Application of MMR involves injection of a low-frequency current into the ground, and measurement of the resulting magnetic field using state-of-the-art lock-in technology to suppress magnetic noise around the centre frequency. Forward and inverse calculations are both two-step procedures, involving intermittent computation of current density. The validity of this framework is demonstrated using synthetic numerical tests. The laboratory experiment reported considers a saturated sand sample placed in a non-conducting container furnished with electrodes. Migration of a conductive salt tracer was monitored over three days using a fluxgate magnetometer mounted on a device that allowed repeat scans of the sample. Data inversion uses any two of the three measured magnetic field components, and is unique in two spatial dimensions. Combination of inverted data for different electrode arrangements allows visualization of temporal and spatial tracer migration, consistent with preferential tracer movement along pathways located near the container walls.