2D cross-hole MMR – Survey design and sensitivity analysis for cross-hole applications of the magnetometric resistivity method

The magnetometric resistivity (MMR) method measures low-level (typically < 1nT) magnetic fields associated with a low-frequency (1 - 20 Hz) electric current impressed into the ground to determine the subsurface resistivity structure. As a step towards the implementation of MMR for cross-hole imaging, in this Ph.D. thesis several aspects of survey design for near-surface applications are discussed. In numerical, laboratory and field studies the potential of MMR for advanced structural characterization and process monitoring at the field scale is assessed. The 2D cross-hole setup considers borehole measurements of the magnetic field as response to borehole current injection; in this case the magnetic field has only one non-zero component (perpendicular to the imaging plane – B$_{y}$). Optimal survey parameters are inferred from numerical studies regarding signal strength, source-generated noise level and resolving power. Modeling of MMR responses over 2D conductivity structures was performed using a newly developed 2.5D FE program MMRMod. It could be proven that current injection via vertical dipoles provides superior signal-to-noise ratio compared to other transmitter configurations. Analyzing resolving power in terms of sensitivity distribution reveals that dipole configurations reflect confined subsurface volumes, advantageous for tomographic surveys and that transmitter-receiver combinations exceeding an offset equal to the borehole separation do not contribute significantly to the overall crosshole resolution. With the assistance of laboratory testing two concepts for solving two major difficulties inherent in cross-hole MMR field surveying are derived: the correction for the arbitrary borehole sensor orientation and the correction for parasitic correlated noise fields induced by the measurement system itself. The (latter) measurement method with phase switching is thereby first-time successfully applied to the processing of MMR data. In addition, the proposed data processing procedure includes modern lock-in-technique and has proven to be an appropriate tool for an effective information extraction from the measured magnetic fields. Finally, cross-hole MMR data were collected during a water infiltration experiment at the Gorgonzola test site. Acquisition and processing are accomplished according to the developed tomographic measurement approach involving multiple-offset transmitter-receiver arrangements and repeated measurements with time (time-lapse mode). Data, obtained during initially conducted background measurement, are qualitatively validated based on two different conductivity models, one of which is obtained from the inversion of independently collected ERT data. Importantly, the comparison of field data with predicted model curves suggests better resolvability of contrasts by MMR than by ERT. Moreover, the analysis of time-lapse measurements reveals a clear spatiotemporal dependence of the anomalous MMRresponse (MMR response with respect to background value) based upon the water saturation