An efficient cross-borehole direct current resistivity monitoring instrument

ABSTRACT Detailed mapping of pumped agents in in situ groundwater remediation is beneficial for ensuring satisfactory coverage of the contaminated volume. Cross-borehole resistivity measurements can provide time-lapse 3D (or 4D) images that highlight the anomaly volume created by the introduction of agents when the injection has a distinctive conductivity contrast, thereby guiding the injection process. However, existing instruments are mainly designed for surface measurements. We develop an efficient cross-borehole resistivity instrument capable of generating data sets from numerous quadrupoles. After inversion, these data reveal 4D subsurface electrical parameters across the remediation zone. We determine its efficiency by acquiring 599,760 quadrupole measurements at a groundwater remediation test site in just 7.5 h. The two data sets collected before and after agent injection clearly delineate the amendment zone, characterized by the lower resistivity of the remediation agent.