Site Characterization Using Electrical Resistivity Tomography after Microbially Induced Desaturation Treatment

Electrical resistivity tomography (ERT) offers a non-invasive method to evaluate subsurface conditions, including changes from bio-mediated ground improvement. This study investigates the use of ERT to map the spatial extent of microbially induced desaturation (MID) at a field trial site in Portland, Oregon, five years after treatment. MID enhances liquefaction resistance by generating biogas in situ, reducing soil saturation, and increasing pore fluid compressibility. In field applications, nutrient solutions are introduced to stimulate native microbes, triggering denitrification reactions that primarily produce nitrogen gas. ERT surveys measured bulk electrical conductivity influenced by increased salinity and decreased saturation. Inverted 3D resistivity models revealed elevated conductivity near the targeted treatment area that decreased with distance. Conductivity profiles from cone penetration tests (CPT) and pore water samples (BAT Probe) supported the fidelity of the ERT inversion results. Within the treated zone, bulk conductivity was lower than pore fluid conductivity, suggesting partial saturation, while in the far field all methods showed good agreement.