2026 Theses Doctoral

Deep Electromagnetic Imaging of Fluid Distribution in Sedimentary Basins

Le, Huy D.

Electromagnetic (EM) geophysical methods have been used extensively in groundwater explorations for mapping freshwater distribution and elucidating the origin, emplacement processes, and sustainability of the aquifers in responses to the change in geological and climatic conditions. Since EM methods are highly sensitive to the electrical resistivity contrast between pore fluids and host lithology, they offer a powerful approach of delineating complex subsurface hydrogeologic systems. This dissertation presents three studies across diverse sedimentary basin settings, including continental shelf, coastal delta, and platform, to characterize fluid distribution and its environmental implications.

On the U.S Atlantic continental shelf, we examine submarine aquifers extending tens of kilometers offshore (chapter 3). This study evaluates the independent and combined resolving power of three EM data types: sparsely sampled seafloor controlled-source electromagnetic (CSEM) data, seafloor magnetotelluric (MT) soundings, and continuously collected surface-towed CSEM data. Using regularized and Bayesian inversion frameworks, we demonstrate that while MT data are good at constraining conductive and deeper structures, both CSEM data types better resolve the more resistive freshwater layers. Specifically, seafloor CSEM data better resolve the aquifer than the surface-towed CSEM data, likely due to their significantly lower noise floor and longer source-receiver offsets. However, the seafloor data are limited to a few deployed stations whereas the surface towed data has continuous coverage along the survey profile. By combining all three EM data types, we find the resistivity is more tightly constrained so that better estimates of the aquifer dimensions and resistivity are obtained. These results suggest that future offshore groundwater EM surveys should collect a hybrid dataset of seafloor and surface-towed data to optimally constrain the aquifer systems and host geology.

In the coastal Ganges-Brahmaputra-Meghna Delta, we utilize deep-sensing MT soundings to map extensive freshwater reserves beneath southwestern Bangladesh (Chapters 4-5). In this region, groundwater security is threatened by overexploitation, saltwater intrusion, and arsenic contamination. In this coastal zone, much of the shallow groundwater is saline, while the availability of deeper fresh groundwater remains poorly understood. MT results along the Pusur River reveal two distinct deep freshwater bodies separated by a high-salinity zone, whereas the Tetulia transect shows a continuous freshwater reservoir spanning the entire 110 km profile. We propose that these aquifers formed during the Last Glacial Maximum sea-level lowstand and are protected by overlying fine-grained sediments. The observed saline gap in the Pusur region is attributed to paleovalley incision during sea-level lowstands, which was subsequently filled by marine sediments during transgression. This work identifies critical freshwater resources for this water-stressed region and suggests that the interplay between past sea-level cycles, sedimentation, and hydrogeological processes demonstrated here may also control the distribution of fresh groundwater in other deltas.

In the final project, we apply MT method to investigate wastewater disposal and induced seismicity in the Pawnee earthquake region, Oklahoma (Chapter 6). Unlike naturally occurring groundwater, wastewater is a highly conductive fluid from oil and gas activities. By deriving resistivity and porosity models and analyzing fracture distribution in core samples, we image the migration pathways and accumulation zones of these injected fluids. The findings indicate that the intra-basement earthquake hypocenters do not host significant brine accumulation, implying that the Pawnee earthquake was remotely triggered by stress transmitted from the upper disposal zones to the deeper basement, rather than by direct wastewater infiltration into the causative pre-existing fault zone. Our results suggest that a majority of injected wastewater may spread laterally via fractures and pores in sedimentary rocks and is less likely to inundate the deeper, impermeable basement. This study enhances our understanding of the fluid-induced earthquakes and the interactions between disposed fluids, surrounding rocks, and fault systems.

Collectively, this research demonstrates the versatility of electromagnetic imaging in addressing scientific questions related to groundwater problems across varying scales and geological contexts.