Abdullah Basaloom, a doctoral candidate in geology and geophysics, will defend their dissertation titled “Assessing and Comparing Geophysical Methods for Resolving Shallow Subsurface Geology.” Their advisor, Dr. Jeremy Maurer, is an assistant professor in the earth sciences and engineering department. The dissertation abstract is provided below.

Geophysical methods provide vital information for hydrogeological investigations of the subsurface. Electrical resistivity methods, in particular, can provide valuable high-resolution information on the shallow subsurface. In this study, we collect, assess, and compare electrical resistivity observations from three methods having different spatial scales and resolutions for investigating hydrogeological structures in the Kansas River Alluvial Aquifer. We collected towed Transient Electromagnetic (tTEM), Electrical Resistivity Tomography (ERT), and utilized existing Direct Push Electrical Conductivity (DPEC) data at three different sites, including over 8 DPEC and lithology wells, 5 ERT profiles, and ~7 line-km of tTEM observations. In this study, we focus on comparing results co-located in space to assess uncertainties and resolution in the obtained resistivity models. Consistent with previous studies, we find general similarity between ERT and tTEM resistivity models at longer length scales (>126 m), including the water table and depth to bedrock, but differ in detail, even for the same dataset using different inversion algorithms. The tTEM method has lower resolution and does not reliably sense high-resistivity bodies (ρ>500 Ω m) while providing greater depth penetration and acquisition speed. ERT provides higher resolution, in our case, due to the denser measurement spacing, and provides better detection of high-resistivity layers. High-resolution DPEC observations show fine-scale variability that is not captured by either of the surface geophysical measurements. Our analysis suggests that tTEM can miss meter-scale highly resistive layers (<3 m), particularly when the layers are more resistive than the background. This is supported by the synthetic forward modeling of TEM data, which indicates that the inherent vertical smoothing in tTEM leads to a smooth representation of the true resistivity structure within the depth dimension. This finding aligns with our observations: tTEM does not detect thin resistive layers that ERT can identify.