Mapping sand layers in clayey till using crosshole ground-penetrating radar

Abstract

Fluid transport through clayey tills governs the quantity and quality of groundwater resources in the Northern hemisphere. This transport is often controlled by a three-dimensional network of macropores (biopores, fractures and sand lenses) within the clayey till. At present, a non-destructive technique that can map and characterize the sand-lens-network does not exist, and full excavation or extensive drilling is therefore the only solution. Acquisition and modeling of crosshole ground penetrating radar (GPR) may provide the answer to this problem. We collected one-and two-dimensional crosshole GPR data at a field site in Denmark between four 8-m-deep boreholes with horizontal distances varying between 2.64-5.05 m. We show that the depth, thickness, and tilt of a coherent sand layer within the clayey till (approximately 0.4-0.6 m thick) as well as the underlying sand formation can be mapped accurately using the GPR data.We identified the sand efficiently as a highly resistive section with high electromagnetic wave velocities, while the clayey till was conductive with lower electromagnetic wave velocities. We found that the exact location of the sand occurrences was better delineated by the increase in amplitude than the increase in electromagnetic wave velocity. We believe that crosshole GPR may contribute significantly to groundwater protection and contaminant remediation initiatives.

Spectral Induced Polarization Characterization of Non‐Consolidated Clays for Varying Salinities—An Experimental Study

“…In geophysics, the most common methods to identify the presence of clay minerals non‐intrusively in the field are electrical and electromagnetic methods (e.g., Auken et al., 2017): direct current electrical resistivity tomography (ERT) (e.g., Batayneh, 2006), induced polarization (IP) (e.g., Lévy, Maurya, et al., 2019; Okay et al., 2013), time‐domain electromagnetics (TDEM) (e.g., Finco et al., 2018), frequency domain (FDEM) electromagnetics (e.g., Spichak & Manzella, 2009), and ground penetrating radar (GPR) (e.g., Looms et al., 2018). However, if clays are usually associated to high electrical conductivity zones, they can be mistaken with highly mineralized pore water when only the real electrical conductivity is considered.…”

Cross-borehole tomography with full-decay spectral time-domain induced polarization for mapping of potential contaminant flow-paths

“…However, in conductive environments, the greater attenuation of the EM signal severely limits the imaging distance between boreholes. Looms et al (2018), succeeded in mapping sand-lenses in a clayey till by full waveform inversion of cross-borehole GPR data, collected at the same gravel pit as will be presented here. The main advantage of the resistivity method in comparison to the GPR method, is that the resistivity method also works in fully saturated zones.…”

Nanoplastic transport in aqueous environments: The role of chemo-electric properties and hydrodynamic forces for nanoplastic-mineral interaction.

“…Pollutants and ENP infiltrate beyond shallow soils and enter the saturated zone via coarser sediment grains, matrix heterogeneities, well-connected macro or micropores and preferential flow paths, which i.e. develop along fractures and cracks (Kessler et al, 2012;Looms Zibar et al, 2018;Mosthaf et al, 2021). Thus, although we lack evidence of nanoplastic occurrence in deeper aquifers, it may be due to analytical limitations (Schwaferts et al, 2019) that their occurrence in shallow and deeper aquifer systems and subsurface reservoirs is overseen.…”