ABSTRACT:

Mapping unconsolidated sediment transport at the Illinois Lake Michigan shoreline (ILMS) is complex, but also vital for sustainable management and use of this dynamic system that has undergone significant redistribution of sand in the littoral transport system in the past. To understand the erosion and accretion processes it is critical to map the ILMS sediments in high spatiotemporal resolution as the system is constantly changing over time. Here we used two geophysical methods, waterborne electrical resistivity imaging (wERI) and sub-bottom profiling, ground-truthed by hydraulic jet probing and historic borings, to map the thickness of unconsolidated sediments along two reaches at the western ILMS. At the two reaches, geophysical methods show that the sediments have not undergone folding and were generally horizontal, and the thickness of the unconsolidated material ranges between 4 to 5 m. Both methods are in agreement with jet probe results which provide direct evidence of loose sediments up to depths of 4 to 5 m below the lakebed. The wERI shows more detailed variation in the sediment and bedrock topography than the other methods. Overall, the geophysical methods, particularly the wERI, appear as effective tools to accurately map the sediment structure along the ILMS at high spatial resolution. Considering the relatively low cost of the operation of geophysical surveys and simplicity of data analyses, wERI and sub-bottom profiling show promising potential for comprehensive and frequent mapping of the ILMS. The methods supplement the limited extent of direct sampling and the lower spatial resolution but great extent of airborne geophysics and provide the information needed for better understanding of sediment transport mechanisms.

ABSTRACT:

Mapping unconsolidated sediment transport at the Illinois Lake Michigan shoreline (ILMS) is complex, but also vital for sustainable management and use of this dynamic system that has undergone significant redistribution of sand in the littoral transport system in the past. To understand the erosion and accretion processes it is critical to map the ILMS sediments in high spatiotemporal resolution as the system is constantly changing over time. Here we used two geophysical methods, waterborne electrical resistivity imaging (wERI) and sub-bottom profiling, ground-truthed by hydraulic jet probing and historic borings, to map the thickness of unconsolidated sediments along two reaches at the western ILMS. At the two reaches, geophysical methods show that the sediments have not undergone folding and were generally horizontal, and the thickness of the unconsolidated material ranges between 4 to 5 m. Both methods are in agreement with jet probe results which provide direct evidence of loose sediments up to depths of 4 to 5 m below the lakebed. The wERI shows more detailed variation in the sediment and bedrock topography than the other methods. Overall, the geophysical methods, particularly the wERI, appear as effective tools to accurately map the sediment structure along the ILMS at high spatial resolution. Considering the relatively low cost of the operation of geophysical surveys and simplicity of data analyses, wERI and sub-bottom profiling show promising potential for comprehensive and frequent mapping of the ILMS. The methods supplement the limited extent of direct sampling and the lower spatial resolution but great extent of airborne geophysics and provide the information needed for better understanding of sediment transport mechanisms.

ABSTRACT:

Electrical conductivity models have been widely used to estimate water content and petrophysical properties of soils in hydrogeophysical studies. However, these models are typically only valid for soils with non-expandable matrices. Soils containing swelling clays are characterized by matrices that expand/contract upon gaining/losing water. In this laboratory study, we demonstrate that soil matrix changes affect the saturation estimation using Archie’s laws. Matrix deformation is not accounted for in Archie’s laws, which were originally introduced for clean sandstone reservoir rocks. A swelling clayey soil sample was fully saturated with a potassium chloride (KCl) solution, then left to dry evaporatively at room temperature. The resistivity of the soil, along with its weight and volumetric changes as a result of shrinkage, were measured daily during drying . Over a course of 11 days, the soil sample decreased in volume by 33%. During this time period, the porosity and saturation of the soil sample were determined as a function of time. The simultaneous evaporation and shrinkage caused a non-linear reduction in saturation with decreasing of water content over time. Application of Archie’s second law leads to erroneous predictions of resistivity if the correction for saturation changes due to shrinkage are not accounted for. Correcting for saturation using the calculated volume reduction results in a power-law relationship with higher R2 value between resistivity and saturation along with more reasonable saturation coefficients.

ABSTRACT:

The mechanical properties of soils play a crucial role in site assessment for construction and infrastructure. Soils with low shear strength can become unstable as a result of natural and/or anthropogenic induced forces. Standard geotechnical methods, such as compressive strength tests, quantify the mechanical properties of soils, but these methods have low spatiotemporal resolution and may involve disruption of existing infrastructure. In contrast, the complex conductivity geophysical method can provide information on spatiotemporal changes in the subsurface in a minimally invasive manner. We investigated complex conductivity signatures resulting from soil deformation and failure during an unconfined compression test. A synthetic soil composed of silica sand (98%) and kaolin powder (2%) was saturated below its liquid limit and packed inside a flexible sample holder custom-equipped with four electrodes under zero confining stress to simulate an unconfined condition. This soil sample underwent a constant and slow rate of compression. Soil stress, strain, effluent volume, along with the frequency dependent real and imaginary parts of the complex conductivity were recorded over distinct time intervals. The first experiment focused on the sensitivity of complex conductivity to soil failure. Imaginary conductivity (equivalent to surface conductivity) abruptly decreased at the failure point (similar to the decrease in stress) compared to the real conductivity signal. The dominant geophysical length scale L2 determined from the complex conductivity spectra (related to pore size) exhibits an inverse linear dependence on compression. The second experiment focused on the complex conductivity during shearing (beyond failure). In this case, the imaginary (or surface) conductivity closely tracked changes in the sample stress. In both experiments, imaginary (or surface) conductivity is highly sensitive to changes caused by rearrangement of soil structure under stress (i.e., deformation and failure). In contrast, the real conductivity is minimally sensitive, and the electrolytic conductivity is insensitive to these changes. Our findings indicate that complex conductivity is capable of tracking mechanical changes of soils under stress and during failure foremost through the surface conductivity.