Combining geoelectrical imaging and X-ray Computed Tomography (CT) for improved hydraulic characterisation of soils

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Soils are the host for hydrological and biogeochemical processes in the unsaturated zone. However, variations in soil structure and hydraulic properties remain difficult to quantify, hence improved physical characterisation at multiple scales is vitally important if we want to truly understand fluid dynamics and the fate of nutrients and pollutants in soils.Current soil imaging methodologies operate at different spatial scales, are sensitive to different physical properties, and have distinctive strengths. Rapid advances have recently been made in two promising, but unconnected fields, namely geoelectrical imaging and X-ray Computed Tomography(CT). Modern geophysical techniques evaluate geophysical properties of soils to infer spatiotemporal models of hydrological properties or states. Novel instrumentation with permanent sensor arrays allows continuous geophysical monitoring of soil volumes in near-real time and with practical resolutions in the cm range on soil columns. Conversely, CT maps variations of spatial attenuation of EM radiation with material densities, which allows examination of the soil porous architecture at the microscopic level. State-of-the-art CT systems achieve much higher spatial resolution than geophysics (~10m voxels on 10mm samples), however accurate segmentation of soil images is non-trivial, a trade-off exists between sample size and resolution, and repeat measurements, e.g. to track moisture dynamics, are time-consuming.Integration of both methodologies has not been attempted so far, however their joint application to quantitative soil characterisation offers great potential for reducing uncertainty in the imaging of preferential flow and estimation of unsaturated hydraulic conductivity. This would benefit studies of agricultural and industrial leaching of contaminants in different soil scenarios.Project aims:Design and undertake pioneering laboratory experiments using concurrent CT and geophysical measurements on soil columns or core;Assess the potential of synergetic imaging by exploiting complementarity;Establish theoretical and quantitative modelling frameworks to explain observed results.Programme of research:The student will explore opportunities arising from typical experimental designs employed in (i)hydrogeophysical laboratory studies on soils undertaken at BGS, and (ii)CT imaging studies of soil pore structure undertaken at the School of Biosciences(UoN). Given practical constraints (scale, resolution, instrumental capability, laboratory space), soil columns/core of ~25cm diameter and ~1 m height will likely form a starting point for synergetic CT and geoelectrical imaging. It is expected that multi-sensor electrical resistivity tomography(ERT) or spectral induced polarisation (SIP)[3] will be the dominant geophysical techniques. The columns will be instrumented with galvanically[2] or capacitively[8] coupled sensor arrays. BGS will provide geoelectrical imaging equipment, & the CT imaging work will be undertaken with UoNs CT scanners.An experimental protocol and measurement strategy will be developed to overcome practical issues (e.g. imaging artefacts). The effect of variations in sample geometry will be assessed and soils of different geological provenance and contrasting texture (sand-dominated versus clay-dominated) and structure (lab-assembled versus field-structured) will be investigated to determine limiting factors of the experimental strategy. Changes in soil water (with and without solutes) distribution will be systematically imaged by both methodologies, in order to quantify their relative sensitivities to these changes. The impact of the synergetic approach on estimating unsaturated hydraulic properties will be assessed and a quantitative modelling framework established. Combined analysis of the data sets using image analysis & computer vision approaches will elucidate relationships between lower-resolution geophysical data and features extracted from CT.