Combining geoelectrical imaging and X-ray Computed Tomography (CT) for improved hydraulic characterisation of soils
Lead Research Organisation:
University of Nottingham
Department Name: Sch of Biosciences
Abstract
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.
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.
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| NE/M009106/1 | 01/10/2015 | 31/03/2024 | |||
| 1799463 | Studentship | NE/M009106/1 | 03/10/2016 | 30/06/2020 |
| Description | To present day, the project reached three important landmarks: 1. Described the potential of using geoelectrical methods for monitoring soil processes and structure [Review article: Published in Geoderma] - https://doi.org/10.1016/j.geoderma.2020.114232 2. Linked soil electrical resistivity with soil x-ray absorption through a mathematical model [Letter article: Published in Journal of Environmental and Engineering Geophysics] - https://doi.org/10.2113/JEEG19-079 3. Column scale non-invasive assessment of soil compaction by combining X-ray CT and ERT: [Original scientific arcticle: Published in Vadose Zone Journal] - https://doi.org/10.1002/vzj2.20109 |
| Exploitation Route | 1. The review study can be used as a reference for future research or industrial initiatives that utilise geoelectrical methods for root zone properties investigation 2. Future studies that employ both X-ray and geoelectrical analysis of soil properties can use the pedophysical relationship obtained to correlate their observations 3. Our methodology offers a more comprehensive view on soil hydrodynamics and allows a direct quantifiable correlation between soil structure and solute flow. This can further serve as a basis for future modeling studies of root zone hydrology. |
| Sectors | Agriculture, Food and Drink,Environment |
| Title | A pedophysical relationship between X-ray Computed Tomography and Electrical Resistivity data |
| Description | The model aims to link two analytical parameters used for soil analysis: x-ray absorption and electrical resistivity. A calibration experiment was designed to express the variation of both parameters with soil water content. Afterwards, the experimental results were fit to previously derived conceptual models found in the literature representative for such dependency. Based on a good fit and a high correlation factor, a new empirical model, which is essentially obtained by mathematically combining the previous models, was derived. |
| Type Of Material | Data analysis technique |
| Year Produced | 2020 |
| Provided To Others? | Yes |
| Impact | Soil X-ray absorption method has been extensively used as a way to investigate soil structure to a high resolution (please consult X-ray CT literature). Geoelectrical resistivity methods have proven very effective in monitoring soil hydraulic processes. Our model aims to link the observations derived from these two methods and subsequently correlate and quantify soil structural properties with soil hydraulic properties. Henceforth, we are making the first steps towards a more robust interdisciplinary interpretation of soil processes. |
| Description | CHAR project placement |
| Organisation | University of Liege |
| Department | Gembloux Agro-Bio Tech (GxABT) |
| Country | Belgium |
| Sector | Academic/University |
| PI Contribution | I joined the CHAR project team for a scientific placement over the course of 3 months. The project is looking at long term impact of charcoal kiln sites on soil hydrodynamics. My role was to correlate soil structure as resolved through X-ray CT scans with electrical properties of soil. Therefore, I sample multiple soil cores, scanned them and processed the data in order to compare the structure of charcoal enhanced soil with a corresponding reference soil from the same fieldsite. These were subsequently correlated with processed Electrical Resistivity Tomography (ERT) 2D time-lapse surveys of soil water content variability. |
| Collaborator Contribution | My collaborators ran multiple other assessments on the same fieldsite, namely: study of pore macroaggregation, monitoring pore solution chemistry and water content variability in time and with depth, running ERT survey acquisitions periodically. |
| Impact | It is a multi-disciplinary collaboration: geophysics, soil chemistry, soil physics, plant science. |
| Start Year | 2019 |