The Capacitive Resistivity Technique for Electrical Imaging of the Shallow Subsurface

Capacitive resistivity (CR) is a novel geophysical technique for the non-intrusive characterisation of the shallow subsurface by electrical imaging. CR is capable of extending the scope of the conventional DC resistivity technique to the urban built environment and other settings where galvanic contact cannot be achieved or where high contact impedances result in poor data quality. Fundamentally, the CR technique is based upon the concept of capacitive coupling between sensors and the ground and a generalisation of the DC four-point array for measuring the resistivity of the subsurface at frequencies in the VLF range. This thesis provides a unified description of CR, including its physical principles, their theoretical formulation and practical implementation in geophysical instruments. In general, the transfer impedance across a capacitive array is a complex function of frequency and geometry. It is shown that a low induction number mode of operation exists where resistivity is proportional to the in-phase component of the transfer impedance. The quadrature component is generally sensitive to a combination of parameters including sensor elevation, dipole offset and ground resistivity. Under the low induction number regime, the electric field is quasi-stationary so that theoretical equivalence with the DC case is achieved and conventional DC interpretation schemes are applicable to CR data. A comprehensive parameter study undertaken in this thesis investigates the applicability of the technique under the specific conditions typically encountered in environmental and engineering site investigation surveys. In those circumstances, practical CR measurements are shown to be limited to an optimal frequency window between 1 kHz and 25 kHz. The condition of low induction numbers imposes further restrictions on the maximum dimensions of the sensor array and the minimum resistivity of the ground. However, a key finding of the parameter study is that even under the quasi-static regime, practical conditions may be such that substantial phase rotations may occur which are exclusively due to the capacitive nature of the technique. Modelling of sensor capacitances is used to demonstrate that the concept of point poles postulated in the quasi-static formulation of CR has a practical realisation in the form of plate-wire sensors. Subsequently, the fundamental concepts of CR are validated experimentally in a series of elementary surveys where the fully complex transfer impedance (amplitude and phase) is measured with a newly developed prototype CR instrument. It is shown that for assessments of shallow subsurface conditions with typical survey parameters and standard geometries, the observed responses are typically in-phase. However, it is also demonstrated that practical circumstances exist under which significant phase rotations can be observed. In such cases, an estimation of apparent resistivity using the in-phase component only is more appropriate than the magnitude-based calculation performed by existing commercial instruments. The nature of the CR technique facilitates the use of towed arrays that allow the dynamic collection of multi-offset apparent resistivity data without the disadvantages of galvanic coupling. This thesis examines the operational characteristics of towed CR arrays and compares data acquired with a range of instruments in a variety of different environments. It is shown that towed-array CR enables the collection of highly repeatable resistivity data at sampling intervals of the order of centimetres. Towing-induced noise is found to be much less problematic than previously found with DC towed-array techniques. It is also demonstrated that high-quality data can be obtained by towed-array CR on artificial surfaces such as tarmac or concrete. Consequently, the technique appears to be particularly suited for assessing the condition of engineered structures such as roads and pavements. Finally, it is demonstrated how multi-offset towed-array CR can be employed for electrical tomographic imaging of the shallow subsurface. Conventional DC resistivity interpretation schemes based on quasi-2D, 2D and 3D inversion algorithms are shown to be applicable to such datasets, provided that some elementary rules are observed with regard to the design of towed-array surveys. Real-time interpretation during data acquisition is shown to be feasible with a continuous vertical electrical sounding (CVES) technique based on a Zohdy-type inversion. Examples of 2D and 3D surveys over shallow targets show the superior quality and resolution of CR datasets compared with conventional DC resistivity

The Capacitive Resistivity Technique for Electrical Imaging of the Shallow Subsurface

Capacitive resistivity (CR) is a novel geophysical technique for the non-intrusive characterisation of the shallow subsurface by electrical imaging. CR is capable of extending the scope of the conventional DC resistivity technique to the urban built environment and other settings where galvanic contact cannot be achieved or where high contact impedances result in poor data quality. Fundamentally, the CR technique is based upon the concept of capacitive coupling between sensors and the ground and a generalisation of the DC four-point array for measuring the resistivity of the subsurface at frequencies in the VLF range. This thesis provides a unified description of CR, including its physical principles, their theoretical formulation and practical implementation in geophysical instruments. In general, the transfer impedance across a capacitive array is a complex function of frequency and geometry. It is shown that a low induction number mode of operation exists where resistivity is proportional to the in-phase component of the transfer impedance. The quadrature component is generally sensitive to a combination of parameters including sensor elevation, dipole offset and ground resistivity. Under the low induction number regime, the electric field is quasi-stationary so that theoretical equivalence with the DC case is achieved and conventional DC interpretation schemes are applicable to CR data. A comprehensive parameter study undertaken in this thesis investigates the applicability of the technique under the specific conditions typically encountered in environmental and engineering site investigation surveys. In those circumstances, practical CR measurements are shown to be limited to an optimal frequency window between 1 kHz and 25 kHz. The condition of low induction numbers imposes further restrictions on the maximum dimensions of the sensor array and the minimum resistivity of the ground. However, a key finding of the parameter study is that even under the quasi-static regime, practical conditions may be such that substantial phase rotations may occur which are exclusively due to the capacitive nature of the technique. Modelling of sensor capacitances is used to demonstrate that the concept of point poles postulated in the quasi-static formulation of CR has a practical realisation in the form of plate-wire sensors. Subsequently, the fundamental concepts of CR are validated experimentally in a series of elementary surveys where the fully complex transfer impedance (amplitude and phase) is measured with a newly developed prototype CR instrument. It is shown that for assessments of shallow subsurface conditions with typical survey parameters and standard geometries, the observed responses are typically in-phase. However, it is also demonstrated that practical circumstances exist under which significant phase rotations can be observed. In such cases, an estimation of apparent resistivity using the in-phase component only is more appropriate than the magnitude-based calculation performed by existing commercial instruments. The nature of the CR technique facilitates the use of towed arrays that allow the dynamic collection of multi-offset apparent resistivity data without the disadvantages of galvanic coupling. This thesis examines the operational characteristics of towed CR arrays and compares data acquired with a range of instruments in a variety of different environments. It is shown that towed-array CR enables the collection of highly repeatable resistivity data at sampling intervals of the order of centimetres. Towing-induced noise is found to be much less problematic than previously found with DC towed-array techniques. It is also demonstrated that high-quality data can be obtained by towed-array CR on artificial surfaces such as tarmac or concrete. Consequently, the technique appears to be particularly suited for assessing the condition of engineered structures such as roads and pavements. Finally, it is demonstrated how multi-offset towed-array CR can be employed for electrical tomographic imaging of the shallow subsurface. Conventional DC resistivity interpretation schemes based on quasi-2D, 2D and 3D inversion algorithms are shown to be applicable to such datasets, provided that some elementary rules are observed with regard to the design of towed-array surveys. Real-time interpretation during data acquisition is shown to be feasible with a continuous vertical electrical sounding (CVES) technique based on a Zohdy-type inversion. Examples of 2D and 3D surveys over shallow targets show the superior quality and resolution of CR datasets compared with conventional DC resistivity

Multi-sensor core logging (MSCL) and X-ray computed tomography imaging of borehole core to aid 3D geological modelling of poorly exposed unconsolidated superficial sediments underlying complex industrial sites: an example from Sellafield nuclear site, UK

The 3D characterisation of geology underlying complex industrial sites such as nuclear plants is problematic due to the presence of the built infrastructure that restricts or in some cases completely prevents access for geologists to the subsurface environment. Outcrops are rare, geophysics surveys are often impossible (particularly at nuclear plants where activities such as vibroseis are frowned upon due to their effect on the infrastructure itself), and boreholes are often the only way to obtain subsurface data. Yet, with sedimentary deposits in particular, geotechnical logging undertaken to specific standards sometimes misses key information that could have been used to directly inform the creation of geological 3D models. Multi-sensor core logging (MSCL) and X-ray computed tomography (XCT) undertaken on core obtained from a borehole within the Sellafield nuclear plant, is used to illustrate the potential for the techniques to contribute significantly to the creation of 3D subsurface geological models, particularly where access is restricted, such as within nuclear industry locations. Geophysical characteristics are recorded and used to reassess and enhance geotechnical descriptions, leading to the modification of existing unit boundaries or the creation of new ones. A new sedimentary log was created and this was used in a comparison with existing logs and nearby historic exposures, and as the basis for an illustration of industrial site to regional correlation

Time-lapse capacitive resistivity imaging: a new technology concept for the monitoring of permafrost

The British Geological Survey, in partnership with the Universities of Sussex and Bonn, is investigating and seeking to prove a new technology concept for the non-invasive volumetric imaging and routine temporal monitoring of the thermal state of permafrost (Figure 1), a key indicator of global climate change. Capacitive Resistivity Imaging (CRI), a technique based upon a low-frequency, capacitively-coupled measurement approach (Kuras et al., 2006) is applied in order to emulate Electrical Resistivity Tomography (ERT) methodology, but without the need for galvanic contact on frozen soils or rocks. Recent work has shown that temperature-calibrated ERT using galvanic sensors (Figure 2) is capable of imaging recession and re-advance of rock permafrost in response to the ambient temperature regime. However, the use of galvanic sensors can lead to significant practical limitations on field measurements due to high levels of and large variations in contact resistances between sensors and the host material as it freezes and thaws Figure 3). The capacitive technology developed here overcomes this problem and provides a more robust means of making high-quality resistance measurements with permanently installed sensors over time. Reducing the uncertainty associated with uncontrolled noise from galvanic sensors increases the value of time-lapse ERT datasets in the context of monitoring permafrost

Predicting the movements of permanently installed electrodes on an active landslide using time-lapse geoelectrical resistivity data only

If electrodes move during geoelectrical resistivity monitoring and their new positions are not incorporated in the inversion, then the resulting tomographic images exhibit artefacts that can obscure genuine time-lapse resistivity changes in the subsurface. The effects of electrode movements on time-lapse resistivity tomography are investigated using a simple analytical model and real data. The correspondence between the model and the data is sufficiently good to be able to predict the effects of electrode movements with reasonable accuracy. For the linear electrode arrays and 2D inversions under consideration, the data are much more sensitive to longitudinal than transverse or vertical movements. Consequently the model can be used to invert the longitudinal offsets of the electrodes from their known baseline positions using only the time-lapse ratios of the apparent resistivity data. The example datasets are taken from a permanently installed electrode array on an active lobe of a landslide. Using two sets with different levels of noise and subsurface resistivity changes, it is found that the electrode positions can be recovered to an accuracy of 4 % of the baseline electrode spacing. This is sufficient to correct the artefacts in the resistivity images, and provides for the possibility of monitoring the movement of the landslide and its internal hydraulic processes simultaneously using electrical resistivity tomography only

Potential of geoelectrical methods to monitor root zone processes and structure: a review

Understanding the processes that control mass and energy exchanges between soil, plants and the atmosphere plays a critical role for understanding the root zone system, but it is also beneficial for practical applications such as sustainable agriculture and geotechnics. Improved process understanding demands fast, minimally invasive and cost-effective methods of monitoring the shallow subsurface. Geoelectrical monitoring methods fulfil these criteria and have therefore become of increasing interest to soil scientists. Such methods are particularly sensitive to variations in soil moisture and the presence of root material, both of which are essential drivers for processes and mechanisms in soil and root zone systems. This review analyses the recent use of geoelectrical methods in the soil sciences, and highlights their main achievements in focal areas such as estimating hydraulic properties and delineating root architecture. We discuss the specific advantages and limitations of geoelectrical monitoring in this context. Standing out amongst the latter are the non-uniqueness of inverse model solution and the appropriate choice of pedotransfer functions between electrical parameters and soil properties. The relationship between geoelectrical monitoring and alternative characterization methodologies is also examined. Finally, we advocate for future interdisciplinary research combining models of root hydrology and geoelectrical measurements. This includes the development of more appropriate analogue root electrical models, careful separation between different root zone contributors to the electrical response and integrating spatial and temporal geophysical measurements into plant hydrological models to improve the prediction of root zone development and hydraulic parameters

Electrical resistivity tomography determines the spatial distribution of clay layer thickness and aquifer vulnerability, Kandal Province, Cambodia

Despite being rich in water resources, many areas of South East Asia face difficulties in securing clean water supply. This is particularly problematic in regions with a rapidly growing population. In this study, the spatial variability of the thickness of a clay layer, controlling surface – groundwater interactions that affect aquifer vulnerability, was investigated using electrical resistivity tomography (ERT). Data were acquired along two transects, showing significant differences in the imaged resistivities. Borehole samples were analyzed regarding particle density and composition, and linked to their resistivity. The obtained relationships were used to translate the field electrical resistivities into lithologies. Those revealed considerable variations in the thickness of the clay layer, ranging from 0 m up to 25 m. Geochemical data, highlighting zones of increased ingress of surface water into the groundwater, confirmed areas of discontinuities in the clay layer, which act as preferential flow paths. The results may guide urban planning of the Phnom Penh city expansion, in order to supply the growing population with safe water. The presented approach of using geophysics to estimate groundwater availability, accessibility, and vulnerability is not only applicable to Kandal Province, Cambodia, but also to many other areas of fast urbanization in South East Asia and beyond

On the field estimation of moisture content using electrical geophysics‐the impact of petrophysical model uncertainty

The spatiotemporal distribution of pore water in the vadose zone can have a critical control on many processes in the near-surface Earth, such as the onset of landslides, crop yield, groundwater recharge, and runoff generation. Electrical geophysics has been widely used to monitor the moisture content (θ) distribution in the vadose zone at field sites, and often resistivity (ρ) or conductivity (σ) is converted to moisture contents through petrophysical relationships (e.g., Archie's law). Though both the petrophysical relationships (i.e., choices of appropriate model and parameterization) and the derived moisture content are known to be subject to uncertainty, they are commonly treated as exact and error-free. This study examines the impact of uncertain petrophysical relationships on the moisture content estimates derived from electrical geophysics. We show from a collection of data from multiple core samples that significant variability in the θ(ρ) relationship can exist. Using rules of error propagation, we demonstrate the combined effect of inversion and uncertain petrophysical parameterization on moisture content estimates and derive their uncertainty bounds. Through investigation of a water injection experiment, we observe that the petrophysical uncertainty yields a large range of estimated total moisture volume within the water plume. The estimates of changes in water volume, however, generally agree within (large) uncertainty bounds. Our results caution against solely relying on electrical geophysics to estimate moisture content in the field. The uncertainty propagation approach is transferrable to other field studies of moisture content estimation

Geophysical-geotechnical sensor networks for landslide monitoring

Landslides are often the result of complex, multi-phase processes where gradual deterioration of shear strength within the sub-surface precedes the appearance of surface features and slope failure. Moisture content increases and the build-up of associated pore water pressures are invariably associated with a loss of strength, and thus are a precursor to failure. Consequently, hydraulic processes typically play a major role in the development of landslides. Geoelectrical techniques, such as resistivity and self-potential are being increasingly applied to study landslide structure and the hydraulics of landslide processes. The great strengths of these techniques are that they provide spatial or volumetric information at the site scale, which, when calibrated with appropriate geotechnical and hydrogeological data, can be used to characterise lithological variability and monitor hydraulic changes in the subsurface. In this study we describe the development of an automated time-lapse electrical resistivity tomography (ALERT) and geotechnical monitoring system on an active inland landslide near Malton, North Yorkshire, UK. The overarching objective of the research is to develop a 4D landslide monitoring system that can characterise the subsurface structure of the landslide, and reveal the hydraulic precursors to movement. The site is a particularly import research facility as it is representative of many lowland UK situations in which weak mudrocks have failed on valley sides. Significant research efforts have already been expended at the site, and a number of baseline data sets have been collected, including ground and airborne LIDAR, geomorphologic and geological maps, and geophysical models. The monitoring network comprises an ALERT monitoring station connected to a 3D monitoring electrode array installed across an area of 5,500 m2, extending from above the back scarp to beyond the toe of the landslide. The ALERT instrument uses wireless telemetry (in this case GPRS) to communicate with an office based server, which runs control software and a database management system. The control software is used to schedule data acquisition, whilst the database management system stores, processes and inverts the remotely streamed ERT data. Once installed and configured, the system operates autonomously without manual intervention. Modifications to the ALERT system at this site have included the addition of environmental and geotechnical sensors to monitor rainfall, ground movement, ground and air temperature, and pore pressure changes within the landslide. The system is housed in a weatherproof enclosure and is powered by batteries charged by a wind turbine & solar panels. 3D ERT images generated from the landslide have been calibrated against resistivity information derived from laboratory testing of borehole core recovered from the landslide. The calibrated images revealed key aspects of the 3D landslide structure, including the lateral extent of slipped material and zones of depletion and accumulation; the surface of separation and the thickness of individual earth flow lobes; and the dipping in situ geological boundary between the bedrock formations. Time-lapse analysis of resistivity signatures has revealed artefacts within the images that are diagnostic of electrode movement. Analytical models have been developed to simulate the observed artefacts, from which predictions of electrode movement have been derived. This information has been used to correct the ERT data sets, and has provided a means of using ERT to monitor landslide movement across the entire ALERT imaging area. Initial assessment of seasonal changes in the resistivity signature has indicated that the system is sensitive to moisture content changes in the body of the landslide, thereby providing a basis for further development of the system with the aim of monitoring hydraulic precursors to failure