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

Jointly reconstructing ground motion and resistivity for ERT-based slope stability monitoring

Electrical resistivity tomography (ERT) is increasingly being used to investigate unstable slopes and monitor the hydrogeological processes within. But movement of electrodes or incorrect placement of electrodes with respect to an assumed model can introduce significant resistivity artefacts into the reconstruction. In this work, we demonstrate a joint resistivity and electrode movement reconstruction algorithm within an iterative Gauss–Newton framework. We apply this to ERT monitoring data from an active slow-moving landslide in the UK. Results show fewer resistivity artefacts and suggest that electrode movement and resistivity can be reconstructed at the same time under certain conditions. A new 2.5-D formulation for the electrode position Jacobian is developed and is shown to give accurate numerical solutions when compared to the adjoint method on 3-D models. On large finite element meshes, the calculation time of the newly developed approach was also proven to be orders of magnitude faster than the 3-D adjoint method and addressed modelling errors in the 2-D perturbation and adjoint electrode position Jacobian

Time-lapse monitoring of climate effects on earthworks using surface waves

The UK’s transportation network is supported by critical geotechnical assets (cuttings/embankments/dams) that require sustainable, cost-effective management, while maintaining an appropriate service level to meet social, economic, and environmental needs. Recent effects of extreme weather on these geotechnical assets have highlighted their vulnerability to climate variations. We have assessed the potential of surface wave data to portray the climate-related variations in mechanical properties of a clay-filled railway embankment. Seismic data were acquired bimonthly from July 2013 to November 2014 along the crest of a heritage railway embankment in southwest England. For each acquisition, the collected data were first processed to obtain a set of Rayleigh-wave dispersion and attenuation curves, referenced to the same spatial locations. These data were then analyzed to identify a coherent trend in their spatial and temporal variability. The relevance of the observed temporal variations was also verified with respect to the experimental data uncertainties. Finally, the surface wave dispersion data sets were inverted to reconstruct a time-lapse model of S-wave velocity for the embankment structure, using a least-squares laterally constrained inversion scheme. A key point of the inversion process was constituted by the estimation of a suitable initial model and the selection of adequate levels of spatial regularization. The initial model and the strength of spatial smoothing were then kept constant throughout the processing of all available data sets to ensure homogeneity of the procedure and comparability among the obtained V S VS sections. A continuous and coherent temporal pattern of surface wave data, and consequently of the reconstructed V S VS models, was identified. This pattern is related to the seasonal distribution of precipitation and soil water content measured on site

Time-lapse monitoring of fluid-induced geophysical property variations within an unstable earthwork using P-wave refraction

A significant portion of the UK’s transportation system relies on a network of geotechnical earthworks (cuttings and embankments) that were constructed more than 100 years ago, whose stability is affected by the change in precipitation patterns experienced over the past few decades. The vulnerability of these structures requires a reliable, cost- and time-effective monitoring of their geomechanical condition. We have assessed the potential application of P-wave refraction for tracking the seasonal variations of seismic properties within an aged clay-filled railway embankment, located in southwest England. Seismic data were acquired repeatedly along the crest of the earthwork at regular time intervals, for a total period of 16 months. P-wave first-break times were picked from all available recorded traces, to obtain a set of hodocrones referenced to the same spatial locations, for various dates along the surveyed period of time. Traveltimes extracted from each acquisition were then compared to track the pattern of their temporal variability. The relevance of such variations over time was compared with the data experimental uncertainty. The multiple set of hodocrones was subsequently inverted using a tomographic approach, to retrieve a time-lapse model of VP for the embankment structure. To directly compare the reconstructed VP sections, identical initial models and spatial regularization were used for the inversion of all available data sets. A consistent temporal trend for P-wave traveltimes, and consequently for the reconstructed VP models, was identified. This pattern could be related to the seasonal distribution of precipitation and soil-water content measured on site

Construction, management and maintenance of embankments used for road and rail infrastructure: implications of weather induced pore water pressures

Understanding the age and construction quality of embankments used for road and rail infrastructure is critical in the effective management and maintenance of our transport networks, worth £billions to the UK economy. This paper presents for the first time results from full-scale, carefully controlled experiments on a unique model embankment conducted over the 4-year period between 2008 and 2011. It combines point location and spatially distributed measurements of pore water pressures and water content with outputs from hydrological modelling to draw conclusions of significance to both ongoing research in this field and to the asset management practices of infrastructure owners. For researchers, the paper highlights the crucial importance of transient permeability and soil water retention behaviour of fill materials in controlling the magnitude and distribution of pore water pressure in response to climate and weather events. For practitioners, the work demonstrates that there are significant differences in pore water pressure behaviour across the embankment, which is influenced by construction-related issues such as compaction level, aspect and presence of a granular capping material. Permeability was also observed to vary across the embankment both spatially and with depth, being dependent on degree of saturation and macroscale effects, particularly within a ‘near surface zone’. It is proposed that this ‘near surface zone’ has a critical effect on embankment stability and should be the focus of both ongoing scientific research and inspection and monitoring as encompassed by asset management regimes

The impact of across-slope forest strips on hillslope subsurface hydrological dynamics

Forest cover has a significant effect on hillslope hydrological processes through its influence on the water balance and flow paths. However, knowledge of how spatial patterns of forest plots control hillslope hydrological dynamics is still poor. The aim of this study was to examine the impact of an across-slope forest strip on sub-surface soil moisture and groundwater dynamics, to give insights into how the structure and orientation of forest cover influences hillslope hydrology. Soil moisture and groundwater dynamics were compared on two transects spanning the same elevation on a 9° hillslope in a temperate UK upland catchment. One transect was located on improved grassland; the other was also on improved grassland but included a 14 m wide strip of 27-year-old mixed forest. Sub-surface moisture dynamics were investigated upslope, underneath and downslope of the forest over 2 years at seasonal and rainfall event timescales. Continuous data from point-based soil moisture sensors and piezometers installed at 0.15, 0.6 and 2.5 m depth were combined with seasonal (~bi-monthly) time-lapse electrical resistivity tomography (ERT) surveys. Significant differences were identified in sub-surface moisture dynamics underneath the forest strip over seasonal timescales: drying of the forest soils was greater, and extended deeper and for longer into the autumn compared to the adjacent grassland soils. Water table levels were also persistently lower in the forest and the forest soils responded less frequently to rainfall events. Downslope of the forest, soil moisture dynamics were similar to those in other grassland areas and no significant differences were observed beyond 15 m downslope, suggesting minimal impact of the forest at shallow depths downslope. Groundwater levels were lower downslope of the forest compared to other grassland areas, but during the wettest conditions there was evidence of upslope-downslope water table connectivity beneath the forest. The results indicate that forest strips in this environment provide only limited additional sub-surface storage of rainfall inputs in flood events after dry conditions in this temperate catchment setting

Monitoring rock freezing and thawing by novel geoelectrical and acoustic techniques

Automated monitoring of freeze-thaw cycles and fracture propagation in mountain rockwalls is 23 needed to provide early warning about rockfall hazards. Conventional geoelectrical methods 24 such as electrical resistivity tomography (ERT) are limited by large and variable ohmic contact 25 resistances, requiring galvanic coupling with metal electrodes inserted into holes drilled into 26 rock, and which can be loosened by rock weathering. We report a novel experimental 27 methodology that combined capacitive resistivity imaging (CRI), ERT and microseismic event 28 recording to monitor freeze-thaw of six blocks of hard and soft limestones under conditions 29 simulating an active layer above permafrost and seasonally frozen rock in a non-permafrost 30 environment. Our results demonstrate that the CRI method is highly sensitive to freeze-thaw 31 processes; it yields property information equivalent to that obtained with conventional ERT and 32 offers a viable route for non-galvanic long-term geoelectrical monitoring, extending the benefits 33 of the methodology to soft/hard rock environments. Contact impedances achieved with CRI are 34 less affected by seasonal temperature changes, the aggregate state of the pore water (liquid or 35 frozen), and the presence of low-porosity rock with high matrix resistivities than those achieved 36 with ERT. Microseismic monitoring has the advantage over acoustic emissions that events were 37 recorded in relevant field distances of meters to decameters from cracking events. For the first 38 time we recorded about 1000 microcracking events and clustered them in four groups according 39 to frequency and waveform. Compared to previous studies, mainly on ice-cracking in glaciers, 40 the groups are attributed to single- or multiple-stage cracking events such as crack coalescence