A former dolerite quarry and landfill site was investigated using 2D and 3D electrical resistivity tomography (ERT), with the aims of determining buried quarry geometry, mapping bedrock contamination arising from the landfill, and characterizing site geology. Resistivity data were collected from a network of intersecting survey lines using a Wenner-based array configuration. Inversion of the data was carried out using 2D and 3D regularized least-squares optimization methods with robust (L1-norm) model constraints. For this site, where high resistivity contrasts were present, robust model constraints produced a more accurate recovery of subsurface structures when compared to the use of smooth (L2-norm) constraints. Integrated 3D spatial analysis of the ERT and conventional site investigation data proved in this case a highly effective means of characterizing the landfill and its environs. The 3D resistivity model was successfully used to confirm the position of the landfill boundaries, which appeared as electrically well-defined features that corresponded extremely closely to both historic maps and intrusive site investigation data. A potential zone of leachate migration from the landfill was identified from the electrical models; the location of this zone was consistent with the predicted direction of groundwater flow across the site. Unquarried areas of a dolerite sill were imaged as a resistive sheet-like feature, while the fault zone appeared in the 2D resistivity model as a dipping structure defined by contrasting bedrock resistivities.

/pdf/electrical-resistivity-tomography-applied-to-geologic-3nk2inpzwh.pdf

Electrical resistivity tomography applied to geologic, hydrogeologic, and engineering investigations at a former waste-disposal site

Author(s): Whiteley, JS; Chambers, JE; Uhlemann, S; Wilkinson, PB; Kendall, JM | Abstract: Geophysical monitoring of landslides can provide insights into spatial and temporal variations of subsurface properties associated with slope failure. Recent improvements in equipment, data analysis, and field operations have led to a significant increase in the use of such techniques in monitoring. Geophysical methods complement intrusive approaches, which sample only a very small proportion of the subsurface, and walk-over or remotely sensed data, which principally provide information only at the ground surface. In particular, recent studies show that advances in geophysical instrumentation, data processing, modeling, and interpretation in the context of landslide monitoring are significantly improving the characterization of hillslope hydrology and soil and rock hydrology and strength and their dynamics over time. This review appraises the state of the art of geophysical monitoring, as applied to moisture-induced landslides. Here we focus on technical and practical uses of time-lapse methods in geophysics applied to monitoring moisture-induced landslide. The case studies identified in this review show that several geophysical techniques are currently used in the monitoring of subsurface landslide processes. These geophysical contributions to monitoring and predicting the evolution of landslide processes are currently underrealized. Hence, the further integration of multiple-parametric and geotechnically coupled geophysical monitoring systems has considerable potential. The complementary nature of certain methods to map the distribution of subsurface moisture and elastic moduli will greatly increase the predictive and monitoring capacity of early warning systems in moisture-induced landslide settings.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018RG000603

Geophysical Monitoring of Moisture-Induced Landslides: A Review

https://escholarship.org/content/qt2tw802pg/qt2tw802pg.pdf?t=pexpjc

Assessment of ground-based monitoring techniques applied to landslide investigations

Geophysical inversion methods are used as part of an interpretation process that seeks to differentiate geologic units. To improve the reliability of geologic differentiation based on recovered images from geophysical inversions, we have developed a multidomain clustering inversion algorithm that can incorporate statistical petrophysical data into a deterministic geophysical inversion framework through the use of the fuzzy c-means clustering technique. Petrophysical data are treated in the parameter domain in the same way that geophysical data are treated in the spatial domain, and these two different types of data are simultaneously inverted in their respective domains through the minimization of a single common objective function. The resulting physical property model honors the geophysical and petrophysical data and therefore can represent the earth better than geophysical inversion models that solely honor the geophysical data. Geophysical inversion and geology differentiation are generally tr...

Multidomain petrophysically constrained inversion and geology differentiation using guided fuzzy c-means clustering

Landslides pose significant risks to communities and infrastructure, and mitigating these risks relies on understanding landslide causes and triggering processes. It has been shown that geophysical surveys can significantly contribute to the characterization of unstable slopes. However, hydrological processes can be temporally and spatially heterogeneous, requiring their related properties to be monitored over time. Geoelectrical monitoring can provide temporal and volumetric distributions of electrical resistivity, which are directly related to moisture content. To date, studies demonstrating this capability have been restricted to 2D sections, which are insufficient to capture the full degree of spatial heterogeneity. This study is the first to employ 4D (i.e., 3D time-lapse) resistivity imaging on an active landslide, providing long-term data (three years) highlighting the evolution of moisture content prior to landslide reactivation and showing its decline post reactivation. Crucially the time-lapse inversion methodology employed here incorporates movements of the electrodes on the unstable surface. Although seasonal characteristics dominate the shallow moisture dynamics during the first two years with surficial drying in summer and wetting in winter, in the months preceding reactivation, moisture content increased by more than 45 % throughout the slope. This is in agreement with independent data showing a significant rise in piezometric heads and shallow soil moisture contents as a result of prolonged and intense rainfall. Based on these results, remediation measures could be designed and early-warning systems implemented. Thus, resistivity monitoring that can allow for moving electrodes provides a new means for the effective mitigation of landslide risk.

/pdf/four-dimensional-imaging-of-moisture-dynamics-during-m0tej7olct.pdf

Four-dimensional imaging of moisture dynamics during landslide reactivation

Electrical Resistivity Tomography (ERT) has been used to map the 3D spatial distribution of waste and leachate concentrations within a closed and unconfined landfill Borehole sampling and a 2D ERT

The Use of 3D Electrical Resistivity Tomography to Characterise Waste and Leachate Distribution within a Closed Landfill, Thriplow, UK

A novel method for the effective identification of bedrock subsurface elevation from electrical resistivity tomography images is described. Identifying subsurface boundaries in the topographic data can be difficult due to smoothness constraints used in inversion, so a statistical population-based approach is used that extends previous work in calculating isoresistivity surfaces. The analysis framework involves a procedure for guiding a clustering approach based on the fuzzy c-means algorithm. An approximation of resistivity distributions, found using kernel density estimation, was utilized as a means of guiding the cluster centroids used to classify data. A fuzzy method was chosen over hard clustering due to uncertainty in hard edges in the topography data, and a measure of clustering uncertainty was identified based on the reciprocal of cluster membership. The algorithm was validated using a direct comparison of known observed bedrock depths at two 3-D survey sites, using real-time GPS information of exposed bedrock by quarrying on one site, and borehole logs at the other. Results show similarly accurate detection as a leading isosurface estimation method, and the proposed algorithm requires significantly less user input and prior site knowledge. Furthermore, the method is effectively dimension-independent and will scale to data of increased spatial dimensions without a significant effect on the runtime. A discussion on the results by automated versus supervised analysis is also presented.

Distribution-based fuzzy clustering of electrical resistivity tomography images for interface detection

Two new ERT array optimisation strategies are presented that attempt to maximise the spatial resolution of the image for a given number of measurements. Synthetic modelling studies and inversions are used to assess the performance of the optimised arrays against arrays of standard Wenner-Schlumberger and Dipole-Dipole configurations. For the same number of configurations, the optimised arrays are found to perform at least as well as, and mostly better than, the standard arrays.

Comparison of the Spatial Resolution of Standard and Optimised Electrical Resistivity Tomography Arrays

Earthworks ground model development using surface wave surveys

Many parts of the UK’s rail network were constructed in the mid‐19th century long before the advent of
modern construction standards. Historic levels of low investment, poor maintenance strategies and the
deleterious effects of climate change have resulted in critical elements of the rail network being at
significant risk of failure. The majority of failures which have occurred over recent years have been
triggered by extreme weather events. Advance assessment and remediation of earthworks is, however,
significantly less costly than dealing with failures reactively. It is therefore crucial that appropriate approaches
for assessment of the stability of earthworks are developed, so that repair work can be better targeted and
failures avoided wherever possible. This extended abstract briefly discusses some preliminary results from an
ongoing geophysical research project being carried out in order to study the impact of climate or seasonal
weather variations on the stability of a century old railway embankment on the Gloucestershire Warwickshire
steam railway line in Southern England.

/pdf/assessing-climate-effects-on-railway-earthworks-using-masw-2yfc34o3t1.pdf

Assessing climate effects on railway earthworks using MASW

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