An Automated time-Lapse Electrical Resistivity Tomography (ALERT) system has been developed for the long-term monitoring of coastal aquifers. This ALERT system has been permanently installed in the River Andarax, Almeria, Spain to monitor and manage the impact of climatic change and landuse practice on the underlying Quaternary aquifer. An electrode array, nearly 1.6 km long, has been buried below the normally dry river bed with electrode take-outs at regular intervals of 10 m. The maximum depth of investigation is about 160 m below ground level. An unmanned, permanent control station, in a secure location, allows the aquifer to be interrogated remotely from the BGS office in the UK. Volumetric geoelectric images of the subsurface can be obtained "on demand" or at regular intervals; thereby eliminating the need for expensive repeat surveys. The entire process from data capture to image on the office PC is fully automated and seamless. The ALERT technology can provide early warning of potential threats to vulnerable water systems such as over-exploitation, rising sea levels, anthropogenic pollutants and seawater intrusion. The electrical images obtained (in space and time) are interpreted in terms of the hydrogeologic features including the seawater-freshwater interface. The timely detection and imaging of groundwater changes can help to regulate pumping and irrigation schemes.

The optimum processing technique (2D vs. 3D inversion) to interpret and visualize parallel and/or orthogonal two-dimensional surface Electrical Resistivity Tomography data collected from archaeological sites is examined in this work. A simple modification of a standard resistance-meter geophysical instrument was implemented in order to collect parallel two-dimensional sections along the X-, Y-or XY-direction in a relatively short time, employing a pole-pole array.The sensitivity analysis showed that the distance between the parallel 2D lines must be smaller or, at the most, equal to the basic inter-electrode spacing in order to produce reliable 3D resistivity images of the subsurface. This was confirmed by modelling and inversion of both synthetic and real data.Direct comparisons of the quasi-3D images, resulting from a posteriori combination of the inverted 2D sections, with the full 3D inverted resistivity models indicated the superiority of the 3D inversion algorithm in the reconstruction of buried archaeological structures, even in complex archaeological sites. Due to the inherent three-dimensionality of many archaeological targets, quasi-3D images suffer from artefacts. The combination of a single survey-direction with a full 3D processing and interpretation scheme is adequate to image the 3D subsurface resistivity variation in detail. Furthermore, the implementation of a quasi-Newton Jacobian matrix update technique reduced the processing time by one-half without any significant loss of accuracy and resolution.

In this work a study of the effect of surface topographical variations on several dc resistivity arrays is presented. A 2.5-D finite‐element modeling scheme is used to examine the performance of several arrays over buried features that exist below a range of different topographical terrain contexts, such as valleys, hills, and steep slopes. A mesh‐generating algorithm allows a realistic representation of terrain topography. The results confirm that topographical variations can have a significant impact on the field resistivity data values for all resistivity arrays. Further, topographical variations can be treated flexibly using a realistic resistivity forward modeling process. Thus, topographic effects, to the degree they can be modeled, are predictable and should be taken into account when designing surveys and interpreting data.

Electrical Resistivity Tomography (ERT) has been applied in single boreholes to study leachate generation within a disused and unconfined landfill. Several boreholes were drilled in the landfill to characterize the waste and to establish the concentration and spatial variability of the leachate. To assist this study, six of these boreholes were instrumented by installing an electrode array on the outside of the PVC casing prior to completion. Resistivity measurements were taken every month from December 1999 to November 2000 to monitor the seasonal hydrochemical changes in the Chalk immediately beneath the landfill base. The results have been modelled using a novel reconstruction scheme tailored to the requirements of single-hole data. Apart from the expected matrix infiltration, the results suggest that contaminant transport in the Chalk may also occur as transient pulses in narrow fractures, when the waste is re-activated by changing groundwater levels. The study highlights the advantages of a volumetric imaging technique such as ERT to complement and optimise hydrochemical sampling.