P. Tsourlos

Automated monitoring of coastal aquifers with electrical resistivity tomography

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 land-use practice on the underlying Quaternary aquifer. An electrode array, nearly 1.6 km long, has been buried below the normally dry riverbed 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 effect of terrain topography on commonly used resistivity arrays

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.

Efficient ERT measuring and inversion strategies for 3D imaging of buried antiquities

ABSTRACT 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.

A large scale geophysical survey in the archaeological site of Europos (northern Greece)

Abstract The results of a large scale exploration of an archaeological site by geophysical means are presented and discussed. The operation took place in the site where the ruins of the ancient city of Europos are buried. This site is in northern Greece. Resistivity prospecting was employed to detect the remnants of wall foundations in the place where the main urban complex of the ancient city once stood. The data were transformed in an image form depicting, thus, the spatial variation of resistivity in a manner that resembles the plane view of the ruins that could have been drawn if an excavation had taken place. This image revealed the urban plan of the latest times of the life of the city. Trial excavations verified the geophysical result. Magnetic prospecting in the same area complemented the resistivity data. The exact location of the fire hearths, kilns and remnants of collapsed roofs were spotted. Magnetic gradient measurements were taken in an area out of the main complex of the ancient city and revealed the location of several kilns. One of these locations was excavated and a pottery kiln was discovered. The resistivity prospecting in one of the graveyards of the ancient city showed anomalies which were expected and corresponded to monumental tombs. The locations of a few of them were excavated and large burial structures were revealed. Ground probing radar profiles were measured over the tombs which showed pronounced resistivity anomalies, so far unearthed. The relatively high resolving ability of the method assisted the interpretation in the sense that a few attributes were added. In the presented case, it was concluded that a particular tomb consists of two rooms and that it is roofless.

Non-destructive DC resistivity surveying using flat-base electrodes

The application of flat-base electrodes to geoelectrical measurements is examined. This study is motivated by the inability to apply geoelectrical techniques in environments where conventional spike electrodes cannot be inserted into the ground. The performance of flat-base electrodes is examined in various environments, using different measuring modes. It is shown that flat-base electrodes can be satisfactorily used in most cases, producing data that are almost identical to the measurements obtained using standard electrodes. Several case studies in various urban locations in Greece, where flat-base electrodes have been successfully employed, are also presented. The results indicate that flat-base electrodes provide the advantage of a fully non-destructive application and, therefore, the extension of geoelectrical methods to environments that, otherwise, would not have been considered suitable.

4D time-lapse ERT inversion: introducing combined time and space constraints

Time-lapse Electrical Resistivity Tomography (ERT) can be used to characterize dynamic processes occurring in the subsurface of the Earth. It involves the installation of a permanent array of electrodes to monitor changes in resistivity associated with changes in pore-water properties (salinity, temperature, water content) or porosity (compaction or dilation). The interpretation of time-lapse data is complicated by both the presence of noise in the data and the influence of low sensitivity in parts of the model. A uniform space and time constraint is not able to address this problem. In this work, we propose a new approach to distinguish noise-related artefacts to true changes in resistivity, while at the same time addressing the problem of the lack of sensitivity of electrical resistivity tomography with depth. We propose transforming the space and time constraints to be active, meaning that the regularization parameters are distributed rather than being uniform for the entire model. This way, both time-related noise (assumed to be random) in the data and the lack of sensitivity are addressed and we can incorporate prior information in a natural way into the inversion scheme. Using this strategy, the inversion scheme is able to favour areas where the expected changes are likely to occur while filtering out areas where no changes should occur. The favoured areas can be either selected from a preliminary analysis of the data, or by incorporating other types of prior information into the system based on the process that is monitored.

A large-scale magnetic survey in Makrygialos (Pieria) Greece

A large-scale magnetic survey was conducted in the archaeological area of Makrygialos. The site was threatened due to the construction activities carried out in the area, as part of the national highway re-route project. Geophysical prospection contributed to the archaeological evaluation of the site, which was based mainly on the salvage excavations that took place prior to and after the geophysical survey. Magnetic prospecting was applied on a routine base, in order to cover a large area in a short period of time. Also, magnetic susceptibility was used to acquire detailed information of the stratigraphy of the ditches revealed by the excavations. The Le Borgne contrast was calculated and was used as an index of the magnitude of the magnetic anomalies. Geophysical data were processed by a number of filtering techniques, including the removal of regional trends and Hanning smoothing. Fourier transformation was applied and bandpass filtering procedure was based on the examination of the power spectrum of the data. In addition, two-dimensional inversion filtering was performed at specific parts of the data set, in an effort to rectify the significant geophysical anomalies of the site and obtain more information about their width and magnetization. The results of the geophysical survey were able to highlight a system of three curvilinear ditches, which excavation data suggested were probably dug during the Neolithic period. Various linear and geometrical anomalies, related to subsurface structures, are included among the other geophysical features encountered at the site. The geophysical prospecting techniques were able to map more than 60,000 m2 of the site, a large portion of which has now been destroyed by the construction activities for the national road. In this way, geophysical maps can be used as a valuable source of information for the future study of the site. The present case study illustrates the impact of geophysical exploration in the management of archaeological sites threatened by large-scale construction projects.

IP4DI: A software for time-lapse 2D/3D DC-resistivity and induced polarization tomography

We propose a 2D/3D forward modelling and inversion package to invert direct current (DC)-resistivity, time-domain induced polarization (TDIP), and frequency-domain induced polarization (FDIP) data. Each cell used for the discretization of the 2D/3D problems is characterized by a DC-resistivity value and a chargeability or complex conductivity for TDIP/FDIP problems, respectively. The governing elliptic partial differential equations are solved with the finite element method, which can be applied for both real and complex numbers. The inversion can be performed either for a single snapshot of data or for a sequence of snapshots in order to monitor a dynamic process such as a salt tracer test. For the time-lapse inversion, we have developed an active time constrained (ATC) approach that is very efficient in filtering out noise in the data that is not correlated over time. The forward algorithm is benchmarked with simple analytical solutions. The inversion package IP4DI is benchmarked with three tests, two including simple geometries. The last one corresponds to a time-lapse resistivity problem for cross-well tomography during enhanced oil recovery. The algorithms are based on MATLAB^(R) code package and a graphical user interface (GUI).

Time-lapse Monitoring in Single Boreholes Using Electrical Resistivity Tomography

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 volumetri...

Measurement and inversion schemes for single borehole-to-surface electrical resistivity tomography surveys

Several measurement arrangements involving different electrode arrays are considered for the case of single borehole-to-surface ERT surveys. Synthetic data tests indicate that standard measuring and inversion techniques can result in reconstructed images that suffer from artefacts. Based on the measurement sensitivity analysis, we propose a modified inversion scheme that compensates for the symmetric sensitivity of the in-hole measurements and improves the quality of the image. The new scheme has been tested using both synthetic and field data. The best results were obtained using pole–dipole or dipole–dipole arrays. Finally, some recommendations are made to improve the efficiency of field measurement configurations.