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Application of Two Dimensional Electrical Resistivity Tomography (ERT) For Moisture Detection in Thessaloniki’s Rotunda Pillars and Three-Dimensional ERT Modeling Using Optimized Electrode Arrays

Prodromos Louvaris, Panagiotis Tsourlos, Gregory Tsokas, George Vargemezis, Nectaria Diamanti, Konstantinos Polydoropoulos et Georgia Zacharopoulou
p. 179-181

Résumé

– Moisture detection inside pillars using electrical resistivity tomography (ERT).

– Three-dimensional ERT modeling using non-conventional arrays.

– Jacobian matrix optimization method.

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Texte intégral

This work was accomplished on behalf of “EKATY: Innovative imaging of the subsurface of archaeological sites and the interior of structural elements of monuments in 3 and 4 Dimensions”. The project runs within the framework of the Operational Programme Competitiveness, Entrepreneurship and Innovation 2014-2020 (EPAnEK), Special Actions “Aquaculture” – “Industrial Materials” – “Open Innovation in Culture”, Τ6ΥΒΠ-00211.

1Electrical Resistivity Tomography (ERT) is widely known for providing valuable information for numerous archaeological prospection problems in two and three dimensions. The purpose of this work was to detect possible moisture inside one of the main pillars of Thessaloniki’s Rotunda (Region of Macedonia, Greece), a Unesco heritage monument using various geophysical methods, including two and three dimensional (2D & 3D) ERTs, as well as the Ground Penetrating Radar method (GPR).

22D ERT lines were initially performed directly on the wall, in order to show the pillar’s current state using 23 electrodes with conventional arrays such as the dipole–dipole and multi-gradient (Fig. 1). Holes for fixing the electrodes, approximately 6 mm wide, had to be drilled in the mortar of the wall. The electrodes were placed along a single line with inter-electrode spacing set to 0.6 m, encircling a large part of the pillar to obtain the maximum possible coverage. To minimize contact resistance, a small amount of bentonite clay was injected into each hole. The inverted results presented here delineate internal areas of low resistivity (Fig. 2), which are interpreted as regions of increased moisture; the GPR sections obtained over the same line have confirmed these findings.

Figure 1. Archaeological site plan (rotunda, Thessaloniki). ERT and GPR measurements were conducted by the eastern and northeastern structural pillars.

Figure 1. Archaeological site plan (rotunda, Thessaloniki). ERT and GPR measurements were conducted by the eastern and northeastern structural pillars.

Figure 2. Inverted results of 2D ERT located by the northeastern pillar, obtained using the multi-gradient array.

Figure 2. Inverted results of 2D ERT located by the northeastern pillar, obtained using the multi-gradient array.

3Besides, synthetic ERT models simulating a 3D grid of electrodes covering the pillar were generated prior to the on-site installation in order to optimize the measuring and installation scheme. The synthetic data were calculated using conventional and non-conventional arrays involving comprehensive and optimized protocols, following the circled pillar involving four lines of 12 electrodes each. The specific setup of electrodes in the presence of extreme topographical variation demands a stringent filtering process (Loke et al., 2014; Wilkinson et al., 2008) to calculate the optimized protocols. For this reason, an algorithm was developed to compute optimum measurements using already known techniques (Fig. 3). Some of the existing optimization techniques require the calculation of the Jacobian and Resolution matrices in order to compute the optimized protocol (Stummer et al., 2004; Wilkinson et al., 2006), which can be a time-consuming process. The method that was preferred among others due to its rapid protocol computation depends only on the Jacobian matrix (Athanasiou et al., 2009). This optimization technique does not require the calculation of the Resolution matrix and consequently makes it a time-saving method, giving the opportunity to rapidly recalculate or alter the existing optimum measurements. Synthetic data inversions are very promising and helpful in deciding the measurement geometry, which will be implemented in the field to operate in a time-lapse monitoring framework.

Figure 3. Cumulative Jacobian for calculated optimized 3D arrays, involving four lines with 12 electrodes each.

Figure 3. Cumulative Jacobian for calculated optimized 3D arrays, involving four lines with 12 electrodes each.
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Bibliographie

Athanasiou, E.N., Tsourlos, P.I., Papazachos, C.B., Tsokas, G.N., 2009. Optimizing Electrical Resistivity Array Configurations by Using a Method Based on the Sensitivity Matrix. Near Surface 2009, Dublin: 7-9.

Loke, M.H., Wilkinson, P.B., Uhlemann, S.S., Chambers, J.E., Oxby, L.S., 2014. Computation of optimized arrays for 3-D electrical imaging surveys. Geophysical Journal International, 199 (3): 1751-1764.

Stummer, P., Maurer, H., Green, A.G., 2004. Experimental design: Electrical resistivity data sets that provide optimum subsurface information. Geophysics, 69 (1): 120-139.

Wilkinson, P.B., Meldrum, P.I., Chambers, J.E., Kuras, O., Ogilvy, R.D., 2006. Improved strategies for the automatic selection of optimised sets of electrical resistivity tomography measurement configurations. Geophysical Journal International, 167: 1119-1126.

Wilkinson, P. B., Chambers, J.E., Lelliott, M., Wealthall, P., Ogilvy, R.D., 2008. Extreme sensitivity of crosshole electrical resistivity tomography measurements to geometric errors. Geophysical Journal International, 173: 49-62.

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Table des illustrations

Titre Figure 1. Archaeological site plan (rotunda, Thessaloniki). ERT and GPR measurements were conducted by the eastern and northeastern structural pillars.
URL http://journals.openedition.org/archeosciences/docannexe/image/9443/img-1.jpg
Fichier image/jpeg, 525k
Titre Figure 2. Inverted results of 2D ERT located by the northeastern pillar, obtained using the multi-gradient array.
URL http://journals.openedition.org/archeosciences/docannexe/image/9443/img-2.jpg
Fichier image/jpeg, 768k
Titre Figure 3. Cumulative Jacobian for calculated optimized 3D arrays, involving four lines with 12 electrodes each.
URL http://journals.openedition.org/archeosciences/docannexe/image/9443/img-3.jpg
Fichier image/jpeg, 1,6M
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Pour citer cet article

Référence papier

Prodromos Louvaris, Panagiotis Tsourlos, Gregory Tsokas, George Vargemezis, Nectaria Diamanti, Konstantinos Polydoropoulos et Georgia Zacharopoulou, « Application of Two Dimensional Electrical Resistivity Tomography (ERT) For Moisture Detection in Thessaloniki’s Rotunda Pillars and Three-Dimensional ERT Modeling Using Optimized Electrode Arrays »ArcheoSciences, 45-1 | 2021, 179-181.

Référence électronique

Prodromos Louvaris, Panagiotis Tsourlos, Gregory Tsokas, George Vargemezis, Nectaria Diamanti, Konstantinos Polydoropoulos et Georgia Zacharopoulou, « Application of Two Dimensional Electrical Resistivity Tomography (ERT) For Moisture Detection in Thessaloniki’s Rotunda Pillars and Three-Dimensional ERT Modeling Using Optimized Electrode Arrays »ArcheoSciences [En ligne], 45-1 | 2021, mis en ligne le 16 août 2021, consulté le 29 janvier 2023. URL : http://journals.openedition.org/archeosciences/9443 ; DOI : https://doi.org/10.4000/archeosciences.9443

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Auteurs

Prodromos Louvaris

Corresponding author, Laboratory of Exploration Geophysics, Aristotle University of Thessaloniki

Panagiotis Tsourlos

Laboratory of Exploration Geophysics, Aristotle University of Thessaloniki

Articles du même auteur

Gregory Tsokas

Laboratory of Exploration Geophysics, Aristotle University of Thessaloniki

George Vargemezis

Laboratory of Exploration Geophysics, Aristotle University of Thessaloniki

Articles du même auteur

Nectaria Diamanti

Laboratory of Exploration Geophysics, Aristotle University of Thessaloniki

Konstantinos Polydoropoulos

Laboratory of Exploration Geophysics, Aristotle University of Thessaloniki

Georgia Zacharopoulou

Hellenic Ministry of Culture, Ephorate of Antiquities of Thessaloniki

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