Materials and Structures/Mat~riaux et Constructions, Vol. 29, April 1996, pp 158-163 Evaluation of electrical resistivity maps for ancient masonry D. Van Gemert, H.Janssens, F. Van Rickstal Department of Civil Engineering, Katholieke Universiteit Leuven, De Croylaan 2, B-3001 Heverlee, Belgium ABSTRACT R~SUM~ The use of electrical resistivity maps for the evaluation of ancient masonry is a well-known non-destructive mea- suring technique. However, these maps do not only contain the information sought, i.e. the localisation of the inhomogeneities, but also the influence of the physical lay- out and the geometrical boundaries on the measurements. These elements partly hide the desired information. Therefore, computer programs are developed in which this particular boundary effect is incorporated. This leads to maps of relative differences in resistivity that are a picture of the real anomalies in the structure. The evaluation and interpretation of the electrical resistivity maps are thereby made much simpler. I , , , / )l , L'utilisation de cartes de reszsttvlte pour l evaluatzon de la ma~onnerie ancienne est une technique non-destructive d~jh bien connue. Cependant ces cartes contiennent non seulement l'information d~sir~e, c'est-h-dire la localisation des inhomog~nSit~s, mais aussi l'influence des bords g~om~triques. Ces ~l~ments masquent partiellement l'information de'sirSe. On a d~velopp~ des programmes qui prennent en compte cet effet de bord. Cela donne des cartes de diff&ences relatives qui sont une image des anomalies r~elles de la structure interne. Par voie de consequence, ~. / . ~z , z 9 9 , z 1 mterpretatton et I evaluation des cartes de resmwtte ~lectrique se trouvent simplifi~es. 1. INTRODUCTION An electrical resistivity map is a useful instrument to analyse ancient masonry for restoration purposes [1]. The information on the internal state of the structure visualised in an electrical resistivity map is partly masked due to the influence of the limited dimensions of the analysed masonry element. This observation led to the idea of making maps in which the influence of the boundaries would be eliminated. The resulting maps can then directly be correlated to the real properties or ano- malies of the structure. The influence of the geometrical boundaries on a resistivity measurement can be formulated in a parametrical way. The underlying theory is based on the image theory. In this theory, every interface between two materials with different material properties is considered as a semi-transparent mirror that transmits a part of the current and reflects the rest. The formulae are translated into computer programs and the validity of the procedures is tested using case studies. 2. INFLUENCE OF GEOMETRICAL BOUNDARIES A boundary in this context is not only an external boundary, but also the interface between two materials with a different electrical resistivity. The image theory states that each boundary acts as a semi-transparent mirror for the electrical current [2,3]. One part of the current lines penetrates this mirror, while the other part is reflect- ed. At any point in the structure, the electrical current can be considered as being the sum of the current provided by /2 / / : ' : Editorialnote ..... Prof. Dionys Van Gemert isa RILEM Senior Member. He is involvedin the workofRILEM Technical Committee 151-APC on Adhesiontechnology in concrete engineering - Physicaland Chemicalaspects. For severalyears, Prof. Van Gemerthas sewedon the Scientific Committee ofMaterials and Structures. 0025-5432/96 9 RILEM 158