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The term "geoelectric null-array" is used for direct current electrode configurations yielding a potential difference of zero above a homogeneous half-space. This paper presents a comparative study of the behaviour of three null-arrays, midpoint null-array (MAN), Wenner-γ null-array and Schlumberger null-array in response to a fracture, both in profiling and in azimuthal mode. The main objective is to determine which array(s) best localise fractures or best identify their orientation. Forward modelling of the three null-arrays revealed that the Wenner-γ and Schlumberger null-arrays localise vertical fractures the most accurately, whilst the midpoint null-array combined with the Schlumberger null-array allows accurate orientation of a fracture. Numerical analysis then served as a basis to interpret the field results. Field test measurements were carried out above a quarry in Les Breuleux (Switzerland) with the three null-arrays and classical arrays. The results were cross-validated with quarry-wall geological mapping. In real field circumstances, the Wenner-γ null-array proved to be the most efficient and accurate in localising fractures. The orientations of the fractures according to the numerical results were most efficiently determined with the midpoint null-array, whilst the Schlumberger null-array adds accuracy to the results. This study shows that geoelectrical null-arrays are more suitable than classical arrays for the characterisation of fracture geometry.

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the local and regional structural pattern of volcanic edifices strongly controls the space distribution of electrical resistivity. Here we report on the structural context of the western part of the Las cañadas caldera of tenerife (Lcc) thought to have initiated the formation of the caldera. Using a new dataset of 11 audiomagnetotelluric tensors we emphasize the resistivity distribution of Ucanca caldera and propose a major revision of its extension. We find that Ucanca caldera has a limited westwards extent and that El cedro sector is a depression margin of the caldera. According to the extent of hydrothermalized rocks at the base of the Lcc wall and the distribution of Pico teide -Pico Viejo vents, we constrain the location and size of Ucanca caldera. the interpretation of these results also constrains the extension of the Icod Valley and proposes a headwall located below the Pico teide -Pico Viejo complex.

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