International audienceThe karst environment is one of the most challenging in terms of groundwater, engineering and environmental issues. Geophysical methods can provide useful subsurface information in karst regions concerning , for instance, hazard estimation or groundwater exploration and vulnerability assessment. However, a karst area remains a very difficult environment for any geophysical exploration; selection of the best-suited geophysical method is not always straightforward, due to the highly variable and unpredictable target characteristics. The state of the art is presented, in terms of the contributions made by geophysical methods to karst-system exploration, based on extensive analysis of the published scientific results. This report is an overview and should be used as a preliminary meth-odological approach, rather than a guideline

The term "null array" is introduced for those electrode configurations where the measured potential difference is zero above a homogeneous half-space when using a measuring dipole M 0 N 0. Different types of null arrays (three-electrode, Schlumberger, and dipole axial/equatorial null arrays) and their corresponding traditional arrays are studied. It was shown in a field study carried out in a karstified limestone area covered by thin sediments that it is possible to obtain geologically meaningful results with null-array techniques. The main features of the null-array data are as follows. (1) Nullarray data appear to be more spatially variable than the classical data. The spatial variability provides information about the presence of karstic fractures in the subsurface; (2) The null-array anomalies caused by nearly vertical karstic fractures in the limestone basement do not decay with depth as quickly as the classical array anomalies. (3) The strike direction of the fractures is much less ambiguous than that found by using classical arrays. Nevertheless, the depth variation of the basement is more reliably observed in geoelectric anomalies obtained using traditional arrays. Therefore a joint use of classical arrays and their corresponding null methods is recommended, because the combined methods provide more information about the subsurface structure.

The knowledge of size, density and both orientation and vertical distribution of fractures as well as their opening and filling material or overburden thickness is a valuable contribution to estimating hydraulic conductivity and to evaluating the vulnerability and protection strategy of karst aquifers. To obtain some of these parameters and to ascertain high‐permeability zones, the Very Low Frequency‐Electromagnetic Gradient (VLF‐EM GRAD) method was applied, together with Radio Frequency‐Electromagnetics (RF‐EM), Radiomagnetotellurics (RMT), Geoelectrical Tomography and refraction seismics, over a karstic terrain in the Swiss Jura Mountains. In this area, karst springs infiltrate a porous aquifer. This survey investigated the highly permeable karst structures, which provide fast water‐infiltration pathways into the karstic flow network. A dye tracer test validated the geophysical results. The results show the efficiency and reliability, particularly of the VLF‐EM GRAD method, for high‐resolution investigation at shallow depths and for its potential for fast data acquisition over large surfaces at catchment area scale without ground contact.

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