© 2013 European Association of Geoscientists & Engineers 283 * guy.senechal@univ-pau.fr Near Surface Geophysics, 2013, 11, 283-291 doi: 10.3997/1873-0604.2013008 Ground-penetrating radar investigation inside a karstified limestone reservoir Guy Sénéchal 1* , Dominique Rousset 1 and Stéphane Gaffet 2 1 IPRA, CNRS-FR2952, Université de Pau et des Pays de l’Adour, France 2 UMR GEOAZUR 6526, UNS/CNRS/OCA, France Received October 2011, revision accepted January 2013 ABSTRACT In this paper, we present ground-penetrating radar (GPR) investigations performed along a 3.7 km long tunnel located inside a lower Cretaceous limestone massif of south-eastern France. This frac- tured massif is mainly characterized by water circulation and karstic structures. This kind of geo- logical formation contains a large part of the fresh underground water resources of the world and is also considered as an analogue of Middle East oil reservoirs. Since tunnel walls are covered by thick reinforced concrete, direct geological observations are impossible. After some preliminary tests, the entire tunnel was investigated using 250 MHz shielded anten- nas. Data are generally of very good quality, with reflection time up to 400 ns (down to 18 m under the tunnel floor with a velocity of 9 cm/ns). We correlate the GPR signal along the tunnel with surface geological observations: the upper part of the investigated formation (Bedoulian) displays prominent stratigraphic reflectors while the lower part (Barremian) does not. Numerous diffractions are observed in both formations and can be related to karstic features. These investigations allow to better constrain the geological context along the tunnel, necessary for future hydrogeological studies. We conclude that this tunnel offers a unique opportunity of performing GPR measurements within a karstified limestone massif. Moreover, hydrogeological contexts are often related to thin networks of water circulation that require a high resolution of investigation. All these considerations make GPR a very well adapted tool for hydrogeological purposes. Nevertheless, investi- gations from the surface are commonly affected by the shallow subsurface conditions (weathered zone) and a direct access inside the limestone massif itself induces strong advantages in terms of data quality (Van Vorst et al. 2011). The Low Noise Interdisciplinary Underground Science and Technology Laboratory (Laboratoire Souterrain à Bas Bruit – LSBB – http://www.lsbb.eu) of Rustrel is a decommissioned military facility. It constitutes an exceptional opportunity to study water circulation inside an unsaturated area of a karstic structure since galleries are directly located inside the limestone porous and fractured reservoir. Nevertheless, even if the tunnel allows to access inside the shallow and deep unsaturated areas of the reservoir, the direct observation of the local geology is now limited to unpublished studies produced during the 1960s by SEREB (Society for the Study and Realization of Ballistic Missiles) prior to the creation of the Strategic Missile Group of the ground based French nuclear dissuasion task operating from 1971. Consequently fur- ther hydrogeological studies need detailed characterization of the geological context using geophysical investigations. GPR imag- INTRODUCTION A water circulation study in the ground is a current problem extensively investigated globally, for example for water resource purposes. Karst systems contain important groundwater resourc- es for the global supply of drinking water (Ford and Williams 2007). Karst formations cover a significant part of territories: 33% in France (Zwahlen 2003) and 10% of the global surface (Ford and Williams 2007). Bakalowicz (2005) pointed out the high potential of karst as a water reservoir around the Mediterranean. Karstic structures constitute a specific case among various geological contexts. Hydrogeological studies are commonly based on field observations, including coloration, in order to better understand the subsurface pattern followed by water flow (Fleury et al. 2007; Garry et al. 2008; Perineau et al. 2011). Such surface observations can be widely supplemented by geophysical investigations. These measurements are performed using various approaches, such as seismic, DC resistivity or ground-penetrating radar (GPR). Beyond the capacity to charac- terize the geological context in terms of geometry, these tools are widely used because of their sensitivity to the water content (Yedlin et al. 2009; Yedlin et al. 2010), for a complete review of the methods see (Chalikakis et al. 2011; Maufroy et al. 2011).