Ground-penetrating radar investigation inside a karstified limestone reservoir,
Résumé
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 fractured massif is mainly characterized by water circulation and karstic structures. This kind of geological 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 antennas. 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.