Detection of ancient mine voids by using geophysical methods: the case of Castel-Minier

Summary The mining operations of Ouels, also named Castel-Minier, close to the village of Aulus-les-Bains in the Pyrenees, was mainly effective between the 13th and 16th centuries. It was the most important mine in the French Kingdom during the 14th century, and was operated for silver, cupper and lead. Today, it is a site for archaeological studies and the archaeologists are specially interested in the cavities left by the exploitation. It is with the aim of retrieving the voids left in a remote area that geophysical facilities are requested, following the discovery of a 20th century plan attesting to the existence of a shaft and a vein in this area. Two geophysical methods have been implemented: Electrical Resistivity Tomography (ERT) and microgravimetry. The first has been used on two parallel profiles spaced 15 m apart, and allowed the detection of a very high resistive body that could fit a cavity at less than a depth of 5 meters. On the other side, microgravimetry revealed two anomalies. It seems to confirm the one seen by ERT, while a second anomaly may attest to a deeper cavity hypothetically lying at 15 to 20 m depth. These hypotheses are compatible with the archaeological expectations.