f The Application of Geophysical methods to resolve a fascinating archaeological query

The most interesting cultural characteristics in Sardinia are defined by the presence of more than seven thousand ancient buildings called ‘nuraghe.’ They are three thousand years old. Their impressiveness bears witness to the great capability of building of the ancient Sardinian population. In fact Sardinia is the only part of the world where ‘nuraghe’ occur. The nuragic civilization constructed varied building forms: towers with ‘tholos’ rooves, huts, sacred wells, multi-towered castles and so on. Well construction in very soft, fractured rocks and in coastal areas also demonstrated the great capability in searching for water, a rare resource on the island of Sardinia. The ‘Cuccuru su Nuraxi’ well in the village of Settimo S.Pietro near Cagliari the capital of Sardinia, is of particular interest. The well is more than thirty metres deep, two metres large and is covered by granite and sandstone blocks, fifty centimetres thick . The well head is eleven metres deep, in a room with a tholos roof. The well and the room are underground. A vertical stair leads to the summit of a hill where nuraghe artifacts are found (figure 1). From the exterior, the system appears as a regular conical hill formed by marl rocks. The context is formed by Miocene marl. Other irregular conical hills are found. Beyond archaeological interest, construction details along with the geological constitution of the hill pose some fascinating queries: Is the well an isolated sacred well, or is it inserted into a majestic castle and is it a water supply well? Was the well constructed upwards or downwards? Is the hill natural or artificial? What methods have they used to find water (there is still water in the well!) under marl formation? To answer these questions , crucial for the planning of archaeological excavations, geophysical surveys are carried out. Taking advantage of the conical form of the hill, we executed: three horizontal seismic tomographies at different levels; many gravimetric `Nettleton' profiles; a pole- pole electrical survey. The seismic tomographies show a relative high velocity zone only in the section around the well (figure 2) and constant velocity in the other sections. The tomographies were made with different shots-geophones array to verify the anomaly zones. They probably show the well in side a larger nuragic structure. Nettleton profiles have been utilized to determine the density of the hill (1.88 g/cm3), as well as to identify differences from right trend probably corresponding to voids. The well is very well indicated and a zone of 1.5 g/cm3 seems to indicate an old excavation (a trench ?). The pole-pole electric survey shows resistive anomalies which can be attributed to nuragic walls around the known well.