f Multimethod geophysical survey on a karst area

A geophysical survey with several techniques was carried out on a karst area in southern Sardinia (Italy), where a large sinkhole, 15m deep and 18m wide suddenly occurred. The sinkhole was situated at a distance of 50m from a railway and therefore it could represent an impending danger. The aim of the geophysical survey was to verify wether the sinkhole or other karst phenomena were someway extended up to the railway's lay-out. The geology of the area is very complex since, even at very shallow depths, shows the results of several events among which two tectonic cycles, compressive and distensive respectively, erosion processes, two continental sedimentary cycles. The main lithotypes which characterise the area, from bottom to surface are as follows (Atzeni et al., 1994): 1) fractured Paleozoic limestones and dolomites with water circulation and karst phenomena, 2)Tertiary continental sedimentary formation mainly made up of sandstones and clay marls, 3)Quaternary sediments. As shown by several test holes, in the study area the thickness of Quaternary overburden is in the order of 1-5m and that of Tertiary sediments in the order of at least 5-20m. The Paleozoic basement, which outcrops no far, was found in one test hole only, at a depth of 12m.