f Combined geophysical methods for cavity detection in highly inhomogeneous material over karstic basement. A case history

Sometimes the need for an immediate action for applying a geophysical investigation is of vital importance particularly in cases where remedial measures are necessary to be conducted soon after the termination of field surveys. An example is given in this paper for an off-river reservoir at an elevation of approximately 80m above sea level, few hundred meters from the coastline, in the southwestern part of the Chania prefecture, in Crete island, Greece. The reservoir was under construction, partly by excavation and partly by filling the depressions, when the authors were called to investigate the reservoir area. They were faced with a sizeable subsidence in the form of an almost perfect cylindrical cavity having a diameter of 10 m and a height of 9 m, downstream of the embankment. The surface material is highly inhomogeneous and basically is composed of brown silty sand, gravel of very variable composition and huge blocks of rock. This overburden layer lies above a karstic breccia formation which is easily eroded due to the solution of gypsum present in the initial composition of breccias and extends up to sea level. In such a geologic environment subsidence phenomena are prone to be developed due to weathering factors that continuously enlarge the cavities by the downward flow of meteoric water and widening the existing karstic openings in the basement, as it has been shown in the past (Sowers 1984 and Newton 1984).