In spite of the widespread development of high-resolution seismic reflection techniques during recent years, the detection of underground cavities remains fundamentally unsolved. Except for some general principles, each case requires a particular study. Until now, the successful location of small (in regard to wavelength) cavities has been performed in an indirect way, i.e. by means of the detection of the cavity-induced perturbations of the seismic image of the regular geological structure (which should be known in advance) [1],[2],[3],[4] The direct detection of cavities by means of the reflection originating from the top of a void, though theoretically possible, gives no results in practice. [5]. This paper presents the successful application of the high resolution seisrnic reflection for the indirect detection and location of abandoned old chalk-pits, appearing in the north of France and being now unexploited. Some positions of these voids which form a network of tunnels are mapped, whereas others are unknown and represent a real danger for both population and building activities. The tunnels occur typically in chalk at a depth of 8-15 m and are followed by a water table and a man layer (Fig.1).


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