Application of the "impact echo method" to the detection of underground cavities part II: field experiments

This paper presents the practical verification of the theoretical results obtained in Part 1. The "impact echo method" (IEM), is applied in order to verify the possibility of detection of holes in concrete and cavities in soiI. The advantages of IEM are its simplicity and low cost. It only requires one sensor, an impulsive seismic source and a single channel acquisition system. An original time-frequency analysis procedure which enables the automatic research of an optimum temporal window of observation is used. The joint procedure including acquisition and processing is further referred as Time Frequency Analysis of Seismie Reflection (TFASR). The field experiments were performed on sites where the positions of voids are known. In the case of concrete, the experiments were undertaken on a slab with three thicknesses (0.2m, 0.35m and 0.5m) within which some pipes of different diameters and polystyrene blocks were hidden at various depths. In the case of real underground cavities, the experiments were conducted in an "ideal" setting, because the cavities are located near the surface, in chalk. Their depth varies between 3.5 m and 8 m, and their size varies between 4 m and 12 m.