Research of intact and vulnerable stalagmites in caves of Belgian Ardennes, a contribution to seismic hazard assessment in Belgium - First tests.

A good assessment of the long-term seismic hazard is necessary for the management of critical infrastructure, Seveso or nuclear industries. One way to do this is to evaluate ground motions necessary to break candlestick stalagmites because the presence of undamaged candlestick stalagmites in a specific cave provides invaluable information about the level of ground motions that have not been reached locally.

The knowledge of the transfer function of ground motion inside cave compared to the outdoor is necessary to transpose these underground observations to the surface and thus to reassess seismic hazard levels. Effects related to the depth, the size of the rooms and the substratum of the stalagmites must be considered.

We undertook an investigation in the Han-sur-Lesse cave (Belgian Ardennes) at less than 100 km E/SE of the Rhine Graben. The first tests of the methods envisaged already give a good overview of the possibilities and precautions to take to avoid damaging the stalagmites. We conducted a 3D scan of stalagmites to measure their shape precisely and to model their natural resonance frequency. The installation of seismic sensors on an metal post (traffic sign post, without sign) without forced excitation shows the feasibility of studying the natural resonance frequency with seismic noise. Finally, simplified numerical modelling of a stalagmite as well as analytic tests show the influence of sensors on frequency measurements (because of their weight) and suggests limiting this effect by putting the sensors lower and favour lighter sensors as much as possible.