Studying the eigenfrequencies of candlestick stalagmites in the Han-sur-Lesse cave

Depending on their shape, some candlestick stalagmites have an eigenfrequency in the band of regional earthquake ground motions. Hence, an earthquake could break them if the ground movement is strong enough. Therefore the eigenfrequencies of speleothems are fundamental parameters in the study of their response to earthquakes. Such a study is currently in progress in the cave of Han-sur-Lesse (Ardenne, Belgium). The eigenfrequencies of the stalagmites were determined from the recording of ambient seismic noise.

To do this, a three-component seismic sensor able to record very weak movements and adaptable to the various geometry of stalagmites was manufactured from a Raspberry Shake 3D Personal Seismograph (RS). The RS initially integrates three orthogonal velocity sensors (Sunfull PS-4.5B), the digitizer, and the Raspberry Pi computer into a single plexiglass box. The RS was split and distributed around the stalagmite. The three geophones were separated from the acquisition system by modifying and extending the geophone wiring. Finally, to guarantee the orthogonality of the horizontal sensors, a 3D-printed support was created. This new configuration reduces the stresses caused by the sensor by distributing its weight around the stalagmite. It also reduces the impact of the sensor on frequencies measurements and the risks for fragile structures by reducing the global weight of the sensor attached to the stalagmite.

The eigenfrequencies of 16 candlestick stalagmites in the Han-sur-Lesse cave were determined thanks to this new configuration of seismic sensors. The polarization of the motions associated with these frequencies was also deducted. Moreover, the daily and weekly variation of ambient noise and transient events like quarry blasts, earthquakes or flooding events in the cave can be registered thanks to the two-weeks recording period of measurement for each stalagmite. Finally, these observations are complemented by the comparison of seismic movement at the surface and in cave, the study of the possible impact of the room and the substrate as well as the shape, mechanical properties and age of the stalagmites. These analyses will provide important information validating seismic hazard.