Measuring the eigenfrequencies of candlestick stalagmites with a custom 3D-printed sensor modified from a Raspberry Shake

The eigenfrequencies of speleothems are fundamental parameters in the study of their response to earthquakes. To study these, the seismic ambient noise is measured by three-component seismic sensors adapted to the geometry of the speleothems. This method is currently being studied in the Han-sur-Lesse cave (Ardenne, Belgium). A previous study (Martin et al. ; 2020) was carried out with a SmartSolo IGU-16HR 3C sensor on an imposing 4.5 m tall stalagmite. This approach demonstrated the feasibility and interest of studying the eigenfrequencies of stalagmites from ambient noise. However, this sensor was too heavy for use on thin and slender stalagmites. The challenge was to find and adapt a lighter sensor able to record very weak movements while being easily adjustable to the various shapes of the stalagmite and securely attachable on these to reduce the impact of the sensor on frequencies measurements and the risks for the fragile structure. A solution was found by using a Raspberry Shake 3D Personal Seismograph (RS) that initially integrates three orthogonal velocity sensors (Sunfull PS-4.5B), the digitizer, and the Raspberry Pi computer into a single plexiglass box. The RS has the advantage of being less heavy while being composed of three weak motion geophones. After a comparison study, this sensor gives similar results for eigenfrequency and polarization analyses. However, the use of this new sensor on thin and slender stalagmites requires the creation of suitable support. The RS was split and distributed around the stalagmite. The geophone wiring was modified and extended to separate the geophones from the acquisition system. A 3D-printed support was created to guarantee the orthogonality of the horizontal sensors while reducing the stresses by distributing the weight of the sensor around the stalagmite. This new configuration allowed determining the eigenfrequencies of 16 thin and slender stalagmites in the Han-sur-Lesse cave (Ardenne, Belgium) and the polarization of the motions associated with these frequencies. Moreover, a two-week recording period allows to measure the daily and weekly variation of ambient noise and transient events like earthquakes, quarry blasts or flooding events in the cave.