Monitoring underwater seismic activity in the Campi-Flegrei caldera with fiber optic seismic hydrophones

In the present study, we have successfully developed and tested a fibre optic seismic hydrophone. These hydrophones use a fibre coil interferometer in Michelson configuration carefully wrapped around a sensitive composite cylinder. The materials and dimensions of the hydrophone were carefully determined by numerical analyses to achieve zero resolution in the sea state. The hydrophones were installed in an underwater module at a depth of 40 metres and integrated into the existing national seismological monitoring system of the Campi Flegrei caldera. In particular, the hydrophone was installed in a submarine module onto the seabed in the Gulf of Pozzuoli and the optoelectronic readout system was placed on an instrumented geodetic buoy. During the field tests carried out in the area, several earthquakes occurred. The seismic activity of the caldera was successfully recorded by the fibre optic hydrophones and the corresponding spectrograms have been here reported. To evaluate the performance of the fibre optic hydrophone, the recordings were compared with those of a piezoelectric reference hydrophone. The comparison of the traces recorded by the optical and reference hydrophones reveals a high level of similarities with cross-correlation higher than 85 %. Additionally, the seismic sequence was used to retrieve the sensor responsivity in the frequency range 1–80 Hz. We found a responsivity of about -300 nm/Pa enabling an average noise floor level down to 100μPa/√Hz. Overall, the comprehensive results of the field test have clearly demonstrated the effectiveness of the fibre-optic hydrophones for geophysical and volcanological monitoring purposes.