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Fiber optic hydrophones for geophysical and volcanological monitoring

Here, we report on the development and field test of a fiber optic seismic hydrophone. The sensing configuration exploits a fiber coil interferometer in Michelson configuration, wrapped around a sensitive composite cylinder. The materials and the size of the hydrophone were selected by numerical analysis in order to offer performance suitable to operate with resolution down to the sea state zero in underwater environments. The sensing system was integrated in the seismological monitoring system of the Campi Flegrei caldera. During the field trials, we detected several earthquakes occurred in the area and compared the results with a reference piezoelectric hydrophone. The seismic sequence was used to retrieve the sensor responsivity in the frequency range 1-80Hz. The sensing system exhibited a responsivity of about -300nm/Pa and an average noise floor level down to 100µPa/√Hz. The reported field trial demonstrated the capability of optical fiber hydrophones to operate for geophysical and volcanological monitoring.