Field demonstration of an optical fiber hydrophone for seismic monitoring at Campi-Flegrei caldera

An Innovative Fiber Optic Hydrophone for Seismology: Testing Detection Capacity for Very Low-Energy Earthquakes

“…The responsivity evidently depends on the combination of the elastic and geometric properties of the FOH. The novel FOH was designed [4] to operate at low frequencies (below 100Hz) and was fabricated in a rugged fashion, equipped with pressure hydraulic compensation mechanics for underwater operation. Depending from consideration about the environmental and the self-noise of the interrogation system (intrinsic to system composed by the optical fiber and the electronics that convert the light in an electric voltage signal followed by an ADC), a target responsivity of 346 • 10 6 nm/µPa has been estimated (see 'Results and Discussion' section).…”

Innovative Photonic Sensors for Safety and Security, Part III: Environment, Agriculture and Soil Monitoring

“…The increasing demand of capillary and high-performance monitoring stations all over the world continuously requires technological advancements in the sensing devices and monitoring systems. In the next sections, we describe our recent research activities devoted to the development of optical fiber sensing systems for geophysical and volcanological monitoring [ 70 , 71 ].…”

“…Nonetheless, as highlighted by a recent review from Meng et al [ 102 ], there is a clear lack of low frequency hydrophones (specifically operating below 40 Hz), nor are they deployed on the seabed and demonstrated in operative conditions for vulcanological monitoring. To note, we reported in 2022 a field demonstration of fiber optic hydrophones for earthquakes monitoring in real operative scenarios [ 71 ]. Several works deserve to be mentioned for the achieved performances, even if most of them have been developed for SONAR applications (i.e., in the “acoustic” frequency range) and not for seismic monitoring applications (i.e., <100 Hz).…”

“…An acoustic wave with a wavelength larger than the hydrophone size behaves similar to a hydrostatic pressure for the hydrophone; that is, a uniform force is exerted on the external surface such as to cause a deformation of the complaint mandrel layer. The deformation involves either an expansion or a compression of the mandrel, which results in a strain in the optical fiber around it [ 71 , 109 ].…”

“…The responsivity of the FOH under the applied pressure can be expressed: where ∆ l f is the fractional variation in fiber length with respect to the applied pressure P , and is the radial displacement. To determine , which enables us to evaluate the responsivity, we performed a full 3-D numerical simulations by finite element method [ 71 ]. Specifically, the hydrophone design was devoted to obtaining a sensitivity as high to possess self-noise levels below sea-state zero and to offer a robust package for underwater operation.…”