Acoustic Positioning for Deep Sea Neutrino Telescopes with a System of Piezo Sensors Integrated into Glass Spheres

Position calibration in the deep sea is typically done by means of acousticmultilateration using three or more acoustic emitters installed at knownpositions. Rather than using hydrophones as receivers that are exposed to theambient pressure, the sound signals can be coupled to piezo ceramics glued tothe inside of existing containers for electronics or measuring instruments of adeep sea infrastructure. The ANTARES neutrino telescope operated from 2006until 2022 in the Mediterranean Sea at a depth exceeding 2000m. It comprisednearly 900 glass spheres with 432mm diameter and 15mm thickness, equipped withphotomultiplier tubes to detect Cherenkov light from tracks of chargedelementary particles. In an experimental setup within ANTARES, piezo sensorshave been glued to the inside of such - otherwise empty - glass spheres. Thesesensors recorded signals from acoustic emitters with frequencies from 46545 to60235Hz. Two waves propagating through the glass sphere are found as a resultof the excitation by the waves in the water. These can be qualitativelyassociated with symmetric and asymmetric Lamb-like waves of zeroth order: afast (early) one with v_e ≈ 5mm/μs and a slow (late) one withv_ℓ≈ 2mm/μs. Taking these findings into account improves theaccuracy of the position calibration. The results can be transferred to theKM3NeT neutrino telescope, currently under construction at multiple sites inthe Mediterranean Sea, for which the concept of piezo sensors glued to theinside of glass spheres has been adapted for monitoring the positions of thephotomultiplier tubes.