mermaid (Mobile Earthquake Recording in Marine Areas by Independent Divers) is a more established recent alternative (Simons et al. 2018) may hold promise for extending the range of existing seismic arrays (e.g. Repurposing ocean-bottom telecommunication optic fibers for distributed acoustic sensing (e.g. The logistical difficulties and high costs of installation and data recovery of these devices render such methods not viable for filling vast gaps in the ocean with sufficient station density for seismic tomography. 1993) and ocean bottom seismometers (e.g. Approaches to mitigate this problem include installing moored hydrophones (e.g. Global seismic tomography, the imaging of 3-D wave speed structure inside the Earth (Ritsema & Lekić 2020 Tromp 2020), is data-limited by the sparsity of oceanic stations (Romanowicz 2008). Pacific Ocean, Seismic noise, Infrasound 1 INTRODUCTION These produce secondary microseisms at acoustic frequencies between 0.1 and 1 Hz according to the well-known frequency-doubling mechanism. The noise record is extremely well explained, both in spectral shape and in temporal variability, by the interaction of oceanic surface gravity waves. We relate the temporally varying noise spectral density to a time-resolved ocean-wave model, WAVEWATCH III. After removing all seismic signals from the record, and also those from other transient, dominantly non-seismic, sources, we are left with the infrasonic ambient noise field recorded at 1500 m depth. Only 10 earthquake records had been automatically reported by the instrument-the others were deemed low-priority by the onboard processing algorithm. The observed signals contain seismoacoustic conversions of compressional and shear waves travelling through crust, mantle and core, including P, S, Pdif, Sdif, PKIKP, SKIKS, surface waves and hydroacoustic T phases. Using time-domain, frequency-domain and time-frequency-domain techniques to comb through the time-series, we identified signals from 213 global earthquakes known to published catalogues, with magnitudes 4.6–8.0, and at epicentral distances between 24° and 168°. We present a unique 1-yr-long data set of sound recorded at frequencies between 0.1 and 20 Hz in the South Pacific around French Polynesia by a mermaid float that was, in fact, recovered. Reporting seismograms in near-real time, within hours or days after they were recorded, the instruments are not usually recovered, but if and when they are, their memory buffers can be read out. The instruments are programmed to detect and transmit acoustic pressure conversions from teleseismic P wave arrivals for use in mantle tomography. A fleet of autonomously drifting profiling floats equipped with hydrophones, known by their acronym mermaid, monitors worldwide seismic activity from inside the oceans.

Spectral comb sound reference signals