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can be made to complement and extend the triggered and automatically reported datasets.
The spreading out of MERMAID deployments did allow for a unique opportunity. After about one year of continu- ous operation in the Pacific, one MERMAID was recovered and its buffer was read out. Access to the continuous time series helps us understand detection sensitivity and eval- uate the performance of the triggering algorithm. The complete record includes the first-arriving seismoacoustic P waves of our primary interest but also later arriving wave types such as the purely hydroacoustic, or T, waves. The P waves mostly travel in the mantle before being converted to acoustic energy in the ocean. The T waves for the most part travel in the water layer itself, and they are thus not useful for imaging of the solid Earth.
Excising all those “signals,” we are left with the “noise” soundscape of the marine environment at 1,500 m depth, which enables us to study its seasonal variability. Figure 8 shows two noise power-spectral densities for different months. The sensitivity of the hydrophone decreases steeply below 0.1 Hz, and, in Figure 8, filtering has removed the signal above 10 Hz. The very low frequency (VLF) acoustics band above 5 Hz largely contains human- made noise, mainly from shipping. The broad peak of power is caused by wind and wave action at the ocean surface. Such noise is also observed at the ocean floor and even through reverse conversion of acoustic pressure to elastic waves on distant land stations. It constitutes the dominant “secondary microseismic” noise contribu- tion that Bradner set out to observe in situ back in the
1970s. We have come full circle. A group of MERMAIDs modified to report ambient noise spectra rather than earthquake signals was commissioned by Lucia Gualtieri at Stanford University (Stanford, CA) for deployment in the Mediterranean and the Atlantic in the year to come.
United We Dive: EarthScope Oceans
The MERMAID project evolved from a single proto- type to a fledgling fleet of promising newcomers to a now 50-strong array of robust third-generation units with a lifetime of 5 years that is currently floating about, collecting acoustic data for seismological science. The story of how we got there is one of selfless collabora- tion across generations, between individuals, institutions, and nations. The EarthScope Oceans consortium (see www.earthscopeoceans.org) was founded to coordinate
efforts worldwide. It intends to shepherd projects into the international arena where globally relevant, and mutu- ally agreed upon, decisions can be made on instrument development, science objectives, data management, and outreach activities, much like the land-based seismological academic community is doing today. Indeed, EarthScope Oceans already deposits data and metadata with the Data Management Center of the Incorporated Research Institu- tions for Seismology (IRIS) in Seattle, WA.
The Fourth Generation and the
Great Beyond
MERMAID’s fourth generation will carry more than just acoustic sensors, and the instruments will become fully programmable, even reprogrammable, midmission. The computer language developed for this purpose, MeLa (Bonnieux et al., 2020), is the bridge between engineers worried about hardware constraints, computer scientists specialized in the design of low-power embedded sys- tems, and the multiple end users who will simply want to focus on maximizing scientific data return. Obser- vation modes will be able to flexibly switch between, for example, earthquake observation, noise record- ing, whale call identification, and profiles of salinity or biogeochemical measurements. A mobile app, Adopt-A- Float, exists to animate classroom outreach activities. By including seismologists and acousticians, biologists and bioacousticians, physical and chemical oceanographers, meteorologists and climate scientists, and others, Earth- Scope Oceans will become even more multidisciplinary. Collaboration with other ocean-observing programs will enable cost-effective instrument deployment and oppor- tunistic recovery by nonspecialized vessels, such as cruise ships or pleasure yachts.
In the years to come, MERMAID will carry more instrumental payload, dive deeper, travel farther, and live longer. Many hands (on few decks) will make light work. Consider yourselves invited to join the commu- nity. Sound, after all, is an Essential Ocean Variable (see acousticstoday.org/ocean-soundscapes).
Acknowledgments
We owe a debt of gratitude to the editor, Arthur N. Popper, whose tireless advocacy on behalf of the readers of Acous- tics Today helped us immeasurably to improve the writing and presentation of this article. We also thank Micheal Dent and Helen A. Popper for numerous constructive edits.
Summer 2021 • Acoustics Today 49