Page 43 - Summer 2021

not drift). Instead of measuring ground displacement directly, hydrophones now listened for ground motion conversions to acoustic water pressure variations.
Although these approaches jumpstarted the use of hydroacoustics for earthquake seismology, subsurface moorings were expensive and surface sonobuoys were noisy and had short lifetimes. As a result, the idea of having any seismological instrumentation drift with the currents at depth, let alone at the surface, was largely abandoned. Earthquake signals picked up by newer float models incorporating both geophones and hydrophones did continue to get reported through the late 1980s, but design improvements of ocean-bottom seismometers ultimately took the science of instrumenting oceanic areas for global seismology in an altogether different, and successful, new direction (Suetsugu and Shiobara, 2014). Semipermanent hydrophone arrays in the oceans today continue to play a role in Nuclear Test Ban Treaty verification (Bradley and Nichols, 2015), but they do not reliably detect earthquake arrivals.
Nevertheless, the costs of deploying and recovering ocean- bottom sensors remain prohibitive, and the vision of a long-lived network of easily launched, passively drifting low-cost hydrophones to detect and report distant earth- quakes lived on. Guust Nolet, at Princeton University in New Jersey, carried on building the science case. In the decades since the earliest forays, battery technology leapt forward, the Global Positioning System enabled precise surface location and timing anywhere in the oceans, and satellite communication matured to the point where com- mercial systems provided coverage anywhere on Earth, allowing for near real-time data transmission.
Ocean-float technology also came of age (Gould, 2005). By the mid- to late 1990s, the development of freely drifting repeatedly diving “profiling” floats brought together the oceanographic community into worldwide collaboration. Fast forward, and as of this article, some 4,000 so-called
“Argo” floats (see argo.ucsd.edu) are surfacing every 10 days, collectively returning almost 400 conductivity-tem- perature-depth profiles per day for oceanographic and climatological research. Additionally, novel biological and geochemical sensors (Riser et al., 2018) have vastly widened the range of instrumental capabilities compared with what was originally envisioned.
Fifty Mermaids
In the early 2000s, John Orcutt and Jeff Babcock at the Scripps Institution of Oceanography in La Jolla, CA, spearheaded the development of a system for
“Mobile Earthquake Recording in Marine Areas by Independent Divers” (MERMAID). They equipped an oceanographic float with a hydrophone recording package, and the first-generation MERMAID was born (Simons et al., 2006).
The development of MERMAID is told starting in First Sound. Spoiler alert! Here’s how this story ends. Fifty years after Bradner et al.’s (1970) visionary instru- ment, 50 third-generation MERMAIDs, each with a life span of about 5 years, have been launched in the Pacific, freely drifting at depth but surfacing for data transmission and reporting seismograms triggered by earthquake sources around the world, ready for seismo- logical analysis (see Figure 2). A recent modification designed to report acoustic spectral densities for the environmental analysis of marine sound is set to debut in the Mediterranean this year.
A consortium, EarthScope Oceans, now coordinates autonomous midwater robotic seismometry worldwide while hatching ambitious plans for other applications of hydroacoustics and ocean observation writ large. Here, we provide a brief history and a current status report and share our dreams for the future.
Figure 2. Location of last surfacing of the instruments in the South Pacific Plume Imaging and Modeling array of third-generation Mobile Earthquake Recording in Marine Areas by Independent Divers MERMAID instruments, an international project coordinated by the EarthScope Oceans consortium (see www.earthscopeoceans.org). The legend identifies every instrument’s institutional owner. Image courtesy of Jonah N. Rubin.
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