Page 47 - Summer 2021

Figure 5. Design schematic of the third-generation MERMAID float. Image courtesy of Yann Hello and OSEAN SAS.
Future versions will sustain performance to full-ocean depths and may be equipped with “landing” capabilities such that MERMAID can become a temporary ocean- bottom hydrophone. This will allow some researchers to shift their focus from the global recording of distant earthquakes to monitoring local and regional events and listening for landslides, cracking glaciers (Deane et al., 2019), and other nonseismic signals.
Five Years
With Yongshun John Chen and his team from SUSTECH in Shenzhen, China, in the lead, the French, Japanese, and US partners joined forces. Their attention has turned to the volcanic islands of Polynesia including Tahiti, Samoa, and Marquesas among many others. What lies beneath them? A broad region of anomalously slow-wave velocities in the mantle deep under the South Pacific may feed these volcanoes via conduits of hot uprising rock that may stretch all the way from the core-mantle boundary to the surface (French and Romanowicz, 2015). High-resolution tomography is needed to understand the fine-scale nature of this planetary hot-rock plumbing system. The 50 MERMAIDs launched so far (Figure 6) will give scientists the large-aperture array necessary to perform detailed imaging at depth.
We have arrived at the point where acoustics ends and seismology begins. The analysis pipeline (Simon et al., 2020), focuses on matching the seismograms reported by MERMAID to known earthquakes such that seismic wave speeds can be measured. Figure 7 shows seis- mograms from two distant earthquakes recorded in the Pacific. The solid travel-time curve is a prediction, based on a reference Earth model that contains only depth-based subdivisions of wave speed and no lateral variations. The seismic waves arrive around the predicted times, but the signal of the three-dimensional Earth with its internal wave speed variations lies in precisely how closely these measurements agree with the predictions. Seconds matter!
One Year of Noise
MERMAID is a low-cost solution to instrumenting the oceans for seismology, most of all because deployment is extremely straightforward. No specialized equipment or personnel is required, and recovery is possible but not man- datory if it is not cost effective. Hours after deployment at sea, the data files start accumulating on the scientists’ desk- top computers. A running buffer of one year is maintained, and, thanks to two-way communication capability, requests
Figure 6. Launch of a MERMAID in the waters around Tahiti. No specialized equipment is needed. Once waterborne, MERMAID is completely autonomous and reports earthquake records via satellite, on average every 6.25 days. Photograph courtesy of Frederik J. Simons.
Summer 2021 • Acoustics Today 47