A fully autonomous, long-lived marine geophysical instrument, the Son-O-Mermaid, was developed and with this grant, it will be tested at sea in three month-long deployments offshore the Bahamas, where deep water is easily accessible and ocean conditions generally favorable and well-characterized. Son-O-Mermaid is a freely drifting buoy that (1) derives energy from wave action, enough to power (2) a vertical array of hydrophones suspended from a compliant cable connected to a damping plate below the waves, (3) a full-ocean-depth echo sounder, (4) GPS for location and timing accuracy, (5) an IRIDIUM satellite communication unit for near real-time data transfer, and an (6) on-board digitizing and processing unit. No similar instrument exists. The predecessor instrument, MERMAID, differs in fundamental aspects from our Son-O-Mermaid. MERMAID is wholly lacking (1), (3) and the array aspect of (2). The Son-O-Mermaid capabilities (4)-(6) are always "on": the instrument is self-powered and as a whole never completely submerged at depth. The Son-O-Mermaid is to be deployed by untrained personnel from ships of opportunity, which gives it an extra advantage over conventional (e.g., ocean-bottom, tethered,moored) approaches. Son-O-Mermaid was first and foremost designed for whole-Earth seismological (tomographical) applications. The hydrophone array configuration eliminates non-propagating noise and suppresses surface multiples. The fathometer helps in determining the travel time of teleseismic waves with maximal accuracy. It will also deliver data for the high-resolution study of Earth's global bathymetry, and because Son-O-Mermaid is a drifting instrument, surface currents will be another byproduct.
Geophysical measurements made in (not just of) the oceans are rare because of accessibility, logistics, and costs. While the continents are densely covered with sophisticated instrumentation, there are few alternatives, if the data need to be collected in the oceans, other than visiting the ocean floor or using shipboard measurements. For global seismology, the paucity of oceanic data is extremely detrimental, and to many other branches of Earth sciences, oceanic data are simply essential. Physical, chemical and biological oceanography will all be served by the availability of low-cost, autonomous, and global data collection strategies. Other than hydrophones and depth sounders, many other instrument types can be envisaged as payload. It also goes without saying that hydrophones will detect a whole range of acoustic signals in the oceans, from the crackling of glaciers and calving ice sheets to the calls of whales and other marine fauna, from the faint rumble of large, rare, and distant earthquakes to the swarms of ridge or transform-fault events. Lastly, while we are after geophysical signal, the study of acoustic noise in the ocean is a discipline with many intrinsically interesting scientific questions as well as practical and societal applications.
