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A recent example of the IMS hydroacoustic network provid- ing a service beyond its original mission purpose of detecting nuclear detonations was its role in finding the lost Argentine submarine ARA San Juan. Triangulation of the intense sound generated by the San Juan, presumably from the rapid col- lapse of its hull when it exceeded its crush depth, was the crucial piece of evidence that led to its discovery, 920 meters below the ocean’s surface. Although the sinking of the ARA San Juan was a tragedy, the methods used to find it provide an example and source of future opportunity. Hydroacoustic triangulation of the San Juan was possible due to the pre- cision of the IMS recordings and a reliable estimate of the ocean climate, an example being the World Ocean Atlas (see www.nodc.noaa.gov).
Quoting the study by the National Research Council (2011, p. 108) on climate change-related technical issues impact- ing naval operations, “The U.S. Navy and other world navies have invested large sums to acquire field measurements of temperature and salinity, as well as bathymetry, to produce climatological “atlases”... [and while] it would be comforting to assume that climate-induced ocean changes will be slow, and that the impact on current data atlases will be minimal... not enough is known about climate change to be assured of these assumptions. Although it was possible to triangulate the San Juan to within a few kilometers with these atlases, the inaccuracy ultimately boils down to an ill-constrained state estimate of the ocean climate. In fact, a correction to the errors for a known source location provides a constraint for estimating the climate of the ocean, a process known as acoustic thermometry (or tomography). As shown here, the impulse signal from the San Juan is imprinted with a sig- nature of the oceanography it propagated through. Further review and analysis of the IMS record of ocean sound may be an important sentinel of the extent and rate of climate change and global warming.
Acoustic tomography is one example as to why ocean sound has been defined as an essential ocean variable (see goosocean.org). Ocean sound serves as an indicator pertinent to physical, chemi- cal, and biological oceanographic processes. The remote static measurements of the IMS stations provide a natural laboratory and historical data bank to study low-frequency ocean noise (Bradley and Nichols, 2015) and infer temperature (Sabra et al., 2016). The most intense sounds in the hydroacoustic recordings are generated by catastrophic events (underwater earthquakes, landslides, and volcanic eruptions), including ones that were devastating to humans such as the Sumatra-Andaman earth-
Figure 2. Left top: in 60 days, 5 Hugin autonomous underwater vehicles surveyed 21,000 km2 of the seafloor with their side-scan sonars. Left bottom: 230-kHz backscatter image in which the ARA San Juan was clearly identified, resting on a small ridge in a ravine 920 m deep. Right: an enlargement of the San Juan debris field. Image courtesy of Ocean Infinity.
quake of 2004. Analysis of that event and its potential for early warning tsunami detection were, in part, a motivating factor to put IMS recordings of ocean sound into the public domain.
The Search for the ARA San Juan
“On the night of 14 November 2017, facing rough seas, the
commanding officer reported a water entry (apparently through the snorkel) that had caused a short circuit in the forward battery compartment. A fire followed, but it was con- trolled by the crew. The San Juan then was ordered to change course and return directly to her home port, Mar del Plata, Argentina” (Villán, 2019, p. 1393). The last transmission received from the San Juan was at 1019 Coordinated Univer- sal Time (UTC) on November 15, 2017. Two and a half hours later, the submarine exceeded its maximum depth rating as it sank to the bottom of the ocean and was crushed by extreme water pressure. An intense sonic impulse was generated by the compression phase of the hull collapse, lasting roughly 35 milliseconds based on acoustic and forensic analyses of the USS Scorpion submarine, which, too, after suffering a battery fire, lost buoyancy and sank beyond its crush depth in 1968 (Bruce Rule, personal communication, 2018).
The sound generated by the catastrophic event of the sinking of the San Juan was detected by 2 IMS hydroacoustic stations, one over 6,000 km away in the mid-Atlantic Ocean and the other 8,000 km away in the Southern Indian Ocean. CTBTO
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