Source triangulation utilizing three-dimensional arrivals: Application to the search for the ARA San Juan submarine

On Nov. 15, 2017, an intense acoustic event coincident with the disappearance of the Argentine navy submarine, ARA (Armada Argentina) San Juan, was recorded on the hydroacoustic network established to enforce compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Analysis by Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) scientists, based on two hydroacoustic and one seismic detection, provided a likely origin within an error ellipse of 19 km by 12 km; analysis based solely on the main arrival detected at the two hydroacoustic stations gave an error ellipse of ∼500 km by ∼25 km [Nielsen, Zampolli, Le Bras, Mialle, Bittner, Poplavskiy, Rozhkov, Haralabus, Tomuta, Bell, and Grenard, in European Geosciences Union General Assembly, Vol. 20, EGU2018-18559 (2018)]. The large major axis depends on uncertainty in establishing the event time, while the minor axis depends on precision in the ocean state estimate used to model propagation speed. This paper demonstrates how three-dimensional (3-D) propagation features can also be used in source triangulation, in particular when no seismic detection is available. A mode-based 3-D propagation model is implemented to reconstruct the propagation path of a 3-D arrival bathymetrically refracted from the continental slope. This additional arrival provides a third (virtual) station to decouple the location and time of the event and triangulate the event. This improvement is commensurate with the CTBTO analysis, but does not rely on the additional seismic station detection.