Detection of underwater explosion at very long-range

The channelling efficiency of the deep ocean sound channel (often referred as SOFAR channel) allows long range propagation of acoustic waves over a few thousands kilometres. Consequently, strong T-waves, referred to a third arrival on seismic waves, are commonly observed on both underwater and coastal receivers, when an oceanic earthquake or an underwater explosion occurs, even for small events. Many insights could be provided by these records, once they could be interpreted. To analyse these signals, the authors propose a new hybrid numerical scheme, combining in a single approach two powerful techniques, a ray tracing method completed by the Maslov summation and a finite difference scheme, which allows the authors to model T-wave underwater propagation and complex interactions with obstacles. In their modelling, before the T-waves arrive in front of an obstacle such as a continental shelf, an island shore or a seamount, the propagation is resumed on a vertical artificial boundary and the acoustic wave field is considered as the input for a finite difference method. An experiment released in the South Pacific and its modelling illustrates the key role played on the envelope shape of T-waves by the source depth, the SOFAR channel propagation, the bathymetry of the continental slope and the continental geological structure. Their approach gives a straightforward interpretation of the SOFAR channel propagation and may provide a phase identification, as well as a source characterization capability. A time domain study of the recorded data on seismic station (or on a hydrophone) may give an evaluation of the charge weight of an underwater explosion. Therefore, this technique is particularly relevant for monitoring the Comprehensive Test Ban Treaty in oceans