Indian Ocean Hydroacoustic Wave Propagation Characteristics

Abstract : The channeling efficiency of the Deep Sound Channel (often referred as the Sofar channel) allows long range propagation of hydroacoustic waves over a few thousands of kilometers. Strong T-waves, referring to a third arrival on seismic waves, are commonly observed on underwater receivers (hydrophones stations) and on coastal receivers (T-phase stations), when an oceanic earthquake or an underwater explosion occurs, even for small events. Consequently, to insure the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT), the hydroacoustic network of the International Monitoring System (IMS) uses five T-phase stations and six hydrophones. At the end of 2001, three hydrophone stations--HA1 at Cape Leeuwin, HA4 at Crozet, and HA8 at Diego Garcia will continuously send their data to the IMS. These data will also be available at National Data Centers. Then, using these data it will be possible 1) to refine the network detection capability 2) to estimate the network localization precision and 3) to estimate the transmission loss of the hydroacoustic propagation and the hydroacoustic-to-seismic conversion at the T-phase stations. To prepare this evaluation, we are studying the underwater propagation in the region of the Indian Ocean and in the South Atlantic Ocean using modeling approaches. The first part of this paper gives a general view of the variation of the bathymetry, the sound speed propagation and the Sofar channel axis in the Indian Ocean. In particular, it is shown that there is a strong sound speed profile variation, from the North to South Indian Ocean, due to a cold water front coming from the Sub-Antarctic Ocean. In a second part, three areas are defined in the region of the Indian Ocean and in the Antarctic Ocean. In each of them, a typical sound speed profile has been considered to estimate numerically the underwater transmission loss characteristics. The underwater blockage effect due to underwater seamounts are also investigated in this part.