Operational Impacts On ASW by South China Sea Sand Dunes

Abstract

Very large subaqueous sand dunes were discovered on the upper continental slope of the northern South China Sea (SCS) in the spring of 2007 in water depths of 160 m to 600 m. While subaqueous dunes are found in many locations throughout the world’s oceans and coastal zones, these particular dunes appear to be unique for two principal reasons: their location on the upper continental slope and their distinctive formation mechanism (approximately 30 episodic, extremely energetic, large amplitude, high frequency, trans-basin non-linear internal wave (NLIW) events each lunar cycle). These NLIWs and sand dunes are important acoustical features, as they will cause operationally significant anomalies in the acoustical field, which will impact the performance of both active and passive sonar systems. The acoustical impact is a function of frequency, source/receiver depth, NLIW and sand dune amplitude, and source-receiver azimuth relative to the wave fronts and dune crests. At propagation angles nearly parallel to the NLIWs, short time scale transmission loss anomalies are as high as 10 dB; at propagation angles perpendicular to the sand dune crests, short time scale transmission loss anomalies are as high as 8 dB. These anomalies are significant enough to impact sonar system performance, and knowledge of these NLIWs and dunes inform ASW asset placement and performance assessment.