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WE STUDY THE EFFECTS of a variable ocean environment on the propagation and scattering of sound -- the forward ocean acoustics problem. We also exploit our understanding o these effects to solve the inverse acoustic oceanography problem -- where sound is used as a probe to study the ocean from a few cycles per second (Hz) to hundreds of kHz (1 kHz = 1000 cycles per second). AS LOW-FREQUENCY (10-1000 HZ) SOUND TRAVELS through the ocean, it is altered by changes in ocean temperature, currents, and bottom and surface characteristics. Understanding these effects is part of the North Pacific Acoustic Laboratory (NPAL). In the fall of 2004, NPAL researchers examined how ocean internal waves affect long-range acoustic propagation. We also use coupled mode theory to examin acoustic/elastic wave propagation within the ocean and seabed to understand how seismic wave energy originating in the seabed is converted to very low-frequency acoustic energy that then propagates in the ocean. ENHANCING U.S. NAVY MID-FREQUENCY SONAR (1-10 kHz) operation in shallow water involves theoretical and experimental efforts to better understand and model the physics of propagation and scattering. Ocean acoustics models are improved by including the influence of the rough ocean surface and variable seafloor. These models, which must balance fidelity against computational speed, will be tested in an at-sea experiment off the New Jersey coast in fall 2006. TAKING ADVANTAGE OF OUR KNOWLEDGE of acoustic propagation in shallow water environments, we have begun to study the effects of mid-frequency sound on marine mammal communication, behavior, and health. This includes sound propagation research in the shallow waters of Puget Sound, where bottom topography and sound speed variations in the water can significantly affect the sound levels incident on marine mammals as they move within their environment. The studies are numerically intensive and use parabolic equation approximations that have been tested and are being improved using in-house exact propagation codes. TO PREDICT THE CAPABILITY OF SONAR to detect buried mines at shallow grazing angles, we are seeking a mechanistic understanding of scattering from, penetration into, and propagation within the seabed. Studies have shown that diffraction due to sand ripples is the main mechanism of penetration at shallow grazing angles and frequencies above 10 kHz. The SAX04 ocean acoustics experiment carried out in September-November 2004 examined the use of this penetration mechanism as well as other, lower frequency (1-10 kHz) mechanisms to detect buried mines using Synthetic Aperture Sonar (SAS). 
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