LONG-TERM GOAL Our long-term goal is to develop accurate models for high-frequency scattering from, penetration into, and propagation within shallow water sediments. Reaching this goal requires a better understanding of several fundamental issues pertinent to high-frequency sediment acoustics. These issues include an understanding of the dominant scatterers versus frequency near the sediment surface, the potential need for poroelastic sediment models, the appropriateness of stochastic

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... ns of sediment heterogeneities, the importance of single versus multiple scattering in sediments, and an understanding of the physical and biological processes that determine sediment structure. OBJECTIVES We have five specific objectives. First, identify the dominant backscattering mechanisms (20-300 kHz) and demonstrate that the backscattered field can be quantitatively modeled based on measured sediment properties (20-50 kHz). Second, identify the dominant subcritical penetration mechanisms in sandy sediments and demonstrate that the penetrating field can be quantitatively modeled based on measured sediment properties (10-50 kHz). Third, measure sediment attenuation (10-300 kHz) and sound speed (10-300 kHz) and determine constraints imposed on sediment acoustic models, such as poroelastic (Biot) models. Fourth, obtain measures of acoustic spatial and temporal coherence within the sediment and demonstrate that these quantities can be quantitatively modeled based on measured sediment properties (10-50 kHz). Fifth, measure backscattering levels from buried objects in order to test buried-object-detection (10-50 kHz) modeling accuracy. Same as Report (SAR) 18. NUMBER OF PAGES 8 19a. NAME OF RESPONSIBLE PERSON a. REPORT unclassified b. ABSTRACT unclassified c. THIS PAGE unclassified Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18