Estimating Seabed Interval Velocities from the SBP-120

Short Work Statement: The goal of the proposed two-year program is to develop the methodology to estimate sediment sound speed versus depth and range from SBP-120 data along its survey track. This methodology will provide useful seabed information to NAVOCEANO and provide constraints for the HFBL, BBS (and potentially the LFBL) databases. The seabed often has a profound effect upon Navy sonar systems. Acoustic energy interacting with the seabed is reflected, refracted, absorbed, and scattered by a variety of mechanisms over a wide range of spatial scales. The variety and complexity of seabed mechanisms makes it difficult to predict sonar performance without location-specific seabed information. To this end, several Navy sediment databases have been developed: the Low Frequency Bottom Loss (LFBL) valid from 31-1000 Hz and the High Frequency Bottom Loss database (HFBL) from 1500 – 5000 Hz. LFBL contains geoacoustic parameters (i.e., sound speed, density, and attenuation as a function of depth), HFBL is comprised of nine empirical curves. Both databases predict reflection from the seabed and are useful for supporting mission planning, performance prediction, and training for low to midfrequency passive systems. A new database to support low to mid frequency active systems is being transitioned later in FY16 to the Ocean and Atmospheric Master Library (OAML). This database is called the Bottom BackScattering (BBS) database, with a frequency range from 200-10,000 Hz. This will replace an empirical model (Lambert-McKenzie [1]) used for many decades that predicts no dependence of seabed scattering on location or frequency. The database is generated by estimating spectral parameters (descriptive of layer interface roughness and sediment volume heterogeneities) from long-range reverberation data [2]. The need Recently, there has been concern over the adequacy of the HFBL database and the empirical curves upon which it based. There is interest in considering a more physically realistic foundation, based on geoacoustic properties and possibly including sediment heterogeneity, e.g., roughness. For BBS database advancement there is also a need to estimate the geoacoustic properties. In addition , there is an interest in examining alternative methods for estimating the spectral heterogeneity parameters. Long-term objective To improve the HFBL and BBS seabed databases by providing independent estimates of sediment geoacoustic properties and sediment heterogeneities from the SBP-120. These sediment parameters will complement (and likely provide critical bounds for) parameter estimates derived from long-range transmission loss and reverberation. Goals of the research The goal of the proposed two-year program is to develop the methodology to estimate sediment sound speed versus depth and range from SBP-120 data along its survey track. This methodology will provide useful seabed information to NAVOCEANO and provide constraints for the HFBL, BBS (and potentially the LFBL) databases.