Improved High-Frequency Bottom Loss Characterization

The existing HFBL (High-Frequency Bottom Loss) database has been recognized to be unsatisfactory due to its lack of physical underpinning and inability to provide consistent performance across frequency and space. The aim of the project is to replace the HFBL database with a geoacoustic model that leads to a smooth transition to the LFBL (Low-Frequency Bottom Loss) model at 1 kHz. To this end, this project has been focusing on analyzing the extensive dataset provided by NAVOCEANO to understand its physical nature. While this project is focused on applications, work so far has shown that the effort touches on almost all aspects of basic ocean acoustics for mid-frequencies (1-10 kHz).  Work under this effort will 1) develop and document the software tools that read the NAVO database of measured transmission loss and environment information; set up the acoustic models for particular runs; and do an inversion for geoacoustic parameters. 2) analyze in detail specific subsets of the data where the model/data agreement is good to demonstrate the capability 3) analyze data from the Seabed Characterization Experiments in the New England Shelf area to demonstrate the processes in a new area. Based on work to date, we expect to show that different 'objective functions' (matching TL in third-octave bands, time-series matching, etc.) will work best for different areas. For instance, near-field and/or deep-water measurements are more amenable to matching the full time-series. Further, in deep water complicated sub-bottom layering will be present only in some areas and require particular techniques. The work will address these issues and make recommendations for a general process.