William E. Avera

AQS-20 Through-the-Sensor (TTS) performance assessment

Performance of existing and planned mine hunting sensors is dependent on the environment. When the sea floor is a flat smooth hard sandy surface with no mine like clutter on it, then sensor performance is outstanding and acoustic mine hunting is relatively easy. Introduce clutter, a rough seafloor and a soft muddy bottom, sensor performance is seriously degraded making mine hunting operations extremely difficult to impossible. One must know the environment to know sensor performance. Historical environmental data is important but not sufficient. In spite of painstaking efforts to collect, process and disseminate data, historical information is often missing, outdated or in error. To know sensor performance, near realtime environmental data must be collected to verify, supplement and refresh historical holdings. This paper describes the results of two near real-time end-to-end Through-the-Sensor (TTS) demonstrations conducted in FY05 using AQS-20 data. Critical environmental parameters were extracted from the raw tactical data stream using a TTS approach. Data collected by the AQS-20 was processed for bathymetry, sediment type and % burial. Supplemental data was fused with historical information on scene and used to calculate doctrinal bottom type in NAVOCEANOs Bottom Mapping Workstation. The information was passed to MEDAL where track spacing and hunt times were calculated. NAVOCEANO, in a fast reach back mode using TEDServices, examined the data, added value, and returned it. The impact to the mine warfare community is a true sense of sensor performance.

Sensing shallow seafloor and sediment properties, recent history

Near surface seafloor properties are needed for recreational, commercial, and military applications. Construction projects on the ocean seafloors often require extensive knowledge about strength, deformability, hydraulic, thermal, acoustic, and seismic characteristics for locating stable environments and ensuring proper functioning of structures, pipelines, and other installations on the surface of and buried into the marine sediments. The military is also interested in a variety of seafloor properties as they impact sound propagation, mine impact burial, trafficability, bearing capacity, time-dependent settlement, and stability of objects on the seafloor. Point measurements of sediment properties are done using core samplers and sediment grab devices (with subsequent lab analysis) and in-situ probes. These techniques are expensive in terms of ship time and provide limited area coverage. Sub-bottom acoustic and electromagnetic sensors can provide profiles of near surface sediment information with improved coverage rates. Fusion techniques are being developed to provide areal extent of sediment information from multiple sensors. This paper examines the recent history of techniques used to measure sediment properties in the upper portions of the seafloor and in shallow (< 100 m) water.

Nonhydrostatic Hindcasts of High Amplitude Internal Waves in the Mid-Atlantic Bight

From late July to early September 2006 an intense field program, Shallow Water 2006 (SW06), was conducted in the Mid-Atlantic Bight (MAB) off of the New Jersey coast. The goal of the program is to the determine the environmental processes that affect shallow water acoustic propagation and scattering and to understand the dynamics of the generation and evolution of those processes. A phenomena that dominates much of the hydrodynamics in the coastal ocean are the internal waves. The waves interact with each other, with the topography and with the ambient currents and stratification to form a complex field of baroclinic internal waves of varying frequencies and wavelengths. Many of these waves are nonlinear and have very high amplitudes (exceeding 50 m) hence large energy. The generation, interactions and transformations of these waves is studied with a very high resolution, nonhydrostatic (NRL-MIT) model system of the ocean hydrodynamics. This model is imbedded in a nested hydrodynamic nowcast/forecast system comprised of the global Navy Coastal Ocean Model (NCOM) and a series of higher resolution NCOM domains.

High-frequency acoustic inversion using a military mine-hunting sonar

Summary form only given. Because the AQS-20s Volume Search Sonar (VSS) contains multiple beams at a number of grazing angles, the NRL Sediment Mapping System can be used to invert the backscattered acoustic signal for seafloor sediment parameters. Backscattering strength is computed for each beam and averaged over several pings. A physical backscatter model that predicts backscattering strength as a function of grazing angle is used with a simulated annealing inversion to find the model parameters. This paper discusses the inversion results using VSS data from a test off the coast of Panama City, Florida. The analysis results include sediment-water density ratio, bottom relief spectral strength (roughness parameter), and a volume interaction parameter. From these model parameters, we estimate mean grain size and root-mean square interface roughness. The sediment-water density ratios from the inversion are compared with the solutions from another NRL bottom classification system using a normal incidence impedance-based inversion technique.