8.1
Model-based analyses in support of inverse-acoustic sensing of coastal oceans: implications for coupled ocean-acoustic modeling and forecasting
Paul C. Etter, Northrop Grumman Corporation, Baltimore, MD
This paper identifies and describes models that are appropriate for the planning and analysis of inverse-acoustic sensing experiments in coastal oceans. The objective is to provide model-selection guidance to researchers engaged in acoustics-based remote sensing of coastal environments.
Inverse-acoustic sensing provides useful adjunct methods for obtaining synoptic portraitures of coastal oceans, for monitoring long-term variations in the coastal environment or for performing rapid environmental assessments in remote coastal locations. Inverse methods extract information from a limited set of direct measurements of the physical properties of the coastal ocean and then combine these measurements with theoretical models of underwater acoustics. The objective is to estimate detailed underwater acoustic fields in complex coastal environments from sparse physical measurements using the theoretical models as guides.
Coastal environments are generally characterized by high spatial and temporal variabilities. When coupled with attendant (acoustic) spectral dependencies of the surface and bottom boundaries, these natural variabilities make coastal regions very complex acoustic environments. Specifically, changes in the temperature and salinity of coastal waters affect the refraction of sound in the water column. These refractive properties have a profound impact on the transmission of acoustic energy in a shallow-water waveguide with an irregular bottom and a statistically varying sea surface. Thus, accurate modeling and prediction of the acoustical environment is essential to an understanding of the physical properties of the coastal ocean derived from inverse measurements.
Physical processes controlling the hydrography of shelf waters often exhibit strong seasonal variations. Annual cycles of alongshore winds induce alternating periods of upwelling and downwelling. The presence of coastal jets and the frictional decay of deep-water eddies due to topographic interactions further complicate the dynamics of coastal regions. Episodic passages of meteorological fronts from continental interiors affect the thermal structure of the adjacent shelf waters through intense air-sea interactions. River outflows create strong salinity gradients along the adjacent coast. Variable bottom topographies and sediment compositions with their attendant spectral dependencies complicate acoustic bottom boundary conditions. At higher latitudes, ice formation complicates acoustic surface boundary conditions near the coast. Waves generated by local winds under fetch-limited conditions, together with swells originating from distant sources, conspire to complicate acoustic surface boundary conditions and also create noisy surf conditions. Marine life, which is often abundant in nutrient-rich coastal regions, can generate or scatter sound. Anthropogenic sources of noise are common in coastal seas including fixed sources such as drilling rigs and mobile sources such as merchant shipping and fishing vessels. Surface weather, including wind and rain, further contribute to the underwater noise field. Even noise from low-flying coastal aircraft can couple into the water column and add to the background noise field.
Inverse-acoustic sensing of the ocean utilizes one of three acoustic phenomena: propagation, noise or reverberation. Propagation measurements can be used to infer bulk properties of the water column or characterize the sea floor via matched-field processing, ocean-acoustic tomography or deductive-geoacoustic inversion. Noise measurements can be used in an inverse fashion to characterize surface winds and waves, estimate rainfall rates, image submerged objects (using ‘acoustic daylight') or geoacoustically invert bottom properties. Reverberation measurements can be inverted to image the sea floor, and environmentally adaptive reverberation nulling can be performed using time-reversal mirrors that refocus sound back at the original probe source position with the assistance of model-based calculations.
The current inventory of underwater-acoustic models comprises 125 propagation models, 19 noise models, 23 reverberation models and 33 sonar-performance models. Approximately 18 percent of this inventory is tailored for shallow-water applications. These models are summarized in charts that discriminate attributes useful in inverse applications. Selection criteria based on these attributes are presented in convenient checklists to guide researchers in matching appropriate acoustic-modeling capabilities with inverse-acoustic sensing requirements in the coastal ocean.
When coupled with coastal atmosphere-ocean models, these ocean-acoustic models can function as sophisticated prognostic and diagnostic tools in support of naval operations, offshore industries and oceanographic research. Implications for coupled ocean-acoustic modeling and forecasting in the coastal oceans are discussed.
Session 8, Coastal ocean and atmosphere observations & analyses—II
Wednesday, 12 September 2007, 3:30 PM-5:00 PM, Boardroom
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