Improving accuracy of acoustic prediction in the Philippine Sea through incorporation of mesoscale environmental effects
Freitas, Kimberly M.
Colosi, John A.
Paduan, Jeffrey D.
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An understanding of ocean acoustic fields, their statistics, and relation to the oceanographic environment is the sine qua non of undersea warfare. In the tactically important Philippine Sea, powerful mesoscale eddies can have strong effects on acoustic fields. To quantify eddy effects, a mesoscale sound-speed model was developed and interfaced with a parabolic equation acoustic simulation. Eight combinations of frequency (20Hz/250Hz), wavenumber spectra (Stammer/Lorentzian), and source depth (50m/200m) were simulated through the model. For each combination, the unperturbed transmission loss (TL) curve and composite eddy-field TL curve were compared to assess acoustic variability caused by mesoscale ocean features. Eddies alter acoustic energy by shifting convergence zones, driving energy into the seabed, trapping energy in surface ducts, and increasing scintillation. These effects are greater at higher frequencies and deeper source depths, shifting both the mean TL difference and RMS variability on order 5-10dB. The wavenumber spectrum showed no significant effect on acoustic variability. Eddies also cause horizontal out-of-plane scattering. Ray equations were manipulated to demonstrate that eddy-induced bearing-angle errors can be on order one degree at 500km, increasing as the square-root of range. Target localization errors due to angle error are on order 7km at 500km, increasing as range to 3/2 power.
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