Accession Number : ADA569183


Title :   Modal Theory of Transverse Acoustic Coherence in Shallow Oceans


Descriptive Note : Formal rept. Nov 2008-May 2012


Corporate Author : NAVAL RESEARCH LAB WASHINGTON DC ACOUSTICS DIV


Personal Author(s) : Mignerey, Peter C


Full Text : https://apps.dtic.mil/dtic/tr/fulltext/u2/a569183.pdf


Report Date : 28 Sep 2012


Pagination or Media Count : 42


Abstract : The coherence of a shallow-water acoustic field transverse to the propagation path is represented using adiabatic normal modes, and path-integral theory is applied to fluctuations of mode amplitudes to obtain the coherence function. The problem is formulated in the horizontal plane where ray paths are nearly straight having no interaction with lateral boundaries, and requires the spectrum of environmental fluctuations that are provided by a shallow-water Garrett-Munk internal-wave model. The coherence function of the acoustic field is shown to depend on phase-structure functions of the normal modes. Environmental fluctuations with wavevectors transverse to the acoustic path were found to govern the structure functions. The impact of internal waves on coherence is proportional to the internal-wave energy, and is strongest for the lowest internal-wave modes in conjunction with those acoustic modes having maxima in the vicinity of high buoyancy. The transverse phase-structure function is found to behave like a sinc function, which provides the transition between limiting solutions for small and large hydrophone separations. Results show the coherence predicted by the theory decays too slowly with hydrophone separation to describe the observed ocean.


Descriptors :   *ACOUSTIC FIELDS , *COHERENCE , *SHALLOW WATER , ACOUSTIC WAVES , ADIABATIC CONDITIONS , AMPLITUDE , BUOYANCY , DISPLACEMENT , HELMHOLTZ EQUATIONS , HYDROPHONES , INTERNAL WAVES , PATH INTEGRALS , WAVE PROPAGATION


Subject Categories : Physical and Dynamic Oceanography
      Acoustics


Distribution Statement : APPROVED FOR PUBLIC RELEASE