Acoustic normal mode fluctuation statistics in the 1995 SWARM internal wave scattering experiment

In order to understand the fluctuations imposed upon low frequency (50 to 5 00 Hz) acoustic signals due to coastal internal waves, a large multilaborat ory, multidisciplinary experiment was performed in the Mid-Atlantic Eight i n the summer of 1995. This experiment featured the most complete set of env ironmental measurements (especially physical oceanography and geology) made to date in support of a coastal acoustics study. This support enabled the correlation of acoustic fluctuations to clearly observed ocean processes, e specially those associated with the internal wave field. More specifically, a 16 element WHOI vertical line array (WVLA) was moored in 70 m of water o ff the New Jersey coast. Tomography sources of 224 Hz and 400 Hz were moore d 32 km directly shoreward of this array, such that an acoustic path was co nstructed that was anti-parallel to the primary, onshore propagation direct ion for shelf generated internal wave solitons. These nonlinear internal wa ves, produced in packets as the tide shifts from ebb to flood, produce stro ng semidiurnal effects on the acoustic signals at our measurement location. Specifically, the internal waves in the acoustic waveguide cause significa nt coupling of energy between the propagating acoustic modes; resulting in broadband fluctuations in modal intensity, travel-time, and temporal cohere nce. The strong correlations between the environmental parameters and the i nternal wave field include an interesting sensitivity of the spread of an a coustic pulse to solitons near the receiver. (C) 2000 Acoustical Society of America. [S0001-4966(00)00601-9].