Acoustic Mode Coherence in the Arctic Ocean

• Main Author: Polcari, John J.
• Other Authors: MASSACHUSETTS INST OF TECH CAMBRIDGE
• Format: Text
• Language: English
• Published: 1986
• Subjects: Acoustic Detection and Detectors; Acoustics; *ACOUSTIC ARRAYS; *BEAM FORMING; MEASUREMENT; STATISTICS; COHERENCE; SURFACE ROUGHNESS; EFFICIENCY; THESES; PROCESSING EQUIPMENT; SENSITIVITY; ICE; LIMITATIONS; LEAST SQUARES METHOD; ATTENUATION; PERTURBATIONS; ACOUSTIC SCATTERING; DRAG; AMPLITUDE; DECOMPOSITION; HYDRODYNAMICS; ACOUSTIC DATA; CHANNELS; MULTIPLE BEAMS(RADIATION); ARCTIC REGIONS; AUDIO TONES; UNDERWATER SOUND SIGNALS; LOW FREQUENCIES; ARCTIC OCEAN; ACOUSTIC CHANNELS; ALGORITHMS; TIME INTERVALS; MODAL DECOMPOSITION; Arctic; Arctic Ocean; Arctic Sound
• Online Access: http://www.dtic.mil/docs/citations/ADA216942; http://oai.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADA216942

The dual issues of modal decomposition for tonal sound fields and the temporal coherence of the modal amplitudes are investigated for the case of the central Arctic sound channel at very low frequencies (15-80 Hz). The performance of each of four different modal beamforming algorithms when applied to the vertical array deployed during the FRAM IV Arctic Acoustic Experiment is analyzed. A multiple beam (or decoupled beam) least squares processor produces the most acceptable results for Arctic conditions. The modal decomposition is sensitive to vertical array tilt caused by hydrodynamic drag; a technique for its estimation from the acoustic data is developed. Tonal data taken from both the horizontal and vertical arrays deployed during FRAM IV is analyzed. Horizontal array results confirm the modal amplitudes generated from vertical array data. The rough surface scattering from the ice canopy places an upper limit of 40 Hz on efficient surface duct propagation. Attenuation measurements for the first mode show excellent agreement with predictions made for ice scattering using the method of small perturbations and experimental ice statistics. The high levels of coherence observed (0.95 to 0.99) show that tonal signal propagation in the Arctic channel is essentially deterministic for time periods well in excess of one hour. Theses. Original contains color plates: All DTIC reproductions will be in black and white.