This thesis develops and utilizes a method for analyzing data from the North Pacific Acoustic Laboratory's (NPAL) Basin Acoustic Seamount Scattering Experiment (BASSEX). BASSEX was designed to provide data to support the development of analytical techniques and methods which improve the understanding of sound propagation around underwater seamounts.The depth-dependent sound velocity profile of typical ocean waveguides force sound t o travel in convergence zones about a minimum sound speed depth. This ducted nature of the ocean makes modeling the acoustic field around seamounts particularly challenging, compared to an isovelocity medium. The conical shape of seamounts also adds to the complexity of the scatter field. It is important to the U.S. Navy to understand how sound is diffracted around this type of topographic feature. Underwater seamounts can be used t o conceal submarines by absorbing and scattering the sound they emit.BASSEX measurements have characterized the size and shape of the forward scatter field around the Kermit-Roosevelt Seamount in the Pacific Ocean. KermitRoosevelt is a large, conical seamount which shoals close to the minimum sound speed depth, making it ideal for study. Acoustic sources, including M-sequence and linear frequency-modulated sources, were stationed around the seamount a t megameter ranges. A hydrophone array was towed around the seamount to locations which allowed measurement of the perturbation zone.Results from the method developed in this thesis show that the size and shape of the perturbation zone measured coincides with theoretical and experimental results derived in previous work.
The SPICEX-LOAPEX-BASSEX experiments measured forward scattering from the Kermit-Roosevelt seamounts. LFM and M-sequence signals from 250-Hz moored sources and a 75-Hz ship-deployed source were received with Penn State University’s 162-element FORA array. Receptions were obtained from these sources at ranges of hundreds to 1600 km. Previous work [J. Acoust. Soc. Am. 117, (2549)] discussed the BASSEX forward shadow and patterns of horizontal refraction around the seamount. Additional work [J. Acoust. Soc. Am. 118, (1936)] compared experimental results with theory using a conical section model of a seamount. This presentation concerns the pulse compressed and array processed time series of M-sequence signals in the forward scattered field of the seamounts. Both travel times and angles of arrival are reconciled with 2-D ray-trace models. Results are comparable to refracted rays, which mainly propagate over the seamount. These, however, do not explain additional echoes, which appear in the experiment, possibly due to reflection and/or diffraction. Normal mode and PE methods are used to confirm ray models and identify additional echoes.
The SPICEX-LOAPEX-BASSEX expermiments were executed in the Northeast Pacific to examine several long range, low frequency propagation phenomena. Low frequency sources centered at 68 and 75 Hz with nominal bandwidth of 30 Hz and a source at 250 Hz with 100 Hz bandwidth were deployed transmitting orthogonal M seqeunces and FM chirps. A 64 element towed array cut for 250 Hz from Penn State Univ. was the primary multichannel receiving system. This presentation concerns the BASSEX study of low frequency scattering around the Kermit-Roosevelt Seamount complex which shoals to 900 m near 39 N and 145 W. We examine (i) the forward scattering shadow, (ii) any patterns of horizontal refraction as a function of source and receiver range from the seamounts, (iii) any backscattering from the seamounts, and (iv) the modal content of the signals by travel time methods. In addition, directional spectra of ambient noise were measured.
In the SPICEX-LOAPEX-BASSEX experiments conducted in Fall 2004 forward scattering from the Kermit Roosevelt Seamounts was measured. There were four sources insonifying the seamounts: Kauai, S1 (a moored source), S2 (another moored source), and a ship suspended source at ranges from hundreds of kilometers to 1600 km. The signals were M sequences and LFMs centered at 75 and 250 Hz. These data were presented in J. Acoust. Soc. Am. 117, 2549, which indicated clear convergence zones and horizontal refraction by the seamounts as well as a gradual ‘‘fill in’’ of the scattering with increasing distance from the seamounts. Here, we compare the experimental results to a model of layered conic sections with both point excitations and a modal vertical line arrays. This Kauai, moored and ship suspended sources were deployed by Scripps Institute of Oceanography and the University of Washington, Applied Physics Laboratory as part of the joint experiment.
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