Broadband Modal Beamforming of Acoustic Tomography Signals Acquired by a VerticalArray

摘要

The objective of this thesis is to develop a technique and associated algorithmsto extract the arrival time of modal energy, using a vertical array, from broadband signals, Modal energy arrival time is important to shallow water acoustic tomography because low angle rays, which contain the majority of acoustic energy, are often not resolvable. Tilt compensation is included in the beamforming algorithm to provide a virtual vertical array. A broadband modal filtering technique is accomplished through weighting the frequency components of phase encoded tomographic signals by the spectrum of the mode shapes. A methodology of phase decoding after beamforming was adopted to minimize processing. Initial development and prototyping was done using a parabolic equation model. Further testing was accomplished on real data taken from the Barents Sea Polar Front Experiment, August 1992. Results show consistency over a number of transmitted pulses. Mode energy travel time measurement is simplified due to the distinct arrival structure of beamformed signals.