Efficient Localization in a Dispersive Waveguide : Applications in Terrestrial Continental Shelves and on Europa

by Sunwoong Lee Submitted to the Department of Mechanical Engineering on May 5, 2006, in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Mechanical Engineering Methods are developed for passive source localization and environmental parameter estimation in seismo-acoustic waveguides by exploiting the dispersive behavior of guided wave propagation. The methods developed are applied to the terrestrial continental shelf environment and the Jovian icy satellite Europa. The thesis is composed of two parts. First, a method is derived for instantaneous source-range estimation in a horizontally-stratified ocean waveguide from passive beam-time intensity data obtained after conventional plane-wave beamforming of acoustic array measurements. The method is advantageous over existing source localization methods, since (1) no knowledge of the environment is required except that the received field should not be dominated by purely waterborne propagation, (2) range can be estimated in real time with little computational effort beyond plane-wave beamforming, and (3) array gain is fully exploited. Second, source range estimation and environmental parameter inversion using passive echo-sounding techniques are discussed and applied to Europa. We show that Europa's interior structure may be determined by seismo-acoustic echo sounding techniques by exploiting natural ice fracturing events or impacts as sources of opportunity. A single passive seismic sensor on Europa's surface may then be used to estimate the thickness of its ice shell and the depth of its subsurface ocean. To further understand the seismo-acoustic characteristics of natural sources on Europa, a fracture mechanics model is developed for the initiation and propagation of a crack through a porous ice layer of finite thickness under gravitational overburden. It is found that surface cracks generated in response to a tidally induced stress field may penetrate through the entire outer brittle layer if a subsurface ocean is present on Europa. While Europa's ice is likely highly porous and fractured, our current caculations show that porosity-induced scattering loss of ice-penetrating radar waves should not be significant. Thesis Supervisor: Nicholas C. Makris Title: Associate Professor of Mechanical and Ocean Engineering