Abstract:
Various tomographic approaches are investigated for determining the spatial and temporal characteristics of the current field in the vicinity of a tidally forced coastal front near Stuart Island in Haro Strait, British Columbia (Canada). The front separates water masses with a current differential of 3 kn or more, corresponding to Mach numbers of 0.001 or higher. Three vertical receiving arrays were moored 2 to 4 km apart around the front. A variety of signals were transmitted across the water column: a 100-Hz-bandwidth linear FM sweep centered at 250 Hz, cw signals ranging from 100 to 350 Hz, and very wideband impulses with a strong spectral content from 150 to 850 Hz. Three current inversion schemes are compared: traditional travel time tomography, matched-field tomography, and linear full-field modal tomography. The latter is a novel algorithm based on cross-mode coupling coefficients appearing in the modal representation when an inaccurate or arbitrary mode set is used in place of the exact solution to the Sturm--Liouville problem. Its performance vis-a-vis bandwidth and signal-to-noise ratio is discussed. [Work supported by ONR.]