Progress in acoustic tomography
The Effect of Undersea Ridges on SOFAR Signals
Leon Krige (Oceanography Department, University of Cape Town, Rondebosch 7701, South Africa)
email: Ikrige@mweb.co.za
ABSTRACT
Analysis of shot signals over a 4.39 Megametre line between Cape Town and Ascension shows that reflections off the sea bottom lengthen the duration of the signal at the receiver. Ray trace modelling shows that many reflected eigenrays are probable. The direct and reflected components of the signal are not readily distinguishable by inspection of the signals arriving at the receiver. The arrival time of the reflected rays, unlike that of direct rays, cannot be simply equated to the depth of the ray turning points. Rays with launch angles less than about 12 degrees tend to have small grazing angles and closely mimic rays without reflections. Rays with larger launch angles are reflected more often and are widely scattered in arrival time. Some reflected rays may overlap the arrival time of direct rays and contaminate the temperature information. Since similar ridges occur in most ocean basins, the problem for ocean acoustic tomography will be to identify and avoid this type of contamination.
The test
The ATOC-FACT (Acoustic Thermometry of Global Climate-Feasibility, Ascension to Cape Town) transmission test was done between Cape Town and Ascension island in November 1992 to evaluate the suitability of this path in a global ATOC network.(1,2)
Because of uncertainty about topographical interference by Walvis ridge, St Helena Island and Bonaparte seamount, a number of shots were fired at eight sites in a line extending to the south-west of Cape Town. This produced a fan of paths of which at least some would avoid the undersea obstacles. The signals from the shots were received on five hydrophones at Ascension. (Figure 1).
