Abstract:

In the ATOC program, the source was sited on a slope at a depth of the SOFAR axis so that the signal would excite the axial modes efficiently for long-range propagation. One of the important issues was the elevation of the source off the seafloor. Buoying it on a mooring into the water column presented significantly more engineering challenges than simply resting it on a platform on the seafloor. Here, the impact upon the mode excitation spectrum of the source depth on a rough, elastic, sloping bottom is examined using the new range-dependent OASES code. This code matches vertical wave numbers across range-independent sectors, which may include interface roughness, as it steps out in range. It has been very successful in modeling the related problem of the effect of epicenter depth for T-phase excitation on a sloping bottom [Sperry et al., J. Acoust. Soc. Am. 100, 2641(A) (1996)]. A Munk sound-speed profile is assumed with a minimum at 1000 m and a sloping bottom out to a range of 30 km after which the bottom depth is constant. The field is projected onto the depth-dependent modes to determine their excitation. Also, the group delays are examined to determine the impact on travel times. Initial results suggest that even a modest (100 m) elevation concentrates the power in the low-order modes. In addition to the source elevation, the slope gradient and roughness and the elastic properties of the bottom are important parameters. [Work supported by ONR.]