(Woods Hole Oceanographic Institution, 1981-04)
Macpherson, Mark K.; Frisk, George V.
The effects of normal modes in the bottom on the acoustic field in the ocean are examined. The ocean bottom
model consists of a slow isovelocity layer overlying an isovelocity half-space to simulate the characteristic
sound velocity drop at the water-bottom interface. Attention is focused on the perfectly trapped modes which
are excited in the layer by inhomogeneous waves emitted by a point source in the water column. The relative
normal mode contribution to the total acoustic field in the water is calculated analytically for a near-bottom
source/receiver geometry and evaluated for representative ocean bottom examples. It is shown that, for
combined source/receiver heights less than a wavelength, the field is dominated by the leaky mode
contribution at short ranges ( $ 2 km) and the trapped mode contribution at long ranges ( ~ 2 km). For fixed
bottom parameters, the trapped mode contribution increases exponentially with decreasing combined
source/receiver height. It is also shown that, for a fixed layer wavenumber-thickness product and fixed layer
sound speed, the leaky mode fields at different frequencies are approximately range-scaled versions of the
same field.
