The contribution of normal modes in the bottom to the acoustic field in the ocean

Abstract

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.