Multiple precursors due to leaky head waves

Abstract

Using the parabolic approximation to the wave equation, the influence of head wave propagation along the water–sediment interface on the total field in a range-independent environment can be computed analytically. Results from a numerical PE model are observed to agree nicely with the analytical solution for cw sources. The PE model may therefore be used with confidence to investigate various phenomena associated with head wave effects. A simple travel-time geometrical model can be used to demonstrate that multiple arrivals in the time domain appear ahead of the direct path in the water column due to leaky head waves reflected by the sea surface back to the bottom at the critical angle. A broadband implementation of the PE model in shallow water numerically predicts these precursors quite accurately. Such precursors may not be easily distinguishable in practice and could lead to erroneous estimations of sediment sound speed. The effects of bottom loss and bathymetric range dependence has been investigated. Possible experimental configurations for observing these multiple precursor arrivals are suggested. [Work supported by ONR, Code 11250A.]