Penetration of acoustic waves into sandy seafloors at low grazing angles: the Helmholtz-Kirchhoff approach

Abstract

The Helmholtz-Kirchhoff integral is applied to model the penetration of sound waves into sandy seafloors at grazing angles above and below the critical angle. Although the conditions for the validity

of the Kirchhoff approximation can be limiting, this approximation should be valid at high frequency for

gently undulating seafloor surfaces even at moderate to low grazing angles, providing that the selfshadowing effect is carefully removed. The analytical development of the method is first presented,

followed by numerical examples. Simulations and data acquired at sea are in very good agreement in

the 2-15 kHz band (Maguer et al. [SACLANTCEN Report, SR-287, April 19981). The model predicts,

in agreement with the 2-15 kHz acoustic data, the frequency at which the contributions due to

roughness effects begin to dominate those due to the evanescent wave. Secondary effects such as Bragg interference patterns and the loss of signal coherence with grazing angle or depth are correctly

predicted. The model simulations strongly suggest that roughness of the sediment interface is most

likely the cause of anomalous sound penetration into the seabed.