Seabed acoustic imaging in the surf zone

An acoustic technique for reflection profiling in the surf and nearshore zones is described that provides multiple-free images of beach sediments. Rectangular transmitting and receiving array apertures mounted under acoustic baffles eliminate water multiples when towed within 50 cm of the seabed. Multiple-free profiles of a beach in Cheasapeake Bay are generated in water depths of 8 meters to 0.3 meters. Classifying sediment layers using normal incidence acoustics requires measurements of subsurface amplitudes. Towing sensors close to the seabed requires that the amplitude of a reflection from a sediment layer be corrected for destructive interference caused by differences in the acoustic path lengths when the layer interface falls within the near field of the sonar. A model is developed and tested for calculating the corrections to the subbottom reflection amplitudes.