Vertical line array observations of the angles of intromission into a layered shallow water seabed.

Bottom-reflected acoustic arrivals from muddy seabeds can exhibit incident grazing angle-dependent reflection loss associated with the angle of intromission. Previous studies investigated this phenomenon through bottom-reflection measurements on individual hydrophones. In this Letter, acoustic transmissions from a towed source in two mid-frequency bands (3.5-4.5 and 5.5-6.5 kHz) are analyzed using vertical line array hydrophones deployed on the Washington shelf. Weak bottom-reflected arrivals are observed, exhibiting more complex frequency-dependent behavior than predicted by a homogeneous muddy halfspace. Guided by sub-bottom profiler measurements, a preliminary multilayer analysis shows that interference among reflections from multiple sediment layers provides a plausible explanation.