Analysis of broadband echo soundings over a gassy seabed

Broadband echo soundings, between 1 and 20 kHz, were recorded in shallow water over a fine‐grained sediment near Viareggio, Italy. The data‐acquisition system was configured to provide a calibrated echo, from which the seabed’s reflection coefficient could be determined. Environmental information, including sediment grab samples and photographs of the seabed, was collected coincidentally. The track traversed a previously charted gas front which was apparent in the displayed data. Anomalously high levels of reflection loss were noticed at the edge of the spectral peak. It is hypothesized that nonlinear conversion of acoustic energy is responsible. The low‐energy part of the spectrum is consistent with linear model predictions. An acoustic color‐mapping method [Boyle and Chotiros, J. Acoust. Soc. Am. 99, 2553(A) (1996)] was applied to the data, clearly showing the spectral character of certain features. A discussion will be included on the method’s relevance to remote determination of the seabed’s strucure....