Bottom classification in very shallow water

Bottom classification based on echo features and multivariate statistics is now a well established procedure for habitat studies and other purposes, over a depth range from about 5 m to over 1 km. Shallower depths are challenging for several reasons. To classify in depths of less than a metre, a system has been built that acquires echoes at up to 5 MHz and decimates according to the acoustic situation. The multirate signal processing accurately maintains the echo spectrum, preventing aliasing of noise onto the signal and preserving its convolution spectral characteristics. Trials have been done over sediments characterized visually and by grab samples. The major applications are expected to be in lake, river, and near-shore marine environments where the water is opaque or the information sought is not just surficial.