The role of echo duration in acoustic seabed classification and characterization

Acoustic seabed classification with single-beam sounders involves discriminating among bottom types using attributes of their echoes. One attribute that is rich in sediment information is the duration of these echoes, but it needs to be interpreted with care because it depends on water depth, seabed slope and sounder beamwidth, as well as on seabed characteristics. The interpretation relies on an equation for echo duration, which is the sum of four terms: · Spreading time across the seabed from first contact to the periphery of the beam width. · Duration of the transmit pulse. · Time to penetrate into the sediment to a depth from which volume scatter is no longer significant. · Earlier start times due to shorter path lengths if there is macro-roughness such as rocks and macroalgae. Only the first of these depends on depth or slope. This dependence must be compensated to make maps of seabed classes, rather than maps of classes influenced by bathymetry. Working with echoes collected at many transducer altitudes and tilts over homogeneous patches of seabed, this equation is verified. Being able to predict echo durations from a standard seabed (the standard echo length, SEL) allows effective compensation for the depth each echo came from. Seabed slope strongly affects echo durations, shapes, and amplitudes. Provided the slope is not so severe that there is no specular return, the effect of slope on the echo can be compensated in the same manner. Echo durations from rough seabeds can often exceed the SEL through an effect that can be modeled as widening of the transducer beam. In some cases, the difference between SEL and observed echo durations, called the residual echo duration, can be used to determine the seabed sediment without the need for ground truth.