Sea-trial Validation of an Echosounder for Seafloor Classification and Mapping

Abstract

Abstract In the context of the Italian Scientific Program for Antarctica, a multibeam echo sounder 10 be used for scientific research in the Ross sea (Antarctica) is under development. The instrument implements an original technique for cepstralanalysis of the acoustic echoes, which makes it capable of effectively discriminating different types of seafloor. In the past few decades, several researchers have shown that it is possible to extract information on small-scale topography and material composition of the seabottom from backscattered acoustic waves. Unfortunately, the identification performance of conventional techniques is poor when more than three or four seafloor types are considered. In recent years, many theoretical and experimental studies have shown the effectiveness of cepstral analysis in problems such as voice recognition, ultrasonic characterization of materials, etc. The technique implemented in the echo sounder takes advantage of the recent progress made in these fields. Generally speaking, the instrument will be used in marine geological and/or biological surveying and mapping of large areas. Specific applications could be found in mineral resource surveying, or in seabottom monitoring after oil tanker spills. After describing the echo sounder and the signal processing algorithms, the paper reports the results of field validation of [he instrument. The field data show that the classification technique implemented by the instrument gives a better performance than that of a conventional echo sounder with classification capability. In particular, over a set of six different types of seafloor (mud, sand, gravel, reek, seagrass, and dead seagrass), the identification performance of the instrument was better than 85%. This result shows that the performance of an echo sounder for seafloor surveying can be significantly increased properly using advanced signal processing techniques. Introduction Since sound waves have lower attenuation in water than light and electromagnetic waves, acoustic techniques appear lhe most feasible for remote sensing of large areas of seafloor. Hence, a lot of research has been devoted to understanding how the acoustic backscattering of the seafloor is related to its physical and geological properties. The most immediate way to understand the interaction mechanisms of acoustic waves with the seabed is to analyze simple situations that are not complicated by the effects of elements that may exist in the natural environment (e.g. vegetation, shells or gas bubbles, etc), For example, many studies, considering a sedimentary seabed flat and homogeneous, have been carried out on the effects of acoustic wave veloeity and attenuation in sediment on the acoustic response, and the dependence of these parameters on frequency. The influence of seatloor roughness (on the behavior of the backscattering strength versus the incidence angle) has also been widely investigated 2, but in most cases, multiple scattering, shadowing effects, acoustic penetration, and scattering due to bulk discontinuities were neglected.