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In contrast with remote sensing through the atmosphere, the relatively opaque and inhospitable ocean medium presents unique and formidable challenges. Impenetrable to most forms of electromagnetic radiation, the sea yields a glimpse of its hidden structure mainly through acoustic and optical means. For certain applications, though, an autonomous underwater system may benefit from a perception of its environment in terms of magnetics, gravity, chemical signature, or other sensing modalities. This presentation offers an introduction for researchers unfamiliar with issues in remote sensing underwater. Application areas include machine perception for intelligent underwater systems, large-scale seafloor mapping, survey, and environmental characterization.
The issues associated with the many different sensing modalities are complex and varied, and cannot be fully addressed here. Instead, the focus is on optical and acoustic sensing, two of the more important tools for remote subsea exploration. Sonars, for example, will continue to play an important role in world-modeling for autonomous underwater systems because of the greater range available to acoustic sensors compared with other sensing modalities. However, attempts to automate the interpretation of sidescan sonar data are typically based on two- dimensional image-processing and pattern-analysis techniques. Such sidescan "images" provide only indirect, qualitative, and view-dependent information, since the intensity of the returned signal is a function of both seafloor shape and scattering properties of the bottom materials.
The issues in multisensor, multiscale modeling underwater are illustrated by recent work at the Deep Submergence Laboratory. Several techniques for three-dimensional sonar processing are presented. New split-beam (phase-difference) sidescan sonars uses bathymetric measurements to estimate seafloor backscatter characteristics, and to reduce geometric and radiometric dependencies in the intensity signal. The approach will be applied to a new forward-scan, multibeam bathymetric sonar now under development and a split-beam sidescan sonar for under-ice survey by an autonomous underwater vehicle. Such systems allow quantitative modeling of three-dimensional shape and backscatter characteristics, and can be applied to feature classification and terrain-relative navigation for intelligent underwater vehicles.