HYDRO 2016 Paper 9A2

Forward looking sonar mosaicing: A new approach for underwater inspection in low-visibility conditions

Natàlia Hurtós, Pere Ridao


Vehicle operations in underwater environments are frequently compromised by poor visibility conditions. The perception range of optical devices is heavily constrained in turbid waters, thus often complicating navigation and mapping tasks in environments such as harbours, bays, or rivers. A new generation of high-frequency forward-looking sonars that provide acoustic imagery at near-video frame rates have recently emerged as a promising alternative for working under these challenging conditions.

We have proposed an end-to-end mosaicing framework tailored to the characteristics of forward-looking sonar imagery in order to build consistent overviews of underwater areas regardless of water visibility. Our solution targets versatility: it enables the generation of acoustic mosaics that involve roto-translational motions and comprise different vehicle tracklines; it is suitable for a wide range of scenarios, from feature-rich areas to environments with scarcity of features; it can be applicable on data collected with minimally instrumented vehicles; and it allows both offline and real-time operation. Moreover, the resulting mosaics provide a significant improvement of the signal-to-noise ratio and resolution with respect to the individual sonar images.

The presented system stands up as a valuable tool for short-range high-resolution acoustic mapping (up to centimetre level, depending on the employed sonar) but also opens new avenues for more challenging applications. The capability of being localised within a real-time generated acoustic map of the seafloor holds great potential for providing visual support to ROV pilots under low-visibility conditions (to ensure coverage of the area and immediate location of targets of interest), or in general to any AUV or ROV application that can benefit of context awareness regardless of the visibility conditions. The proposed framework has been validated with several experiments in the context of relevant field applications such as harbour monitoring or mapping of underwater archaeological sites.

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