Digital terrain mapping of the underside of sea ice from a small AUV

In April 2007 the first three-dimensional digital terrain mapping of the underside of sea ice to be done from an ice-launched autonomous underwater vehicle (AUV) was carried out at an ice camp in the Beaufort Sea using the Gavia vehicle, which could be launched and recovered through 3 x 1 m holes. Gavia was equipped with a GeoSwath 500 kHz interferometric sonar system, which yielded swaths of typical width 80 m. The imagery showed the morphological distinctions between first-year (FY) and multi-year (MY) ice undersides, the contrast between the shapes of FYand MY ridges, and the appearance of refrozen leads. Ice drafts were validated by drilling, and it was found that ridge slope statistics and local probability density functions (PDFs) of draft could be derived with high precision. The special opportunities offered by access to 3-D mapping are pointed out.