Investigation of under-ice thermal structure: small AUV deployment in Pavilion Lake, BC, Canada

Within the past decade, autonomous underwater vehicles (AUVs) have seen increased application in oceanographic research with limited limnological application. In 2006, the University of British Columbia Environmental Fluid Mechanics (UBC-EFM) group began field trials of the 'UBC-Gavia' in two lacustrine systems (Kelly and Pavilion Lake, B.C., Canada) and one estuarine system (Loch Etive, Scotland). One of the driving forces behind using AUV platforms for scientific surveys is the ability of these vehicles to reach environments that are impractical to reach by conventional, ship-based, means. Exploration of the world's polar environments is currently the basis of several AUV-based research groups (e.g. Autosub Under-ice program), with AUVs having successfully completed long distance surveys under sea-ice in both Arctic and Antarctic waters. One of the long-term objectives of the UBC-EFM group is to deploy UBC-Gavia under polar lake ice at selected sites both in the Canadian High Arctic and in Antarctica. In February 2007 a series of under-ice experiments were conducted in Pavilion Lake, B.C. with the primary objective of investigating under-ice thermal structure. Results demonstrate horizontal heterogeneity in the thermal structure of the lake indicative of convective circulation in the form of a negatively buoyant plume. In addition, novel techniques of AUV deployment, navigation and recovery developed for this project are described. Pavilion Lake, B.C. was selected as the field site for development of AUV deployment techniques during a two-week span in February 2007. This site was selected because ice-cover is relatively thin (<50 cm) and there is relatively little snow cover during an average year (<5 cm). In contrast to some recent under-sea-ice AUV applications (e.g. Autosub Under-ice Program, SHEBA), mission operations were conducted from a hole in the ice surface (1times3 m) rather than an ice edge, polynya or lead. Three different deployment techniques were used; a direct surface start, depth trigger start and remote trigger start. Testing included executing mission-tracks of increasing complexity at varying depths in the water column with additional runs conducted with the vehicle ballasted in an inverted orientation in order to make sonar and photographic observations of the ice underside. Vehicle positioning was achieved by two acoustic long baseline (LBL) transponders moored in the ice at fixed positions reference by GPS. A disadvantage of this system was the limited range (~1 km2) required for valid triangulation of the vehicle. Missions were also executed using an inertial navigation system (INS) coupled with a Doppler velocity log (DVL). A combination of the LBL and INS/DVL systems is proposed for sea-ice deployments where a moving frame of reference (the sea-ice) is an additional degree of complexity. Since the vehicle does not typically return to the deployment hole, several search and recovery techniques were tested and will be reviewed in this paper.