Model supported AUV freshwater distribution surveys in the upper layers of deep fjords

The Trondheim Fjord is characterized by a high degree of stratification, considerable density variations and freshwater fronts important to the vertical mixing of freshwater and nourishments, and thus significant to the fjord system. These conditions are modelled in a numerical model. Two AUV surveys were conducted in the Trondheim Fjord, Norway in June 2001 and April 2002 to complement and validate the numerical simulations. The Danish AUV Maridan 150 was used in the operations. Salinity, temperature, and current data were necessary to investigate the validity of the numerical models. A CTD unit and ADCPs pointing in both vertical directions formed the AUV payload. In the first survey the vehicle traversed the fjord to produce horizontal density and current profiles. Observations made during these runs placed focus on fresh water front structures, which were investigated more closely in the subsequent survey. The paper presents the survey setup and execution along with experiences made underway.