AUV Freshwater Distribution Surveys in Deep Fjords

Sammendrag

Researchers in the MODTEQ group at NTNU in Norway have developed numerical models for a particularly interesting area of the Trondheim fjord characterized by colliding water masses, a high degree of stratification and considerable density variations. Two AUV surveys have been conducted in the modelled waters to physically validate these models; one in June 2001 and the other in April 2002. The Danish AUV Maridan 150 was used in both surveys, and base of operations was a 60-foot twin-hull vessel. A CTD unit and ADCPs pointing in either vertical direction formed the AUV payload. In the June 2001 survey, the vehicle traversed the fjord along straight lines of constant depth 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 second survey. The paper presents the survey set-up and execution along with experiences made underway, and concludes with a series of considerations to be made prior to oceanographic surveys of similar phenomena. The ways in which the surveys complement and enhance the investigative method of numerical modelling, and supplement remote sensing are discussed.