Reconstruction of a wave-induced ice break-up using unmanned aerial vehicles and stereo-DIC methods

Ocean waves can trigger sea ice fractures. This process plays an important role on the evolution of the floe size distributions in the marginal ice zone (MIZ). However, the fracture of sea ice by waves is still poorly constrainted by observations, as it is difficult to precisely forecast, it mostly occurs on short time (minutes) and spatial scales (meters), and are often happening during extreme weather conditions. In order to better understand the fracture of ice by waves, we built on the work of Dumas-Lefebvre et al. [1] and designed an experiment where waves are generated by an icebreaker nearby a continous ice sheet. This experiment was realized in the Saguenay Fjord, Québec, Canada, in February 2024, in the context of the Transforming Climate Action (TCA) program. The continuous 12-cm thick layer of ice that recently formed was fully characterised before the CCGS Amundsen generated wave by sailing at a speed up to 15.8 knots. Wave propagation and ice break-up were recorded by wave buoys placed on the ice surface and by three unmanned aerial vehicles (UAV) in stationary flights overseeing the ice from different angles.

Using digital image correlation (DIC), geometric projections and rectifications, we developed a method to recover the three components of the waves velocity field from UAV observations. The accuracy of the method has been tested using the buoy signals as a reference, showing a quantitative agreement with a relative error of about 5%. Thanks to this stereo-DIC method, we obtained the full wave velocity field over a grid of 106x75 meters with a spatial resolution of 0.8 pix/m and a sampling frequency of 30 Hz. This reconstruction method offers a precise, high spatio-temporal sampling needed for future characterization of sea ice fracture, or any other dynamics of textured surfaces.

References:

1. Dumas-Lefebvre, Elie and Dumont, Dany Aerial observations of sea ice breakup by ship waves TheCryosphere (2023)