Both methods depend on the change in speed of ocean waves as they move beneath ice. The first employs tiltmeters attached to ice in the central Arctic; these register the motion of long-period waves in the ocean beneath. With periods of 20 to 30 seconds, the waves have an amplitude of just one mm or so. Typically they form during storms in the mid-Atlantic, before bouncing off the coast of Morocco and travelling to the Arctic.
Detecting the waves' arrival time from the storm enables researchers to calculate the change in their speed as they move beneath the ice. This wavelength change can then be converted into a figure for ice thickness. Such infra-gravity waves are also of interest to oceanographers because their period is longer than any open ocean swell, explained Wadhams.
The second technique measures the thickness of the pancake ice that forms at the edges of the open ocean. In this case, scientists can take advantage of synthetic aperture radar data from satellites to track the motion of waves across the ocean and beneath the ice. Again, mathematical analysis provides a figure for its thickness.
Although the techniques provide only average values for ice thickness and are not recommended as primary measurement methods, they could provide valuable extra information to complement that from research expeditions, satellites, and submarines.