Current techniques

We can learn a great deal about sea ice from satellite and aircraft surveys – its extent, its type, its surface features. But its thickness is hard to measure by remote sensing, because the brine cells in the ice give it a high electrical conductivity such that electromagnetic waves do not easily penetrate. The radio-echo sounding methods which have been used to measure the thickness of terrestrial ice sheets and glaciers cannot therefore be used for sea ice.

So far, five direct techniques have been commonly employed for measuring ice-thickness distribution. In decreasing order of total data quantity, they are:

1. submarine sonar profiling;

2. moored upward sonars;

3. airborne laser profilometry;

4. airborne electromagnetic techniques;

5. drilling.

Submarine sonar profiling

Most synoptic data to be published so far have been obtained by upward sonar profiling from submarines. Beginning with the 1958 voyage of Nautilus (Lyon, 1961; McLaren, 1988), which was the first submarine to the North Pole, many tens of thousands of kilometres of profile have been obtained in the Arctic by US and British submarines, and our present knowledge of Arctic ice-thickness distributions derives largely from the analysis and publication of data from these cruises. Problems include the necessity of removing the effect of beamwidth where a wide-beam sonar has been employed (Wadhams, 1981), and the fact that the data are sometimes obtained during military operations, which necessitates restrictions on the publication of exact track lines. For the same reason, the data set is not systematic in time or space.