Arctic

The two features peculiar to the polar environment that most strongly influeace underwater sound are the permanent ice cover and the velocity structure in the water. Ice movement generates background noise and the ice modifies propagation, particularly at high frequencies, by scattering waves from the rough ice boundaries. Sound velocity is a function of temperature, salinity, and pressure. The relationship between these variables in the central Arctic Ocean is such that sound velocity is generally an increasing function of depth from the surface tb the bottom. Such a velocity profile is found only in polar waters. The sound velocity structure is remarkably uniform both as a function of location and time of year. Sounds are transmitted to great ranges in this natural arctic waveguide or sound channel by upward refraction in the water and repeated reflection from the ice canopy. A two-pound explosion of TNT has been heard at ranges exceeding 1,100 km. (700 miles). The surface sound channel of the Arctic is the polar extension of the deep sound channel or SOFAR channel of the nonpolar Ocean6 (Ewing and Worzel 1948), but the arctic signals are often quite different fronl those observed in the deep channel, largely because of the predominance of lowfrequency waves in the Arctic. The arctic sound channel is of considerable importance to the Navy because of the possibility of long-range detection and communication. That ocean also provides an ideal test area for new concepts of signal detection and processing because of the easy access to the sound channel and the permanence of installations located on ice islands. The purpose of this paper is to review our present knowledge of underwater sound obtained from experiments made aboard drifting ice stations in the central. Arctic Ocean and to recommend future research in this field. I shall present a summary of the results of experiments made by Lamont-Doherty Geological Observatory of Columbia University; these results have been published by Kutschale (1961), Hunkins and Kutschale (1963), Hunkins (196% Hunkins (1966), and Kutschale (1968). Many of our experiments were conducted in cooperation with the U.S. Navy Underwater Sound Laboratory, the Pacific Naval Laboratory of Canada, and AC Electronics Defense Research Laboratories of General Motors Corporation. Results by workers from these laboratories have been published by Marsh and Mdlen (1963), Mellen and Marsh (1963), Milne (1964), Buck and Green (1964), and Buck (1968). Drifting ice stations provide an ideal platform for research on underwater sound. These stable platforms over deep or shallow water are far removed from ship traffic and they provide a lafge surface area for detector arrays. Detectors