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Echo-sounding, seismic-reflection, and magnetic measurements were made across the Chukchi cap in the Arctic Ocean from drifting station Charlie in 1959. The Chukchi cap is 130 km wide at the line of crossing and its minimum depth is 246 meters. A microrelief of 5 to 30 meters on the top is attributed to gouging by icebergs. Seismic reflection results indicate that the top of the Chukchi cap is thinly covered with unconsolidated sediment but that small basins at the foot of the western margin may be filled to depths of I kin. A magnetic anomaly with a maximum peak-to-trough amplitude of 1600 gammas, associated with the western margin, is attributed to a large ridge in a basement rock of high susceptibility. Magnetic diurnal variations show a characteristic morning maximum at 1800 GMT. It is concluded that the Chukchi cap is a dissociated section of continental shelf. Introduction. Drifting station Charlie was established and maintained in the Arctic Ocean during 1959 by the U.S. Air Force as a floating pack ice station for scientific research. It was a United States contribution to the International Geophysical Cooperation--1959 and was originally known as Alpha II. The drift of the ice station, under the influence of wind and current, carried it over the Chukchi cap, a submarine feature which rises from ocean depths greater than 2000 m to less than 300 m. The location The research programs were in operation from June 1959 until January 1960, when the camp was evacuated at 77ø05'N, 168ø38'W, after the floe had been heavily fractured during a storm. The programs were under the supervision of several different organizations. The U.S. Weather Bureau made surface and upper-air meteorological observations. Researchers from the University of Washington studied micrometeorology, physical oceanography, and ice petrofabrics. Underwater sound transmission was investigated by the U.S. Navy Underwater Sound Laboratory. Lamont Geological Observatory of Columbia University conducted studies in marine geophysics, marine biology, sedimentation, and underwater sound propagation. Cromie [1961_] reported on the preliminary results of the 235

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Strong hydroacoustic signals from underwater explosions as small as 2 pounds of TNT have been detected in the Arctic Ocean at distances up to 1150 km. Hydrophones at depth and geophones on the ice surface were used as listening devices. The signals propagate in the Sofar channel, whose axis in the Arctic Ocean occurs approximately at the surface, but the character of the signals differs markedly from that typical of the nonpolar oceans, largely because of the predominance of low‐frequency waves in the Arctic. The character of the arctic Sofar signals is best explained by normal‐mode wave propagation in a channel bounded by the surface and by the zone of increasing velocity in the upper several hundred meters of water. Irregularities in the ice boundaries apparently strongly attenuate the high‐frequency waves but have a negligible effect on the amplitudes of the low‐frequency waves. At least two normal modes were observed. In each normal mode the waves have a nearly sinusoidal appearance with periods decreasing from about 120 to 30 msec in the first mode and from 70 to 40 msec in the second mode. A good quantitative fit between experimental data and theoretical dispersion curves has been obtained. The signals were detected in both deep and shallow water.