The paper presents the main results of the field investigations using multi-frequency (150 MHz, 270 MHz, 400 MHz and 900 MHz) ground-penetrating radar profiling and georadar sensing by the common depth point method. The investigations have been carried out on the fast sea ice of Nella Fiord (Eastern Antarctica) during the 2016/17 austral summer field season. The studies have included the seawater salinity measurements. The sounding of about 1 m thick sea ice have proved to be most efective at the radar frequencies which correspond to the wavelengths in a meter range. Besides the intensive reflections from the lower edge of sea ice, the ground-penetrating radar data have revealed a boundary between fresh and salt water whose position has been corroborated by salinity measurements. The ground-penetrating radar common depth point technique has served as a basis for the subsurface velocity model showing that efective dielectric constant for sea ice equals 10, which is accounted for intense near-surface internal melting and for the ice being exceedingly saturated with fresh water. The efective dielectric permittivity of the fresh water layer is found to be 75 that may have been prompted by roughness of the lower ice boundary.