DIELECTRIC-CONSTANT AND LAYER-THICKNESS INTERPRETATION OF HELICOPTER-BORNE SHORT-PULSE RADAR WAVE-FORMS REFLECTED FROM WET AND DRY RIVER-ICE SHEETS

Analysis of short-pulse radar data taken over river ice sheets in late spring reveals that radar signal penetration of an ice sheet and determination of its thickness are not necessarily prevented by the presence of surface water or internal melting. Radar data for both wet and dry ice were extracted from surveys performed from a helicopter operating at an altitude of about 2-7 m and a speed of about 5 m/s over the Connecticut River near Windsor, Vermont. The radar used a broadband wavelet of several nanoseconds duration at a center frequency of about 500 MHz. By use of plane wave theory, the dielectric constant of ice is interpreted from the amplitudes of reflections from a solid ice sheet. This verification of plane-wave interpretation and the lack of dispersion seen in wet ice bottom returns are then used to interpret data from segments of a wet ice sheet survey for both ice thickness and surface water depth. Other ice sheet segments, for which the data allow only sporadic interpretation of ice depth, are interpreted as having internal melting. It is concluded that ice thickness can be determined for surface water layers less than about 8-mm thick for the particular wavelet used so long as the ice interfaces are not severely rough.