Sonar mapping of the underside of pack ice

Abstract

An underice survey was carried out in a pack ice field near Fletcher's Ice Island (T-3) by means of a 48-kHz Kelvin Hughes transit sonar. The transducer was lowered through a hole, and discrete underice scans were made through 360°. Five sets of data have been collected and transformed into polar coordinate mosaics. The echo length, the shadow zone, the arrival time of echoes, and the sounding depth provide the basic information for interpretation. A complete underice map has been constructed to show the distribution of the underice features. The keel depths of the ridges have been estimated. A surface survey that included a cross-ridge survey was carried out. The top and bottom features have been compared. The average ratio of the peak top elevation to the keel depths is 1:7.6.