SEA-SURFACE SIGNATURES RELATED TO SUBAQUEOUS DUNES DETECTED BY ACOUSTIC AND RADAR SENSORS

Side-scan sonar records and radar images of the Lister Tief in the Ger man Bight of the North Sea have been analysed. The radar data show sig natures on the sea surface which are related to irregularities in the submarine seabed. Some side-scan and radar data from the test area wer e taken at different dates, but at the same tidal phase and under comp arable weather conditions. Existing one-dimensional models of the rada r imaging mechanism predict extremes in radar backscatter above maximu m slope regions of subaqueous dunes. However, the acoustic data obtain ed during the ebb tidal phase do not always show an enhanced backgroun d noise and backscattering strength modulation directly above maximum slopes of the dunes. A large variation of the position of background n oise has been observed. The experimental acoustic data contradict the results of existing radar imaging models. The sonographs showed that r egions with increased background noise at close range (<5 m) are often associated with signatures of enhanced backscatter at ranges farther away (<40 m) or at lower grazing angles (<30-degrees). We conclude tha t the modulation of scattering strength can be attributed to regions o f air bubbles generated by turbulence and breaking water waves. Simula tions of the radar cross-section modulation above the large slopes of dunes are too large to remain within the bounds of the weak hydrodynam ic interaction theory in the relaxation time approximation. Therefore, this theory is not applicable in the sea area of the Lister Tief. Fur thermore, the hydrodynamic mechanism of standing waves or stationary s urface deformations associated with dunes is discussed.