ACOUSTIC SCATTERING LOSSES IN THE GREENLAND SEA MARGINAL ICE-ZONE DURING THE 1988-89 TOMOGRAPHY EXPERIMENT

Using arrival amplitude data from the 1988-89 Greenland Sea tomography experiment, the effects of ice scattering at 250 Hz in the marginal i ce zone (MIZ) are estimated. The resolution of the arrivals is examine d first, showing that arrivals are resolved as rays from about 8 degre es to 13 degrees launch angle and a normal modes for low angles, below about 4 degrees launch angle. The under-ice reflection losses of reso lved rays and modes are then considered using two methods. First, the ratio of arrival amplitudes during ice-free and ice-covered periods is estimated. Second, the ratio of members of a ray group with different reflection numbers is used, showing similar (but not identical) resul ts to the first method. For higher grazing angle reflections, fast fie ld calculations using the program SAFARI are observed to model the dat a reasonably, using a combination of measured and historical ice param eter values. For the low angle arrivals, the amplitudes of the arrival s are observed to depend strongly upon both the details of the surface mixed layer and the ice cover itself. Using reasonable mixed layer es timates from both data and model inputs, it is observed that part of t he amplitude reduction seen in the data is indeed due to ice, though t he exact amount is sensitive to the details of the mixed layer.