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 ice zone (MIZ) are estimated. The resolution of the arrivals is examined first, showing that arrivals are resolved as rays from about 8 degrees to 13 degrees launch angle and a normal modes for low angles, below about 4 degrees launch angle. The under-ice reflection losses of resolved 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) results to the first method. For higher grazing angle reflections, fast field calculations using the program SAFARI are observed to model the data reasonably, using a combination of measured and historical ice parameter values. For the low angle arrivals, the amplitudes of the arrivals are observed to depend strongly upon both the details of the surface mixed layer and the ice cover itself. Using reasonable mixed layer estimates from both data and model inputs, it is observed that part of the amplitude reduction seen in the data is indeed due to ice, though the exact amount is sensitive to the details of the mixed layer.