Measuring sea ice draft and coverage with moored upward looking sonars

Presented is a method to derive ice draft and coverage from acoustic measurements made with moored Upward Looking Sonars (ULSs), sounding the sea surface remotely from below. The method was developed on the basis of two-year long time series obtained from four locations in the Weddell Sea. It takes into account the variations of sound speed and density that occur between the target and the instrument, the variations of the surface air pressure, and a possible bias of the mean ice draft within the ensonified window, which results from the combined effect of beam spreading and skewed ice distributions; ice and open water are differentiated by their specific echo amplitude signatures. The residual total error in the determined mean ice draft is estimated as 4 cm, compared to an initial error of almost 90 cm in the original uncorrected data. Suggestions for further instrumental improvements are made. (C) 1998 Elsevier Science Ltd. All rights reserved.