Beamwidth effects on sea ice draft measurements from U.K. submarines

Single-beam upward-looking sonars mounted on British submarines have provided a vast amount of Arctic sea ice thickness data since the early 1970s. In this paper we develop a systematic procedure to estimate the bias introduced in the measurement of the ice draft by the standard procedure of taking only the first arrival of the returned echo. Because the magnitude of this error varies significantly with the depth of the submarine, the beamwidth of the sonar and the topography of the underside of the ice, it has to be calculated for each individual transect. Based on data collected during a submarine cruise in the Arctic Ocean in the winter of 2004, we estimate that for conic beams of semi-angles 3° and 6° the observed drafts are typically 7–20% and 15–35% higher than the real drafts. In view of the size of these errors, much higher than previously reported, we argue that beamwidth corrections must always be taken into account when measuring sea ice draft from below.