Two‐dimensional mapping of sea‐ice keels with airborne electromagnetics

Abstract

Airborne electromagnetic (AEM) equipment can be used to sense sea ice thickness by interpreting the AEM data to obtain the distance from the towed bird that holds the EM system to the ice/seawater interface. The ice thickness itself is obtained by subtracting from that quantity the distance from the bird to the upper ice surface, as determined by a laser altimeter. To interpret AEM data acquired over sea‐ice keels, we first solve the forward problem using an integral equation approach to the Neumann boundary‐value problem. In this approach, we assume that sea ice is an insulator and that seawater is a perfect conductor. When the ice keel is two‐dimensional, the pertinent equations can be transformed into the wavenumber domain along the strike direction, resulting in the rapid numerical computation of the AEM response. By compiling numerical‐model results, we constructed an interpretation chart that relates the parameters of the observed AEM response anomaly to the geometric variables of the ice keel. The strike length of the ice keel should be about three times the bird height above the Ice‐water interface, so that the assumption of two‐dimensionality holds. The use of the chart has been verified by interpreting field AEM data.