Helicopter-towed electromagnetic (HEM) induction sounding systems are typically used for geologic surveys. More recently, HEM systems have been used for the remote measurement of sea-ice thickness and shallow sea bathymetry. An important aspect of this remote sensing technology is the area, or footprint, in which the secondary field is predominantly generated by induced currents. A knowledge of the size of the footprint is important to understanding the accuracy of HEM sounding results over lateral variations in relief or conductivity. Conventional wisdom among workers in the field held that the footprint diameter is a few times the HEM antenna altitude. We confirm this view using airborne measurements over sea ice to calculate the footprint size/antenna altitude ratio. These findings are compared to various theoretical estimates and are found to be in reasonable agreement. For a vertical coaxial coil antenna arrangement, the apparent footprint diameter was found to be about 1.3 times the antenna height above the sea-ice/water interface, and for a horizontal coplanar coil figuration the ratio is about 3.8 times the antenna height.

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