Helicopter electromagnetic data map ice thickness at Mount Adams and Mount Baker, Washington, USA

Ice-thickness measurements critical for flood and mudflow hazard studies are very sparse on Cascade Range (North America) volcanoes. Helicopter electromagnetic (HEM) data collected to detect hydrothermal alteration are used to determine ice thickness over portions of Mount Baker and Mount Adams volcanoes. A laterally continuous inversion method provides good estimates of ice <100 m thick over water-saturated and altered regions where the resistivity of the basement is <200 Ωm. For areas with ice overlying fresh, resistive rocks with small resistivity contrasts between ice and rock, ice thickness is not well resolved. The ice thicknesses derived from HEM data are consistent with the previous drillhole data from Mount Adams and radar data from both volcanoes, with mean thicknesses of 57 m for Mount Adams and 68 m for Mount Baker. The thickest ice on Mount Baker rests on the gentle lower slopes whereas the thickest ice at Mount Adams lies on the flat summit. Ice volume calculations suggest that Mount Baker contains ∽710 × 106 m³ of ice in the HEM survey area, with a crude estimate of ∽1800 × 106m³ for the entire volcano. Ice volume on Mount Adams is 65 × 106m³ in parts of the HEM survey area and ∽200 × 106m³ overall.