Passive Radar Sounding of Firn Aquifers: Geophysical Constraints and Sensitivity

Firn aquifers retain liquid meltwater within the near-surface layers of ice sheets and ice shelves, which may influence mass balance and subglacial hydrology. Despite their importance, measuring changes in firn aquifer water storage remains a challenge using existing satellite, airborne, and ground-based active radar methods, largely due to the significant spatial and temporal variability of firn aquifers. Complementary to active radar techniques, passive radar sounding is an advancing radioglaciological method that does not transmit its own signal for echo detection, but instead receives and correlates ambient radio emissions from the Sun to detect subsurface reflections, including those from firn aquifers.

In this presentation, we investigate the geophysical constraints (e.g., firn temperature, saturation, density, and depth) that govern the sensitivity of passive radar sounding to detect firn aquifer water table fluctuations. Our analysis highlights simulation-based, site-specific case studies representative of firn aquifer environments in Greenland, Svalbard, and Antarctica. Using realistic firn properties and expected solar geometry throughout the year, we evaluate signal attenuation, depth sensitivity, and expected echo time delays to identify seasonal observation windows for passive sounding.

Our results show that passive radar sounding can achieve sufficient signal-to-noise ratio and depth sensitivity to support monitoring on daily to seasonal timescales, particularly during and after the summer melt season when the most rapid changes in firn aquifers are likely to occur. These results further highlight the conditions under which passive sounding could enable quasi-continuous monitoring of firn aquifer dynamics and address a key gap in current cryospheric observational strategies.