Active Source Seismic Investigation of Firn Aquifer Structure in Southeastern Greenland

The perennial storage of water in the firn of southeastern Greenland has been observed since 2011. In the summers of 2015 and 2016, our team deployed to the ice sheet upstream of Helheim Glacier and performed an extensive series of geophysical and hydrological measurements of the firn subsurface structure and associated aquifer system. As part of these experiments, we carried out a series of active source seismological refraction profiles to probe the base of the aquifer and quantify the volume of water present. We analyze the resulting P-wave travel times and surface wave dispersion using a joint Transdimensional Hierarchical Bayesian analysis to obtain 1-D velocity profiles for P and S-waves beneath each refraction site. We find the base of the aquifer lies 25-35 meters beneath the subsurface, which is in agreement with the depth found in ice cores obtained at two of the seismological investigation sites. The transition in seismic velocity associated with the top and base of the aquifer are modeled as a Voigt-Reuss-Hill medium consisting of air, ice, and liquid water to obtain the change in water content with depth, which is compared to the measured core water contents, ground penetrating radar measurements of the depth of the top of the aquifer in the firn, and water volumes estimated from magnetic resonance sounding of the aquifer structure. Elucidating the geometry of and volume of water stored within these recently discovered aquifers is vital for determining the hydrological structure and stability of the southeastern Greenland ice sheet.