Mapping Ice Buried by the 1875 and 1961 Tephra of Askja Volcano, Northern Iceland Using Ground-Penetrating Radar: Implications for Askja Caldera As a Geophysical Testbed for in Situ Resource Utilization

Eruptions of the Askja Volcano in Northern Iceland in 1875 and 1961 blanketed the caldera with rhyolitic and basaltic tephra deposits, respectively, which preserved layers of seasonal snowpack as massive ice. Askja serves as an operational and geophysical analog to test ground-penetrating radar field and analysis techniques for in situ resource utilization objectives relevant to the martian and lunar environments. We conducted ground-penetrating radar surveys at center frequencies of 200, 400, and 900 MHz to map the thickness and extent of tephra deposits and underlying massive ice at three caldera sites. We identified up to 1 m of tephra preserving up to 4.4 m of massive ice. We measured the real dielectric permittivity of the overlying tephra and the total attenuation at each frequency of the tephra and ice. A key objective of our investigation was to determine if attenuation (or loss) could be used as an additional diagnostic signature of massive ice preserved at depth when compared to ice-free stratigraphy. Loss rates of the ice-rich subsurface decrease with increasing ice thickness relative to the overburden, which may constitute a possible signature. Attenuation also increased with increasing frequency. The tephra, ice, and other volcanic deposits at each of our three caldera sites and the ice-free, pumice-mantled 1961 Vikrahraun lava flow exhibited consistently low loss rates at all frequencies. This result highlights the ambiguity associated with identifying the unique signature of ice within low-loss stratigraphies, a possible challenge for its identification in the martian or lunar subsurface using radar. The Askja Volcano in Northern Iceland is considered to be a planetary analog for other terrestrial worlds such as Mars and the Moon. We conducted ground-penetrating radar surveys of the Askja caldera where seasonal snowpack was buried by eruptions of low-density ash and tephra in 1875 and 1961. This erupted volcanic material protected the snowpack long-term, where it later densified into thick layers of ice. We successfully mapped up to 4.4 m of this ice preserved beneath up to 1 m of erupted material. Transmitted radar signals decay naturally as their distance from the source increases. Some materials such as water ice are less conducting than others and are therefore less lossy to this transmitted signal. We used the ice and tephra deposits at Askja as a test case to determine if large quantities of buried ice would result in a detectable signature that indicates its presence when compared to ice-free regions. We found that increases in ice layer thickness relative to the overlying volcanic material result in columns of material with bulk properties that are less dissipative to the radar signal and therefore may indicate a signature of buried ice in the subsurface. Multi-frequency Ground-penetrating radar (GPR) surveys identified massive ice up to 4.4 m thick buried by up to 1 m of tephra from two Holocene eruptions of Askja GPR readily maps vertical and horizontal extents of subsurface ice and tephra overburden; ice concentration transitions are not detected Ice-rich and ice-free sites present a similar attenuation; ice-rich sites demonstrate a lower attenuation rate with increasing ice thickness