Using Surface NMR Nuclear Magnetic Resonance to Determine Sediment Structure and Properties Beneath Thermokarst Lakes

Abstract:

Thermokarst lakes form following the subsidence of ice-rich permafrost terrain and concurrent infilling of the depression with water. Areas of unfrozen sediment, called taliks, can form under lakes that have a mean annual bottom temperature greater than 0°C. Taliks are thought to play an important role in permafrost hydrology and carbon cycling. Sub-lake taliks can extend to depths of tens of meters, making them difficult and costly to measure by direct methods such as boreholes. Surface nuclear magnetic resonance (NMR) provides an unambiguous measurement of the liquid water content associated with unfrozen taliks; however, limitations of the measurements and the interpretation of the data still remain. Forward models were created to test the effect of varying water column thicknesses on the ability to resolve the depth and water content of taliks with parameters similar to geometries measured in late-winter field studies in the Arctic and Boreal regions of Alaska. The results of the forward modeling show that talik depth resolution decreases with a larger water column thickness, a shallower talik depth, and a lower water content. These results place constraints on our field measurements, with lakes greater than 9 m deep yielding potentially misleading inversion results. Field data was collected at a small, 4.6 m deep thermokarst lake near Fairbanks, Alaska, which had a known subsurface structure including talik depth, and available talik sediments from cores. Known subsurface structure was not used to inform the inversion model to determine how well talik depth could be resolved in lakes with unknown structure. With constraints placed on ice and water thickness, which are easily measured in the field, the blocky inversion model was able to accurately resolve the talik depth and water content for lakes less than 9 m deep.