Long-term refraction seismic monitoring: a reliable method to detect ground ice loss at mountain permafrost sites

Abstract

Geophysical monitoring becomes more and more popular in permafrost environments due to its remarkable success to detect permafrost thawing and spatio-temporal changes in the ground ice content. Mostly geoelectric methods such as Electrical Resistivity Tomography (ERT) are applied due to the strong differences in the electrical properties between frozen and unfrozen state. However, seismic properties also change markedly upon freezing/thawing and time-lapse refraction seismic tomography (RST) has been shown to be applicable to permafrost over smaller time scales (e.g., Hilbich 2010). The reason why only few studies employ long-term seismic monitoring in permafrost is probably due to the higher logistical effort required. At two Swiss permafrost monitoring sites (Schilthorn and Stockhorn) yearly RST surveys are conducted using the same setup for more than 15 years, in addition to standard borehole temperature, climatic and ERT measurements (www.permos.ch). The monitoring aim is to image the interannual changes of the thickness of the active layer as well as differences in ice content within the permafrost layer below. Additional long-term observations are available from RST (and contemporary ERT) surveys from several mountain permafrost sites in Norway that were initially conducted to characterise permafrost conditions around boreholes drilled in 1999/2008 (Juvvasshoe/Jotunheimen), and 2007/2008 (Iskoras/Finnmark, Guolasjavri/Troms, and Tronfjell, cf. Isaksen et al. 2011, Farbrot et al. 2013). These surveys were repeated with the same geometry in 2019 after 11 years in northern Norway, and after 8 and 20 years in southern Norway. As for the Swiss sites, temperatures from all these boreholes show a clear warming trend over the last 1-2 decades (Etzelmüller et al, 2020, 2023). We here present the observed long-term changes in electrical resistivity and seismic P-wave velocity based on a) annually repeated measurements in the Swiss Alps, and b) on long-term repetition in northern and southern Norway. The geophysical changes are related to the observed borehole temperature increase during the same period (Etzelmüller et al. 2023) and analysed with respect to climate-induced thawing. We evaluate the advantages and disadvantages of seismic monitoring compared to the more standard ERT monitoring. Finally, the results are also analysed with respect to their suitability for future ERT-seismic joint inversion approaches in a monitoring context.