ALBERT

Description: Climate-induced warming increasingly leads to degradation of high-alpine permafrost. In order to develop early warning systems for imminent slope destabilization, knowledge about hydrological flow processes in the subsurface is urgently needed. Due to the fast dynamics associated with slope failures, non- or minimally invasive methods are required for inexpensive and timely characterization and monitoring of potential failure sites to allow in-time responses. These requirements can potentially be met by geophysical methods usually applied in near-surface geophysical settings, such as electrical resistivity tomography (ERT), ground-penetrating radar (GPR), various seismic methods, and self-potential (SP) measurements. While ERT and GPR have their primary uses in detecting lithological subsurface structure and liquid water/ice content variations, SP measurements are sensitive to active water flow in the subsurface. Combined, these methods provide huge potential to monitor the dynamic hydrological evolution of permafrost systems. However, while conceptually simple, the technical application of the SP method in high-alpine mountain regions is challenging, especially if spatially resolved information is required. We here report on the design, construction, and testing phase of a multi-electrode SP measurement system aimed at characterizing surface runoff and meltwater flow on the Schilthorn, Bernese Alps, Switzerland. Design requirements for a year-round measurement system are discussed; the hardware and software of the constructed system, as well as test measurements are presented, including detailed quality-assessment studies. On-site noise measurements and one laboratory experiment on freezing and thawing characteristics of the SP electrodes provide supporting information. It was found that a detailed quality assessment of the measured data is important for such challenging field site operations, requiring adapted measurement schemes to allow for the extraction of robust data in light of an environment highly contaminated by anthropogenic and natural noise components. Finally, possible short- and long-term improvements to the system are discussed and recommendations for future installations are developed.

Description: Geophysical methods are widely used to investigate the influence of climate change on alpine permafrost. Methods sensitive to the electrical properties, such as electrical resistivity tomography (ERT), are the most popular in permafrost investigations. However, the necessity to have a good galvanic contact between the electrodes and the ground in order to inject high current densities is a main limitation of ERT. Several studies have demonstrated the potential of refraction seismic tomography (RST) to overcome the limitations of ERT and to monitor permafrost processes. Seismic methods are sensitive to contrasts in the seismic velocities of unfrozen and frozen media and thus, RST has been successfully applied to monitor seasonal variations in the active layer. However, uncertainties in the resolved models, such as underestimated seismic velocities, and the associated interpretation errors are seldom addressed. To fill this gap, in this study we review existing literature regarding refraction seismic investigations in alpine permafrost permitting to develop conceptual models illustrating different subsurface conditions associated to seasonal variations. We use these models to conduct a careful numerical study aiming at a better understanding of the reconstruction capabilities of standard and constrained RST approaches. Our results demonstrate, that the incorporation of structural constraints in the inversion and the usage of constrained initial models help to better resolve the geometry and the velocity structure of the true models. Moreover, we present the successful application of this extended constrained approach for the inversion of refraction seismic data acquired at Hoher Sonnblick (Austria) by incorporating complementary information obtained from the modelling of ground-penetrating radar (GPR) signatures. In conclusion, our study shows the potential of an extended constrained RST to improve the reconstruction of subsurface units and the associated seismic velocities in a permafrost environment, permitting to reduce the uncertainties in the interpretation of the imaging results.