Spatio-temporal Permafrost Monitoring on Mt. Zugspitze (Germany) with Seismic Interferometry and Wavefield Gradients

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Permafrost crucially affects the stability of rock walls. In particular, globally rising temperatures promote permafrost degradation potentially resulting in hazardous failure of rock walls. Thus, various techniques including electrical resistivity tomography and borehole temperature logging have been employed to monitor permafrost. However, the commonly used methods are typically laborious, expensive, and are limited in the spatial extent and/or temporal resolution. In contrast, by continuously sampling the ground motion, seismology yields high temporal resolution and spatial insights, as the seismic waves travel through the subsurface. This has been extensively harvested for the monitoring of various subsurface processes through seismic interferometry. Yet, only a few exploratory experiments were conducted in the realm of permafrost monitoring.In this project, we aim to monitor and image permafrost on Mt. Zugspitze (Germany) with seismic interferometry. From a seismological perspective, the occurrence of permafrost strongly affects seismic velocities of the subsurface, hence, we expect spatial velocity contrasts (from frozen to unfrozen rock) and temporal velocity variations due to permafrost dynamics (thaw and refreeze). We will install three seismic arrays along the permafrost affected ridge, which will consist of standard seismometers allowing to derive also rotational ground motions. In addition, we will periodically (at least once per quarter year for one week) add a rotational motion sensor to each array and measure ground-motion induced strain every few meters along a fiber-optic cable to be deployed along the ridge. Rotations and strains, the wavefield gradients, constitute new observables with new opportunities for seismology.We plan to extract continuous time series of seismic velocity variations between the arrays for at least two years. In addition, the unprecedented spatial sensor coverage of strain sensing along the fiber-optic cable will allow us to locate the velocity variations and to image the spatial extent of permafrost. The wavefield gradients will help to better detect the permafrost boundaries, as they are more affected by near-receiver structure than the classically measured translations, i.e. the wavefield. As wavefield gradients just start to be incorporated in common seismic deployments, experience is limited. We will therefore develop the methodology with numerical modeling of seismic wavefields and their gradients, thereby establishing a close connection between theory and field experiments allowing to optimally exploit the advantages of wavefield gradients. The objective of this project is to decipher permafrost dynamics, i.e. thaw, refreeze and general degradation, through improved spatio-temporal monitoring. This will help to better detect rock instabilities caused by permafrost degradation and to improve our understanding of the dynamic interplay between the atmosphere and the Earth.