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

多年冻土对岩壁的稳定性有着重要的影响。特别是，全球气温上升促进永久冻土退化，可能导致岩壁的危险破坏。因此，包括电阻率层析成像和钻孔温度测井在内的各种技术已被用于监测多年冻土。然而，通常使用的方法通常是费力的、昂贵的，并且在空间范围和/或时间分辨率方面受到限制。相比之下，通过连续采样地面运动，地震学产生高的时间分辨率和空间洞察力，因为地震波穿过地下。这已被广泛收获，通过地震干涉测量监测各种地下过程。然而，在多年冻土监测领域，只有少数探索性的实验进行。楚格峰（德国）地震干涉测量。从地震学的角度来看，冻土的发生强烈影响地震速度的地下，因此，我们预计空间速度对比（从冻结到未冻结的岩石）和时间的速度变化，由于冻土动态（解冻和再冻结）。我们将沿着受永久冻土影响的山脊安装三个地震阵列，这些地震阵列将由标准地震仪组成，允许获得旋转地面运动。此外，我们将定期（至少每季度一次，为期一周）增加一个旋转运动传感器到每个阵列和测量地面运动引起的应变沿着光纤电缆部署沿着山脊每隔几米。旋转和应变，波场梯度，构成新的观测与地震学的新机会。我们计划提取连续的时间序列的地震速度变化之间的阵列至少两年。此外，应变传感器沿着光纤电缆的空前空间覆盖范围将使我们能够定位速度变化并对永久冻土的空间范围进行成像。波场梯度将有助于更好地检测永久冻土边界，因为它们比经典测量的平移（即波场）更受近接收器结构的影响。由于波场梯度刚刚开始纳入普通地震部署，经验有限。因此，我们将开发的方法与地震波场及其梯度的数值模拟，从而建立理论和现场实验之间的密切联系，允许最佳地利用波场梯度的优势。该项目的目标是通过改进时空监测，破译永久冻土动态，即解冻、再冻结和一般退化。这将有助于更好地探测永久冻土退化造成的岩石不稳定性，并增进我们对大气与地球之间动态相互作用的理解。

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.