Urban Shallow Subsurface Imaging and Hazard Assessment with Distributed Acoustic Sensing in Athens, Greece

In urban environments, existing telecommunication fiber-optic cables can enable geophysicists to characterize the shallow subsurface and conduct monitoring using Distributed Acoustic Sensing technology.

Athens is a metropolitan city with a population of 3.5 million potentially exposed to severe seismic hazards. Here, we use data from a 23-km long "dark" fiber provided by the Hellenic Telecommunications Organization (OTE) in the northern suburbs of Athens. The data was acquired over 36 days in autumn 2021.

We explore an ambient noise interferometry processing workflow for DAS, starting from a 1-km segment of the fiber and extending to other parts of the dataset. From the seismic noise correlations, we extract dominant Rayleigh wave dispersion measurements. We apply a trans-dimensional MCMc inversion method from surface wave dispersion to produce vertical 1D shear velocity models. We observe a low-velocity layer in the shallow (20 to 40 m) subsurface and show its lateral variations through the mapped 2D velocity models with high resolution (meter-scale). The velocity model is well-constrained in the top 100 meters, and it allows us to estimate site amplification effects that improve our understanding of urban hazards in Athens.