High-Resolution Subsurface Imaging for Hazard Assessment in Urban Areas Using Ambient Noise and Dark Fibers: a Case Study in Istanbul

Urban areas are very vulnerable to the effects of geohazards, with a high potential for human life and financial loss due to their high population density and advanced infrastructure. Obtaining high-resolution, subsurface information in urban areas is critical to understand and mitigate the effects of these hazards. In these densely populated centers, the shallow Earth is most stressed, as human activities continuously change their properties and interfere with natural processes. Geophysical surveys often face limitations in urban environments due to logistical constraints (anthropogenic activities, legal restrictions, and risks of equipment theft, among others). Repurposing unused, existing telecommunication optical fibers (so-called dark fibers) as sensing arrays offers a promising alternative to traditional geophysical methods, enabling subsurface imaging at high spatial and temporal resolution in densely populated areas. 

The Megacity of Istanbul (Turkey), situated in one of the most tectonically active regions in the World, represents an ideal case for exploring the potential of using dark fibers for subsurface investigations in urban areas. Since May 2024, we have been using Distributed Acoustic Sensing (DAS) to continuously record passive seismic data along a 17 km-long dark fiber crossing the Kartal district in the metropolitan area of Istanbul with the aim to explore the potential of this technology for seismic hazard assessment. Our objective is to apply passive seismic interferometry approaches to DAS ambient seismic noise data to identify hidden faults and generate high-resolution urban-scale subsurface velocity models that can contribute to a better understanding of structures and material properties and their association with seismic risk. Using dark fibers in urban contexts presents multiple challenges, including vast data volumes, a complex noise environment, and unconventional geometries. To address these issues, we develop tools to maximize the potential of dark fibers by effectively utilizing opportunistic ambient noise sources. We evaluate valuable sources, such as train tremors and other traffic, and assess their effectiveness for DAS-based passive seismic interferometry in a complex array setting. Our final objective is to develop efficient approaches to achieve imaging at high spatial and temporal resolution, providing insights that could help mitigate geohazard risk in Istanbul and other similar urban areas.