Distributed Fibre Optic Sensing for High-resolution Subsurface Investigation in Istanbul and the Marmara Sea

Understanding the structure, physical properties and dynamics of the shallow subsurface at high resolution is critical for evaluating geohazards and exposure. However, detailed characterization and monitoring of subsurface structures and processes remains a challenge, specially in urban environments and offshore areas where subsurface access is very limited. Fibre-optic sensing deployed on existing, unused (“dark”) telecommunication networks offers an unprecedented opportunity to investigate the subsurface at high resolution in an efficient and sustainable way. In particular, Distributed Acoustic Sensing (DAS) enables seismic measurements at a spatial resolution of a few meters over tens of kilometers and at a temporal resolution of a few milliseconds, which would be cost prohibitive and non-viable using conventional seismic sensors.

We explore the potential of using DAS deployed on onshore and offshore dark fibres for subsurface imaging and monitoring in the metropolitan area of Istanbul and the eastern Marmara Sea region (Turkey). Istanbul sits 20 km north of the North Anatolia Fault Zone (NAFZ), one of the World’s most active faults. The NAFZ presents complexities in fault geometry near the city, which are not well understood, and data are lacking on the presence and geometry of hidden faults underneath the urban area. Besides, many of the fast-growing districts of the metropolitan area are experiencing deformation processes, such as subsidence and landslides, and are prone to strong co-seismic shaking and liquefaction due to underlying soft sediments and shallow hydrological systems. To monitor seismicity and deformation processes in the eastern Marmara Sea, GFZ is operating the Geophysical Observatory of the Northern Anatolian Fault (GONAF) in collaboration with the Turkish Civil Disaster Emergency Authority (AFAD), consisting of seven boreholes equipped with seismometer strings and partly with strainmeters. With our study, we expand and strengthen the observatory by integrating fibre-optic sensing technologies and developing advanced approaches for DAS-based high-resolution imaging and process monitoring.

Since May 2024, two dark fibres have been interrogated by two DAS instruments deployed in a telecommunication facility in the Istanbul district of Kartal. One of the fibres is fully onshore; it is 17 km long and traverses part of this densely populated district, crossing almost perpendicularly the under-investigated Kartal Fault. The second fibre crosses the coastal portion of Kartal and runs on the ocean floor connecting four of the Princess Islands, which host GONAF stations. Ambient seismic noise from both natural (i.e. ocean waves) and anthropogenic (traffic) origin have been continuously recorded, and several local and regional earthquakes have been captured by both fibres. We are applying ambient seismic noise interferometry approaches and investigating the properties of earthquake wave propagation to obtain high-resolution images of the subsurface along the urban-coastal area, to illuminate potential hidden faults and retrieve detailed information on material properties and their relationship with site response and ground deformation. Ultimately, our study aims to provide approaches for leveraging dark fibre in densely populated coastal areas for efficient subsurface investigations for improved geohazard assessment.