ALBERT

Description: In recent years, it has become clear that Distributed Acoustic Sensing (DAS) is capable of capturing many seismic signals, including earthquakes across a range of magnitudes. Given the vast networks of fibre optic cables beneath urban areas, there is great potential for the utilisation of existing telecommunication fibres for the purpose of seismic monitoring and hazard analysis. However, it is crucial that we first understand the capabilities and potential limitations of applying DAS in this way.In late 2021, we collected over one month of urban DAS data beneath North-East Athens, Greece, using ~ 24 km of telecommunication fibre owned by the OTE group. This urban dataset contains a broad range of signals, including local anthropogenic activities, as well as numerous seismic events.We build a catalogue of events observed within the DAS data, and assess differences between our DAS event catalogue and that of the regional seismic network managed by the National Observatory of Athens (NOA). This includes an analysis of to what extent sensitivity to events is influenced by event magnitude, depth, distance and azimuth. The dense spatial sampling offered by DAS facilitates the use of novel array processing methods, which can be used to detect smaller events. We determine whether the benefits of using DAS and applying such methods within a regional seismic network justify the additional costs.We also explore the influence of the exact fibre deployment and local geological conditions on the seismic events recorded, and how this information can be considered for future DAS earthquake-monitoring experiments.

Description: We use 24 km of telecommunications fiber in Athens, Greece, to explore the use of ambient noise correlations and the construction of a shallow 2D velocity model. Models of the shallow subsurface are especially important for hazard and site-response characterisation, yet traditional measurement campaigns in dense, urban environments can be challenging. Using pre-existing telecommunications fiber provides an efficient and easy way to construct such high-resolution models.The fiber was provided by the Hellenic Telecommunications Organization (OTE), over 36 days in the Autumn of 2021, and in cooperation with the National Observatory of Athens (NOA). Using a Silixa iDAS interrogator, we measure strain-rates every 4 meters along the fiber to provide dense observations of wave propagation. Strong anthropogenic noise gives clear Rayleigh-wave signals in cross-correlations in the frequency band of 5-30 Hz. We discuss our efforts to efficiently stack multiple virtual sources’ record sections and construct dispersion curves. We then invert for numerous 1D velocity profiles of the top ~100 m, comparing different approaches, and finally present a 2D model over the entire line.

Description: Distributed Acoustic Sensing (DAS) has become a popular method of observing seismic wavefields: backscattered pulses of light reveal deformation with meter-scale resolution along a fiber-optic cable. In contrast, a few newer systems transmit light through a cable and collect integrated phase delays over the entire cable, such as the Microwave Frequency Fiber Interferometer (MFFI). The outstanding advantage of these integrated systems is that they can be interrogated over significantly longer distances of hundreds or thousands of kilometers, may be used at the same time as live telecommunication, and can be significantly cheaper. However, they provide only a single time series representing strain over the entire length of fiber. In the first part of this contribution, we present a theory for integrated fiber optic sensing. This enables a quantitative comparison of these new systems with established DAS measurements. Furthermore, we note that sensitivity to deformation depends critically on local fiber curvature, which offers opportunities to mimic distributed measurements with integrated systems. In the second part, we present the first results of a quantitative, head-to-head comparison of a DAS and the integrated MFFI system using pre-existing telecommunication fibers in Athens, Greece.