Subsurface Imaging in Urban Areas With Ambient Noise Using DAS and Seismometer Data Sets: Granada, Spain.

Distributed acoustic sensing (DAS) is an innovative technology with great potential for acquiring seismic data sets in urban areas. In this work, we check the suitability of a DAS data set acquired in Granada (Spain) for retrieving subsurface reflectivity from ambient noise. The fiber‐optic is a pre‐existing underground telecommunication cable that crosses the city from Northwest to Southeast. We use a 10 hr recording of strain rate from a 2020 experiment to obtain seismic reflections using the autocorrelation method. We compare the DAS results with reflections obtained from seismic ambient noise recorded in nine seismometers deployed close to the fiber‐cable for 7 days in November 2022. The novel approach proposed in this study for the identification of the reflections is to use autocorrelations after bandpass filtering for specific central frequencies and to check the stability of the signals over a broad frequency band. Microtremor Horizontal to Vertical Spectral Ratio (MHVSR) measurements at a total of 14 stations, five of them outside the city, help to constrain the reflection interpretation. These include one station at the borehole that reaches the basement in the Granada Basin crossing all the Cenozoic units. We use the legacy sonic log to obtain a relationship between frequencies of MHVSR peaks and depth. Autocorrelation and MHVSR methods give consistent results delineating bedrock depth deeper than 1,000 m in Granada. These results confirm that DAS can provide valuable subsurface information in urban areas. Plain Language Summary: Urban subsurface information is of paramount importance to understand how the underground affects the development of cities (groundwater or hazards such as earthquakes). One of the most useful methods to improve the knowledge of the city underground is seismic technique but installation of vibration sensors can be difficult in an urban area. We use in this work a cutting‐edge technology called Distributed Acoustic Sensing (DAS) that uses fiber‐optic cables in the city, such as preinstalled telecommunication cables, as a high number of seismic sensors. We compare the seismic results obtained from the measurements in the fiber‐optic cable and from conventional seismometers. The analysis of the vibrations acquired with DAS and seismometers provide reflections, like echo signals, from the subsurface changes. In particular we obtain a reflection coming from the contact between sediments and bedrock. The depth of this contact is similar with the two type of measurements reaching 1,400 m depth below the city. This confirms that this new technology can be used in cities to study their subsurface. Key Points: Distributed Acoustic Sensing holds great potential for urban seismic data acquisitionAutocorrelation and Microtremor Horizontal to Vertical Spectral Ratio methods consistently reveal bedrock depth exceeding 1,000 m in GranadaA novel approach is developed to analyze autocorrelation results for retrieving subsurface reflections [ABSTRACT FROM AUTHOR]