Near-surface characterization using Distributed Acoustic Sensing in an urban area: Granada, Spain

Abstract

The Granada Basin in southeast Spain is an area of moderate seismicity. Yet, it hosts some of the highest seismic hazards in the Iberian Peninsula due to the presence of shallow soft sediments amplifying local ground motion. In urban areas, seismic measurements often suffer from sparse instrumentation. An enticing alternative to conventional seismometers is the Distributed Acoustic Sensing (DAS) technology that can convert fiber-optic telecommunication cables into dense arrays of seismic sensors. In this study, we perform a shallow structure analysis using the ambient seismic field interferometry method. We use a DAS array field test in the city of Granada obtained on August 26th and 27th, 2020, using a telecommunication fiber. In addition to the existing limitations of using DAS with unknown fiber-ground coupling conditions, the complex geometry of the fiber and limited data recording duration further challenge the extraction of surface wave information from the ambient seismic field in such an urban environment. Therefore, we develop an ad-hoc processing scheme in which we incorporate a frequency-wavenumber (f−k) filter to enhance the quality of the virtual shot gathers and related multi-mode dispersion images. We are able to employ this dataset to generate several shear-wave velocity (VS) profiles for different sections of the cable. The shallow VS structure shows a good agreement with different geological conditions of soil deposits. This study demonstrates that DAS could provide insights into soil characterization and seismic microzonation in urban areas. In addition, the results contribute to a better understanding of local site response to ground motion.