Seismic observation and subsurface imaging using an urban telecommunication optic-fiber cable

Seismic observation and subsurface imaging are of key importance in mitigating the seismic hazards and exploiting subsurface spaces in urban areas. However, the urban seismic observation is, to some extent, hindered by more and more populations and facilities in urban areas. To develop new methods for urban seismic observation, we carried out several field experiments in Binchuan city, Yunnan Province. In the experiments, we applied the distributed acoustic sensing (DAS) technique to an urban telecommunication optic-fiber cable to monitor ground motions. The DAS technique was rapidly developed and applied to seismic investigations of different scales during the past several years. Binchuan city is located in the Binchuan basin, which is one of the largest sedimentary basins in western Yunnan. The Binchuan basin is well studied and equipped with seismic instruments, including a Fixed Airgun Signal Transmission Station (FASTS). The FASTS can radiate highly repeatable seismic signals with seismic energy equivalent to a magnitude 0.7 earthquake; the seismic signal can be used to test the sensitivity of seismic instruments during the seismically quiet period. During the experiment, a 5.2 km long optic-fiber cable buried along two local roads was parted into ~1000 sensing channels with a channel spacing of several meters, and the cable vibration was continuously digitized with a sampling rate of 5000 sps (samples per second). The DAS registered the ground motions originating from various sources. We clearly identified some vibrations moving with a speed of ~ 40 km/h, which likely originate from moving vehicles along the road. During the experiment, we fired 111 airgun shots. The seismic signal from each single airgun shot was invisible in the DAS records, but clearly emerged after stacking. The airgun signals recorded by the DAS show similar features as recordings from co-located short period seismometers. To test the feasibility of imaging the subsurface with DAS, we also hammered the ground at 48 points along the road with an 8 kg hammer. The DAS clearly recorded the hammer signal traveling with apparent velocities of 300−400 m/s to a distance of ~100 m. Furthermore, we reconstructed the empirical Green’s function from continuous DAS record. We are able to retrieve high-quality surface wave signals and corresponding dispersion curves (the relationship between surface wave velocity and frequency) from 20-min long noise recordings. Then based on the dispersion curves, we obtained the shear wave velocity structure of the uppermost 200 m. The shear wave velocity of the uppermost layer is important for the soil classification in engineering seismology. The shear wave velocities of the uppermost 200 m range from ~280 to 430 m/s, indicating a class II soil in the Binchuan basin. Our preliminary results revealed the feasibility of seismic observation using an urban telecommunication optic-fiber cable, which provides a novel method for many urban seismic applications, such as fine subsurface structure imaging, traffic monitoring, and earthquake early warning.