Land subsidence monitoring in sinking coastal areas using distributed fiber optic sensing: a case study

A number of coastal areas have been suffering from severe land subsidence, which draws worldwide attention. Quantifying the subsidence and the contribution of each compacting stratum is crucial to study its development mechanism. In this paper, the distributed fiber optic sensing (DFOS) technique based on Brillouin scattering was adopted to monitor land subsidence in a 100-m-deep borehole located in Tianjin, China. Vertical strain profile was obtained by a kind of fixed-point cable embedded in the borehole, and the DFOS-based land subsidence system successfully achieved a 2-year-period in-situ investigation of the soft soil. The results revealed that the land subsidence rate was 21.6 mm/a after 2017, and the strata deformation measurements were refined up to each 5-m-thickness in vertical direction. The compression strata were localized at shallow strata (3.4-38.4 m), and the dominant contributors were soft soil strata at depth of 3.4 m to 18.4 m that the contribution of every 5 m thick stratum from top to bottom was 34.4%, 27% and 19.1%, respectively. The subsidence and strata contribution obtained by DFOS were in good agreement with those of extensometers. The groundwater fluctuations and additional loading may be the significant triggering factors of the compaction of the soft soil. This study showed that the DFOS-based measurement is an effective approach for land subsidence monitoring and will be a supplement to existing techniques in coastal areas.