Monitoring of vertical deformation response to water draining-recharging conditions using BOFDA-based distributed optical fiber sensors

Land subsidence is a geological disaster, which has a variety of inducements. This paper presents the results on the investigation of the soil deformation in land subsidence with a combination of laboratory and field experiments using the distributed fiber optics sensing (DFOS) technology. The laboratory experiment was mainly carried out to study the soil deformation in draining-recharging cycles based on Brillouin optical frequency domain analysis (BOFDA). In the field experiment, the BOFDA technology and some grating sensors were applied in a borehole in the Yangtze River Delta region, to monitor the pore pressure in certain specific strata and the change in strata parameter of the full borehole with the data that monitored 2 years. The result of the laboratory experiment demonstrated that the soil structures changed in the draining-recharging cycles. The soil gradually tends toward elastic deformation. The field experiment results show that the main deformation occurs at the aquitards adjacent to the pumping aquifer. The compression amount in the vertical distribution is not uniform which is related to the distance from the pumping aquifer and the soil properties. The compressive deformation of each layer is closely related to the change in groundwater level. The result proposes that the BOFDA technology is suitable for monitoring land subsidence caused by seasonal water-level fluctuations and dewatering projects. It is also of great significance to investigate land subsidence mechanisms.