Estimation of land subsidence potential via distributed fiber optic sensing

Abstract

Land subsidence is a global phenomenon that derives from natural processes such as auto-compaction and tectonic activity, as well as anthropogenic activities that are exemplified by withdrawal of water, oil or other fluids from the subsurface. Fluid withdrawal-induced subsidence has become a threat in densely populated zones such as coastal areas, especially in this era of growing climate change impacts when hydrological changes may not enable sufficient recharge of aquifers to replenish withdrawn water. It is important to estimate settlement for design of counter-measures: a challenge in areas of complex sedimentary history and geomedia stratification. Herein is presented the methodology and results of configuration of the Logistics Model using measured settlement data to predict future settlement in the coastal town of Tianjin, P.R. China with validation using continuous strata deformation data obtained through borehole distributed fiber-optic sensing. The cumulative subsidence over the October 2017-December 2019 period reached 56 mm with an estimated potential limit of 96 mm, leaving a balance of 42.0% of the total for attainment in the future. Subsidence of the area is estimated to reach the “stability subsidence stage” by 2050. In the G06 borehole depth range of 3.4–18.4 m which comprises clayey silty sand and silty fine sand, the measured deformation accounts for 58.5% of the total while the residual (future) deformation is 37.6% of that total. The depth ranges of 0–3.4 m (shallow stratum) and 18.4–38.4 m (deep stratum) constitute the “priority layer” because the strata are still in the “accelerated subsidence stage” that is characterized by high subsidence rate estimates for the future. Thus, the projected high subsidence rates of the two layers warrant the design and implementation of proactive countermeasures.