Detection of Sinkhole Formation by Strain Profile Measurements Using BOTDR: Simulation Study

Abstract

The sudden collapse of sinkholes in the Dead Sea area represents a serious threat to infrastructure in the area. The formation of these sinkholes has been shown to be directly correlated with the drop in the Dead Sea water level which is accompanied by a corresponding lowering of the groundwater level and permits the penetration of low-salinity groundwater into coastal areas. This water causes dissolution of the salt layers which results in the formation of subsurface voids that develop into collapse sinkholes. Various tools and measurement methods have been investigated in order to attempt to detect the formation of sinkholes, but to date, there is no method capable of providing early warning of possible collapse. This paper investigates the use of fiber-optic Brillouin optical time-domain reflectometry (BOTDR) or Brillouin optical time-domain analysis (BOTDA) for such detection. Brillouin optical time-domain reflectometry or analysis (BOTDR/A) is an optical measurement technique that provides distributed measurements of strain along tens of kilometers of conventional optical fibers, based on the Brillouin frequency shift of backscattered light. The rationale for this approach is that the formation of an underground cavity causes strains in the soil which can be detected using a fiber-optic cable buried at a shallow depth. A closed-form solution for the expected surface sinkhole-induced strain profile attributable to spherical voids in elastic-plastic soil is developed, validated, and evaluated against more realistic conditions. The model is then used to develop a procedure that can differentiate between signals induced by a sinkhole and signals caused by disturbances. The suggested procedure uses wavelet decomposition to filter out the disturbances and extract the sinkhole contributions. This procedure is evaluated with signals measured in the field during a 50-day period on which were superimposed theoretical strains based both on the closed-form model and finite-difference analysis of more realistic cases. This analysis showed that in the depth range investigated (up to 50 m), detection can be achieved while the subsurface cavities are still stable and early enough to enable the implementation of countermeasures.