Vertically Distributed Sensing of Deformation Using Fiber Optic Sensing

Abstract

Vertical deformation can be revealed by various techniques such as precise leveling, satellite imagery, and extensometry. Despite considerable effort, recording detailed subsurface deformation using traditional extensometers remains challenging when attempting to detect localized deformation. Here we introduce distributed fiber optic sensing based on Brillouin scattering as a geophysical exploration method for imaging distributed profiles of vertical deformation. By examining fiber optic cable‐soil interaction we found a threshold in confining pressure to achieve a strong cable‐soil coupling, thus validating data collected from a borehole‐embedded fiber optic cable deployed in Shengze, southern Yangtze Delta, China. Clear‐cut strain profiles acquired from November 2014 to December 2016 allowed us to pinpoint where compaction or rebound was actively occurring and examine strain responses at various locations along the entire cable length. We suggest that distributed fiber optic sensing can complement with extensometry and remote sensing techniques for improved monitoring of vertical deformation.

Toward Distributed Fiber‐Optic Sensing of Subsurface Deformation: A Theoretical Quantification of Ground‐Borehole‐Cable Interaction

“…Despite lack of standardized backfilling procedure, current practice indicates that FO borehole backfilling materials are of similar stiffness as surrounding strata and so E b and μ b are considered to be within 0.01–40 GPa (Plinninger et al, 2010) and 0.15–0.45, respectively. The radius of borehole, r b , is assumed to range from 25 to 200 mm (Kogure & Okuda, 2018; Zhang et al, 2018), and the ground‐borehole interface adhesion coefficient, k b , from 1 to 1,000 GPa/m (Xiang & Wang, 2018).…”

“…Like the ground‐borehole interface adhesion coefficient, reporting of typical ranges for the gradient of monitored FO strain is particularly scarce in the literature. However, a DSS system deployed in Shengze (southern Yangtze Delta, China) for groundwater‐related settlement monitoring (Zhang et al, 2018; Figure 4a) has provided huge data sets to constrain the strain gradient. A near 2‐year data set (total amount of data N = 36,133) reporting strain values measured from the aforementioned 1 mm radius tight‐buffered, TPU‐jacketed cable is analyzed (Figure 4b).…”

“…Red marker indicates borehole location (30°53′23.96″N, 120°40′47.11″E). See Zhang et al (2018) for details on field deployment of fiber‐optic instrumentation. (b) Strain gradient estimated using a near 2‐year data set (total data amount N = 36,133).…”

“…To obtain subsurface movements, field instrumentation represented by strainmeters, tiltmeters, and extensometers is commonly utilized (Amoruso et al, 2015; Peltier et al, 2006; Sleeman et al, 2000). These instruments can provide strains/displacements of subsurface formations but are currently limited by discretely instrumented measuring points, if more spatial detail is needed (Zhang et al, 2018).…”

“…In fact, the assumption that the monitored FO strain sufficiently approximates the ground strain is implicit in many practical case studies. However, the strain extracted from the demodulated signal is not fully representative of that experienced by the ground, due primarily to the interface slippage and/or the interface shear transfer within the ground‐to‐cable system (Zhang et al, 2018). Previous research has examined the ground‐cable interface shear behavior under the pullout scenario, with particular emphasis on the progressive failure of the interface (Iten et al, 2009; Zhang et al, 2016).…”