## 3 Distributed Optical Fibre Sensing Applied to Geo-Hydrological Applications
### Ground Subsidence and Earth Fissure Monitoring  
A particular type of soil movement is the lowering of ground surface due to sinking and settling. Among the many natural phenomena responsible for subsidence, we can cite the removal of underground water and fluids and natural consolidation. Anthropic phenomena such as underground mining are also associated with subsidence. The subsidence--occurring at depth--may cause large soil surface tension and the formation of large cracks in the ground, referred to as earth fissure. A traditional approach in subsidence monitoring consists of using borehole extensometers that measure the vertical displacement at depth, and few examples of borehole extensometers have been implemented through distributed optical fibre sensing [219, 220]. The sensing approach is only apparently similar to that proposed above for landslide monitoring by optical fibre inclinometer:  
Figure 20: (**a**) Cumulative displacement curves measured by a BOTDR-based optical fibre inclinometer, and, on the right, the colour map of the strain field in time and space; (**b**) Displacement curves measured by the optical fibre inclinometer vs. a standard gauge. Reproduced with permission from [214]. indeed, vertical displacement is of interest here. In particular, [PERSON] et al. [220] successfully tested three types of commercial cables (10 m-fixed point, metal-reinforced, and polyurethane sheath cable), interrogated using a BOTDR interrogator with 1 m spatial resolution, in a 200 m-long borehole in Shenzen, Suzhou (China). The borehole was filled with fine sand-gravel-bentonite after the cables were installed, and no strategies for temperature compensation were adopted. The same research group investigated the strain effect in the soil during the drainage-recharge cycle in a small-scale physical model with the aim of investigating deformation laws in pumping and artificial groundwater recharge processes involved in subsidence trends [221]. In the experiment, the strain exerted by volumetric compression and rebound of the soil during water drainage and recharge cycle, respectively, was clearly detected. To improve the coupling effectiveness, they used a hytrel sensing cable integrated with 4 cm-diameter and 1 mm-thickness plexiglass disks every 10 cm, which was interrogated using a BOTDA interrogator with high spatial resolution (Figure 21). Figure 21: (**a**) Sensing cable used for measuring soil strain during a drainage–recharge cycle, with vertically deployed fibres; (**b**) Time variation of water content and displacements, measured using a BOTDA scheme, during the drainage and recharge cycle for different soils. Reproduced with permission from [221]. Earth fissure has been addressed in a recent work [222] in which--according to the authors--the cable was engineered to assure durability, fracture resistance, stability, and strength. The structure of the cable contained three layers from the core to the outer coating materials: the bare-optical fibre, a polyurethane coating, and a spiral-shaped metal sheath. Five centimeter-long sections of cable were further encased into an aluminium alloy tube and into a heat-shrinkable tube (10 cm-long) at regular distance. These raised portions of cable were used to anchor the cable to the ground with opportune nails. The distance between anchors was fixed to 2 m after careful consideration, and the cable was pre-stressed during installation (Figure 22). The strain was finally measured with a BOTDR interrogator, and additional strain-free cables were measured for temperature compensation. Overall, the measurement campaign--carried out in a rice field in Jiangsu province (China)--lasted more than three years, and two main ground fissures (with a maximum strain value of 360 \(\mu\)\(\varepsilon\)) were detected and measured. However, many tiny collateral cracks were not (Figure 22).