## 3 Distributed Optical Fibre Sensing Applied to Geo-Hydrological Applications
### Soil Leves and Embankments Monitoring
#### 3.1.1 Distributed Temperature Sensing for Soil Levees and Embankments
The mechanism is similar to that exploited for leakage detection: different levels of soil moisture lead to different thermal conductivity, and in turn to a differential temperature behaviour for heating and cooling cycles (Figure 8b) or for seasonal temperature variations. In particular, it was shown that the accuracy of the active method [120; 129; 120] was sufficient to qualitatively assess the local degree of saturation and the volumetric heat capacity. Nonetheless, optical fibre sensing systems provide lesser accuracy than traditional probes, such as dielectric acquameters or electrical time domain reflectometers: for this reason, [PERSON] et al. [130] proposed to integrate the temperature deviation over time. This method significantly improved the accuracy of soil water content measurements. Despite the strong effort made in the measurement of soil moisture, which also has important applications for other affine sectors (e.g., water management, agriculture, and landfill monitoring), field applications are still challenging. At the moment, mandatory calibration routines can be reliably implemented in a homogeneous soil, like that of small-scale physical models, but not in heterogeneous soil, like that of the large part of real applications [131]. Regarding Brillouin-based DTS, very few examples can be found in the literature, and they mainly regard physical models [132; 133; 134]. In all of these works, the cross-sensitivity of Brillouin scattering to both temperature and strain was not considered, nor were opportune compensation strategies adopted, as it is implicitly assumed that no significant strain field variations occur in the surrounding soil. While this condition can be opportunely controlled and verified in laboratory tests, this is not the case for real installation. In one case, the authors used the Brillouin frequency shift due to both temperature and strain indistinctly to reveal seepage and settlements occurring inside a physical model of an embankment dam [135]. Nonetheless, they highlighted the importance of discriminating between temperature and strain for field applications because the accuracy needed to monitor temperature fluctuation may be severely affected by the strain perturbation. Finally, [PERSON] et al. used an OFDR interrogator to measure the temperature field inside a small-scale physical model (Figure 9) with an unprecedented spatial resolution to investigate the feasibility of the technique for seepage detection in physical models [136]. The use of an OFDR interrogator, with its high spatial resolution, was indeed mandatory due to the limited size of the model. The authors confirmed the feasibility of the approach, but pointed out the need to further reduce any strain experienced by the fibre during the tests. Although most of the applications and papers dealt with short-term measurement campaigns [107; 137] or experiments in physical models [123; 125; 138; 139], there are some examples testifying years-long measurement campaigns [100; 106; 140]--all employing Raman-DTSs. For example, within the framework of the project IJKdijk, [PERSON] et al. [106] analysed the data collected over a measurement period of one year to test the reliability of long-term data analysis approaches. Another example is that of the Lovon field test [100] where, according to the authors, the fibre was still working well after six years from the installation. Regarding long-term campaigns aimed at measuring the soil moisture via active DTS methods, [PERSON] et al. [131] reported two severe obstacles hampering the application of subsurface DTS as well as other subsurface thermal probes. The first hindrance was a hysteretic response of the DTS sensor. The second obstacle was an evolution through time of the relationship between soil moisture and temperature fluctuation due to soil structure healing. Overall, both obstacles were due to an ameliorated thermal coupling between the soil and the cable over time.