## 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  
According to [PERSON] and [PERSON] and [PERSON] (1990), the application of fibre-optic Raman-DTSs to leakage detection in levees was proposed in the middle of the 1990s, with the first field test installation made in France in 1995 (1995). The principle of installation is shown in Figure 7.  
After that first installation, many others followed (1995); (1995)  
temperature field can be affected by several factors other than those directly imputable to existing filtration flows. Great effort was paid to analytically and numerically model the thermal response of levees and to implement data analysis models, often supported by experiments carried out in small- or medium-scale physical models of levees. More rarely, those models were validated in real and experimental test sites. Among the data analysis models, we mention the dissimilarity or alarm approach [109; 110], the source separation approach [111; 112], and the impulse response method [113; 114]. Each of them works on a different time scale (i.e., daily, monthly, or annually, respectively), and is aimed at a different monitoring purpose; i.e., short-term/early warning, medium- and long-term monitoring, respectively [115]. Other existing methodologies deal with general signal processing techniques [116; 117]. Alternatively, the so-called \"heat-up\" or \"active\" thermometric method was proposed [118; 119]. This method is also called the active heated fibre optics method [120], and is intended specifically in the cases of insufficient temperature gradients between reservoir water and measurement point or small seasonal temperature variations of the reservoir water. It consists of measuring the temperature field dynamics along an electrically heated optical fibre during heating and cooling phases [121]. The method should allow for identification of those regions that--due to the water flow--present a different thermal conductivity and thus a differential temperature behaviour for heating and cooling (see Figure 8a). Several minutes or hours of alternate or direct current voltage are needed to produce this heating, and the electric cable is typically integrated into the same fibre cable. The use of a separate electrical cable kept at a constant distance from the optical fibre was investigated with interesting results [123; 124; 125]. Nonetheless, it is required that the optical fibre and the electrical cable be separated by a constant distance all along the cable--a condition not so easy to achieve in practical applications. Notably, the active method requires very large electric power that ranges from 3 to 15 W/m or more, and therefore it is usually applied over short distances not longer than a few kilometres, employing large-diameter cables (up to 2 cm). According to [PERSON] et al. [107], an electric power from 3 to 5 W/m is adequate for the aim of simple leakage detection. More than 10 W/m are needed instead to quantify the distributed flow velocity whose measurement accuracy is affected by the thickness of the cable [126]. Moreover, better accuracy was observed at higher electrical power. Figure 8: Fibre-optic cable heat-up in time (colours correspond to different time stamps). (**a**) Leakage is detected at the section where the temperature drops due to a larger thermal conductivity value of the soil; (**b**) Non-saturated soil region is detected at the section showing a larger temperature increment corresponding to a smaller thermal conductivity value of the soil. Reproduced with permission from [122]. Some authors also investigated the effectiveness of passive [127] and active [118; 128] approaches for estimating soil thermal conductivity and soil moisture. These works only partially address the application to embankments and levees monitoring, but are still very relevant to the topic because, as stated above, these parameters influence the shear strength of the levees.