## 5 The ECOAL Project Implementation
### Results  
#### 5.2.1 Temperature Measurement  
A comprehensive set of temperature measurements in the coal waste pile was gathered over several weeks. Figure 10A depicts a measurement of the temperature profile for the complete fiber length at a given moment of the day on 28 February 2015. Each point is measured twice, obtaining a symmetrical profile but also measurement redundancy. The temperature data are easily extracted from the measured BFS by using the sensitivity of the BFS to temperature. Figure 10B shows the temperature profile of 250 m of fiber length obtained during one week, for which two measurements were acquired consecutively in the same day. The two profiles obtained during the same day (the orange and gray lines in Figure 10B) show good agreement between them, highlighting the repeatability of the BOTDA system. The other temperature profile (blue line) was performed a week before the others. As can be seen, some temperature peaks present in the blue line do not appear in the orange and gray lines. Figure 10: **(A)** Temperature profile along the optical fiber in the coal pile, in the normal route of the cable and in the return route; **(B)** details of the one-week evolution of the temperature profile along the optical fiber in the coal pile. Figure 9: (A) View of the field installation of the fiber optic temperature cable; (B) one of the structures where the gas sensors/point temperature sensors were installed. The burning pattern of the coal waste pile is a dynamic process, quite often changing with time in a non-foreseeable way. This happened with the temperature level in one of the spots for gas sensing (the right one in Figure 7C), which increased to a value of 270 \({}^{\circ}\)C (Figure 11A) not compatible with the operation of the gas sensors. Therefore, at this stage the activity was focused on the other spot, which shows a much lower temperature, as seen in Figure 11B, where the temperature measured by the FBG system is shown at three points, located at depths of 1.3, 1.4, and 1.5 m, over approximately 10 days (from 25 February to 5 March 2015). Even at a reduced scale, the dynamics of the burning coal pile is also imprinted on this data, with a temperature amplitude variation of -18 \({}^{\circ}\)C observed in the period considered. The figure also shows that a depth increase of 20 cm causes a temperature rise of \(\sim\)4 \({}^{\circ}\)C.  
As mentioned at the end of Section 4.1, a hybrid low-finesse Fabry-P\(\acute{\text{e}}\)rot interferometer was developed for temperature sensing, based on a four-bridge double-Y-shape-core microstructured fiber, designed within the consortium (Figure 12A). Figure 11: (**A**) Thermographic temperature measurement in one of the gas sensing spots; (**B**) temperature evolution in the other gas sensing spot at depths of 1.3 m (1), 1.4 m (2), and 1.5 m (3). Figure 12: Views of the cross section of the fibers projected for temperature (**A**) and gas sensing (**B**). The sensing head was protected by a ceramic capillary cap with an inner diameter of \(\sim\)127 \(\upmu\)m in order to prevent any damage during the installation process. Several tests were made in the laboratory before installation, verifying the performance of the temperature MOF-PF sensor with this cap. The cap did not influence the sensor's sensitivity or time response to temperature variations. The interrogation of the FP sensor was performed using a commercial FBG interrogator (Smartec SM125, SMARTEC, Lugano, Switzerland), with MatLab software enabling the use of FFT remote real-time (scan frequency of 1 Hz) analysis. By monitoring the phase of the FFT, temperature variations are characterized without the negative influence of noise or amplitude fluctuations [49]. The MOF-FP optical spectrum and its FFT (magnitude) are shown in Figure 13.  
This technique allows us to multiplex simultaneously in a single channel several sensors (the authors have used up to five successfully), thereby reducing the economic cost of the system.