# IoT Sensor Challenges for Geothermal Energy Installations Monitoring: A Survey
## 2 Sensor Technologies
### Temperature Measurement
#### 2.1.1 Static Installations  
Static installations deploy thermistors, thermocouples, fibre optic thermometers or DTS technology. Thermistors are sensors capable of measuring high-quality temperature-depth information from boreholes up to 100 m or more below the Earth's surface [19]. Thermocouples are installed to measure temperatures at the inlets and outlets of geothermal heat exchangers [20]. Wired optical sensors provide continuous measurements with high-resolution temperature profiles along pipelines [18] and are often attached to the outer surface of the pipe wall or located within the underground structure itself [21].  
DTS technology is used for temperature measurements along various types of underground geothermal structures. Measurements are based on detecting light from Rayleigh, Raman or Brillouin backscattering phenomena [15]. DTS technology measures temperatures using optical fibres that provide a continuous temperature distribution profile of the entire cable [22]. This method is able to achieve highly accurate temperature measurements over long distances, up to 30 km. This technology is very sensitive for monitoring temperature, as it is characterized by a spatial resolution of 1 m at an accuracy of \(\pm 1\,^{\circ}\)C and a resolution of 0.01 \({}^{\circ}\)C [23]. This technology works by detecting thermal changes along the optical fibre that manifest as molecular or lattice vibrations in the material. In addition, thermal changes cause local changes in the refractive index. Changes in the refractive index are then manifested as the inelastic scattering of transmitted light. Molecular vibrations at high frequencies around 10 THz cause Raman scattering. Brillouin scattering is caused by low-frequency vibrations in the range of 10 to 30 GHz. This scattering can then be used to measure the temperature along the fibre [24].  
DTS technology is widely applied to inspect subsurface thermal properties. DTS can be used in various applications, such as investigating the hydro-geological conditions of downholes or determining the geological properties of aquifers. Especially in deep boreholes, its application provides notable benefits over discrete-point temperature measurements [5]. DTS is widely used in long-term monitoring to determine the contribution of groundwater flow and thermophysical characteristics to subsurface heat transfer and storage [16]. This type of technology is used to monitor the conditions inside well bores to optimize expensive workover procedures [17].  
\begin{table}
\begin{tabular}{l l l} \hline
**Author, Source** & **Geothermal Structure** & **Methodology** \\ \hline [PERSON] et al. [16] & Geothermal exchange borefield & DTS \\ [PERSON] et al. [17] & Geothermal well & DTS \\ [PERSON] et al. [5] & Downhole & DTS \\ [PERSON] et al. [18] & Borehole heat exchanger & Autonomous wireless sensor \\ [PERSON] et al. [13] & Borehole heat exchanger & Geoball and GEOsniff \\ \hline \end{tabular}
\end{table}
Table 2: Overview of technologies applied to temperature measurement in underground geothermal structures.