# Monitoring and Data Analysis in Small-Scale Landslide Physical Model
## 3 Review of Landslide Monitoring Equipment
### Geotechnical Monitoring System
#### 3.2.1 Water-Related Sensors  
All water-related sensors are from the manufacturer METER Group (METER Group AG, Munchen, Germany), including the two data loggers we have obtained specifically for collection of data from these sensors. Data loggers are METERS' ZL6 six-channel loggers with integrated GPS and ZENTRA application which can be accessed from tablets or smartphones. METER data loggers have sensor logging interval from 5 min to 12 h and can return the average or accumulation of the one-minute sensor readings across a chosen measurement interval. All of the sensors from the METER Group have low power requirements, which is ideal for long-term experiments on soils in laboratory or in the field. Soil moisture sensors are called TEROS 10 and TEROS 12. TEROS 10 series are simpler, low-cost sensors that measure volumetric water content (VWC), while TEROS 12 series additionally has the ability to measure temperature and electrical conductivity of soil. Both types of sensors use an electromagnetic field to measure apparent dielectric activity (\(\epsilon_{\text{a}}\)). Frequency capacitance technology is used to supply a 70 MHz oscillating wave to the needles; the difference being that TEROS 10 only has two stainless-steel needles and TEROS 12 has three stainless-steel needles (Figure 5). Needles charge according to the dielectric of the material, and charge time is proportional to substrate dielectric and substrate VWC. Charge time is measured and outputs raw value based on the substrate \(\epsilon_{\text{a}}\), which is then converted to VWC by a calibration equation [61; 62]. While installing the sensors, it is necessary to avoid installation near large metal objects that can affect their function and distort readings. If the sensor is installed in a vertical position, more soil depth is integrated in the soil moisture measurement. In horizontal installation, the sensor provides soil moisture measures at more discreet depths. Ferrite core on the sensor cable works as an isolation from interferences and potential noises from the system; therefore, this section of the cable from the sensor to the core should not have anything attached [61; 62]. In addition to VWC and temperature measurements, TEROS 12 monitors electric conductivity (EC). Soil moisture (VWC) is measured between needle 1 and needle 2 and electric conductivity between needle 2 and needle 3. Embedded thermistor located in the central needle takes temperature readings and responds faster to temperature changes than its similar sensor TEROS 11, which is without EC measurements. Output temperature is in degrees Celsius, unless otherwise stated in the data logger program [61; 62]. Another advantage of the TEROS 12 sensors is their measurement volume of 1010 mL, which is a big volume for the size of the sensor. In Table 6, ranges of volumetric water content, temperature, and electric conductivity are stated. The soil water potential sensor that is used in the model is TEROS 21, although it is not considered as a standard tensiometer sensor due to the lack of ability to measure positive pore pressure. All soil water potential measurement techniques measure the potential energy of water in equilibrium with water in the soil in accordance with the Second Law of Thermodynamics. After coming into hydraulic contact with the soil, the sensor water potential comes into equilibrium with the water potential of the soil [63]. TEROS 21 sensors measure the water potential and temperature of soil with two porous ceramic discs (Figure 5). The sensor measures moisture changes of ceramic discs sandwiched between stainless-steel screen and the circuit board. By introducing a material with known pore size distribution and achieving equilibrium with the soil, soil water