## 2 State-of-the-Art (Method)
### The Use of Electromagnetic Sensors for Environment Monitoring
#### 2.1.2 Monitoring Parameters with Inductive Coils  
On the other side, we can find the use of inductive sensors to obtain environmental data reported in several scientific articles. These inductive sensors are based on the creation of magnetic fields, which interact with the near environment, producing changes in the generated current. [PERSON] et al. [28] developed, in 2010, a system that can measure water salinity based on inductive coils. The proposed conductivity sensor is composed of two coils. One is the powered and the other the induced coil. Their proposal is a solenoidal sensor covered with 1-dodecanethiol for protecting of the corrosion. Then a temperature sensor is used to adjust the values of conductivity to salinity. Another author, such as [PERSON] et al. [29], developed a system based on two coils for monitoring the conductivity in aquifers. They studied different configurations of prototypes based on different criteria: changes in the number of spires maintaining the spires relationship; changes in the relation of spires; changes in the wire diameter; and changes in the coil diameter. The paper concludes that the best prototype is composed of 80 spires in the induced coil, and 40 spires in the PC, the used copper and coil diameter is 0.4 mm, and the Polyvinyl Chloride (PVC) tube 25 mm. In addition, [PERSON] et al. [20] developed an inductive coil for monitoring the inorganic fertilisers in the irrigation water. This sensor creates an electromagnetic field that is sensitive to the conductivity changes. They use different samples between 0 g/L and 45 g/L. Finally, they concluded that the sensors have an excellent correlation with a low average error of 2.15%. [PERSON] et al. [30] designed a salinity sensor system. The initial laboratory testing shows that the salinity sensor system is functional and can be used to display salinity data on a given map. Other authors, as [PERSON] et al. [25] proposed a non-contacting electrical conductivity sensor for remote, hostile environments. They developed an inductive sensor that uses a single turn transmitter and receiver loops to generate and detect eddy currents in the materials. The results showed that the mechanical design of the sensor makes it insensitive to temperature and pressure changes, and accelerations, impact, and abrasion. Therefore, it is operable in remote, hostile environments, such as deep boreholes  
The use of coils to measure in other environments, such as soil, is also well-known. [PERSON] et al. [19] developed a low-cost nitrate detection soil sensor. This system is based on a combination of capacitive and inductive electromagnetic fields for monitoring the content in agricultural soil and contamination. The proposed system concludes that the developed sensor will be able to detect various elements of contamination, and also an improved design of the sensor can be researched. These prototypes can  
Figure 1: Presentation of the operation and variables of an induction coil in an aqueous medium. be used as a tool for water source monitoring in the farm where the nitrate level should not exceed 10 mg/L. Meanwhile, [PERSON] et al. [31] presented a low-cost moisture sensor-based on inductive coils. They tested the inductive coils in different sorts of soils. Besides, they powered the coil using a voltage of 10 peak-to-peak volts. The experiment concluded that the best sensors work in 229 kHz with a correlation model of 0.75. Furthermore, the inductive coils as soil moistures sense have been compared with capacitive sensors in [32]. In this comparative study, [PERSON] et al. demonstrated that for a range of temperature from 1 to 20 \({}^{\circ}\)C, the temperature of soil has almost no effect on the Vout of inductive coils, which is an advantage front the capacitive sensor in which case the effect of temperature is notable.