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In agriculture, soil properties play a significant role in efficient crops cultivation. The major soil characteristics determine the harvest. Thus, it is highly important to analyze agricultural lands and measure the main parameters such as hydrological characteristics, pH, nitrogen and carbon concentration, soil structure and texture, and others (Weil, Brady, & Weil 2016, p. 4). Typically, important soil parameters are measured in laboratories or by special sensors and sensor networks (Srbinovska et al. 2014, p. 297). However, these technologies require a lot of time and efforts, while the use of radar system might provide the needed data for significant land surface more efficiently (Esetlili & Kurucu 2016, p. 23). Nowadays, modern radar systems are used to determine the main soil characteristics. According to Esetlili & Kurucu (2016, p. 23) and Persico (2014, p. 6), the following two types of radar systems are used with this purpose: synthetic aperture radar (SAR) and ground penetrating radars (GPR). Therefore, it is highly important to determine the principles of work, main possibilities and limitations, as well as current use of synthetic aperture radars and ground penetrating radars for soils quality estimation.
Synthetic Aperture Radar System
Principle of Work
According to Wolff (n.d.), SAR is a system assembled on a flying in the air or space platform for earth surface investigation. To create an image, one large antenna on a moving (usually, flying) platform is used. Transmitting antenna of the radar sends a signal and receiving antenna receives a reflected signal. It is one cycle of SAR work. During the procedure, the data of individual cycles are combined. As a result, it is possible to create the high-resolution image of the area. Thus, the parameters of the soil are determined by characteristics of reflected signal which might be different for different types of earth surface (Esetlili & Kurucu 2016, p. 24).
Possibilities and Limitations of Synthetic Aperture Radars
The majority of researches postulated that SAR radar system can be efficiently used for estimating the soil moisture (Zwieback, Hensley, & Hajnsek 2015, p. 77; Zribi, Gorrab, & Baghdadi 2014, p. 63). Besides, soil texture (in particular, the content of sand) is also possible to determine by this system (Ceddia et al. 2017). It is possible to suppose that SAR system can be used for soil maps with determined creation due to its high efficiency and reliability.
Moreover, recent researches (Bartsch et al. 2017; Esetlili & Kurucu 2016; Poggio & Gimona 2017;) have shown that radar system can be used for estimating other important soil characteristics. In particular, Poggio and Gimona (2017, pp. 1108-1109) used in their research a combination of optical and radar sensors for soil properties studying. They determined the following characteristics: depth of soil layer, pH value, organic carbon concentration, saturation, presence of K, Na, Mg, and Ca ions, soil texture, and bulk density. The authors concluded that the use of such system might be the most efficient for the determination of soils characteristics. The investigation of Bartsch et al. (2017, pp. 5467-5468) confirmed the possibility of using SAR system for organic carbon content estimation.
In the investigation of Esetlili and Kurucu (2016, pp. 34-35), the authors determined the following soil characteristics: texture, lime, content of organic matter, pH, moisture, and content of calcium carbonate using the SAR images. The author estimated that SAR could be used for successful determination of soil moisture, texture, and organic matter content. These parameters are significant for crop cultivation. However, other investigated parameters did not have a significant effect on the obtained SAR images characteristics. Consequently. it could be considered as a limitation for the SAR system usage in the agriculture. It is possible to underline the importance of further investigation for estimating the effect of other parameters on the SAR images characteristics.
Summarizing, SAR radar system can be successfully used for estimating soil moisture, texture, content of organic carbon, presence of important ions, and saturation. In addition, it is possible to create land maps with these parameters. However, certain limitations were estimated as well. In particular, there is no significant data that SAR system can be used for estimating soil pH. This area requires further investigation.
Ground Penetrating Radars
Principle of Work
GPR allows looking underground. The radar consists of transmitting and receiving antennas, central unit, and computer. Central unit of a system creates electromagnetic signal that is transmitted into the soil by the first antenna. When a signal meets an object in the soil or a zone of different humidity, it reflects back, and the second antenna receives this signal. Typically, both antennas are composed into a rigid structure and move together to analyze the soil (Persico 2014, pp. 1-2). The characteristics of received signal are different for soils with different texture and moisture.
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Possibilities and Limitations of Ground Penetrating Radars
In general, GPRs are also used to determine physical parameters of the soil, mostly, moisture (Tran et al. 2014, p. 3131). In addition, GPR data provides a possibility to create the hydrodynamic model of the soil which is highly important for harvesting (Tran et al. 2014, p. 3145). GPRs soil testing allows determining objects in the soil and underground water sources, which is important for planning the field usage (Persico 2014, pp. 8-10). Except for hydrological parameters, with the use of GPR, it is also possible to determine the soil electrical conductivity and dielectric permittivity (Jonard et al. 2013, p. 310), soil clay content, hard pan, bulk density, and electrical resistance (Liu et al., p. 535).
Besides, such radar can be assembled on a cultivator. In this case, it is possible to conduct measures during the soil cultivation (Persico 2014, p. 10). Such technology might allow determining soil moisture, density, thickness, and texture. All these parameters are crucial for crops cultivation. Moreover, it is possible to improve the quality of soil in case of necessity based on the test results.
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The possible limitation of this radar system is that in the literature, there was no data that GPR could be used for soil chemical compounds concentration or pH value determining. Such parameters are crucial for harvesting. Thus, in the present time, applying the GPRs for agricultural soil investigation requires additional laboratory tests of the soil quality because radars cannot provide all the needed information. Perhaps, further investigation for improving the technology is needed.
GPR systems is commonly used for studying soil moisture and texture. They provide reliable data that can be used for improving the efficiency of agriculture. However, further researches are required to estimate the possibility of using this system for studying other soil parameters.
In conclusion, both described radar systems have a significant potential for the usage in agriculture. Currently, radars are mostly used for the determination of soils moisture and texture as well as hydrological researches. However, as it was discussed above, it is possible to expand the list of measured parameters. The synthetic aperture radar system was used for the determination of soil pH, basic ions concentration, organic carbon, texture, homogeneity, and structure. Besides, SARs might be used for lands mapping because it is possible to investigate the large areas in a short time. Ground penetrating radars can be used in places. It is possible to assemble them on the agricultural machines and to control the soil characteristics during the process of cultivating and seeding.
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However, to apply these promising technologies, further investigation should be conducted that will help determine the efficiency and accuracy of data obtained by radar systems. In particular, it is important to investigate if radar systems can be used for soil pH, organic carbon and nitrogen concentration, and texture estimating. Based on this necessity, the following research hypotheses can be formulated: (1) for ground penetrating radar system, characteristics of received signals is different for soils with different pH value, organic carbon and nitrogen concentration, and texture; (2) high-resolution images obtained from synthetic aperture radar system are different for lands with soils with different pH value, organic carbon and nitrogen concentration, and texture.