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Types of Case Studies for Electrical Geophysical Methods

Electrical parameters measured with geophysical methods in situ are related with different soil properties, easily measured, and can be used to study many soil problems. Different principles of applications should be considered for three types of problems.

The first-type problems are the monitoring of a soil property, which is only one to vary during the measurements. In such problems the measured electrical resistivity or potential can directly indicate the change in the soil property in situ. Such principle was utilized for measuring differences in peat soil compaction under seasonal road and monitoring soil melting in spring (Pozdnyakova, 1999).

The second-type problems include investigations of soil properties, which predominantly influence the measured electrical parameters. Therefore, the measured electrical parameters usually show strong relationships with such properties even in field conditions. For example, since the variation in stone content influences the soil electrical resistivity much stronger than variation of any other properties in soils of Crimea Peninsula, the VES method was able to accurately outline the layers with different stone contents in these soils and estimate the volumetric content of stones (Pozdnyakov et al., 1996; Pozdnyakova, 1999). Pollution by petroleum products highly increases the electrical resistivity of Gelisols in northwest Siberia, while salty mining solutions decrease resistivity of the soils. Therefore, methods of EP, VES, and NEP could be used to map pollution in these soils (Pozdnyakov et al., 1996; Pozdnyakova, 1999). Extreme dryness of Histosol in some seasons highly increases the electrical resistivity at the top of the profile, whereas variation of soil water content around field capacity usually does not alter the typical profile distributions of electrical resistivity in the soils (Pozdnyakova et al., 1996; Pozdnyakova, 1999). Disturbance of soils changes of the measured electrical resistivity in soils of humid area significantly enough to detect hidden burial places for forensic and archeological applications

The third-type problems require careful considerations of the relationships between many soil properties and electrical parameters measured in situ. Although soil electrical parameters depend simultaneously on many soil properties, such as salt, water, humus or stone content, CEC, texture, and temperature, in many situations the influence of some soil properties can be considered negligible if they vary around their maximum, based on Boltzmann's distribution law. For example, soil water content close to the field capacity does not practically influence the change in electrical resistivity (Figure 3). Therefore, in situ measurements of the electrical parameters of soils in humid areas is not influenced by water content variation and can be used to evaluate elluvial-illuvial horizons in soil profile and more stable soil properties, such as CEC, soil texture, and humus content (Figure 4). On the other hand, the high variation of soil water content within the whole possible range in the profiles of alluvial soils in Astrakhan' area allows locating the groundwater table (Pozdnyakova et al., 2001). The simultaneous influence of various soil properties on the measured electrical conductivity were successfully studied with the methods of geostatistics, which consider not only intervariable but also spatial relationships (Pozdnyakova and Zhang, 1999).