## 1 Introduction  
Geophysical methods have been used for many years in prospecting for industrial and energy resources, geological and engineering surveys, as well as geotechnical surveys. Electrical resistivity, electromagnetic and seismic methods are commonly used in practical and research applications which include: geological [1; 2; 3; 4] and hydrogeological [5; 6; 7] investigations of the subsoil, technical conditions of hydrotechnical structures and other civil engineering structures [8; 9; 10; 11], testing of porosity [12; 13; 14; 15; 16; 17], moisture content [18; 19] and saturation degree [12; 15; 20; 21] of the subsoil, as well as monitoring of landfills, and agricultural and post-industrial areas [22; 23; 24; 25; 26]. A geophysical survey comprises the measurement of physical properties, usually at the ground surface level without disturbing the soil structure, followed by comprehensive processing and interpretation of the recorded data to gather information on the soil type and structure. Its advantages include a non-invasive nature, low costs and quick results, as well as an ability to create a spatial subsoil model without the use of heavy equipment. One of the methods used for many years in subsoil investigations is electrical resistivity tomography. The method is based on the assumption that soil and rocks, as well as other materials, conduct electricity. As shown by previous studies [12; 15; 19; 21; 27], the method is affected by the soil moisture content and water saturation, its porosity and contamination level of water in the soil pores, in particular with salt [15; 28; 29] and NAPL (non-aqueous phase liquid) compounds [19; 30], and change of stress conditions [8; 16]. Extensive development of survey methods in the recent years, both in geophysics and geotechnics, has allowed for the modification of the existing measuring instruments. New instruments have been developed, including the RCPTU (resistivity cone penetration test with pore pressure \"u\" measurements) probe [30; 31; 32], for measuring electrical resistivity and other common parameters. These tests are mostly used in geotechnics to assess the condition of, e.g., a structure's foundation [33; 34], hydrogeological conditions [35; 36] or physico-mechanical properties of the soil [37]. Since the electrical resistivity of some minerals (e.g., quartz and silica minerals) is high and the soil skeleton plays the role of an isolator, the electric current flows through the soil voids filled with water. In coarse grained materials the results of resistivity tests may be significantly affected by water mineralization, and its saturation degree and porosity. If the water content (saturation degree) is constant, soil resistivity generally becomes a function of salts dissolved in the soil water solution and soil porosity. The principal empirical equations concerning relationships between soil resistivity and its physical parameters were presented by [PERSON] [12] and others [2; 10; 18; 21; 38]. In most porous and clay-free materials, the resistivity of the saturated material may be described as:  
\[\rho_{\text{bSAT}}=\rho_{\text{f}}\text{F}=\rho_{\text{f}}\text{-a}\cdot \text{n}^{-\text{m}}, \tag{1}\]  
where \(\rho_{\text{bSAT}}\) is the electrical resistivity of soil in fully saturated conditions [\(\Omega\cdot\text{m}\)], \(\rho_{\text{f}}\) is the electrical resistivity of fluid in pore spaces [\(\Omega\cdot\text{m}\)], F is the formation factor [-], n is the porosity [-], and a and m are the empirical constants (depending on pore shape and cementation). In soils containing clay minerals a double layer phenomenon, which introduces an additional conductivity called surface conductance to the system [17; 39], should additionally be considered. Surface conductance is a special form of ionic transport occurring at the interface between the solid and fluid phases of the system.