# Effect of Non-Homogeneous Soil Characteristics on Substation Grounding-Grid Performances: A Review
## 4 Effect of Vertical Two-Layer Soil Characteristics on Grounding System Performances  
_Electrodes Spacing (a), the Distance between Electrodes and Two-Layers Interface (d) and Electrodes Angle (\(\beta\)) on Apparent Resistivity (\(\rho_{a}\))_  
Commonly, before measuring the actual soil parameters, horizontal soil layers, either two-layers soil or several soil layers are recommended. For representing more realistic scenarios, vertical-layer soil may be considered. This will assist in clarifying the features of soil properties and their measurements [48; 61; 62; 63; 64; 65; 66; 67; 68; 69]. Otherwise, in calculating the soil parameters and grounding system designs, the error in assuming soil structure would probably result in a malfunction. The calculation and analysis of vertical two-layer soil structures are more complex than horizontal two-layer soil structures. As a result, a comprehensive and extensive analysis of grounding performance in vertical two-layer soil structures has yet to be performed.  
The general apparent resistivity expression \(\rho_{a}\), in vertical-layered soil, as in Equation (11) is suggested in [48] to determine the relationship between the locations of 4 electrodes in [PERSON]'s method and the apparent resistivity. According to Equation (11), \"a\" is the distance between the four electrodes (m), \(\rho_{1}\) is the resistivity of the first soil layer (\(\Omega\)m), k is the reflection factor between first and second soil layers, \"d \" represents the standard distance between the first electrode and the border between the first and second soil layer and the angle between a perpendicular line to the soil border and the line where four electrodes are positioned is represented by angle, \(\beta\). The parameters of vertical soil structure are shown in Figure 13.  
There are two cases (Case 1 and Case 2) that were analyzed based on the apparent soil resistivity using Equation (11) in [67]. The analysis was conducted to examine the effects of varying electrode spacing 'a', and the distance from two-layer soil interface 'd' and the  
Figure 13: Vertical two-layer soil structure [48; 67].  
angle '\(\beta\)' between a perpendicular line to the soil border and the line where four electrodes are positioned on the apparent resistivity. The results are summarized in Table 8.  
[PERSON] et al. [62] reported important research on the position of the current electrode on the grounding system in vertically layered soil where the grounding resistance is determined by the 0.618 DCG procedure for grounding resistance calculation in the analysis. In the analysis, the DCG technique refers to the distance between the test current electrode and the grounding system. The fall-of-potential approach was used to examine the apparent grounding resistance curves in various measurement routes. The investigation found that if the measuring lead is positioned parallel to the vertical soil border, which is within the engineering range, the measurement error of the 0.618 techniques is extremely minimal. Even if the distance between the grounding system and the potential electrode is longer, the findings in [62] concluded that positioning the potential electrode on the opposite side of the current electrode in all measurement paths is not recommended because it leads to a measurement error of more than 10%.