Soil resistivity can be extracted in a grid pattern in a site location. With grid resistivity data, you can have an iso-resistance map of the area. For better evaluation, it’s best to compare the iso-resistance graphs with water level data. In this way, we can get a general idea about resistivity and corrosivity. Moreover, this is useful for cathodic protection design too. Soil resistivity for the area as amps or a general guide is not sufficient. For quantitative analysis and use in cathodic protection calculations, like the calculation of anode-to-earth resistance, we need to have a value for the whole area or for each zone.
Calculation Methods
To get a soil resistivity value for an area, there are two methods of calculation:
- Arithmetic average: This is the most straightforward approach. This way, you can make a simple average and use it in your calculation. For example, if you have soil resistivity of 100, 70, and 50 Ω m, the average value for calculation is 73.3 Ω m.
- Logarithmic average: You can find this method in some reference books. This way, you need to average the logarithm of the values, and then 10^MEAN AVERAGE VALUE is the answer. So in the mentioned example, we first calculate the average of Log(100), Log(70), and Log(50), which is 1.848=(2+1.845+1.7)/3. As a result, the average soil resistivity is 10^1.848=70.5 Ω m.
Selecting the method depends on our assumption about the soil resistivity and its usage for further steps. In CCPTools, it is possible to calculate the soil resistivity for an area using both methods. Based on the reference book’s recommendation, we think the logarithmic average is more precise.