12 www.megger.comEffect of Temperature on Earth ResistivityNot much information has been collected on the effects of temperature. Two facts lead to the logical conclusion that an increase in temperature will decrease resistivity: (1) water present in soil mostly determines the resistivity, and (2) an increase in temperature markedly decreases the resistivity of water. The results shown in Table VI confirm this. Note that when water in the soil freezes, the resistivity jumps appreciably; ice has a high resistivity. The resistivity continues to increase an temperatures go below freezing. Temperature C F Resistivity (Ohm-cm) 20 68 7,200 10 50 9,900 0 32 (water) 13,800 0 32 (ice) 30,000 -5 23 79,000 -15 14 330,000*For sandy loam; 15.2% moistureTable V: Effect of Temperature on Earth Resistivity*Seasonal Variations in Earth ResistivityWe have seen the effects of temperature, moisture, and salt content upon earth resistivity. It makes sense, therefore, that the resistivity of soil will vary considerably at different times of year. This is particularly true in locations where there are more extremes of temperature, rainfall, dry spells, and other seasonal variations.From the preceding discussion, you can see that earth resistivity is a very variable quantity. If you want to know what the value is at a given location, at a given time of year, the only safe way is to measure it. When you use this value for survey work, the change in the value, caused by changes in the nature of the sub-soil, is the important thing; from the variations in resistivity you can obtain useful survey results.
