A resistivity meter measures the flow of electric current through the ground to determine soil composition, water content, and the presence of underground structures. The Wenner method is commonly used, and the meter can also measure salts for corrosion prevention and grounding equipment efficiency. It can locate underground structures by detecting changes in soil resistance.
A resistivity is an electronic device used to measure the flow of electric current through the ground by probes inserted at regular intervals. Resistivity is defined as the amount of resistance to current flow, so the device can determine different soil compositions based on the readings. These meters are used for soil surveys, groundwater location and even the detection of underground structures.
When electric current passes through dry soil, there is very high resistance because the soil or dirt is not a good conductor. The presence of water can allow the electric current to flow relatively freely and the resistivity meter will show much lower values. A series of probes can then provide a visual idea of soil composition by preparing graphs from repeated measurements.
Four probes can be placed in a straight line, with current applied to the two external probes and measured to the internal ones. This is known as the Wenner method, invented by Dr. Frank Wenner of the US Bureau of Standards, and is a common method for measuring resistivity. The probes or rods should be of equal spacing with the distance between them three to four times their length. A resistivity meter is designed to measure wider ranges of current and resistance than a standard electrical multimeter.
In addition to determining water content, electrical measurements also measure salts that contribute to corrosion. A resistivity meter is often used to measure soil conditions near buried pipes and other metal structures. Protection bars, called anodes, can be placed at regular intervals to reduce corrosion. The anodes corrode before the structure and any stray electric current is directed to earth through the anodes.
Another use of soil resistance measurement is to determine the efficiency of electrical grounding equipment. Electrical substations, transmission towers, and buildings may have electrical grounding or lightning protection bars. A resistance survey will help determine the best locations for grounding equipment and the efficiency of installed systems. Changes in resistivity over time can show when grounding systems need to be repaired due to corrosion or damage.
A resistivity meter can determine the location of underground structures or artifacts. The electrical resistance of the soil changes as open areas such as underground rooms or tombs are introduced. An electrical resistivity survey is prepared by placing the probes in regular patterns. Differences in meter readings will show the presence of underground objects that are different from flat ground.
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