If you need to go deeper than a metal detector, resistivity is the answer! What is resistivity? According to the dictionary, resistivity is “a measure of the resisting power of a specified material to the flow of an electric current”. For our purposes, resistivity is the measure of an electrical current, in Ohms, that is sent into the ground via 2 to 4 metal probes. When we encounter an anomaly, such as a cave or a large metal object, these readings will change dramatically.


Let’s say that you are looking for a cave… You would start by getting a reading of the ‘normal’ ground conditions by taking a reading near your search area. This serves as a ‘ground balance’ so to speak. For example, you might start with your probes at 10 feet apart and take a reading. With the probes 10 feet apart, your effective depth will be approximately 5 feet. Next, you would move the probes outward to 15, 20, 25, etc and get readings from each, noting that the depth of each scan is roughly ½ of your probe spacing. An example of ‘normal’ ground readings at each depth might be something like 115 Ohms, 125 Ohms, 150 Ohms, 200 Ohms, etc. The Ohms should go up slightly the farther your probes are apart.

Next, you would move into your search area and perform the same array, repeating the same probe spacing from your ‘control’ array, and note the differences in your Ohms readings. If your readings come back similar to your ‘control’ readings, you can move the array over 5 or 10 feet and repeat the process, always comparing your readings to the control scan. If you actually encounter the cave you are looking for, the Ohms reading will go up drastically due to the fact that the air in the cave is more resistant to the electrical current than the surrounding ground. In that case, the Ohms readings would look something like this: 115 Ohms, 125 Ohms, 500 Ohms, 750 Ohms, etc. So, in this example, the cave would start at approximately 10 feet deep and continue at 12.5 feet. If you wanted to estimate the size of the cave, you would keep spreading your probes until the Ohms readings go back into the normal range. This would indicate that you have now gone under the cave. If the readings were to keep going really high, this could mean that you are in a very large cave.

The same example would be true for metal or water except that the Ohms readings would drastically drop instead of increase due to the fact that metal and water are more conductive, making them less resistive. Resistivity can also be used for mineral prospecting, archeological searches, and geophysical surveying, to name a few.

If you seek targets that are beyond metal detector range, the EZ RES PRO is the answer. It is an affordable Professional Geophysical Resistivity Survey Meter that was purpose-built by Terra Exploration Group to withstand the extreme conditions encountered in the field. It can reach depths of 165 feet, or more (in the right soil conditions and/or with additional optional wire), and is also one of our “Tough Series” detectors. This means that we actually use it professionally, it is tried and tested in the field, and performs as advertised. There are no Gimmicks with the Terra EZ RES PRO resistivity survey meter, only results!

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To find out more about resistivity check out the Terra EZ Res Pro product page.

Guest Writer: Robert Leonard
Terra Exploration Group