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Can you explain the 1/20 probe depth rule when conducting a Wenner 4-pin Soil Resistivity Test?

Hi Than and David,

Thank you for your question regarding probe depths while conducting a Wenner 4-point soil resistivity test, it is our pleasure to help.

Here is some information on Wenner 4-Pin Soil Resistivity Testing:  https://www.esgroundingsolutions.com/about-electrical-grounding/what-is-soil-resistivity-testing.php

When conducting a Wenner 4-point soil resistivity test, we need to consider the effects that the “Sphere-of-Influence” will have on our test, in two (2) ways:

1. The distance our test is being conducted from any buried metallic objects, railroad tracks, fence lines, etc.  This distance should be equal or greater than the maximum (“a”) spacing of our test.  In other words, if you are conducting a Wenner 4-point test with a maximum probe spacing of 60-meters (a 180-meter traverse), there should be no interfering objects (fence, buried metal pipes, etc.) within 60-meters of any part of our test.

2. The probes we use to conduct the test, will have their own sphere-of-influence that they will generate based on the depth they are driven in to the earth.  For hand calculations, the probe depth may not exceed 1/20 of the spacing of the Wenner test.  Advanced computer algorithms can adjust for these differences, but the 1/20th rule is a good one.

Here is some more information on the sphere-of-influence:  https://www.esgroundingsolutions.com/about-electrical-grounding/grounding-electrode-sphere-of-influence.php

The bottom line is that deeper depth probes are not better, they are worse.  In fact, some people actually use a heavy weight or a pile of heavy chain with a little salt water, and don’t drive any probes in to the earth at all!  We try not to go much deeper than 6-9 inches for probe depth, and when we conduct the short range readings we actually only go 2-inches into the earth.

Keep in mind that arguably the second most important spacing’s for the Wenner soil resistivity test, are the very short spaced readings.  You should be taking measurements starting at a 6-inch (0.15 meters) spacing and incrementing in interval size by a ratio no greater than 1.5, with a 1.33 ratio preferred.  The probe depth for a 6-inch spaced Wenner measurement should be no greater than 2-inches for the Potential probes, and 4-inches for the Current probes.  This arrangement clearly violates the 1/20 rule.  However, good computer modeling software can adjust for this and still provide good data.

See the following blog for more information: https://www.esgroundingsolutions.com/893

You of course should be using true DC test meters, with test lead cables.  We recommend meters that provide 800 Volt p-p signals with at least 500 mA DC.  See www.agiusa.com for more information.

The last component is of course computer software.  Your raw data and hand calculation are nearly worthless.  In fact, hand calculations will only provide you with what is called “apparent resistivity”, not the actual soil resistivity.  This requires a computer to do properly, and is so complex that it can actually take a high-end processor several minutes of computing time to provide an analysis!  Why even take the test if you are not going to process the data correctly?

The raw data you collect during the Wenner test, must be analyzed and processed in order to develop a soil model (or soil profile).  This soil model will tell you what the resistivities of the soil are at various depths down through the earth at your site.  We recommend the RESAP module from the CDEGS computer program for proper analysis.  See www.sestech.com for more information.

E&S Grounding Solutions can of course help you with the data processing and soil model development.  But if not us, please get someone with the expertise to properly analyze your data.

Best regards,

The Engineering Team at E&S Grounding Solutions


Photo credit: E&S Grounding Solutions

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