Soil Resistivity Testing Methods a popular post. Originally published in 2013 and now updated in 2020.
Wenner 4 Probe test is one of the most common soil resistivity testing methods. It is also part 3 in a series of short posts on Soil Resistivity Testing Methods. Part 1 & Part 3. Together with, the common mistakes encountered. And also, practical advice on how to avoid the Soil Resistivity Testing 10 Common Mistakes.
Soil Resistivity Testing Methods – The Wenner 4 Probe
So soil resistivity testing can be carried out using different methods. Therefore, below describes the Wenner 4 Probe Test Method. Also, its one of the three most popular soil resistivity methods employed, to perform a soil resistivity test:
Wenner 4 Probe Soil Resistivity Testing Method
Wenner array is probably the most labour-intensive of all the ways when performing the longer traverses. Hence, this method can call on up to four people to accomplish the task in a sensible time frame.
On the other hand, it is the optimal Soil Resistivity Testing Method (to date) of choice for Earthing Designs, due to its ratio of received voltage per unit of transmitted current.
Therefore, this means the Wenner Method is considered one of the more ‘reliable’ methods for testing soils to deeper depths.
In the Figure above, the illustration shows how the probe spacings relate to the apparent depth under test, e.g. a 6m probe spacing. As a result, indicates the soil resistivity at a depth of ~6m.
So, do your soil testing using the Wenner 4-probe method specified in IEEE Standard 81 Part 1, BS EN 50522 or BS 7430.
So, for each measurement traverse. The pin spacings (between adjacent probes) need to begin at 6″ to 12”. And increase after that by a factor of approximately 1.5. Then, up to the maximum pin spacing chosen for that traverse.
Therefore it is highly desirable to have 2 to 3 traverses centred at different locations. Likewise, whose maximum probe spacing (between adjacent probes) reaches a distance that exceeds the maximum extent of the substation. For example, its largest diagonal dimension (and any other facility associated with the substation), preferably twice this diagonal dimension or more. While avoiding the influence of buried metallic structures.
Hence, requiring a number of additional shorter traverses (0.15 up to 6m) to obtain data sufficiently representing soil conditions at shallower depths throughout the site.
So, the Wenner 4 Probe Soil Resistivity Test method consists of four-electrode probes; two are for the current injection. And two for potential measurement.
Figure 1 shows the Wenner 4 Probe Test method.
Soil resistivity calculation formula Equation 1: shows the soil resistivity formula associated with the Wenner 4 Probe test method.
Where R is the resistance measured by the machine
a is the spacing of the probe
So, an example of a probe to probe spacings for a Wenner 4 probe configuration is as follows:
When designing to IEC BS EN 50522, there are 14 predefined spacings PER traverse.
Table NC.2 – Recommended Wenner spacings in metres
The above are typical ‘minimum’ spacing-sets. So, the important thing to note, spacings are a ‘series‘ of measurements taken along a single traverse. Thus, providing the appropriate level of granularity for data analysis and inversion.
How Deep Does The Wenner 4 Probe Method Test
Readers often ask, “How deep does the Wenner 4 Probe method test?” The way the Wenner method works is by emitting an electrical signal into the ground via the probes and measuring the returning signal. The probes only penetrate the ground by a few inches, but the electrical signal itself can penetrate many meters.
So, just to reiterate… the probes only physically penetrate a few inches. However, the volume of geology under test is determined by the spacing between each test probe. So, in theory, the testable depth is only limited by the instrument’s strength of the signal and the deployable distance between probes.
BS EN 50522 describes a typical set of probe distances that work on the most size of earth electrodes.
By the way, why not drop us a line or chat for more info on the peculiarities of BS EN50522 verses IEEE Std 81.
Given the capital importance of the soil resistivity data for adequate Earthing, Grounding system design calculations require a well-defined quality control program in the field to demonstrate that readings are valid.
So, when capturing the data from a Wenner Soil Resistivity Test, this data then needs to be processed further:
Soil Resistivity Testing Data Inversion
The measured soil resistivity data needs to be inverted to obtain equivalent multi-layer soils to before using in the subsequent Earthing/Grounding Design.
So, this interpretation requires to account for electrode pin depth. Also, any irregular pin spacings (due to obstacles in the field). And known buried metallic structures. That mildly to moderately distort the measured values.
Therefore, chose one or more suitable soil models for the Earthing Study. From those obtained from all measurement traverses. Also, explain these choices in the final report.
So, currently, the accepted practice for the Data analysis method is to use specialist software tools. Such as CDEGS RESAP or XGS_SRA (From XGSLab) to deliver a 1-D (one dimensional) optimised model. But note, 2-D pseudo sections are not readily usable for Earthing Design. However, the pseudo segments are growing in popularity for geological exploration/investigations. Also, can provide useful insights (3-D finite volume data) for Earthing System Design.
Therefore, it’s wise to have any approximations to the soil model justified for good measure. Account for soil structure model variations due to local, and seasonal variations by developing soil model structure limiting cases.
As a result, it can’t be understated. Just how important reliable, ACCURATE Soil Resistivity data is for the subsequent Earthing Design. Also, it is the absolute foundational requirement that all following safety calculations for touch and step voltages are derived.
Software Tools Update – Jan 2020
Regular readers of this blog will know GreyMatters have championed CDEGS for many years. But now there is a viable and attractive alternative to CDEGS. I explain “Why I chose this alternative” in this article. And compare several past projects, done with CDEGS and the alternative – in this article.