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 Test
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 Test 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, soil testing should be made using the Wenner 4-probe method specified in IEEE Standard 81 Part 1 or BS EN7430.
So, for each measurement traverse. The pin spacings (between adjacent pins) 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 pin spacing (between adjacent pins) 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, a number of additional shorter traverses (0.15 up to 6m) are required to obtain data sufficiently representing soil conditions at shallower depths throughout the site.
So, the Wenner 4 Probe Test method consists of four electrodes; Two are for 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 pin to pin spacings for a Wenner 4 probe configuration is as follows:
| Very Short (m) | Short (m) | Medium (m) | Long (m) |
|
0.1 |
1 |
10 |
100 |
|
0.2 |
2 |
20 |
200 |
|
0.3 |
3 |
30 |
300 |
|
0.5 |
5 |
50 |
500 |
|
0.7 |
7 |
70 |
700 |
As a result, the above are typical ‘minimum’ required spacing-sets. So, the important thing to note, is the spacings are a ‘series‘ of measurements taken along a single traverse. In the above table, only five readings per traverse are shown – in reality; But many more spacings can populate the table. Therefore, providing the appropriate level of granularity for data analysis and inversion.
When designing to EN 50522 (applying as a minimum standard). As a result, there are 14 predefined spacings PER traverse.
By the way, drop us a line or chat for more info on these peculiarities. To BS EN50522 verses IEEE Std 81.
Given the capital importance of the soil resistivity data for adequate Earthing/Grounding system design calculations, a well-defined quality control program is required in the field to demonstrate that readings are valid.
So, when capturing the data from a Wenner Test, this data then needs to be processed further:
Soil Resistivity 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 is to use specialist software tools. Such as CDEGS RESAP 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.
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