Soil Resistivity Methods

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 Posts: 1
 Joined: Mon Jan 09, 2017 2:40 pm
Soil Resistivity Methods
I am not a Geophysicist, however my career in cathodic protection requires the use of some of the testing. I've been trying to understand more about soil resistivity and how I can maximize the data. I have a basic formula for finding the resistivity for a specific strata when using the "wenner 4 pin method", it is: R2=(R1xR2)/(R1R2), R3=(R2xR3/(R2R3), etc. R then gets put into: p=R x a x 2 x (pi)... p=resistivity, R=resistance, a=pin spacing in cm (or difference between the previous pin spacing and the current spacing giving you the thickness of that strata) and (pi) being 3.141593....... Does this look right? And if so, should my resistance value be decreasing with the expansion of the pins (deeper sounding), and only decreasing, with no chance of getting a higher value? I noticed that if I get a value that was greater then the previous value, the strata formula doesn't work. (Because of the negative value in the denominator.) I am eager to learn how to perform this test in a more appropriate manner and be able to gather as much in depth data as I can. Any comments on the questions I have would be greatly appreciated as well as tips on how to simply do the job better. Thanks!
Re: Soil Resistivity Methods
Firstly, welcome!
Can I suggest using more paragraphs to separate your ideas, to make them easier to read?
I'm sure you've already found something similar, but have a quick look at this website: 'http://en.openei.org/wiki/DC_Resistivit ... ner_Array)'
Generally, the 'Apparent resistivity' isn't intrinsically useful  You'll need to 'invert' it to produce meaningful data. (Inversion is essentially the statistical recreation of a geoelectrical model which can reproduce the apparent resistivity values you've measured.)
If it's just the 1D scenario (i.e. you 'expand' your array over a singular point of earth) then you end up with a curve that will increase or decrease depending on the layering and relative conductivity.
I'm not sure about your 'R' values  it looks like you're only measuring 3 values. Where did you get the equation from?
Here is another link which may explain the method better  http://www.geosci.usyd.edu.au/users/pre ... s/Res1.pdf
Can I suggest using more paragraphs to separate your ideas, to make them easier to read?
I'm sure you've already found something similar, but have a quick look at this website: 'http://en.openei.org/wiki/DC_Resistivit ... ner_Array)'
Generally, the 'Apparent resistivity' isn't intrinsically useful  You'll need to 'invert' it to produce meaningful data. (Inversion is essentially the statistical recreation of a geoelectrical model which can reproduce the apparent resistivity values you've measured.)
If it's just the 1D scenario (i.e. you 'expand' your array over a singular point of earth) then you end up with a curve that will increase or decrease depending on the layering and relative conductivity.
I'm not sure about your 'R' values  it looks like you're only measuring 3 values. Where did you get the equation from?
Here is another link which may explain the method better  http://www.geosci.usyd.edu.au/users/pre ... s/Res1.pdf
Re: Soil Resistivity Methods
The Wenner fourpin method, as shown in figure above, is the most commonly used technique for soil resistivity measurementsDavidDettra wrote:I am not a Geophysicist, however my career in cathodic protection requires the use of some of the testing. I've been trying to understand more about soil resistivity and how I can maximize the data. I have a basic formula for finding the resistivity for a specific strata when using the "wenner 4 pin method", it is: R2=(R1xR2)/(R1R2), R3=(R2xR3/(R2R3), etc. R then gets put into: p=R x a x 2 x (pi)... p=resistivity, R=resistance, a=pin spacing in cm (or difference between the previous pin spacing and the current spacing giving you the thickness of that strata) and (pi) being 3.141593....... Does this look right? And if so, should my resistance value be decreasing with the expansion of the pins (deeper sounding), and only decreasing, with no chance of getting a higher value? I noticed that if I get a value that was greater then the previous value, the strata formula doesn't work. (Because of the negative value in the denominator.) I am eager to learn how to perform this test in a more appropriate manner and be able to gather as much in depth data as I can. Any comments on the questions I have would be greatly appreciated as well as tips on how to simply do the job better. Thanks!

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 Posts: 1
 Joined: Thu Sep 28, 2017 10:26 pm
Re: Soil Resistivity Methods
I agree to this one .chrisine wrote: ↑Sat Mar 04, 2017 1:48 pmThe Wenner fourpin method, as shown in figure above, is the most commonly used technique for soil resistivity measurementsDavidDettra wrote:I am not a Geophysicist, however my career in cathodic protection requires the use of some of the testing. I've been trying to understand more about soil resistivity and how I can maximize the data. I have a basic formula for finding the resistivity for a specific strata when using the "wenner 4 pin method", it is: R2=(R1xR2)/(R1R2), R3=(R2xR3/(R2R3), etc. R then gets put into: p=R x a x 2 x (pi)... p=resistivity, R=resistance, a=pin spacing in cm (or difference between the previous pin spacing and the current spacing giving you the thickness of that strata) and (pi) being 3.141593....... Does this look right? And if so, should my resistance value be decreasing with the expansion of the pins (deeper sounding), and only decreasing, with no chance of getting a higher value? I noticed that if I get a value that was greater then the previous value, the strata formula doesn't work. (Because of the negative value in the denominator.) I am eager to learn how to perform this test in a more appropriate manner and be able to gather as much in depth data as I can. Any comments on the questions I have would be greatly appreciated as well as tips on how to simply do the job better. Thanks!