Soil Resistivity - Probe Depth

[Modula2]

Has anyone seen references on the effect of probe depth vs probe spacing using the Wenner 4 point method. It is easy to see statements that spacing to probe depth should be 10 or 20 to 1, and formulae are available when spacing is not much greater than probe depth. However, exact backup is not easy to find, and I am thinking about empirical as well as analytical observations. If it was 1 metre spacing and 1 metre depth, what would happen and why? That is what I am trying to find out. Having been presented with such data, I am trying to assess if it is useful.

 

[Humble2000]

Refer to IEEE 81, there is a formula to relate probe depth to spacing. If th eprobe spacing to depth ratio is high enough simplification in formula can be made.

 

[Modula2]

That's what I said also: formulae are available. But why choose 1 metre probe depth instead of 100 or 200 mm? If the formulae give the same result, then easier to do 100 mm. And if 1 metre, then why not 2 or 3 m. On what basis would the decision be made as to depth?

 

[jghrist]

Sometimes you need to drive the probe deeper to get good soil contact. Most resistivity testers will provide an error indication if the contact resistance is too high to achieve accurate readings.

 

[sslobodan]

Probe depth should go on the depth of the grounding system so you can get more accurate reading. Considering that depth is usually 1m (top of grounding system) . That is why that data is in IEEE Code. I did some probing on very problematic terrains in my days (While i was working on another job) And I was able to get pretty accurate readings with smaller depths. Engineering in practice is more like cutting out that 3 decimals that will show up if the measurement was probed as it should. For real life you can go with 1.8ohm not needing to know whether it is 1.79845 or 1.812245 Ohms but what you should take in consideration is self resistivity of gear and cables and probes. It can make serious mistakes (100% or more) if not counted on. I was doing my readings with some digital German device (cant remember which one) With cable length it required (100 - 200m) and sonda resistivity mistake I needed to count on was 2.3 ohms as i recall. Considering that allowed resistivity of grounding system is 4ohms it could made quite mess in calculations. And ohm meter with small resistivity would suffice to measure the self resistivity you need to count on, with mistake les to analog more to digital (it would be perfect that you have Whitston bridge so you can do it )

 

[jghrist]

Some misconceptions in sslobodan's post, assuming resistivity measurements with the Webber 4-point method (most common):

The measured resistivity is the average at a depth equal to the horizontal spacing of the probes. The depth at which the resistivity is measured is not dependent on the probe depth.

The lead lengths are not part of the measurement and the resistance does not have to be deducted from the measurement.

Resistivity is measured in ohm-meters, not ohms.

 

[jnunner]

In section 13.3 of IEEE 80, there is a formula to determine the resisitivity of the soil based on the measured resistance. This formula is dependant on the depth of the probes. If the value of the probe depth is much less (say 1/20th) of the probe spacing, then the effect of the probe depth is negligible and the formula becomes simplified to

p = 2 * pi * a * R. (where a is the probe spacing and R is the measured resistance).

Some software (such as CDEGS) will account for the probe depth when calculating resistivity. To perform the calculations yourself, refer to equation 44 in IEEE 80.

I hope this helps.