Soil Resistivity 1

[nsong]

Hi Guys,

I am working on a project that requires reviewing some soil resistivity data. The values I've got are around 800,000 ohms-cm, they were measured in damp to moist silty, clayey sands. They seemed pretty high, are these values possible? I have read literatures suggesting that the maximum values are around 500,000 ohms-cm. Where could I find some sound references?

Thanks for your help!

 

[MRM]

Hi nsong,
I see that you're new to Eng-Tips-I think you'll find it to be very helpful!

I've got a few comments for you about your resistivity question. A value of 800,000ohms-cm is possible, but not probable for the soil you described. I generally wouldn't expect numbers higher than about 80,000ohm-cm, and more realistic numbers would probably be around 15,000 to 30,000ohm-cm for those soils. Of course I'm only going on a short description of the soils when other factors could come into play.

Here are some suggestions for checking your results. Some of these suggestions depend on the type of meter you’re using.

1. You typically get resistance (ohms) values directly from the meter. In order to get soil resistivity (ohm-cm), be sure you're using the proper equation, and double check your arithmetic and units.
2. Watch where your decimal place is. Be sure you know if you're testing in ohms or kilo-ohms on your meter.
3. Be sure you run several tests not just a single test at one depth. This not only helps you evaluate the site, but it also will help highlight anomalies.
4. Speaking of anomalies in your results, be sure you attempt each reading one or two times, i.e. push the button on your meter and hold it several seconds and repeat. Some resistivity meters can be finicky.
5. Consider the possibility of stray voltage on the site. This could give you odd values.
6. Be sure your meter isn’t indicating a problem with the reading. Some of them will alarm you to something amiss during a test.
7. Make sure the meter has been calibrated in the last year or so.

Does this help at all?

 

[nsong]

Hi MRM,

Thank you so much for your helpful and suggestive reply. You message further encouraged me to look into the issue. We went back to the original field data, and the new Soil Resistivity values we obtained are about 12000 to 20000 ohms-cm.

You are absolutely right, Eng-Tips is a very useful site. Actually I have been browsing here and there within Eng-Tips for a while, but this is my first time to post a question.

Thanks again!

nsong

 

[wildspread]

I have a further question regarding soil resistivities:

The standard units for soil resistivity is ohm*cm. However, the meter I'm using is giving me units of ohm/cm after I use the conversion chart. Is this a unit anyone is familiar with? I'm thinking it may be a type of "average resisitivity?"

Furthermore, the readings are astronomically high (>1,000,000 for a slightly moist sand and gravel). The only thing I can think of is the battery may be low or else I'm in such a dry region not much current can flow. I made sure to get an "intimate" contant with the probes/soil and yet my resistivities are through the roof.

I sent some samples to the lab (Miller Box test) and they are in the expected ranges (300 to 1500 ohm*cm, approximately). I can't quite figure out why my resistivity meter is giving high numbers?!

Thanks for reading my first post ever on this board.

 

[MRM]

wildspread,
I'm sorry I have not noticed your post until now. The units of ohm/cm are not conventional for most reporting purposes. I would check the chart again to see if you can find where that is coming from.

For slightly moist sand and gravel, readings of 1,000,000 ohm-cm are possible. Surface conditions may also have an effect on your readings. What I'm wondering about is your Miller box test; were the results you mentioned, 300 to 1,500 ohm-cm, for that same sand and gravel specimen? For that type of soil, those numbers are suspect. Now maybe it's possible that you can have lower-than-usual resistivity values depending on the concentration of ions within the pore water. Then again, it's only slightly moist.... You've got me curious now, wildspread.

Remember that the resistance values you obtain with the meter, after converting them to resistivity, are actually an average. A weighted average, if you will. Sometimes in reports you'll see the term "apparent resistivity" used. This is because when you obtain resistivity readings for say a 20 foot spacing, that resistivity value represents the weighted average for everything from the surface to 20 feet. If the soils were very uniform with depth, it would be possible to have all the same resistivity data with depth, i.e., your "apparent resistivity" readings would reflect closely the actual resistivity readings you might find at a discrete level within that soil mass.