# Soil resistance surveying - will this work?

A friend of mine has been told that there used to be some interesting buildings on his land (a windmill, apparently) - there is no obvious visible trace of them today - the area is just grassed.

I know there are a number of geophysical survey methods that can reveal traces of features that may once have existed there, or may be buried under the modern surface - and that one of these involves measuring the resistance of the soil.

However, my initial research suggests this is done by laying out a grid of charged wires/pegs and measuring the resistance of the ground with one probe at intervals across the area. I’m sure I’ve seen it done more simply than that with a handheld rig that just had two probes, about 18 inches apart that were pushed into the soil, with the resistance being measured between those two points (and this being repeated across a grid of co-ordinates)

So… initial experiments suggest that soil resistance between two points spaced 18 inches is within the range measurable by my cheap electronic multimeter. If I knock up a simple rig with two stainless steel pins, say four inches long and spaced 18 inches apart, and use this to measure resistance across a grid of points - is there any reason why that can’t possibly produce some sort of meaningful result?

Obviously I’d only be measuring the resistance of the surface soil, but there should be some sort of relationship between the surface properties and what lies beneath, shouldn’t there?

Fairly sure the Time Team crowd used an arrangement similar to what you describe. So if you trust one of Baldrick’s cunning plans, well…

That said, I’m not sure if a meaningful signal will be in the first or fourth decimal place of your resistance measurements, or can even be extracted at all. Very dry soil or sand will basically max out the range of the meter, so what you’ll be measuring are wet soil resistances. This has various components - the contact resistance between the probes and the soil is an additional contribution to the soil resistance itself and may dwarf it. High frequency resistance measurements can get around this because there’s a small contact capacitance in parallel with the contact resistance,. High frequency essentially bypasses the contact resistance through the capacitance. Geophysics isn’t my field though: this comes from my corrosion background. I’ll try Googling some relevant terms and get back to you.

The best I could find after a short search was this:http://www.martechcon.com/SoilResist3c.PDF. Googling “wenner array” gets you a lot more. Basically, its recommended to use fOUR electrodes - two to pass current through the ground and a pair measuring the voltage between them. Two-elecrode systems are unlikely to work well.

Have you tried looking at an aerial view instead? Often you can see shapes from above (or from, say, a roof) that you can’t see on the ground.

yes, a long-range ocular as mentioned above might be better.

resistivity surveys are generally used for determining soil composition, moisture and water quality. it also helps identify the underlying rock. the most profitable use of resistivity survey is in geothermal exploration. geothermal water has more ions in it so it’s more conductive.

when looking for underlying structures, it’s better to used seismic reflection (bounced waves) or other geophysical methods such as magnetic surveys, gravity, induced polarization.

but the thing you’re looking for is a man-made structure buried in soil, not in rock that is very near the surface. if an eagle’s view is not possible as mentioned in the above post, follow this treasure hunter’s method:

take a narrow but strong length of steel rod maybe 3/16 inch diameter and around 4 feet long. sharpen one end and securely attach the other end to a wooden handle (make sure the handle’s secure enough to yank the rod out of 3 feet of soil.)

walk along a grid in your field and start sticking the rod down to a depth of 3 feet. better to mark off your grid so that you can follow up in between the earlier probes. i once joined a group of treasure hunters exploring in the jungle. it takes a good eye to choose a good spot on which to sink the probe. in three days, we found broken glass and pottery whose age none of use was qualified to determine. it was fun all the same.