# Resistivity in archaeology

1. Apr 13, 2013

### Johntoby

Hi all, I am writing an assignment at the moment, asking me to report on the use of resistivity in identifying objects under the ground.
I've gotten confused as to whether resistivity is used at all - surely resistance is all that is needed? For example, in any multi probe resistive survey, the resistance is measured between points to give a map of resistances and then show areas of abnormality. Where does resistivity come into it?

Thanks very much

2. Apr 13, 2013

### tiny-tim

Welcome to PF!

Hi Johntoby! Welcome to PF!
Different materials have different resistivity … can you write something about that?

3. Apr 13, 2013

### mikeph

Resistivity is a local property: specifically it is the magnitude of electric field required to produce 1 amp per unit cross section of current in the material. It doesn't depend on the length or thickness of the material, so it is a good measure of the intrinsic resistance of a certain material to electric current.

Resistance is a global property: it's the voltage needed to make 1 amp of current to flow between two pre-defined points. Clearly, this depends on where the points are, how thick the conductor is, how long it is, etc. It's typically what you measure.

Why is this useful ? Resistivity gives you clues about what is underground. If you know the resistivity is low in certain areas, then it's likely you might find some metal in there because metal has a lower resistivity. So if you put two probes either side of this metal, you'll measure a lower resistance than if you put two probes between a lump of coal!