How resistivity works

There are many better explanations of the physics and other considerations about resistivity (check the links below).

This is therefore just a general starter.

Our equipment has a twin probe configuration. It has a mobile frame with two probes connected to the meter mounted on it. The meter is also connected by a 50m long cable to two remote probes.

The basic idea is that you measure the resistance to an electrical current passing between two points in the ground. In our system, the two points are one of the probes in the frame and one of the remote probes. click to get a bigger image (click for a bigger image)

To carry out a resistivity survey over a 20m x 20m grid, the remote probes are fixed in the ground a long way – at least 15m – from the grid. The frame is then inserted into the ground in each square of the grid consecutively, recording the resistance measurement. The electrical resistance of the ground is basically dependant on the moisture content and the concentration of dissolved salts in the water.

At the extremes it all falls apart, if there is not enough moisture to connect the two probes the resistance is always infinitely high, if the soil is waterlogged then the resistance is virtually zero. In between those extremes the resistance varies enormously, but on one site at one time it usually falls within a moderate range. This moderate range is then influenced by the precise conditions between and beneath the moveable probes. If there is a local increase in the amount of humus in the soil, more water will be retained and electrical resistance will be lower. If there is building rubble, there will be less moisture retained and resistance will be higher.

These two situations would occur with a filled in ditch and buried foundations respectively. You do need to be aware though, that there may be other factors at play. For example:

A filled in ditch may actually still be draining better than the surrounding soil and therefore contain less moisture and show as a high resistance. Broken or freshly ploughed soil resistance may be influenced by the degree of contact between broken lumps of mud. Deliberate salting of the ground with fertilisers or lime to mark out pitches can reduce the resistance measured.

click to get a bigger image (click for a bigger image)

At the end of the day you end up with a large square, representing your surveyed area, with resistance measurements for each part of it, a bit like a chess board with a number in every square. To most people this would be meaningless, but if you convert the numbers to shades of grey or other colours then patterns start to emerge. This is very similar to the shape of a hill or valley appearing when you make contours from an area full of spot height measurements.

Links:

Geophysics - mainly geological

Resistivity the story of our kit

Geophysical Methods

Archaeological geophysics

And a book:-

CHRIS GAFFNEY & JOHN GATER 2003
Revealing the buried past: geophysics for archaeologists
Stroud: Tempus