Resistivity

Resistivity (also known as Earth Resistance) is a non-invasive geophysical survey technique that allows archaeologists to map buried features without excavation.  The survey technique measures the resistance of the soil to an electrical current passed between electrodes, which reveals variations in moisture content and composition that indicate buried features. It is particularly good at identifying stone structures, walls or foundations.  Resistivity is not as quick as other geophysical surveying techniques such as Magnetometry, but is not impacted by the presence of modern iron objects like fencing, refuse, underground cables and pipelines or overhead lines and so can be carried out on sites where magnetometry may not be possible. 

Methodology

The Group operates a R M Frobisher TAR-3 Resistance Meter.

Systematic Survey - We will usually mark out a system of 30m x 30m grids (using highly accurate GPS equipment). The person operating the meter, together with a helper to make sure the cables don't get tangled on any vegetation, walk backwards and forwards across each grid, (aided by guide lines or ropes laid on the ground and moved by two other people).  

The meter takes a series of readings at regular intervals of the voltage of the small electrical current passing between the electrodes on the meter and a remotely placed electrode (at least 15 m from the grid being surveyed), from which the meter can calculate variations in the resistance of the soil to the flow of the electrical current.  Don't worry if its your first time, its a lot easier than it sounds and all training will be given on the day by the project lead.

Data Mapping Results 

The resistance is affected by soil moisture and the material present.

• • Buried stone walls or foundations tend to be drier and more resistant to the current, showing up as areas of high resistance.

  • Features with higher moisture, such as ditches or pits with soil infill, show lower resistance.

Using specialist software we can use these resistance measurements tocreate a map of the subsurface resistivity, showing potential archaeological features, with high resistance areas shown white and low resistance areas shown black.  

Best Conditions

Resistivity is most effective under specific conditions: 

No crop cover: It is easiest to survey fields where there is no crop cover (normally after harvesting or over grassland). Where the Group has attempted to survey land with taller vegetation, the results have not been as successful.

Weather: Our surveys are weather dependent - the resistance meter is a sensitive piece of electronic equipment, so we do not operate in the rain.

Seasonality:  As resistivity is dependent on the moisture content of the soil, it is best carried out in autumn and spring, avoiding the winter months when the ground can be waterlogged and summer when the soils become too dry.