Resi methods

Full training on how to use the equipment will be given in the field, which all makes a lot more sense when you see the equipment, but here is a background to the technique:

Resistivity

Resistivity is a useful technique for archaeologists which uses the measurements of voltages associated with rocks, soils and archaeological features. This is measured using the resistance meter.

Soil resistance and water: Most rock-forming minerals and soils are poor conductors of electricity and hence ground currents are therefore mainly carried by ions moving in pore waters. The electrical resistance of the soil is therefore almost entirely dependant on the amount and distribution of water within it. This is why you would never use this method during or soon after a long period of drought, or immediately after/during heavy rain. (In the former case there is no moisture to allow the current to flow; the latter case will take the current along the water-soaked horizon and hence time is needed for water to penetrate the ground through to the archaeological interest levels.)

Some soils and geological areas are better for this technique than others. Dissolved salts in the soil moisture will help resistance measurements by increasing the conducting property. Hence, chalk is an excellent area for resistivity surveys.

The machine set-up:

The four-probe system is a way of estimating the true measurements accurately. Two probes are set to one side as the ‘tester’ probes (called the remote probes). These measure the background values, so that the two mobile probes (on the machine) measure their area of detection better. The two tester probes are usually set c. 1 m apart (but they can be between 0.3 to 2 m), and the two mobile probes are set to 0.5 m apart on the frame. With this set up the remote probes must now be placed in the ground at least 15 m from the nearest point of the grid. However, you can change the probe spacing if you want to and you must do this at the start of the survey – never change spacing during a survey. For instance, if you widen the spacing the machine will see deeper layers, but the drawback is that you lose the ability to see small features.

In a uniform soil (that is, an area of no archaeology) the soil current will be evenly distributed around the probes. In an area of high resistance (e.g. a wall) the currents will be deflected as the currents will be forced to flow around instead of through the feature. Hence walls will be positive features (higher values) than the background level. These will therefore be shown as white on the resistivity plots. In low resistance areas a high proportion of the current will be ‘taken into’ the feature, hence a low voltage is recorded. This is seen as a negative feature on plots - dark to black areas).

Normal readings (in ohms):

<30 poorly drained, deep topsoil, clays or wet ground.

30 to 200 normal soils

>200 well drained dry topsoil, thin or sandy soils.

The operator sets the machine range to suit the site conditions according to the above range – this is set before the start. If you do not set this then the machine will switch the range automatically – which will take ages!

Which filter to select?

This is usually straight-forward and the choice is rural, semi-rural or urban. If unsure then switch the machine on – a flickering of 0.1 division is OK. But if the reading fluctuates by a number of ohms, then change the filter level.

Grids: grids are usually laid out as 20 x 20 m as this is the default on the download programme. The start point of the grid is always the bottom left hand corner and all grids to be merged on the same plot are to have the same orientation relative to each other (i.e. you cannot start walking the next grid east-west if you already started previous grids north-south). Grids are always walked in a zig-zag, that is you walk up one line and back down the next one, and so on. After G1 go on to G2 (starting bottom left corner again.) A 20 x 20 m grid will take about 30 to 40 mins to complete.

Grid layout for nine 20m x 20 m grids:

G3	G4	G9
G2	G5	G8
G1	G6	G7

Start / Baseline A

The key rules to resistivity:

	Never use in drought conditions (no current = no measurement. The best times are the Spring and Autumn).
	Never use immediately after rain (poor or no measurements of the archaeological layer).
	Never use in the rain as the meter is not waterproof!
	Never use on waterlogged soils (current will flow along the water layer, not the feature).
	Cautious use in areas with a very high water table (winter is not a good time for this type of survey in high water table regions).
	Insert the probes fully into the ground to ensure a good connection.
	Never thrust the probes into the ground – any shock or vibration will be carried to the meter and damage it – especially vulnerable in stony or hard ground!
	The remote probes must be positioned so that they are more than 30 times the mobile probe spacing, e.g. mobile probes set to 0.5 m means the remote probes must be more than 15 metres from the nearest part of the survey.
	Watch out for electricity pilons as these cause enormous noise and mean you will need to change the filter level as you approach.
	Watch out for drift – values slowly drifting up and down – the machine is being overwhelmed by noise and is a disaster for the survey. Check filter level and check distance from remote probes.
	When you have finished a grid never go on to the next one unless 100% happy. Errors can only be changed now, not after the start of the next grid.
	Never open the meter housing yourself to look for source of any error – this needs an expert’s attention.
	Under no circumstances use the equipment in a thunderstorm or even if they are forecasted.
	Never use the equipment next to electrical installations.
	Note that cables are a trip hazard and do not put the probe spikes through your foot if you can help it!
	Have fun!