ABINGDON ARCHAEOLOGICAL GEOPHYSICS

RESISTIVITY

Equipment   

Our CIA/TR systems meter is used with a frame which has its probes 0.5 metres apart and a pair of remote probes which are placed at least 15 metres from the survey area. A second frame also is available which has the probes at 1 metre apart, (in which case the remote probes need to be at least 30 metres away). This second arrangement is used where the overburden is expected to be over a metre deep. 

Expectations

 One realistically cannot expect to find much below 1.5 metres although large ditches can show up at that depth and even greater depths if the soil conditions permit. Above that depth and it can be expected to find walls, floors and similar areas where the moisture content of the features differs from that of the soil surrounding them.  

Ground Conditions

Resistivity is always sensitive to ground conditions and this is principally the amount of moisture there is in the soil. In general the autumn and winter months can be better than the summer as in the summer there can be so little moisture that it is hard to gain the definition required for a successful survey.  In low lying areas where saturated soils are a problem the summer months may be better than the winter ones.

Resistivity surveys are affected by leakage from electric cables and the reverse current under electricity supply lines. This can drastically reduce the rate at which a site can be surveyed. See the Requirements page for details.

Grids

 For initial surveys we usually use 20 metre by 20 metre squares with readings every metre along the lines and 1 metre intervals between the lines. This gives 400 readings per square.

This is usually good enough to locate areas of buildings and similar and to give an indication as to their layout, although once a building is located a survey at 0.5m reading spacing should be carried out.

Coverage

Approximately 13 squares of 400 readings per square can be completed in one day, on level unobstructed short grass, using our equipment and two people.

 A more detailed survey can be carried out with lines at 0.5 metre intervals and readings every 0.5 metres. This would then give 1, 600 readings a 20 metre by 20 metre grid. As this method is four times as dense it takes approximately four times as long to carry out and thus the site coverage is reduced. It does however show a good degree of detail when ground conditions permit. 

Processing

This is carried out using the T R Systems software as this is fairly robust and can enable initial processing to take place or site which then enables one to establish whether a survey should be reoriented during its progress. The end result for the applicant is a bitmap image showing a grayscale plan of the resistance of the area. ArcheoSurveyor is also used as this, although slower, can give the end plot in a variety of file types and colours which can highlight additional features.