ABINGDON ARCHAEOLOGICAL GEOPHYSICS
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.
AS a general principle, a 0.5metre mobile probe spacing will read at 0.5 metres depth and a 1m mobile probe spacing will read at a 1 metre depth. For shallow sites a 0.25 metre spacing may be best as otherwise one can end up recording the geology under shallow remains. Often we do an initial survey of a small area with different mobile probe spacings to seek to ascertain the depth at which remains show best.
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.
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.
For initial surveys we usually use 30 metre by 30 metre squares with readings every metre along the lines and 1 metre intervals between the lines. This gives 900 readings per square. This is because the same grids can be used for magnetometry.
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.
Approximately 6 30x30 metre grid squares of 900 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 3,600 readings a 30 metre by 30 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.
This is carried out using TerraSurveyor as this is can enable initial processing to take place or site which then enables one to establish whether a survey area should be adjusted during its progress. The end result for the client is an image showing a grayscale or colour plan of the resistance of the area.
Resistivity profiles or pseudo-sections
These are carried out by having a line of probes across an area of interest. A current is passed betwen these at different spacings and this can, when processed using a programme called Res2dinv, produce a profile which indicates the depth of features. This is useful when estimating the resources needed for an excavation and can enable features to be investigated without large excavations being needed. This method can go down to some 5 metres or so.