MAGNETOMETRY
A Bartington Grad 601/2 fluxgate gradiometer in use.
Equipment and Method
We use our own Bartington 601/2 equipment which has two 1 metre long gradiometer tubes spaced 1 metre apart. These each contain two vertical axis fluxgate magnetometers, one at each end of the tube. This enables the top detectors to reject the large scale atmospheric magnetism and isolate the very slight readings which are caused by archaeological features. We chose this equipment as it does not have the dead zones of caesium based magnetometers and can collect data at a faster rate and thus enables more detailed surveys than overhauser based magnetometers.
The survey method normally used is a 30 metre by 30 metre grid and these are surveyed with 1 metre intervals between the lines surveyed and either 4 or 8 readings are taken per metre along the lines. The readings are taken in zig zag mode. Bushes and other obstructions to the survey grids add significantly to the time taken and cost.
Surveys can also be carried out with 0.5 metre spacing between the lines but this takes approximately twice as long and the data is often not much better than with the larger line spacing interval.
Coverage
Approximately 10-15 30 by 30 metre grids can be carried out in a day using two or more people depending on the layout and number of obstructions in the survey area.
Processing
This is done using ArcheoSurveyor as some initial processing can be carried out on site and thus influence the direction of further work. This programme is good at removing the half metre stagger which almost everyone gets when using the Bartington logger system.
Ground Conditions
Magnetometer surveys are dependent upon having soils which have magnetic properties and also having features within them which have different magnetic properties to the surrounding soils.
It is difficult to assess which soils are most likely to show results. Some boulder clays and sandstones are not very good but the best way of finding out may initially be to carry out a small survey over a known site and to compare the magnetometer data with that from, say, air photographs.
The topoil often has slight magnetic properties and the strength of readings declines with distance from the anomaly which causes them. These factors combine to give stripy data in recently ploughed fields and in potato fields with their ridges. These field conditions are therefore best avoided. Similarly areas of nettles can lead to the magnetometers being raised over the nettles and this adversely affects the quality of the data.
Sites with metal fences and iron pipelines and other ferrous rubbish can have so much interference as to obscure the archaeology in the magnetometery results - see the Requirements page for more detail. It has also been found that previously flooded areas such as the Fenlands can have naturally occurring iron minerals which can obscure the archaeology.
Expectations
As magnetometry detects very slight variations to the Earths magnetic field, this depends upon how much the ground has been magnetically enhanced and how deep the features are. For most features a depth of approx 1 metre is the best that can reasonably be expected. However deeper features which are very magnetic, such as pottery kilns, can show up at 2 or even 3 metres deep if the ground conditions are right. This method is generally good at finding ditches, kilns and burnt areas but is less good at finding stone buildings, postholes and shallow spreads. Its advantage is that it is quite rapid and is less influenced by the moisture content of the soil and its salinity than resistivity and ground penetrating radar.