Detailed survey using Magnetometer / Gradiometer

Magnetic Fields and Gradients

It is well known that the Earth has a large magnetic field which can be measured by a magnetometer. In the U.K. this field has a value in the order of 48,000 nano Tesla (nT)

Geological and buried manmade features or objects can cause local variations in the earths magnetic field which can also be measured with a magnetometer. However, if two readings are taken at each station, one vertically above the other, a magnetic gradient can be established thus removing the large background effect of the Earth fields. Using sensitive instruments such as a Geoscan FM36 or a Scintrex Envimag, magnetic variations as small as 0.1nT can be detected. With new instruments such as the Scintrex Smartmag using caesium vapour sensors, gradiometer sensitivity can be increased to 0.01nT.

The Scintrex Smartmag gradiometer in use	A Fluxgate Gradiometer survey over an Iron Age enclosure at Llanbethery. The prominent features detected are cut features such as ditches and pits. The enclosure appears to have been reinforced during the Roman period with a larger ditch. In addition we can also see strong positive / negative spikes that are usually associated with ferrous objects.	An operator using a Bartington 601/2 collecting two lines of data simultaneously

Buried Objects

What causes these local variations in the magnetic field? Objects that have been heated have their magnetic properties markedly changed. These are called 'thermoremanent' effects. These effects can be very large and strong such as an igneous dyke, through to a horse shoe nail in the top soil causing a 'ferrous' spike. Features with thermoremanent magnetisation can be easily found with gradiometers so that objects such as brick walls, foundations, steel or clay pipes, hearths, kilns and ferrous artifacts associated with archaeology will stand out from the background magneticlevels.

'Cut' Features

In addition to thermoremanent effects more subtle changes in magnetisation can be detected. Top soil is generally more magnetic than its parent subsoil, particularly in high magnetically susceptible soils. This is because of the incorporation of decomposed or burnt particles which are significantly more magnetic. If a ditch has been dug in the past and subsequently silted up with humic soil a weak positive linear magnetic anomaly will be formed. Similarly, where an old embankment has been ploughed out leaving a zone of thinner topsoil, a linear negative feature can be detected.

Example of an Iron Age site showing a series of circular and sub-circular enclosures which subsequent excavation showed to contain huts. The site had obviously been in use over a period of time due to the number of overlain enclosures.