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 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.

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 like manhole covers, reinforced concrete, mine shaft caps (see above), brick walls and steel or clay pipes can be detected in civil engineering applications and hearths, kilns and ferrous artefacts in archaeology.

'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 organic material 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.