Analytical Methods:Field Mag Sus

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Magnetic susceptibility: field analysis
The magnetic susceptibility of a material is a measure of its ability to become magnetised by an external magnetic field and is a function of the mixture of minerals present and their individual magnetic characteristics.

Questions
The magnetic susceptibility of a soil reflects the presence of magnetic iron-oxide minerals such as maghaematite; just because a soil contains a lot of iron does not mean that it will have high magnetic susceptibility. Magnetic forms of iron can be formed by burning and microbial activity such as occurs in top soils and some anaerobic deposits. Magnetic iron compounds can also be found in igneous and metamorphic rocks.

The relationship between iron and burning means that magnetic susceptibility is often used for:
 * Site prospection, to identify areas of archaeological potential prior to excavation.
 * Identifying hearth areas and the presence of burning residues in deposits
 * Explaining whether areas of reddening are due to burning or other natural processes such as gleying (waterlogging).

The relationship between soil formation and magnetic susceptibility means that it can also be used to:
 * Identify buried soils in depositional sequences.
 * Identify redeposited soil materials in colluvial, alluvial or other depositional sequences.

Sampling
If you have a suitable field probe measurements can be taken directly from the section as long as the surface has been thoroughly cleaned and there are no metal objects or electric cables to interfere with the measurement. Magnetic susceptibility can be highly variable so make multiple measurements on each context and throughout the profile / section, and also measure the magnetic susceptibility of local ‘natural’ parent materials.

If a laboratory sensor is available for use in a site hut etc. samples will need to be taken from the section. Each sample should be at least 10 cm3 and if possible plastic, rather than steel, implements should be used for sampling. Samples from a representative part of the context, being careful not to bulk together different deposits, because of high variability replicate samples are a good idea. Samples should be taken from the natural and from throughout the profile, not just from the context of interest.

Analysis
Thoroughly clean and cut back the section to give a flat surface. If measurement are being made of the ground surface remove any surface litter and cut away vegetation where possible.

Ensure that the equipment is fully charged before use (allow to charge overnight and periodically fully discharge the battery and then recharge to preserve the battery life). Allow the equipment to warm up before use and measure the magnetic susceptibility (at least 1m from any obstacles) of the air. The sensor should be placed against the section (sensitivity declines rapidly with distance from the object) and a measurement taken. Re-measure the air then deduct the average of the air measurements from the section reading.

Using a dual frequency laboratory sensor either on-site or during post-excavation analysis can provide additional information about the nature, and hence the source, of the magnetic iron compounds.

Data and interpretation
The magnetic susceptibility of a sample depends on the mixture of minerals it contains and their individual magnetic behaviour. The different types of magnetic behaviour are:

Soils contaminated with iron from sampling tools etc. will have a very high magnetic susceptibility signature becuase of the presence of ferromagnetic minerals.

Igneous rocks contain ferrimagnetic minerals such as magnetite with a high magnetic susceptibility; as a result soils in igneous areas may have high and sometimes noisy background magnetic susceptibility levels, which can make it hard to identify subtle changes due to soil formation for example. Burning soil can result in the formation of maghaematite and other minerals (Le Borgne, 1960), which also give high magnetic susceptibility measurements. You may also get high values associated with the formation of iron sulphide compounds such as greigite (Fe3S4) in waterlogged anaerobic conditions.

Less pronounced enhancements in magnetic susceptibility are also associated with topsoils thought to result form the effects of bacterial and redox processes. Antiferromagnetic minerals such as haematite and goethite are also common in soils.

Paramagnetic and Diamagnetic minerals and materials are also common in soils and have a very low magnetic susceptibility; these minerals may dilute the effects of more magnetically susceptible materials in the sample.

Other things to consider when interpreting field measurements are:
 * The possibility of interference from iron-based materials,
 * The contact between the field probe and the soil; this can affect the volume of soil analysed and hence the reading.
 * The size of the area being analysed compared to the size of the feature or layer of interest.

Related Techniques

 * Laboratory magnetic susceptibility
 * Magnetic remanence