Magnetometry

Magnetometry is a non-invasive geophysical survey technique used by archaeologists that maps variations in the Earth's magnetic field to detect buried archaeological features like ditches, pits, and kilns. It works by sensing different magnetic properties between archaeological remains and the surrounding soil, often caused by heating or iron-rich materials. This allows archaeologists to "see" into the ground without excavation and locate potential areas of human activity over large areas.

This type of survey can cover large areas of land quickly, making it efficient for initial site assessment.

Methodology

The magnetometer (we use a Barrington gradiometer) continuously records the Earth's magnetic field as it is moved across a site in a systematic grid.

We will usually mark out a system of 30m x 30m grids (using highly accurate GPS equipment). The person operating the magnetometer walks backwards and forwards across each grid, (aided by guide lines or ropes laid on the ground and moved by two other people) allowing the machine to take a series of readings of the fluctuations in the magnetic field at regular intervals across each grid.

Data Mapping Results

The equipment identifies areas of different magnetic properties that create "anomalies" or variations from the normal magnetic field. This data can be mapped using specialist software

Identifies buried features:

Burnt features: Kilns, hearths, and furnaces become strongly magnetic after being heated and are easily detected.
Ditches and pits: When filled with more magnetic topsoil from surrounding areas, these features show up as anomalies.
Iron objects: Ferrous objects, such as iron tools or artefacts, will also be detected.

Best Conditions

Magnetometry is most effective under specific conditions:

No crop cover: It is easiest to survey fields where there is no crop cover (normally after harvesting or over grassland) although that's not always possible and the Group has successfully surveyed areas with waist high vegetation.
Weather: Our surveys are weather dependent - the magnetometer is a sensitive piece of electronic equipment, so we do not operate in the rain.
Non-archaeological contamination: Modern iron objects like fencing, refuse, underground cables, pipelines and overhead lines can all cause false readings and must be avoided or filtered out.

Equipment

We will provide all equipment, including the magnetometer, adjustable carrying straps, and guide lines or ropes. However if you do wish to take part in a magnetometry survey you have to be careful what you wear. You must not wear anything that may contain metal! This clothing advice was prepared by John Rainer one of our Magnetometer Project Leads -

Clothing Advice for New Members

Advice to new members:
Line movement requires you to minimise any metal content on your person but if you can't eliminate it, all you have to do is keep 5m clear of the meter user.

Meter use requires you to be free of all magnetisable metal content - zips, jean studs, boot eyelets, jewellery, sleeper rings, studs, the lot - no matter how small. The meter will pick up the tiny stainless steel screws in otherwise non-magnetic titanium glasses and other metal work and this affects the results.

Although gold and silver jewellery is in theory non-magnetic, standing in a field is no place to discover your jewellery is not quite what you thought it was when the meter protests. Also, most brass and copper clothing fittings are magnetisable as they are actually plated steel.

The best clothing is:
Jogging/tracksuit bottoms or similar - make sure there are no metal belt eyelets. Layered t-shirts, sweatshirts and jumpers.

The problem items are:
1. Shoes. You need stout footwear with ankle support. No wellies! It must not have any metal eyelets. I have v. large feet and have to buy expensive boots and drill out the ankle eyelets but there are cheaper options and other members can advise. Some boots have steel plates embedded within the sole. Whatever you choose, we can scan you on site first to make sure your footware is ok.
2. Bras. Most bras contain metal wiring or clips, its recommended you wear an unwired sports bra with a plastic clip.

3. Coats. Virtually all coats have steel components - even those with plastic zips. Duffel coats with horn or wooden toggles are one of the few types that are ok, which is why you see John Gator wearing one on Time Team.
Otherwise, layer up.

Don't worry if its your first time, we'll start you on the ropes until you have a better idea of what's going on. It's a lot easier than it sounds and all training will be given on the day by the project lead. We can also double-check you're metal free before we start with a simple sweep of the machine!

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