Preparation
4. Site visits and geophysics
Archaeology excavation is a destructive process involving digging and disturbing the ground. Sometimes we do have to site a test pit at random but wherever possible we prefer to choose the placing of a test pit in a targeted way. In an open area it may be that we choose the site from evidence on an aerial photo or a LIDAR scan but it is better where possible to do a geophysics scan first.
Before doing geophysics scans we first need to make site visits to decide whether scans are feasible, which technique to use and what obstructions there are.
Geophysics allows us to use one of two techniques to look below the ground in a non-destructive way to reveal structures - sometimes buried walls, foundations or wells and sometimes ditches that have been filled in but still retain more moisture than the surrounding area.
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The scanning equipment we own lets us use two techniques: resistivity and magnetometry.
This is a resistivity machine, costing about £6,000. We scan an area 20 x 20 metres and the machine measures how well electricity flows through the moisture in the soil between some fixed probes and the spiked probes on the machine. If there is something buried under the ground, such as old wall foundations, it obstructs the flow of electricity (i.e. higher resistivity). However, if there was once a ditch that has since been filled in, this tends to retain more moisture than the surrounding area allowing electricity to flow through the ground more easily (i.e. lower resistivity). High resistivity shows as white and low resistivity as black on our scan plots (see below).
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This is a magnetometer or gradiometer. It costs about £11,000. It is not a metal detector. It measures the local magnetic field of the soil particles directly underneath the machine. By doing so it can tell whether the ground has been disturbed.
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Resistivity works best in the spring or autumn when the ground is not too dry and on the other hand not waterlogged. It is not affected by interference from ferrous objects, mobile phones, etc in the way that a magnetometer is. However, it is very hard physical work and it takes about 45 minutes for an experienced and fit team to scan a 20 x 20 metre square.
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Magnetometry works best in open areas where there is no ferrous contamination - we used it extensively this spring on the National Trust estate at Hinton Ampner. Its advantages are that it is much faster and less arduous. An experienced team can scan a 20 x 20 metre square in 6 minutes.
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Both techniques give us a monochrome picture of what is below the ground, so we tend to be looking for shapes and for patches that are much lighter than the surrounding area. Here is a scan result from a garden in Petersfield.
At first glance it looks like a random pattern of blobs. However, the first thing we notice is a vertical strip running through the central part of the scan. (This highlights the importance of our contacting gas and electricity utility companies before we dig, because the maps they sent us showed that this is a mains cable running through the garden to supply some newly-built houses beyond!).