The technology

Extreme Archaeology's one-woman geophysics team, Meg Watters, had the task of using the best available geophysics technology to provide information about buried features and targets for excavation. So what is geophysics technology and how does it work? Meg Watters explains.

There are three main kinds of geophysical survey available to archaeologists. Each of them serves the objective of finding out what lies under the ground without needing to dig it up. The three main techniques are:

• Resistivity
• Magnetrometry
• Ground-penetrating radar

Resistivity

Resistivity surveys measure subtle near-surface variations of electrical resistance. These measurements are taken by injecting an electrical current into the ground and recording the resulting resistance value for a particular location within a survey grid. Once a complete grid of resistance values has been collected, maps are created that can be interpreted based on the resulting image.

For the Extreme Archaeology programmes, a 'twin probe' array was used for the resistivity surveys. This has two mobile probes about 0.5 metres apart, attached to the frame carried by the surveyor, and two remote probes positioned at a certain distance away from the grid. The system can be set up utilizing a multiplexer to enable sampling at multiple depths across the site. For the Extreme Archaeology surveys the mapping was between 0.5 to 1 metre in depth.

Resistivity survey results are determined by the saturation level of the materials in the ground. So they are affected by soil compaction, geological features and other objects that may be buried beneath the surface. Features that can be identified in this way include pits, trenches, foundations, compacted or disturbed surfaces, and changes in soil type.

Magnetometry

Magnetometry, or magnetic survey, measures the variation in the magnetic fields of the earth and buried features across a site. Different soils and features in the ground can be mapped through contrasting magnetic values. Features that can be detected through this process include: ferrous materials, soil affected by human occupation (rubbish pits or middens with organic materials), fired materials such as kilns and hearths, tiles, bricks, and concentrations of ceramics. Differences in soil type or disturbance are also detected through magnetic survey, enabling the identification of ditches, pits, foundations, graves and other features.

Ground-penetrating radar

Ground-penetrating radar (GPR) works by sending short pulses of electromagnetic energy into the ground. This passes through the ground and is reflected back off different materials in different ways to provide an image of what lies beneath the ground.

Unlike resistivity or magnetometry surveys, GPR data is collected as a series of 2D vertical profiles into the earth. The 2D profiles are made up of a number of scans at a particular location that record the response to the radar's electromagnetic waves at different depths beneath the surface. The 2D profiles can then be put together to create a 3D profile of the surveyed area.

This technique was first developed about 10 years ago and has proved invaluable to archaeology. The depth to which the radar can penetrate varies depending on the geology and other conditions in the ground. Drier materials, such as sand or gravel, will permit deeper penetration than moisture-retaining materials, such as clay.