The main geophysical methods used for archaeological survey are briefly described in this section. All involve the systematic collection of measurements at closely spaced intervals across the area under investigation.
This is followed by the visual display of the combined data-sets that are then analysed and interpreted for archaeological information. Geophysical methods rely on a contrast in some physical property to exist between the buried archaeological remains and the surrounding soil.
Magnetometer survey offers the most rapid ground coverage and records a variety of anomalies caused by past human activity. It is therefore the most widely used geophysical survey technique for archaeological investigation
The data is most commonly collected by a fluxgate gradiometer. This instrument records subtle changes in the local magnetic field continuously across the site and allows large areas to be covered in a relatively short time.
Anomalies are produced when buried features exhibit different magnetic properties to the surrounding soil. Archaeological remains such as kilns, hearths and furnaces can be detected due to the alteration of iron minerals by burning into strongly magnetic forms.
Ditches and pits can be located when they have silted up with more magnetic topsoil from adjacent settlements. Magnetometers can detect buried wall footings, but only when the magnetic properties of the stone contrast with the surrounding soil.
Caesium or alkali-vapour magnetometers are more sensitive instruments than standard fluxgate gradiometers. As a result they can be used to detect weaker anomalies from more subtle remains or those buried at a greater depth.
Earth resistance techniques
Earth resistance survey was probably the first geophysical technique to be used in archaeology. It is still widely employed today, primarily for the investigation of sites suspected to contain buried masonry buildings.
Earth resistance measurements are influenced by the local soil moisture content over a site. A small electric current is injected through electrodes pressed into the ground which is conducted through mineral ions dissolved in the water held in the soil.
Generally two types of archaeological features are detected by this method; ditches, filled with moist water- retentive soil (low resistance), and more solid features, like buried foundations or walls, where little or no water is retained (high resistance).
Due to the reliance on soil moisture, earth resistance measurements demonstrate a distinct seasonal variability. In general the most successful results are obtained during the spring and autumn months between arid summer conditions and the water-logged depths of winter.
Crop and parch marks visible through aerial photography are formed by the same influence of soil moisture variation on the vegetation covering the site.
Recently a number of systems have been developed to increase the speed of earth resistance survey. Arrays of spiked metal wheels act as electrodes across a site. Nevertheless, it remains difficult for earth-resistance measurements to currently match the speed and coverage that magnetometer survey offers.
Ground Penetrating Radar (GPR)
Ground Penetrating Radar (GPR) equipment sends a short pulse of radio frequency energy into the ground. It then records the time and amplitude of reflections from any buried remains. Reflections from deeper remains take longer to reach the surface and allow the depth of burial to be estimated. GPR systems can be operated over a range of different surfaces, seeing through tarmac roads and concrete to reveal the underlying archaeology. Within standing buildings, GPR may be the only practical geophysical technique to apply. Vehicle towed GPR systems operate with an array of antenna elements to produce very detailed measurements over large areas.
Closely spaced lines of GPR data can be used to produce a series of highly detailed time slices. These can show plan views of the archaeology at successive depths from the surface. This data may be further processed to produce 3D representations of the subsurface and animated sequences to zoom the viewer into the ground!