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Geophysical survey

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Geophysical survey is a form of archaeological survey, aimed at confirming an archaeological site and giving guidance to later excavation where surface signs are obscure. It is rarely used to find sites as it is time-consuming and narrow compared to other survey methods (surface survey or aerial survey).

It makes use of a number of instrument-based techniques, of which two predominate - resistivity meters and magnetometers.

A resistivity meter passes an electrical current through the ground between two electrodes. A moist soil will offer comparatively low resistance, while drier and denser matter will give a higher resistance reading. In archaeology two, four, or more probes are fitted to a frame, with a waist-high handle for use. The display meter is usually fixed to the handle. Readings are usually taken along pre-surveyed lines at regular intervals, hopefully cutting across any features of interest. This method works best with an flat and well-drained soil with artifacts at a similar depth, natural variations can easily give misleading results. It is best suited to finding strong linear sources like walls or roads by measuring across a suspected placement.

Resistivity meters without penetrating probes are also used. Less sensitive they are faster to employ and give good results in drier ground.

Magnetometer detect variations in magnetic fields, so archaeologists can undertake a magnetic survey. The alignment of naturally occurring magnetic soil particles can be altered by a number of human activities. Simple soil-moving to form ditches or pits can be detected; solid constructions will often contain fewer magnetic material than surround soil; and high heat can realign magnetic particles - indicating the presence of furnaces, kilns or similar.

There are a number of different instruments used, none of which require penetration of the soil, there are three common passive instruments. Proton magnetometers give absolute readings of magnetic strength at the point of use, these are very sensitive instruments and are used to examine grids of points. Proton gradiometers have two detector bottles on a pole, and report the difference in field strength between the low and the high bottle, theses instruments are less sensitive but quick to use and give an 'instant' response. Fluxgate gradiometers give constant readings as they are moved - this allows for rapid examination of an area but the instrument is insensitive and readings can be skewed by soil variations, nearby modern wires and atmospheric disturbances.

Other geophysical instruments include active magnetomers, metal detectors or a pulse induction meter, radar (only good at very dry sites), and sonar. Physical probing of the soil, by augering or 'malleting', is also possible. Dowsing has also been tried.

Geophysical tools were first used in the 1940s, magnetometers were employed from the late 1950s. The development of continuous read instruments and automatic data recording in the 1960s was a significant boost. Although it was the introduction of computers and high-quality printers from the 1980s that made the most of the techniques - moving analysis from lines on graph paper to full colour displays produced by sophisticated filtering programs.

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