Main Archaeological Techniques

The Three Main Archaeological Techniques
By Sharon White

Geological Survey involves the use of a variety of methods and techniques to produce a detailed map of what is below the surface of the area being surveyed. The primary techniques are Magnetometry and Resistivity. Geophysical survey techniques are the most commonly used method of site identification when a suspected archaeological site lies beneath the land surface. They are non-intrusive and are subsequently invaluable when excavation is not possible. Before conducting either a Geological Survey, or a Ground Survey, a grid needs to be created for the area under investigation. Control points are established which allow the site to be synchronized with the National Grid. First of all, a Baseline is established in the prevailing direction of the site. The ends of this are co-ordinated by Triangulation. This fixes the baseline to the surround area, usually by using tape, which is then broken up into suitable units marked by stakes placed in the ground. Once this has been completed points perpendicular to the baseline are set up, again using tapes. In large areas, Pythagorean theory is implemented (A2 + B2 = C2) but when the area being surveyed is smaller, the Pythagorean triangle system is best. All the corners angles of the grid must be at 90° (a square) so the square root of the two sides of the triangle can be calculated, a measurement necessary to determine the hypotenuse of the right angle triangle. The measurements are then used, once the tapes are situated at the two ends of the baseline (often referred to as A and B) so each square can accurately be constructed.

The resistivity of the area being surveyed concerns the measurement of electrical resistance encountered below the surface. The twin electrode is the most useful instrument for testing resistivity. It is a frame containing a meter, batteries (to provide power) and two probes, usually half a metre apart, but this distance can be altered when a deeper survey is required. Two remote probes and a connecting cable are also necessary; this links the probes when placed at least thirty times the distance between the static probes. For example, if the primary, or static probes on the frame were one metre apart (in the instance of a deep survey) the remote probes would need to be placed a minimum of thirty meters from the area under investigation. It is this that makes the process more complicated than other methods, such as magnetometry. The most accurate results can be obtained in the summer months, in the case of temperate climates, as the soil moisture content is lower then, so the equipment can detect differences in resistance more easily.

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