Saturday, November 25, 2017

Geophysics

Geophysics is a term for a range of geophysical techniques which allow investigation of below-ground “features” without excavation, surveying large areas of ground fairly quickly which enables discovery of new sites or the pin-pointing of areas of interest on known sites, where excavation may later be targeted.

The most common techniques are:
• Magnetometry, where a machine measures the earth’s magnetic field and any effects which structures on the ground have had on it, i.e. anomalies in the readings taken will show up buried walls, pits, or trenches (the latter subsequently filled up, but with material that may give a different reading). A “map” of buried features which may not be visible at all on the ground can be plotted out.

• Resistivity, which measures the electrical resistance of the soil, buried features give different readings. Pits and ditches (now filled up, but with different soil to the “natural” surroundings) will have little resistance, but walls of stone or brick give a high resistance. Variations in resistance are again plotted onto a plan of the area showing the potential features.

• Ground-penetrating radar, used now more frequently (e.g. on Time Team), can examine features or deposits which are very deep, metres down. This is helpful particularly for archaeological features in towns.

During our fieldwork for the project, both magnetrometry and resistivity have been used to survey small areas of interest, and we hope to have more geophysics carried out. The cost is high for a relatively small area, so we need to ensure we target these resources carefully. Other preliminary fieldwork, such as fieldwalking and topographical surveying, is essential to achieve this; alternatively, a documentary source might suggest a promising site but it is unusual to be able to identify a small enough area with certainty in this way.

recording the geophysics

The geophysics results are entered into the computer and the results are shown on the screen.

geophysics underway
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