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resistivity
CATEGORY: geology; technique
DEFINITION: The resistance of soil or buried features to the passage of an electrical current, measured during geophysical surveying. Different materials offer varying resistance to electrical currents, depending on the amount of water present. Resistivity is a method used to identify underlying deposits without excavation.
resistivity meter
SYNONYMS OR RELATED TERMS: resistivity detector
CATEGORY: tool
DEFINITION: A geophysical instrument used to measure the electrical resistivity of the earth to identify buried features and structures. Since the resistivity of the soil changes with humidity, humus content, etc., the machine can detect pits, ditches, roads, floors, etc. This is generally done through an array of four electrodes, pushed into the ground surface. Despite their name, resistivity meters do not actually measure resistivity, but ground resistance. Resistivity is this resistance, standardized for the distance between the electrodes in the ground. The instrument consists of a source of electricity (a handle-operated dynamo in the megger earth tester, batteries in the tellohm, a transistor oscillator in the Martin-Clark meter) and a meter to record the results. All systems employ four steel probes connected by cable to the meter, two to carry the activating current, two to pick up the current passing through the ground. Also, the resistance between two roving probes is now compared with that between two distant static ones. Different spacing between the probes is employed in different conditions; where the probes are spaced equally, as in the Wenner configuration, features up to a depth equal to the probe-separation can be detected. Anomalous readings may indicate the presence of archaeological material.
resistivity profiling
CATEGORY: technique
DEFINITION: Measurement of earth resistance at increasing depths across a site, by widening the probe spacings and thus building up a vertical 'pseudosection'
resistivity surveying
SYNONYMS OR RELATED TERMS: resistivity survey
CATEGORY: technique
DEFINITION: A geosurvey survey technique that measures the electrical resistance of the ground for the location of buried features and structures. Any electrical exploration method in which current is introduced in the ground by two contact electrodes and potential differences are measured between two or more other electrodes. It relies on the principle that different deposits offer different resistance to the passage of an electric current depending largely on the amount of water present. A damp pit or ditch fill will offer less resistance, stone wall foundations more, than the surrounding soil. It is one of the most commonly used and least expensive geophysical surveying methods. Readings are taken in a grid-pattern of points all over a suspected site. Variation of resistance through a site is caused mainly by differences in the amount of water contained in pore spaces of deposits and structures. The outline of features may be seen if the readings are plotted as a plan. Although the technique is generally known as 'resistivity surveying', most archaeological surveys use only the ground resistance (in ohms). It compares well with magnetic surveying, as the instruments are simple and cheap and also because modern features such as power cables, iron scrap, and standing buildings do not affect the readings.
soil resistivity
SYNONYMS OR RELATED TERMS: electrical resistivity; soil resistivity surveying
CATEGORY: technique
DEFINITION: A remote sensing technique that monitors the degree of electrical resistance in soils -- which often depends on moisture content -- near the surface. Buried features are usually detected by a differential retention of groundwater.

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