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.
SYNONYMS OR RELATED TERMS: electromagnetic prospecting CATEGORY: technique DEFINITION: A geophysical surveying method used to locate archaeological features and differences in sediment or soil textures. A pulsed induction meter or soil conductivity meter generate electromagnetic waves at the surface of the earth, penetrating it and inducing currents in conducting ore bodies, thereby generating new waves that are detected by instruments at the surface or by a receiving coil lowered into a borehole. This technique only works at a very shallow level, and no electromagnetic instrument is as accurate as the resistivity meter or a proton magnetometer.
geophysics
CATEGORY: related field DEFINITION: The study of the physical properties of the earth -- structure, composition, and development -- such as magnetism, radioactivity, vulcanism, etc. Its applications to archaeology have been to provide dating methods (geochronology) and techniques for exploration (magnetometer and resistivity survey). Some dating techniques, e.g. palaeomagnetism, are based on geophysical properties of the earth. It is a subdiscipline of both geology and physics.
induced polarization technique
CATEGORY: technique DEFINITION: A technique similar to resistivity surveying used for the location of archaeological features. It involves the measurement of transient induced polarization voltage which results from the passing of direct current through the ground via electrodes. The method requires the presence of an electrolytic solution and thus it is the greater or lesser water content of the features, in contrast to the surrounding soil, that allows their detection. A ditch would have a high induced polarization response, while a wall would have a low one.
instrument anomaly
CATEGORY: technique DEFINITION: Any change in soil resistivity to an induced electric current or variations in the magnetic characteristics of soil due to human activities, such as pit or trench digging, wall construction, and fire.
magnetic surveying
SYNONYMS OR RELATED TERMS: electromagnetic surveying CATEGORY: technique DEFINITION: A technique for the location of archaeological features adapted from techniques used in geological surveying. It is based on the fact that features with thermo-remanent magnetism, like hearths or kilns, or features with a high humus content, like pits or ditches, and iron objects, distort the earth's magnetic field from the normal. Instruments such as the proton magnetometer or the differential fluxgate gradiometer are used to measure those disturbances, and by plotting the results, a map of the features can be built. The ways in which the different types of feature distort the magnetic field vary, though they can all be picked up on the same instrument. Hematite or magnetic, present in most clays, have a small magnetic effect when unburnt, since the grains point in random directions and cancel each other out. Once heated to about 700? C or more, the grains line up, increasing the magnetic effect and causing an anomaly in the magnetic field. This thermo-remanent magnetism is also the basis for magnetic dating. The presence of modern iron as in wire fences can cause problems with this technique of location; if the area to be surveyed is clearly crossed with power lines or fenced with iron posts, a resistivity survey may be more suitable. The method of surveying used requires a grid to be measured out on the site and readings to be taken at regular intervals. The nature of the site may prevent such a grid being laid out, for instance if it is heavily wooded, and magnetic survey may not be possible on these sites. It is one of the most commonly used geophysical surveying methods.
soil conductivity meter
CATEGORY: tool DEFINITION: A geophysical instrument used in electromagnetic surveying for the detection of metal, but also for the location of archaeological features such as shallow pits, which have a different conductivity from the surrounding soil. The instrument has a transmitter coil which is fed with a continuous sinusoidal current, and a receiver coil; they are mounted at right angles to each other at opposite ends of a horizontal bar about a meter long. The instrument is designed to pick up differences in conductivity between features and the surrounding soil, i.e. the reverse of a resistivity meter. Resistivity surveying is considered more sensitive and versatile.
sondage
SYNONYMS OR RELATED TERMS: test pit CATEGORY: technique DEFINITION: A deep trench, often of restricted area, to investigate the stratigraphy of a site; an exploratory excavation made to determine whether a thorough excavation is warranted; a genteel term for test pit. A number of sondages may be dug so that the maximum of preliminary information may be gained with the minimum of effort and disturbance. In modern archaeology, this technique of pre-examination of a site is generally replaced by physical methods (e.g. magnetometer survey, resistivity survey), or if applicable, aerial photography, though a more sophisticated version of the technique of sondage digging would be classified as sampling. Sondage may later be enlarged into an area excavation to give more evidence on the cultural levels or building phases disclosed. The term is often associated with the investigation of the deep stratigraphic record of tells in the Near East.
subsurface detection
CATEGORY: technique DEFINITION: Remote sensing techniques carried out at ground level, and including bosing (or bowsing), augering, coring, Lerici periscope, resistivity detector, magnetometer, and radar techniques.
