Whenever there is an electric current, there is a proportionate amount of resistance, which varies depending on the nature of the material being used as a conductor. For instance, resistance can be very low in water or copper wire but very high in stones, air, or pottery. This behavior of electric current can be used to classify the unknown material into class of a good conductor or a poor conductor depending on the level of resistivity depicted. The nature of geological features of relates to the presence of minerals and moisture. The archaeological features beneath the soil vary in moisture content in such places as pits and ditches and may be detected if electric current behavior is applied appropriately.
The figure below shows a typical schematic arrangement of electrical circuit components when being use to determine resistance of unknown material being investigated.
Figure 1 Flow of current in archaeological feature investigation
In theory, the circuit looks like the figure below;
Figure 2 Determination of resistivity of unknown resistance
Where the flowing current, I, can be determined in the reading on Ammeter and potential difference between two probes in connection with the soil can be read from the voltmeter (Witten 4). From the relationship of current, voltage, and resistance, the unknown resistance can be determined. Voltage, V=Current, I×Resistance, R
Resistance, R=Voltage, VCurrent, I (Ω)
Resistivity, ρ=RAl (Ωm)
Nature of Construction or Archaeological Sites
Prior to carrying out excavation in construction or archaeological sites, the characteristics of the soil and rock underneath must be investigated to determine their attributes. One of the techniques used in this investigation is using an ohmmeter to establish the resistivity. Resistivity method works by determining the electro-physical attributes of the ground through passing on an electric current by means of two probes, in which one is stationary, and the other one is mobile. The measurement through these probes in carried out in a certain interval, from which the nature of ground can be determined (Clark 34).
Instead of moving in the conventional straight lines, the flow of current of electric current takes place in a circular manner. As the mobile probe moves over a geological or archaeological feature, a high reading is realized. The output may be presented graphically such that the behavior of the graph would depict the nature of the earth as indicated through varying resistivity. Variations in these readings are caused by the presence of water in the soil. The presence of archaeological features is mapped in the output screen when they are lower and higher resistivity than those of the surroundings. For instance, the presence of a stone foundation may hamper electricity flow whereas such organic deposits in the midden may conduct electricity much faster than the surrounding (Gaffney and Gater 117).
Although the resistance method is useful in archaeological mapping, it is noteworthy that it is no capable of determining depth sand creating vertical profile of the under investigation. In typical set-up, conducting metal probes should be inserted on the ground to collect the reading to the surrounding material. The resistivity meter made of a square frame having four metals placed at 50cm or 100cm apart. In the data logger, the level of resistance depicted by generated graphs produces a plot on how the density and material varies on the ground. The figure below illustrates the output of an investigation into the geological nature of the place.
Figure 3 Example of resistivity output
Clark, Anthony J. Seeing Beneath the Soil. Prospecting Methods in Archaeology. London,
United Kingdom: B.T. Batsford Ltd, 1996. Print.
Gaffney, Chris; John Gater . Revealing the Buried Past: Geophysics for Archaeologists.
Stroud, United Kingdom: Tempus, 2003. Print.
Witten, Alan. Handbook of Geophysics and Archaeology. London, United Kingdom:
Equinox Publishing Ltd, 2006. Print.