With its important role in modern archaeology, this is the first in a series of brief tutorials into the mechanisms involved in geophysical survey.
Although coming from a radio engineering and electronics background, I am not going to bamboozle you with the boring science, or big up the process as some sort of black art.
The two main types of geophysical survey use basic electronic principles to measure data. The simpliest device is the resistivity meter which in reality is nothing more than a posh electrical tester that you might use to test a fuse, or to check a battery and can be bought for about a fiver from Maplins or Ebay. The process of resisitivity or resistance testing is based on the ability of a material to allow an electrical current to pass through it(conduct). Some materials are very conductive such as copper, hence most electrical cables are made of copper, some materials are non conductive such as rubber or plastic, hence the reason electrical cable is covered in rubber or plastic to stop you getting a shock. To test conductivity we simply place two probes from a meter across the material, send a low electrical current through it, and its electrical resistance value will be shown on the meter.
In archaeology soil and clay all conduct electrical current, especially if wet. On a site without any known archaeology and with a undisturbed geology, placing your probes in the ground at a constant distance from each other across the area should give a general background level of conductivity.
Introduce archaeology into the equation and we return to what is conductive and what is not. A ditch is a good conductor because it is likely to be wetter than the surrounding natural, and therefore will give us a low resistance reading. A wall made up of stone or brick is likely to be drier than the surrounding natural, and will therefore be a poor conductor of electricity. and give us a high resistance reading. If however we have had heavy rain, the ground is likely to be saturated even in and around a wall, minimising the recordable difference that the instrument can detect. Resistivity for best effect should be carried out after a period of dry weather, where buried ditches are likely to be the only areas of dampness in the soil matrix, and buried stone walls and any fill in amongst the stones would be drier than the natural. What is the optimum moment to carry out a survey, well that’s the $64,000 question.
The rest of the process is basic fieldwork. Layout a grid of a preferred size, subdivide each grid square into 1m strips, and traverse the strips taking a reading every meter along each strip, which in a 30m x 30 m grid square would result in 900 readings taken in the square.
Those 900 readings are entered into a piece of software which in the most simpliest of terms gives several colour or greyscale values for ranges from the lowest readings through the background readings to the highest readings. These individual colours or greyscale values when the 900 readings are laid out in order they were takenwill highlight anomalies. For instance if a red colour is given to the highest resistance readings, if there is a wall present we might expect to get a red line. If a yellow colour is given to the lowest resistance readings, if there is a ditch present we might expect to get a yellow line.
Of course these things are subject to all sorts of caveats, geophysical survey can highlight archaeology in a given geology to an astonishing degree, in a different geology with the same archaeology it can be as useful as a chocolate poker.
The apparent absence of archaeology in a geophysical survey, does not necessarily mean there is an absence of archaeology on the site. Geophysical survey should be used as part of a fieldwork investigation, it should not be the only fieldwork investigation, and where it is at all practicable, it should be used in conjunction with other geophysical techniques.
Here endeth the first lesson.