Geophysical probing on archaeological sites is a rapid non-invasive, non-destructive data gathering method that does not disturb the topography or the archaeological record of the site.
Investigating Under Buildings
It is especially useful on locations where sites cannot be archaeologically excavated. Some examples of difficult or impossible to excavate sites would include protected or historic buildings such as churches and ancient pyramids, public parks and areas that now lie underneath modern residential or commercial developments.
In some circumstances the nature of the overbuild may totally restrict any archaeological research to only geophysical exploration.
The Beginnings of Geophysical Archaeology
Prior to WW2 some electronic methods had been used in the exploration of oil and other crucial mineral deposits. Because oil revenue dividends were lucrative, companies were happy to release large budgets for geophysical research and development.
World War Two saw an explosion in geophysical sophistication and high quality, land-based, ground-penetrating instruments were developed. The rapid wartime evolution of earth probing equipment washed over into archaeology and was further pushed by the modern paradigm of less interference to the topography, both natural and artificial.
In contrast to the multi-national petroleum giants, archaeological field budgets are extremely modest and cannot afford the high-tech equipment used by oil and mineral exploration companies. However, fortunately for archaeologists, unlike oil, the archaeological record, containing ancient artefacts, does not lie many miles under the earths crust but usually only a few metres underground.
Therefore, less expensive, smaller scale devices can be employed that use the same technology.
Establishing What’s Under the Ground
Archaeologists can be assisted ahead of the main intrusive excavation by establishing digging priorities using geophysical methods. Ground penetrating systems can reveal high yield, artefact-laden subsoil verses empty, artefact-absent, barren ground and save valuable time and money.
Large archaeological sites will often take many decades of careful excavation to complete the data gathering. Geophysical survey permits even large sites to be rapidly explored from above in a matter of days with the results being available for immediate analysis. The evaluation of a geophysical survey may determine whether an expedition will be raised or a current one abandoned.
Methods of Geophysical Survey
Ground Penetrating Radar
GPR was invented, primarily for military purposes, to locate buried mines and underground tunnelling. Archaeologists have adapted this technology for use in above ground and borehole investigations. In borehole surveys radar probes are lowered into previously drilled holes on an archaeological site allowing geophysical data collection at numerous depths. Cavities or caves that are found can be further explored by inserting a video camera down the hole.
Most often, GPR’s are mounted on small, wheeled carts that are hand pulled across the site. The area needs to permit free and easy movement being clear of large rocks, trees, or buildings.
The system displays echoed radar pulses and a trained operator can interpret these as to the target of interest. Some limitations of GPR are the unwanted reflective echoes known as clutter, limited penetration depths, and attenuation caused by clay soils or wet soils with high salt concentrations.
The wide variety of metal detection devices available and their low cost makes them readily accessible to even the enthusiastic amateur archaeologist.
Metal detectors are battery powered, active instruments that radiate a low frequency AC signal into the ground via a transmitting coil. The larger the coil the deeper the penetration of the signal. Any metal object in the signal path will cause an induced current to re-radiate a weaker signal back to the surface and being non-aligned to the initial frequency is easily read by the receiving coil.
Metal detectors have there own limitations. Small metal objects, such as coins, can only be detected to a depth of a few inches. Large objects, such as a steel helmet, could perhaps be detected up to a few metres but no more than four metres. Power lines, high-density-metal surface objects, iron-rich rock, and metallic litter can all cause noise for metal detectors.
Many archaeological sites are unidentified, complex, or too important to just roll in with a digging team and begin to blindly excavate. Some sites will be excavated for many digging seasons and geophysical methods can assist the archaeologist to prioritise the field attack based on the results of a thorough geophysical survey.