In the last two weeks Archaeological Services Durham University have been carrying out geophysical surveys in the few patches of previously un-surveyed ground around Palace Green. In 2009, Historic Scotland commissioned a survey of the grassed area of Palace Green and the tarmac area to the south, but found no evidence for any mass graves. This recent series of surveys has focused on the Master’s and Fellows’ Gardens within the Castle and part of the graveyard north of the Cathedral.
Geophysical survey enables the relatively rapid and non-invasive identification of features below ground that may be of archaeological significance, and it can involve a suite of complementary techniques. Some techniques are more suitable than others in particular situations, depending on site-specific factors including the nature of likely targets; depth of likely targets; ground conditions; proximity of buildings, fences or services and the local geology. In this instance, it was anticipated that cut features such as ditches, pits and possibly mass graves might be present on the site, as well as other types of feature such as wall foundations and trackways. Three complementary geophysical survey techniques were used: geomagnetic, earth electrical resistance and ground-penetrating radar.
This technique involves the use of hand-held piece of equipment called a Fluxgate Gradiometer, which detects and records anomalies in the vertical component of the Earth’s magnetic field caused by variations in soil magnetic susceptibility or permanent magnetisation; such anomalies can reflect archaeological features. While this technique is very effective at detecting certain types of archaeological features it is not without its difficulties. One problem with using this piece of equipment is that it is affected by metal objects; power cables, underground pipes and even zips and buckles on clothing can seriously affect the readings produced.
Electrical Resistivity Survey
This technique measures the resistance to the flow of an electrical current as it passes through the ground. Since resistance is linked to moisture content and porosity, stone features will give relatively high resistance values while soil-filled features, which typically retain more moisture, will provide relatively low resistance values. This type of survey is very appropriate to the type of work being done on Palace Green, given the possible presence of large soil-filled features such as mass graves, as well as wall-footings, hard surfaces and tracks.
Ground Penetrating Radar (GPR)
The final survey technique we used is Ground Penetrating Radar. This technique generates a short high-frequency radar pulse which is transmitted into the ground via an antenna; the energy pulse travels into the ground, is reflected by buried interfaces and the return signal is received by a second antenna. The buried interfaces are associated with different sub-surface materials, which can include features of archaeological or historic interest. The time that elapses between the transmission and return of radar pulses to the surface can be used to estimate the depth of reflectors. Because of this, as well as conducting traditional 2D area surveys, GPR also has a depth component and so can be used to create pseudo 3D models of sub-surface features.
Carrying out the surveys and what happens next
Each of the areas being surveyed were carefully set-out using an extremely accurate GPS so that any newly discovered features can be exactly located for future reference. Each of the different techniques was then applied in turn to the ground. Data was collected by systematically walking up and down rows of parallel lines marked on the ground. When all the data is downloaded into a computer we can create maps of different types of geophysical anomalies, some of which will hopefully reflect archaeological features beneath the surface. The data created from these techniques is now being analysed to determine if there are any features that may represent mass graves or other features of historic interest.
Author: Beth Upex
Beth Upex is an archaeological science technician in the Archaeology Department at Durham. She works mainly in the environmental and human osteology laboratories but also in the DNA and isotope facilities. Her background is in archaeozoology with a particular interest in animal palaeopathology and the application of scientific techniques to solving archaeological questions. Beth is co-ordinating the scientific analysis relating to the Scottish soldiers project.