For many of the archaeological sites and monuments on land within the CHERISH project, what you see on the surface in the forms of walls and earthwork structures is only half the story. For many monuments there are buried features, including wall foundations and ditches, beneath the surface of the soil which represent the full extent of an archaeological site. To discover these features, we could excavate the complete monument, but this would be costly and, as excavation is a destructive process, we want to make sure that it is only used in specific circumstances.
Like a doctor who can use a range of technologies, such as X-Rays, MRI scanners or ultrasound, to peer inside us to diagnose our ailments, archaeologists can use a range of geophysical survey techniques to look beneath the surface of the soil and identify buried archaeology. Geophysical survey is a general term for a range of non-invasive or non-destructive techniques that can be used to detect features buried beneath the surface without digging.
A range of approaches exists, using specialist equipment to measure variations in the physical properties below the surface and identify archaeological features. Depending on the type of survey being conducted, structural building elements, traces of human activity or artefacts can be identified as they return significant anomalies in the data from the background values of the surrounding soil. Archaeologists can use a series of complementary techniques each enabling the measurement of a different property of the soil and potentially identify archaeological features.
Electrical resistance is an active geophysical technique (electrical current is passed through the soil) and relies on the principle that subsurface archaeological features display different electrical properties to those of the surrounding soil. The technique measures the electrical resistance presented by buried features to the flow of an electrical current. Areas which can potentially contain more moisture, such as pits, ditches and organic deposits tend to display a lower electrical resistance as water is a good electrical conductor. In contrast, building foundations and walls, composed of more compacted less porous and permeable materials, normally have a lower moisture content than the surrounding soils and tend to show a higher electrical resistance. These contrasts, where they exist, enable subsurface archaeological features to be detected and mapped. Depending upon the electrical resistance method used, archaeology to a depth of around 0.5–1.0m can be identified.
Electrical resistance surveys are normally carried out using a series of four stainless steel probes which are inserted into the ground. Two of these probes (remote probe) are mounted on a rigid frame 50cm apart, and are systematically walked across the site using similar grids as described above and placed into contact with the ground, recording the local electrical resistance at that spot. A second set of steel probes (fixed) is connected to the frame through a trailing cable and provides the reference or background electrical resistance of the site. This method is somewhat slower than magnetometry but can complement this approach especially in the identification of buried masonry and stone features.
Magnetometry is a passive geophysical technique (i.e. no energy is passed through the soil) and relies on the fact that some minerals, either naturally occurring or produced as a result of human activity, may have magnetic properties. This difference in magnetism can be caused by features such as silted up ditches, burnt areas or hearths, in-filled pits or post holes, walls and other buried remains. The magnetism of these buried archaeological features and objects is very small compared to the natural background magnetic field of the Earth and cause small localised distortions, or anomalies, in the Earth’s magnetic field and can be detected by a sensitive devices known as magnetic gradiometers.
Magnetic gradiometers come in different shapes and sizes, but most consist of a tube detector which measures the variation in magnetic signature below the soil from that of the natural background variation caused by the Earth. These devices are walked systematically across the area you wish to investigate, providing continuously logged measurements of any subsurface remains, with the hope of discovering buried archaeological features. As magnetometers are very sensitive to ferrous metals, operators must ensure that they have no metal on their bodies or clothing which could affect the results
Within the project we are using two different magnetometer devices to find buried archaeological features. The first device which is the standard for many archaeological geophysical surveys is the Bartington magnetic gradiometer. This looks like a pair of rugby posts and consists of two sensors one metre apart which is walked by a surveyor across the archaeological site within systematically laid out grids, often 20m square. Our new device, the Sensys MXPDA, consist of five sensors which are mounted either 50cm or 25cm apart on a cart system with built in GNSS. The Sensys cart system allows for much faster data collection and for greater areas of survey to be carried out within the CHERISH project areas.
Ground Penetrating Radar (GPR)
GPR is a non-destructive technique that fires pulses of electromagnetic energy in the form of high-frequency radio waves into the ground. These radio waves, similar to those used to detect aeroplanes in the sky, are pulsed into the ground surface with a high velocity and the resulting echo is reflected back off different features or layer contacts and they are finally detected back on the surface by a receiver. As the GPR waves propagate though different materials on their way to buried objects their velocity will significantly change depending upon the variable physical and chemical properties of the materials though which the waves are travelling. The reflected waves are collected by the receiving antenna and converted into a radargram, or image of the subsurface structure. Once the surface topography is applied to the data, the radargram reveals the nature and shape of the features that lie beneath the ground
GPR is widely used in archaeology and geomorphology, with a high spatial resolution and a relatively fast survey time. The data is processed with specialised computer software to generate accurate 3D maps and images of the buried archaeological structures and geomorphological structures
There are several other geophysical survey methods, including Electromagnetic Induction methods (EMI), Electrical Resistivity Imaging (ERI), and Magnetic Susceptibility Survey, which can be used independently or complement Magnetometry or Electrical Resistance
Area surveys in identifying archaeological features. When choosing which method to apply, archaeologists must judge what techniques will potentially yield the greatest results based on the buried archaeology, the background soil/geology, and what equipment they have at their disposal.
Interpreting the Results
Once we have collected geophysical data, it is then integrated and analysed by archaeologists looking for areas where anomalies or differences in the data exist. Following analysis, archaeologists must interpret the results and make a decision if they believe the feature identified in the geophysics to be a naturally occurring object through geology or geomorphology, or it is an archaeological feature caused by human activity. The resulting decisions are plotted on a map as potentially buried archaeological features which will lead to our greater understanding of the extents and features of a site.
Once potential archaeological features have been discovered through archaeological geophysics, these areas can be exposed through targeted excavation. Like a surgeon would carry out keyhole surgery using an x-ray or MRI scan, archaeologists can insert targeted excavation trenches into sites which allow us to visibly see and understand the archaeological feature which is causing the geophysical anomaly, whilst not having to open up the whole site.