5.3 Geophysical Survey

Geophysics crosses disciplines, periods and geographic areas and as such is a vital technique within the wider archaeological, and associated environmental, research frameworks. Geophysical survey applied to archaeological sites has a long history, with its first documented use in Britain in 1946 (Gaffney and Gater 2003), with the discipline being discussed and written about, as a research tool, by the late 1950s in the journal Archaeometry. The early 1990s saw a step change in the application of geophysics with surveys becoming more common in rescue archaeology, with the move to developer-paid archaeology and the incentive to gain enhanced information to allow accurate estimation of excavation costs. In 2004 a conference “Going Over Old Ground” (Jones and Sharpe 2006) looked at the state of the application of geophysics in Scottish archaeology which had, and still does, lag behind the rest of Britain. Given the relatively low application of geophysical surveys to archaeological sites in Scotland, the Institute for Archaeologists Geophysics Special Interest Group (GeoSIG) has worked with Scotland's Association of Local Government Archaeological Officers (ALGAO) to try and raise the profile of geophysics in developer funded archaeology through a series of seminars.

Although no standards or guidelines specific to Scotland exist the industry follows those set by the IfA and English Heritage and these have recently been formally adopted by Scotland’s ALGAO: The Institute for Archaeologists IfA Standards and Guidance: Geophysical Survey (http://www.archaeologists.net/sites/default/files/node-files/geophysicsSG.pdf) ; and Guidelines established by English Heritage, Geophysical Survey in Archaeological Field Evaluation (http://www.english-heritage.org.uk/content/publications/docs/geophysics-guidelines.pdf) .

Due to the variety of complex features that may be found below the ground surface, a wide variety of techniques and equipment are available to the surveyor. The table below highlights the main techniques applied to archaeology.

Table 10: Summary of Geophysical Techniques and their application

Technique Features Detected Appropriate Application
Gradiometry Ditches and pits
Made surfaces, metalled roads and trackways
Drains and gulleys
Pottery and tile kilns
Hearths and ovens
Ferrous debris including some slag
Usually the technique of first choice. Suitable for greenfield sites with archaeology up to a meter or so beneath the surface and able to cover large areas in a time and cost effective manner. Igneous geology can reduce its efficacy.
Magnetic Susceptibility Depending on sample interval:
Areas of anthropogenic activity
Ditches and pits
Pottery and tile kilns
A quick, inexpensive technique suited to locating general areas of activity on a landscape level.
Area Resistance Walls, foundations and rubble spreads
Made surfaces
Metalled roads & trackways
Stone coffins, cists and graves
Drains and gulleys
Ditches and pits
A very commonly applied technique ideal for locating foundations, but also able to detect ditches and pits. This technique benefits from not being affected by underlying geology and so is often deployed on sites with underlying igneous or metamorphic geology.
Electromagnetic Survey Ditches and pits
Walls, foundations and rubble spreads
Made surfaces
Metalled roads and trackways
Pottery and tile kilns
Hearths and ovens
Ferrous debris including some slag
Buried metal objects
Buried storage tanks
Utilities
Buried metal objects
Buried storage tanks
Theoretically this technique can measure the same properties as both gradiometer and resistance survey. However, in practice it lacks the clarity of either of these techniques. However, it is particularly effective if the expected archaeology is known to be at least one meter below the surface or where ground conditions preclude the effective use of gradiometry or resistance survey.
Ground Penetrating Radar (GPR) Ditches and pits
Walls, foundations and rubble spreads
Made surfaces
Metalled roads and trackways
Stone coffins, cists and graves
Drains and gulleys
Depth of peat i.e. mapping of palaeo-landscape
Depth to bedrock
Location of voids
GPR is ideally suited to sites where depth information is required. It can be applied to a variety of even ground conditions and can detect to a wide range of depths making it the most versatile technique available, but not the most time or cost effective.
Electrical Imaging (ERT) Ditches and pits
Walls, foundations and rubble spreads
Made surfaces
Depth of peat i.e. mapping of palaeo-landscape
Depth to bedrock
Location of voids
As with GPR, ERT provides accurate depth information. However this is often at the expense of lateral resolution. It is ideally suited to locating large scale archaeological or environmental features, especially where soil conditions e.g. high clay or salt content, limit the efficacy of GPR

Gradiometry

Of all the geophysical techniques available, gradiometry is the most time and cost effective. As such it is the industry standard and usually the technique of first choice.

It is excellent for locating ditches, pits, middens, hearths, kilns and, depending on the soils and materials used, will often show walls and foundations. Although gradiometry is used routinely in Cornwall, Norway and Iceland, there has been much discussion about the efficacy of gradiometry in Scotland, owing to the large amount of igneous geology. In reality there are few areas where this technique cannot be applied. Unfortunately many people dismiss the use of all geophysical techniques based on the potential issues of this one technique.

Although normally used to survey sites in detail, the instrument can be used in scanning mode to identify hot-spots that can be later surveyed in detail. This is especially effective on very large sites (>100ha) when it is not cost effective to survey the whole development area in detail. However, no data is recorded in the scanning phase. It is vital to have a skilled operator to undertake this phase of activity as per the English Heritage and IFA guidelines.

A gradiometer comprises two magnetometer sensors mounted 1m or 0.5m apart on a vertical axis. Each sensor measures the earth’s magnetic field, in nanoTesla (nT), and the instrument records the difference between the observed readings for each sensor. By doing so the instrument is recording subtle changes or anomalies in the earth’s magnetic field caused by material in the top metre or so of the earth’s surface. By measuring the magnetic field in this manner, variations due to large-scale geological variations and diurnal fluctuations are filtered out. Data is normally collected at 0.25m intervals along traverses 1m apart within the series of 20m or 30m grids, which are later merged together.

Many different types of magnetometers and gradiometer are available but the two most commonly used instruments in archaeology in the United Kingdom are the Geoscan Research FM256 and the Bartington 601. These have fluxgate sensors and are often configured as a dual system which is simply two instruments carried together allowing for quicker survey times, with associated cost savings. Continental workers sometimess use large multi-array wheeled systems. These are normally still configured as a gradiometer, but often use alkali vapour or proton precession sensors which tend to be more sensitive. Although use of such systems is increasing in the UK, on most sites uneven ground limits the efficacy of these systems.

Magnetic Susceptibility

Also relying on magnetic properties of the soil, magnetic susceptibility can be used either as a reconnaissance tool to locate possible sites, or as a research tool to aid interpretation of specific features revealed by excavation or the nature of occupation of a site.

Whilst use of magnetic susceptibility has declined in recent times, it still remains a quick effective way of covering large landscapes to locate areas of interest/activity. Measurements can be taken at 20m, or so intervals, to identify hotspots that can be later surveyed in more detail by gradiometry. Unlike gradiometer scanning, it benefits from recording data that can be displayed to show potential areas of enhanced susceptibility and interest.

Laboratory measurement of magnetic susceptibility of soil samples gives data on individual contexts and also relative intensity of occupation.

The Bartington Magnetic Susceptibility system is the one most commonly used in the United Kingdom

Area Resistance Survey

Area resistance survey can provide detailed plans of building foundations and other stone structures, garden features and burials along with ditches and pits. Whilst slower than gradiometry, resistance is a very effective area prospection technique. At its best it can provide ‘building plans’, but can also be an effective application on those types of monument where magnetic enhancement is limited or unlikely to have occurred (e.g. burials and barrows). In addition it is largely unaffected by geology and for its ability to discern such ephemeral features as flower beds, paths and graves.

This technique passes a small electrical current through the ground and measures the resulting resistance. Data are normally collected 1m, or 05m, intervals within 20m or 30m grids which are later merged together.

Although a variety of resistance systems are available, the Geoscan Research RM 15 (soon to be replaced by the RM85) is the industry standard in archaeology. This system enables a variety of electrode arrays to be deployed. It also allows the probe separation to be adjusted enabling different detection depths. In archaeological prospection it is standard to use a Twin Probe array with a 0.5m probe separation giving a maximum detection depth of 0.75m - 1m. Increasing the probe separation will enable deeper features to be detected, but at the cost of lateral resolution.

Ground Penetrating Radar Survey

While gradiometer and resistance surveys provide detailed plans of buried archaeological remains, ground penetrating radar (GPR) survey is the best technique for providing accurate information on the depth and stratigraphy of features. GPR is currently the technique of choice when depth information is required, or when resolution of a complicated area survey is needed.

The technique can be applied in transect or as an area prospection tool enabling three-dimensional modelling of a buried site. In the latter it is normal to collect data at least 0.05m intervals along traverses 0.5m apart to enable detailed time slice maps to be produced. It is also sometimes appropriate to collect data on an orthogonal grid to improve resolution. Selection of the correct antenna, which affects the depth of penetration of the signal and its lateral resolution is important. Although most companies in the UK use systems with a single antenna new systems with multi-frequency antennas arrays are becoming more common.

In Ground Penetrating Radar surveys pulses of electromagnetic energy are directed downwards into the earth. The transmitted wave is affected by variations in the electrical properties of the subsurface, specifically the dielectric constant and the conductivity of the subsurface which, in turn, are influenced by material type, moisture content and pore fluids. Contrasts, or changes, in these properties cause reflection of the energy wave creating an anomaly. The subsurface is mapped by recording the amplitude of this reflected energy and its travel time. The data are recorded as two-way times i.e. how long it takes for the electromagnetic wave to travel to an interface and be reflected back to the receiver. As a result GPR records detailed vertical sections through the ground, which can provide a wealth of stratigraphic information and can define any discontinuities: it is one of the few geophysical techniques that can provide a good estimation of the depth of potential features. The travel times can be converted to depth using an assumed, measured or calculated velocity. The dielectric constant of a material is a direct measure of its water content, which is directly related to the velocity of the electromagnetic energy. However, the velocity is also governed by the electrical conductivity of the subsurface.

It should be noted that GPR is extremely sensitive to the conductivity of the soil and this is dominated by the proportion of clay minerals present. In Scotland, the amount of clay is often very high, the conductivity is therefore high and GPR often does not have any penetration of significance (less than a few centimetres). The technique is highly site dependant.

A wide variety of radar systems are available, the ones more commonly deployed in archaeology include GSSI's SIR 3000, Software and Sensors' Pulse Ekko and Mala systems. As GPR systems are classified as radio systems there are restrictions on the frequency of antennas that can be used, however this does not negatively impact on the use of GPR in archaeology in the UK. Anyone operating a GPR system within the UK needs to hold an OfCom licence and keep a log.

Electromagnetic Survey

Electromagnetic (EM) survey is useful for many near surface applications including mapping soil properties, utilities, and contamination of ground and water tables, identification of buried metallic objects, and depth to bed rock. The technique allows for simultaneous collection of both magnetic susceptibility and conductivity data, and as such is a fast versatile survey instrument. This technique has the same resolution as other resistance surveys but the way in which it is used reduces the ‘apparent resolution’. It is particularly useful on sites were ground conditions preclude the use of gradiometry and resistance survey.

A wide variety of EM system are available, although the ones most commonly used within the UK are Geonics EM31, EM38 and EM61, each looking at different depths.

Electrical Imaging

Electrical imaging systems, enabling Electrical Resistance Tomography (ERT) surveys, are comparable to GPR systems in that they record vertical sections through the ground enabling a three dimensional image to be recorded. Imaging is suitable on sites having soils with a high moisture or clay content where GPR is not suitable, and can also retrieve data from a greater depth. However, it can suffer from limited resolution on some site types.

As with Area Resistance Survey this technique passes a small electrical current through the ground and measures the resulting resistance. However, rather than maintaining a constant probe separation readings are taken at different probe separations result in a pseudo-section; an image of a vertical slice through the ground showing the variation in resistance values. The technique can be applied in transect or as an area prospection tool enabling three-dimensional modelling of a buried site.

Although geophysical survey is routinely used in field based archaeology throughout the world, particularly in the rest of Britain, its use in Scotland is relatively limited. The current lack of application and trust in the use of geophysical survey in Scotland is being considered by ALGAO and the IFA's GeoSIG. However the application of geophysical survey in Scotland, both in research and rescue archaeology, perhaps suffers from having no equivalent of English Heritage's Ancient Monuments Laboratory within Historic Scotland or RCAHMS. All contractors use guidelines from English Heritage (English Heritage 2008) and the Institute for Archaeology (Gaffney et al. 2002). Not only are surveys less commonly applied/commissioned in Scotland, but the total area per survey is generally less (with obvious exceptions). To what degree this is a function of the size of Scottish development is difficult to answer. Innovative techniques, for example Electrical Resistance Tomography and Electromagnetic survey, are also less readily applied.

Table 11: A summary of the suitable application of various techniques

Technique Suitable for
Electrical Resistance Tomography

Voids, Geology (e.g bedrock, peat)

Ground Penetrating Radar

Voids, Geology (e.g bedrock, peat), Services, Tarmac / concrete

Gradiometry Ditches, pits and kilns (if predominantly sedimentary geology), services
Resistance Ditches, pits and kilns (if predominantly metamorphic or igneous geology), Masonry / stonework, Lawns

Unfortunately geophysical survey is viewed primarily as a pre-excavation tool in both research and developer led projects. Geophysical surveys are often viewed as self-contained finite elements within a project and as a result much potential additional information is discarded. More often than not, once the geophysical survey has served its purpose, usually to just locate the trial trenches, it is put aside and not considered again. This is unfortunate as much information could be gained by re-examination of results following excavation and indeed re-examination of survey results in light of excavation may provide further information to the project. In addition, the data often contains a wealth of geomorphological information which could prove useful but is often disregarded.

There are obviously exceptions to this, for example the large scale gradiometer survey of the Heart of Neolithic Orkney World Heritage Site (Card et al. 2007) and various surveys on the Antonine Wall (Stephens et al. 2008). These two examples show two very different approaches. On Orkney an area of some 250ha has been surveyed with the aim of placing the visible monuments in their wider archaeological context. The surveys on the Antonine Wall have taken the form of several individual surveys / survey areas with the aim of answering very specific questions regarding interpretation and construction of different elements of the site.

Unlike many scientific techniques applied to archaeology, geophysical equipment is relatively cheap, portable and readily available. Many universities have dedicated archaeological geophysical survey equipment, such as fluxgate gradiometers and twin probe resistance systems, including Glasgow, Aberdeen and the University of the Highlands and Islands. In addition, other Universities, such as Edinburgh, Stirling and St Andrews, house more traditional geophysical equipment, including GPR, ERT, EM and Seismics, in their Earth Sciences departments which can be used on archaeological sites. In addition, NERC has the Geophysical Equipment Facility, partly housed at Edinburgh's Earth's Sciences Department. While the facility is primarily aimed at NERC funded research projects and studentships, any British groups may apply for loan of the equipment within the pool. The GPR systems, suitable for use on archaeological sites, are especially underutilised and NERC would welcome increased use of the system. NERC also offer training in the use of their equipment and excellent technical support.

Geophysics in archaeology also benefits from the involvement of the commercial sector. There are many commercial archaeological geophysical companies throughout Britain who undertake not only developer led evaluation surveys, but also contribute to research, either in their own right or as part of wider multi-institutional research projects. Within Scotland there are at least six dedicated archaeological geophysics consultancies; CFA Archaeology Ltd, GUARD Archaeology Ltd, O'Grady Geophysical Surveys, ORCA Geophysics, Rose Geophysical Consultants and Wessex Archaeology.

Visualisation and dissemination of geophysical survey data are often disappointing and survey data are not readily available, although this may be improved with the current ALGAO and GeoSIG proposals. Glasgow University (Jones and Sharpe 2006) started a database of geophysical surveys in Scotland but unfortunately this has not been maintained by other groups working within Scotland. Therefore, initially rather than identify areas and directions of research within Scottish archaeology, more basic work is required to provide 'structure' and benchmarks for geophysics in Scotland. Encouragement of innovative techniques when routine geophysical techniques are not consistently or routinely applied may not fall on fertile ground. The recommendations are:

  1. The creation (adoption) of national guidelines – EH and GeoSIG guidelines
  2. The establishment of a searchable database of geophysical surveys (McKeague and Jones forthcoming)
  3. With a strong recommendation of encouragement for the following:
  4. Early involvement of geophysicists in project design
  5. Ongoing discussions with the geophysicists

The outlook in Scottish geophysics is, however, far from gloomy. Geophysical survey is being used more in general landscape studies and this is a clear area of growth. Ground Penetrating Radar is being used more frequently in the study of buried landscapes. In particular, in mapping peat as part of the Scottish Wetland Archaeology Programme (SWAP) projects (Clarke et al. 1999) were survey data can be integrated with invasive techniques such as augering or coring and considered with geochemical studies. This enables true multidisciplinary studies which enhance an understanding not only of the palaeo-landscapes but also add to the knowledge and understanding of the various techniques and their interaction. This is an active area of research at Glasgow University.

The current growth in the use of air-borne and ground based LiDAR provides an ideal opportunity to consider the integration of geophysical and LiDAR data. Aerial LiDAR data has been collected for the entire inner buffer zone of the World Heritage Site in Orkney, and also been collected for Scotland’s other World Heritage Sites: Antonine Wall, Edinburgh Old and New Towns, St Kilda and New Lanark. This was funded by, and is the copyright of, the Centre for Digital Documentation and Visualisation (CDDV). An obvious next step is to integrate existing geophysical and LiDAR data were available. Aside from the interpretive benefits it should allow for a more digestible form of the geophysical data.

Given that commercial archaeological evaluations appear to be strongly led by the presence, or absence, of aerial photographic evidence, there is a clear role for geophysical survey in the evaluation of crop-mark sites in terms of accuracy of transcription, completeness of the information provided by the APs, but perhaps more importantly the state of preservation of such sites in the light of aggressive modern farming techniques. Such studies have been undertaken in the past, but there is scope for more, given the importance local authority curators place on such information.

The growth of marine/water-based archaeology and geophysical survey in the growing renewable sector present a clear area of research – geophysical investigation of intertidal zones enabling 'seamless' land based – water/marine based geophysical surveys and analysis.

In conclusion, it is important to view geophysical survey as a means of detecting and imaging heritage assets, not just part of the developer funded excavators’ toolbox. Geophysics crosses disciplines, periods and geographic areas and as such has much to offer the wider field of archaeology. However, there is a need to build and expand upon the bedrock of geophysical applications and involvement in archaeological research within Scotland.

 

Geophysics Case Study: Earl's Bu, Orkney

Between 1989 and 1991, various campaigns of geophysical survey at The Earl's Bu and its environs added to the body of information known about the site (the early 12th-century seat of Earl Haakon Paulsson, with a round church, a large hall, a Late Norse midden and an earlier horizontal mill), confirming both considerable disturbance and potential structural traces.

Although now over a decade old, the resulting, free publication in Scottish Archaeological Internet Reports by Paul G Johnson and Colleen E Batey is still a good read! 

Survey at Earl's Bu, Orphir, Orkney 1989-91: geophysical work on a Late Norse Estate Complex

Geophysics Case Studies needed?

 

Does Scotland need some new (or old?) big case studies to publicise the wdier advantages and disadvantages of geophysics, not just as a pre-evaluation tool? Maybe if there was to be more surveys and a variety of different techniques to showcase the wide range of archaeological questions that can be answered (even if only in part) then it might be more widely used?