Geophysical Survey

Geophysical Survey

Geophysical survey is employed within archaeology as a non-invasive, non-destructive aid to understanding sub-surface features and conditions. No physical interventions (such as trenches or pits) are required to conduct a survey, since the geophysical information can be collected remotely from the ground's surface. Geophysical survey thus plays a vital role in the evaluation of archaeological areas - allowing the archaeologist to assess a feature or site without risking damage to the resource, or gauge the placement of a trench to maximise its interpretive potential.

From very specialist, scientific beginnings, geophysical survey has become an integral part of the archaeological process, and is now employed widely in both commercial and non-commercial archaeology. The discipline's profile has been raised thanks to this general uptake and its key involvement in television programmes such as Time Team, but it must be remembered that the term 'geophysical survey' covers a range of techniques, each suited to slightly different conditions, situations and budgets. This section will thus briefly outline the field's development and sub-disciplines, and discuss the practicalities involved in its employment. Three of the most widely-used techniques of geophysical survey - resistivity, magnetometry and ground penetrating radar - will also be examined (see menu, left).

Introduction to Geophysics

From early on in the development of organised archaeological study, techniques were being developed to give archaeological practitioners an understanding of sub-surface features without the need for invasive excavation. However, the inaccuracy and poor adaptability of these early techniques meant that decades passed where excavation was the only real way to visualise and understand archaeological sites. But archaeological excavation is essentially a 'one-chance' undertaking, for while it provides unparalleled understanding and tangible evidence it is also ultimately destructive: stratigraphy, relationships and assemblages are destroyed and can never be rebuilt. With hindsight, many archaeological sites may have yielded valuable information with the use of modern analytical techniques, and indeed future generations of archaeologists may lament our under-use of the archaeological resources we destroy through excavation. Thus, techniques that can 'remotely sense' sub-surface features have been gradually developed, especially in the second half of the 20th century, and today their role in both commercial and non-commercial archaeology is vital.

Geophysics is widely-employed branch of 'remote sensing' (a term covering many disciplines from satellite sensing to dowsing!), and spans disciplines such as geology, town planning and environmental studies in addition to archaeology. Indeed, many early developments were geared towards these other fields, but specific advances in technology have helped archaeologists tailor methods and equipment to suit archaeological requirements. Today, a geophysical survey can be conducted relatively accurately and quickly with the aid of automatic data-loggers and precise sensors, but this was not always the case. What follows is a brief historical overview detailing the development of remote sensing, and especially geophysics, within the field of archaeology.

Current legislation, such as PPG16 (Planning Policy Guidance note number 16), promotes non-invasive archaeological investigation where the archaeology itself is not under direct threat from natural conditions or development.

Historical Overview

Pre-1950

1893 / Pitt-Rivers & Sonic 'bosing'
Augustus Pitt-Rivers employs sonic 'bosing' - striking the ground to judge relative sound responses - at Hadley Down, Dorset, UK.
1920s / Phosphate Analysis
Development of phosphate analysis to map areas of habitation and/or use, Scandinavia, Egypt
1938 / Mark Malamphy: USA
Canadian geophysicist Malamphy conducts first systematic geophysical survey using resistivity soundings at Bruton Parish Church, Williamsberg, USA.
1946 / Resistivity at Dorchester, Dorset, UK
First popular use of resistivity for archaeological purposes (and widely considered the first archaeo-geophysical survey) by Richard Atkinson in an attempt to map sites under threat from gravel extraction. Equipment: Megger Earth Tester used in engineering geophysics; Method: 'leapfrog' survey developed for 2d mapping of deposits.

The origins of remote sensing lie with techniques such as dowsing, which has been practiced for many hundreds of years as a way of locating sub-surface resources such as water or minerals. Debate continues over the scientific basis of dowsing, but the technique has certainly been applied to archaeological situations and is the subject of many journal articles and studies (eg. Martin Aitken's 1959 study). It is unclear whether any significant archaeological decisions have been made as a result of dowsing evidence, but as a fore-runner to more proven techniques it is worthy of mention.

The beginnings of organised archaeology saw a new role for remote sensing, with pioneers such as Augustus Pitt-Rivers using bosing at Hadley Down in Dorset, UK. Bosing entails striking the ground with a heavy wooden mallet or a lead-filled container on a long handle to judge relative sound responses, and was found to be an effective method of identifying silted pits and ditches from surrounding natural soils. However, emphasis must be placed on bosing as a relative technique: it does not provide data or evidence that can be compared accurately to other sites (or even other areas of the same site), for sonic responses of the ground can change due to many variables (eg. geology, water content, soil compaction, etc). Bosing is thus useful only within small areas to delimit areas of 'different' ground.

What archaeology required was a technique of accurately obtaining measured evidence that could be analysed and compared, and it was the field of chemistry that provided such an advance with development of phosphate analysis. Used to identify areas of habitation, occupation or use, phosphate analysis allowed archaeologists to detect the phosphorus residues left by such activities via chemical analysis. Developed and tested on archaeological sites in Sweden and Egypt, this geochemical survey method can be used to great effect when employed in a grid layout, and can complement geophyscial forms of survevy. However, such surveys are slow to conduct due to the practicalities of obtaining samples and they are thus usually implemented over relatively small areas.

While this technique was useful for analysing already-identified areas of archaeology, similar advances were being made in archaeological prospection. In 1938, in Williamsburg, USA, exploration geophysicist Mark Malamphy of the Canadian firm Hans Lundberg Ltd conducted what is understood to be the first systematic geophysical survey, using an equipotential map to display the ground resistivity of a colonial-period churchyard. The survey was designed to pinpoint the location of a buried stone vault within the churchyard, but the high-resistivity feature thought to be the vault was instead discovered to be natural soil contrast - opening the possibilities for the technique to be employed to identify soil-based features.

The first popular archaeological use of resistivity survey was by Richard Atkinson, who employed the technique in his investigation of a henge monument in Dorchester, UK. Equipped with a Megger Earth Tester, common to engineering geophysics, he utilised a 'leapfrog' survey developed for 2d mapping of deposits rather than the usual method at the time which involved profiles represented by graphs.

1950s

1950s / Lerici Foundation: Milan, Italy
Archaeological Prospection Section established within the Foundation - an important organization in the development of geophysics in Europe.
1956 / Martin-Clark resistivity meter
First instrument designed specifically for archaeological work: used at Cunetio Roman town, Wiltshire, to trace circuit fortification wall.
1957 / Thermoremnant Magnetisation
Introduction of dating technique by John Belshe for ceramic materials, quickly followed by the application of this principle to kiln sites by Martin Aitken and Edward Hall, Oxford Research Laboratory.
1958 / 1nT Proton Magnetometer
Prototype proton magnetometer, sensitive to 1nT, introduced by Aitken and Hall at Water Newton to search for kiln sites prior to A1 Motorway construction. Discovery that this method also generated responses from infilled pits and ditches.
1958 / Irwin Scollar & Micro-computers
Irwin Scollar at University of Bonn pioneers use of micro-computers in data filtering and mapping.
1959 / Commercial Availability
Martin-Clarke resistivity meters and two versions of proton magnetometers (Maxbleep and Minbleep) available commercially.

The use of such non-invasive survey technologies was also developing on continental Europe, and a major step was the establishment in 1947 of the Institute of Applied Geophysics at Milan Polytechnic - formed from the Centre of Geomineral Prospecting on the initiative of Ing. Carlo Maurilio Lerici. From the mid-1950s the Institute undertook inter-disciplinary studies of the Etruscan necropolises of Tarquinia, Cerveteri and Vulci, and this combination science and humanistic studies led to the formation of the Ing. Carlo Maurilio Lerici Foundation to focus on the development, testing and application of geophysical, geological and remote sensing techniques to archaeological prospection.

Meanwhile, in the UK the first archaeologically-specific geophysics equipment was beginning to develop in the form of the Martin-Clark resistivity meter. An early employment of the new technology was at Cunetio Roman town in Wiltshire, where Martin Aitken and Edward Hall of the Research Laboratory for Archaeology and the History of Art, Oxford, used the equipment to trace the town's circuit fortification wall. While much slower to assemble, operate and process data than modern equipment, these early geophysics machines formed the basis from which more complex models developed.

Alongside the advancements in resistivity-based geophysical survey, another technique was beginning to emerge based on the understanding that buried objects and features produce small distortions in the earth's magnetic field. These magnetic survey methods developed from the principle of thermoremnant magnetisation - the theory that when heated to around 700°C, iron oxide particles align themselves to whatever orientation the earth's changable magnetic field is in at the time. The technique, using a proton magnetometer, proved highly useful for identifying areas of burning, kiln sites and metal objects, and was employed with success by Aitken and Hall at the Roman city of Durobrivae, near Water Newton, UK, prior to the A1 motorway construction. See the following 1965 article (Adobe .pdf format):

Marriner, E.H., The Proton Magnetometer: New Tool for Archaeologists and Treasure Hunters Radio Electronics, September 1965

Geophysics equipment was soon available commercially, with the Martin-Clarke resistivity meter and two versions of proton magnetometer (Maxbleep and Minbleep) leading the way. The magnetometers utilised a gradiometer mode, with two sensors mounted vertically to help eliminate background geomagnetic variations. But advances were not confined to geophysical survey equipment. Irwin Schollar of the University of Bonn, Germany, helped filter and map the geophysical data obtained through survey with the use of micro-computers, ushering in a new era of computer-based analysis that could handle much more data with more accuracy than traditional techniques.

1960s

1960s / Proliferation & Development
A pivotal time for the proliferation of archaeological geophysics, with key developments in several countries, eg:
USA: Elizabeth Ralph at MASCA (Museum Applied Science Centre for Archaeology, Pennsylvania)
France: CNRS under Albert Hesse and Alain Tabbagh (Centre de Recherches Geopysiques, Garchy)
Germany: Rheinisches Landesmuseum, Bonn, (Irwin Scollar)
Italy: Lerici Foundation (Richard Linnington, Carlo Lerici).
Work extensively published in the journals Archaeometry, and Prospezioni Archeologiche.
1964 / Fluxgate Gradiometer
Fluxgate Gradiometer developed by John Alldred (Oxford Research Laboratory). Designed to increase efficiency and speed of recording, but also had the benefit of balancing for background variations.
1967 / Ancient Monuments Laboratory Geophysics Section
Introduction and experimentation with a number of new instruments, including different resistivity arrays (square frame and Wenner). Development of a continuous recording single-operator Fluxgate Gradiometer - the standard for the next 15 years.

The 1960s saw the general proliferation of geophysical survey within archaeology, testified by many key developments from numerous research institutes across Europe and North America. Elizabeth Ralph of the Museum Applied Science Centre for Archaeology, Pennsylvania, USA, undertook key geophysical investigations at the Palace of Malqata in Thebes, Egypt, while Albert Hesse and Alain Tabbagh undertook geophysical research at the CNRS in France.

The Lerici Foundation continued its developmental work in the discipline and communicated with the other major research centres specializing in archaeological prospecting: Pennsylvania University (USA), Oxford Research Laboratory (England), CNRS Geophysical Research Centre, Garchy (France) and the Rheinisches Landesmuseum of Bonn (Germany). The archaeological geophysics community was further cemented with the signing of permanent collaboration and data exchange agreements between these centres.

The discipline had now reached a sufficient level of use and popularity to warrant its own forums, which duely arrived in the form of the journals Archaeometry, and Prospezioni Archeologiche. New advances and projects that incorporated geophysical survey were published extensively via these media, in addition to other, more general archaeological journals.

Among the most important advancements of this era was the Fluxgate Gradiometer, developed by John Alldred of the Oxford Research Laboratory. Not only did this equipment significantly increase efficiency and speed of recording, but it also allowed for adjustments, or balancing, for background magnetic variation.

Further developments, including different resistivity probe arrays such as Wenner and square-frame, were made at the Ancient Monuments Laboratory Geophysics Section in the UK. Also to come out of this institution was the continuous recording, single-operator Fluxgate Gradiometer, which was to become the standard piece of equipment for this type of survey for the next 15 years.

1970s - Present

Universities
Focus change in Britain and Europe from research and development of new instruments and methodologies to the teaching of techniques and geophysics' place in archaeology.
Postgraduate degree in Scientific Methods in Archaeology at Bradford in 1971 under Arnold Aspinall.
Bradphys twin Probe resistivity meter developed, establishing the new generation of equipment still in use today.
Survey equipment coupled with portable field computers (eg. Epson PX20), transforming methodology of data logging and field procedures.
By 2000 nearly all University archaeology depts. teach geophysical survey - new generation of archaeological practitioners who are familiar with geophysical equipment, methods and results.
Commercial & Popular Use
British Gas appoint full-time archaeologist, and in 1979 a full time Geophysicist.
Commercial possibilities presented by new generation of instruments (designed by Roger Walker, Geoscan, 1984) combining large scale data collection with ease of use.
Developer-funded, tender-based commercial archaeology fuels a archaeogeophysics boom, leading to over 450 commercial surveys in 200 alone.
Television exposure, such as Time Team, boosts coverage further, popularising and promoting the discipline to new levels.

Archaeological geophysics in Britain and Europe could be said to have 'come of age' during the 1970s and 80s, with less emphasis placed on technological and methodological development and more focus upon teaching and theoretical research. Universities played a large role in this focus shift, and in 1971 the postgraduate degree Scientific Methods in Archaeology was established at Bradford University for students who wished to follow this specialist path. The degree is still run today, and Bradford University has gained a reputation for cutting-edge scientific research and degree courses in archaeology. Out of this scientific community at Bradford developed companies such as Bradphys, whose twin-prode resistivity meter is still used today.