Neil Linford

The application of geophysical methods to archaeological prospection

The aim of this review is to combine the almost universal fascination we share for our past with the comparatively recent, in archaeological terms, application of geophysical prospection methods. For their success, each of these methods relies upon a physical contrast to exist between the buried archaeological feature and the properties of the surrounding subsoil. Understanding the archaeological origin of such physical contrasts, in terms of density, thermal conductivity, electrical resistance, magnetic or dielectric properties, remains fundamental to an appreciation of the discipline. This review provides a broad introduction to the subject area acknowledging the historical development of the discipline and discusses each of the major techniques in turn: earth resistance, magnetic and electromagnetic methods (including ground penetrating radar), together with an appreciation of more esoteric approaches, such as the use of micro-gravity survey to detect buried chambers and voids. The physical principles and field instrumentation involved for the acquisition of data with each method are considered and fully illustrated with case histories of results from the English Heritage archives.

The age of Stonehenge

Stonehenge is the icon of British prehistory, and continues to inspire ingenious investigations and interpretations. A current campaign of research, being waged by probably the strongest archaeological team ever assembled, is focused not just on the monument, but on its landscape, its hinterland and the monuments within it. The campaign is still in progress, but the story so far is well worth reporting. Revisiting records of 100 years ago the authors demonstrate that the ambiguous dating of the trilithons, the grand centrepiece of Stonehenge, was based on samples taken from the wrong context, and can now be settled at 2600-2400 cal BC. This means that the trilithons are contemporary with Durrington Walls, near neighbour and Britains largest henge monument. These two monuments, different but complementary, now predate the earliest Beaker burials in Britain – including the famous Amesbury Archer and Boscombe Bowmen, but may already have been receiving Beaker pottery. All this contributes to a new vision of massive monumental development in a period of high European intellectual mobility….

The survey and excavation of a bronze age timber circle at Holme-next-the-sea, Norfolk, 1998-9

In 1998 a circle of timber posts within the intertidal zone on the north Norfolk coast was brought to the attention of the Norfolk County Council Archaeological Service. A subsequent programme of archaeological recording and dating revealed that the structure was constructed in the spring or early summer of 2049 BC, during the Early Bronze Age. Because of the perceived threat of damage and erosion from the sea a rescue excavation was undertaken during the summer months of 1999. The structure was entirely excavated, involving the removal of the timbers and a programme of stratigraphic recording and environmental analysis. A survey was also undertaken within the environs of the site which has identified further timber structures dating from the Bronze Age. Detailed examination of the timber from the circle has produced a wealth of unexpected information which has added greatly to our understanding of Early Bronze Age woodworking, organisation of labour and the layout and construction of timber ritual monuments.

A rival to Stonehenge? Geophysical Survey at Stanton Drew, England

The development of geophysical survey remains a spearhead-priority for new research and cultural resource management alike – since geophysics can find and map sites without destroying them. However, there are current weaknesses of sensitivity and resolution – the instruments cannot easily “see” small features like graves and post-holes of which so many ancient sites are principally composed. Great hopes have been invested in caesium vapour magnetometers, which the Centre for Archaeology has been promoting in England – perhaps nowhere with such dramatic success as at Stanton Drew, Somerset. Here, geophysical techniques have brought to light the lines of broad circles belonging to a previously unrecognised henge monument, and the caesium magnetometer showed these circles to be composed of individual pits about 1.4 m in diameter. The fine focus achieved for these buried features augers well for the discovery and preservation of similar sites and monuments in the future.

Mineral magnetic profiling of archaeological sediments

The commercial extraction of mineral aggregates from a 140 ha site at Yarnton, Oxfordshire instigated a continuing programme of archaeological evaluation in advance of the eventual destruction of the site. It was hoped that geophysical survey, particularly rapid gradiometer coverage, would amplify the information recovered from both aerial photographs and trial excavation. Unfortunately, results from those areas of the site on the Thames floodplain proved disappointing and initially were believed to have been hampered by more recent alluvial overburden. Subsequent area excavation revealed a wealth of archaeological remains from the early/ mid-Neolithic to the late Bronze Age and a significant variation in both the depth and distribution of the alluvial deposits. This study aims to investigate the local contrast in magnetic behaviour between samples recovered from excavated archaeological features and the surrounding natural sediment; initially concentrating on a series of detailed magnetic measurements made on fractionated soil samples from two pit-type features. The study confirmed that despite a marginal contrast in magnetic susceptibility a distinguishable magnetic signature was evident within the anthropogenically modified samples and that a combination of measurements, including artificially induced remanent magnetization, provided the greatest degree of discrimination from natural sediments. The results of the detailed investigation were then applied to a more rapid analysis of bulk soil samples from a wider range of archaeological contexts, by examining a variety of samples from a site where there appeared to be little or no magnetic contrast between anthropogenically modified features and the surrounding natural sediments.

Non-destructive techniques in English gardens: geophysical prospecting

Abstract Buried archaeological sites are identifiable from the almost indelible nature of the geophysical record of past human activity beneath the modem ground surface. Geophysical survey can be a most effective technique of archaeological evaluation, although it has only occasionally been applied to former garden features within the British Isles (figure 1 and table 1). Recent reviews of garden archaeology7 concentrate on the data available from topographic, documentary and aerial photographic sources, but do not consider in detail the potential of other methods of non-destructive investigation. There is, perhaps, a perception that garden features are too shallow and ephemeral to leave a clearly detectable geophysical signature.8 While this may often be the case, the value of geophysics in this rapidly developing subject has not been adequately acknowledged. This paper illustrates, through case-study examples, the success that has been achieved to date and the potential that such techniques offer for th...

Chasing aeroplanes: developing a vehicle-towed caesium magnetometer array to complement aerial photography over three recently surveyed sites in the UK

Aerial photography combined with airborne lidar can often provide information on the location of archaeological sites at a near landscape level of coverage. Ground-based geophysical techniques may then be deployed to complement the aerial survey, particularly where the soils or land use may not be ideal for either producing crop marks or preserving topographic features. This paper describes the development of a vehicle-towed caesium magnetometer array, from an original handpushed system, to allow high-density datasets to be rapidly acquired over large areas required to provide a meaningful comparison at the scale demanded by the aerial survey results. Technical details of the system are presented together with methodological considerations for both data acquisition in the field and appropriate post processing to obtain high-sensitivity field measurements over more weakly magnetized sites. Results are presented from a number of recent collaborative research projects within the English Heritage Remote Sensing Team to illustrate the benefits of a combined aerial and ground-based approach to mapping the historic environment.

New approach to directional filtering of near-surface magnetic data

Topographical anomalies or compressed areas of soil caused by ploughing or more significant relic features such as ridge and furrow produce a response in near-surface magnetic surveys that are usually identified by their repetitive, linear pattern. While they are accurate recordings of the subsurface magnetic properties and micro-topographical features of the site, it is often the anomalies due to deeper features that are the primary focus of the survey. These target anomalies can be masked by the nearsurface pattern and it is therefore often preferable to remove these from the final presentation of the data. Two routines used for removing these features are common in commercial processing software. These are the directional pass/reject and cosine-taper filters. While these filtering techniques can dramatically improve the clarity of the data image, it is shown here that they make significant changes to the data that remain. As interpretation of near-surface magnetic data moves beyond image analysis towards more quantitative methods, it is important to ensure that the final processed data set represents as close as possible the response to the subsurface features of interest. Here an alternative filtering routine dependant on both the azimuth and power-content of the anomalies is proposed that overcomes the problems encountered by the traditional techniques. It is shown that patterns of agricultural linear anomalies can be removed from the data without significantly changing the properties of the desired responses and therefore quantitative interpretation can be subsequently carried out without the data being significantly compromised by the choice of previous processing techniques.

Processing stepped frequency continuous wave GPR systems to obtain maximum value from archaeological data sets

Stepped frequency continuous wave ground penetrating radar (GPR) systems allow highly detailed data sets to be collected across a wide bandwidth using multi-element array antenna, such as the 3d-Radar GeoScope system. Although the final presentation of results is similar to time domain systems, the correct processing of the initial frequency domain data acquired in the field is essential to obtain the maximum information from the site. Consideration of the variation of bandwidth with depth, reduction of noise and background subtraction to minimise loss of data quality is required. This paper explores these themes with relation to data from archaeological sites and also considers the data processing challenges presented by high density (e.g. 0.075m × 0.075m) multi-hectare surveys.

Excavation and Survey at Watchfield, Oxfordshire, 1983–92

Geophysical survey and excavation have demonstrated that iron age settlement features and early Anglo-Saxon burials discovered during road construction in 1983 were part of an extensive multi-period site, now a Scheduled Ancient Monument. This paper presents the results of the archaeological investigations. The bulk of the report is concerned with the early Anglo-Saxon burials, which included a grave containing a balance, weights and runic inscription. The evidence for late mesolithic, neolithic—early bronze age, iron age and post-medieval activity is also discussed, and the site is considered in its regional and wider context.