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2008

Geophysical Survey in Archaeological Field Evaluation

Preface to the Second EditionThese guidelines are intended to help archaeologists, particularly curators, consultants and project managers, to better understand and engage with the techniques of geophysical survey, for the best results. It is hoped too that practitioners of geophysical survey will find them helpful and that, altogether, the guidance can contribute to raising the consistency and quality of geophysical survey in archaeological field evaluation. Geophysical survey in archaeology continues to flourish. As of 2006, it is estimated to be a component of at least 23.4% of all evaluations arising from planning applications (http://csweb.bournemouth.ac.uk/aip/aipintro.htm). The techniques are also finding an increasing role in the presentation and interpretation of archaeological sites, in contributing to archaeological and forensic research, and in helping to satisfy the demand for media coverage of archaeological subjects. Geophysical survey has a wider academic and professional forum than was the case several years ago. Since its inauguration in 1995 at Bradford University in the UK there has subsequently been a succession of biennial conferences on Archaeological Prospection, held in Japan, Germany, Austria, Poland, Italy and Slovakia, and attended by an ever more cosmopolitan variety of specialists in geophysics and remote sensing. The Environmental and Industrial Geophysics Group (EIGG) of the Geological Society has similarly hosted a continuing series of biennial one-day meetings devoted to recent research in the subject. The journal Archaeological Prospection, initiated in 1994, has gone on to establish itself as the main vehicle for publication of relevant research and case studies; and an International Society for Archaeological Prospection (ISAP) was initiated in 2003 (http://www.archprospection.org). Archaeological geophysics is now a component of undergraduate teaching in at least 12 universities, although the only post-graduate degree courses devoted to the subject are the MSc in Archaeological Prospection at the University of Bradford (http://www.brad.ac.uk/ archsci/msc_ap.htm) and the MSc in Archaeological Geophysics recently offered at Orkney College of the University of the Highlands and Islands (http://www.orkney.uhi.ac.uk/courses/archaeology/ geophysics-at-orkney-college-uhi). Despite the increasing familiarity with methods and techniques, and a growing number of practitioners, geophysical survey can be both a very technical subject, as well as a fertile area for continuing innovation, commercial2

exploitation, and integration with other prospecting disciplines. It is clear from our consultations that in these circumstances there remains a need for independent guidance, which the following document is intended to provide not only for curators of the archaeological resource, but also for others who need to know about the potential and pitfalls in more detail. Our purpose here is above all dedicated to bettering the consistency and quality of geophysical survey in evaluations, especially those arising from development proposals. Much of what was presented in the first edition remains valid and will be re-iterated here. There are, however, changes reflecting shifts in thinking and approach that have taken place over the last few years. To take one example, the debate on the efficacy of topsoil magnetic susceptibility as an aid to evaluation, which was very topical in the early 1990s, has moderated now that it is increasingly accepted that detailed magnetometer coverage is preferable, and more feasible, over yet larger areas. More importantly, there have been changes in geophysical instrumentation, technology, methodology and software, all of which are having an impact on the choice and performance of geophysical survey under varying conditions. A particular example is the great improvement in the virtues of ground penetrating radar (GPR), now that software and computing power enable both greater coverage and production of more comprehensible display and interpretation. Another significant development, following the influential example of European practice, is the increasing awareness and availability of alkali-vapour as well as fluxgate magnetometers. Both types of magnetometer, as well as other types of sensor, are now being deployed as arrays on mobile platforms, with considerable potential to raise the versatility and speed of ground coverage. Other areas of rapidly advancing progress include the further integration of geophysical data within Geographical Information Systems (GIS), which has in turn increased the need for consistency of data geo-referencing and archiving. In parallel, there are a growing number of survey projects that seek to integrate ground-based prospecting methods, together with remote sensing technologies such as lidar, to maximise interpretative and analytical potential. That said, wetlands, alluviated and urban environments persist as challenges to geophysicists. While not relevant here, but to be the subject of future guidance from English Heritage (forthcoming 2008) it is worth noting that the remote examination of the shoreline and seabed is a growing

imperative now that maritime archaeological conservation is in the ascendant. The first edition of this guidance was published in 1995, and this revision is offered in the hope of maintaining a balanced and independent view on best practice in the context of progress since then. With the benefit of much positive advice, comment and discussion from many colleagues, for whose patience and advice we are very thankful, we hope we have improved the content, and its presentation and clarity. As the document is available on line (http://www.english-heritage.org.uk/upload/ pdf/GeophysicsGuidelines.pdf) we expect to make future revisions and updates more immediately and easily in future and would, as ever, welcome comment and advice towards these.

ContentsPreface to the Second Edition . . . . . . . . . 2 Part I Standards for Geophysical Survey 1 Introduction . . . . . . . . . . . . . . . . . . . . . 3 2 Guidance . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1 Justification for survey . . . . . . . . . . . . . . 3 2.2 Fieldwork . . . . . . . . . . . . . . . . . . . . . . . . 4 2.3 Data treatment . . . . . . . . . . . . . . . . . . . . 4 2.4 Data interpretation . . . . . . . . . . . . . . . . 4 2.5 The survey report . . . . . . . . . . . . . . . . . 4 2.6 Dissemination . . . . . . . . . . . . . . . . . . . . . 5 2.7 Data archiving . . . . . . . . . . . . . . . . . . . . . 5 2.8 Competence of survey personnel . . . . 5 Part II Geophysical Survey and Planning 1 Archaeology and planning . . . . . . . . . . . 6 2 MoRPHE . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1 Start-up and planning . . . . . . . . . . . . . . 6 2.2 Execution . . . . . . . . . . . . . . . . . . . . . . . . 6 2.3 Closure . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3 Briefs and specifications . . . . . . . . . . . . 7 3.1 The brief . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2 The specification . . . . . . . . . . . . . . . . . . . 8 4 The survey report . . . . . . . . . . . . . . . . 9 4.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.2 Introduction . . . . . . . . . . . . . . . . . . . . . . 9 4.3 Methods . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . 9 4.6 Site location plan(s) . . . . . . . . . . . . . . . . 9 4.7 Data presentation plots and plans . . . . . . . . . . . . . . . . . . . 10 4.8 Plots of raw data . . . . . . . . . . . . . . . . . 10 4.9 Plots of processed data . . . . . . . . . . . . 10 4.10 Interpretative diagrams . . . . . . . . . . . 10 5 Dissemination . . . . . . . . . . . . . . . . . . . 10 5.1 Sources of information . . . . . . . . . . . . 10 5.2 Dissemination requirements . . . . . . . . 11 6 Archiving . . . . . . . . . . . . . . . . . . . . . . . 11 7 Legal considerations . . . . . . . . . . . . . . 11 7.1 Access . . . . . . . . . . . . . . . . . . . . . . . . . . 11 7.2 Metal detectors . . . . . . . . . . . . . . . . . . 12 7.3 Geophysical survey . . . . . . . . . . . . . . . 12 Contributors . . . . . . . . . . . . . . . . . . . . . 60 Acknowledgements . . . . . . . . . . . . . . . . 60 Appendix I Related standards, codes and guidance . . . . . . . . . . . . . . . . 56 Appendix II Contacts . . . . . . . . . . . . . . . 57 Appendix III Useful websites . . . . . . . . . 58 Appendix IV List of consultees . . . . . . . 59 References . . . . . . . . . . . . . . . . . . . . . . . 50 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . 54 2 Analysis of geophysical data . . . . . . . . 41 2.1 Data processing . . . . . . . . . . . . . . . . . . 41 2.2 Data display . . . . . . . . . . . . . . . . . . . . . 45 2.3 Data interpretation . . . . . . . . . . . . . . . 49 Part IV Practitioners Guide to Good Practice 1 Application of techniques . . . . . . . . . . 19 1.1 The survey grid . . . . . . . . . . . . . . . . . . . 19 1.2 Magnetometer survey . . . . . . . . . . . . . 20 1.3 Earth resistance (resistivity) survey . . . . . . . . . . . . . . . . . 24 1.4 Ground penetrating radar . . . . . . . . . 28 1.5 Electromagnetic methods . . . . . . . . . . 34 1.6 Topsoil magnetic susceptibility survey . . . . . . . . . . . . . . .36 1.7 Other geophysical methods . . . . . . . . 37 1.8 Metal detecting . . . . . . . . . . . . . . . . . . 40 1.9 Geochemical methods . . . . . . . . . . . . 40 1.10 Remote sensing . . . . . . . . . . . . . . . . . 41 Part III Guide to Choice of Methods . . 13 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . 13 2 Choice of geophysical survey . . . . . . . . 13 3 Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . .