Book Review

Review by Ben Gourley
Department of Archaeology, University of York, UK
(Email: rbg4@york.ac.uk)

Magnetometry for archaeologists is the latest addition to the teaching literature devoted to near surface geophysics as applied in archaeology. The volume is the second in a series introducing field practitioners and students to theories and methods of archaeological geophysics (Volume 1, written by Lawrence B. Conyers and published in 2004, was devoted to Ground Penetrating Radar). By providing a more in-depth treatment of the subject than can be found in general texts, this book is directed at both those with little or no background knowledge and those who want to probe more deeply into the subject. It essentially comprises two parts: the first considers core concepts of magnetism and its physical presence in the world we occupy; the second is devoted to issues surrounding the archaeological application of magnetic survey.

In order to set the scene - or perhaps to ward off unsuspecting readers who may have chanced upon the book in the library - the authors jump straight in with a discussion of the core concepts and physics of magnetism. Discussion is brief, but of an appropriate depth, and readers encounter a 'to the point' discussion punctuated by equations and schematic diagrams reinforcing the text. Despite the complexity of the subject, the authors have clearly gone to great pains to make it accessible to non-specialists and some, like myself, will be thankful for this as they can return to this initial discussion time and again throughout the book. The second part of this first chapter is devoted to archaeological causes of magnetism. Here remnant magnetism and the various mechanisms behind magnetic enhancement of archaeological soils and features are discussed in depth. The classification of the magnetic susceptibility of background soils and geology is also considered, as is the fundamental requirement for the presence of magnetic susceptibility contrasts for the determination of archaeological signals.

Chapter 2 comprehensively presents the various classes of magnetometer, their historical development, their mechanics and their uses in archaeology. Helpfully, a summary of the characteristics of each type appears at the end of each description, providing a convenient way of comparing types of magnetometers commonly (or in some cases, less commonly, but increasingly) used in archaeology.

Perhaps most useful for those practitioners wishing to employ and interpret magnetic survey but who are less familiar with types of response, Chapter 3 describes the nature and physical form of magnetic anomalies as detected from various near surface geophysical methods. This begins with the description of the isolated dipole response and moves on to a discussion of more complex archaeological features. The treatment here is in the form of diagrams and graphs which illustrate the responses to the same anomaly from different magnetometers, but also a discussion of the decrease in signal strength of magnetic flux with the increase in depth of anomaly. The chapter ends with a usefully detailed characterisation of magnetic noise encountered in archaeological magnetic survey.

Chapters 4 and 5 outline common field procedures and processing applications respectively. After three fairly science-heavy chapters, the authors take the reader in a more field-oriented direction, steering the discussion towards practical considerations. For those with some experience of either geophysical field procedures or data processing this will be familiar terrain. Nevertheless the text provides a good reminder of set-up procedures for different instruments and highlights key problems and potentials in both areas.

The authors devote the second half of the book to chapters entitled 'Archaeology and data interpretation', and 'Magnetometry and archaeology: the future'. In the former, the authors examine five broad categories of site with case studies illustrating the range of archaeology investigated with magnetic techniques. The five include: 'Settlements', 'Industrial sites', 'Ritual and religious sites', 'Gardens', and 'Igneous and ferrous anomalies'. The case studies are well chosen and care has been taken not to 'overdo' the examples; indeed the authors state outright their feeling that '...images and case studies can become repetitive.' Agreed, but given the attention to magnetic anomalies in Chapter 3, I felt that more examples to illustrate the archaeological practicalities of concepts would not have gone amiss. Lastly, I thought it peculiar that 'Landscape' was not included as one of the primary categories - especially as gardens were included - since examples of large scale magnetic applications across landscapes are becoming increasingly important (the authors do raise this subject very briefly in their conclusions).

In the ten pages devoted to future developments and in the concluding remarks, various new forms of instrumentation are introduced: GPS enabled multi-sensor configurations, Caesium Vapour magnetometers and high hopes for developments of SQUID (superconducting quantum interference device) systems. Finally, the major subjects of 'Analysis and visualisation of large data sets' and 'Landscape's are raised briefly in the last four pages. The brevity of treatment comes as something of a surprise given the importance of developments in both fields and the authors' own contributions to these subjects. (On a related note, the absence of discussion of associated topics, such as geophysical metadata, archiving, and publication also seemed unusual). It may be that these topics are better treated in their own right in another context.

Magnetometry for archaeologists will certainly appeal to practitioners and neophytes alike. It will no doubt find its place amongst other formative texts on archaeological geophysics and sit comfortably with more detailed treatments. It covers the subject in a clear and coherent manner and should be welcomed for that reason alone. It also leaves the appetite whetted for a third volume in the series, devoted to issues only touched upon here and to how to deal with all this data. In the meantime, I shall certainly be reaching for it frequently for reference and teaching.