Ground Penetrating Radar: An Introduction for Archaeologists. Lawrence B. Conyers and Dean Goodman. AltaMira Press, Walnut Creek, London and New Delhi, 1997. 232 pp., 58 figures, 8 color plates, 6 tables. Price: $54.00 (cloth), $26.95 (paper). ISBN 0-7619-8927-7.
Reviewed by Stephen Ball, Glenn A. Black Laboratory of Archaeology, Indiana University, Bloomington, IN 47405 USA
Geophysical remote sensing techniques are experiencing a pronounced revival among archaeologists. Although part of the overall archaeological tool kit since the late 1960s, they have sometimes been viewed as suspect and not a viable alternative to excavated or surface collected data. Recently, the increasing costs of archaeological excavation, curation, and a greater emphasis on preservation have added to the appeal of remotely sensed data. Furthermore, the development of GIS has familiarized archaeologists with the manipulation of digital data, and landscape archaeology has provided new conceptual frameworks for the integration of geophysical techniques. The easy availability of fast, powerful computers and software for the filtering, analysis and display of digital data has created a situation wherein the average archaeologist, with some study, can legitimately derive useful archaeological information from geophysical data.
Ground Penetrating Radar: An Introduction for Archaeologists by Conyers and Goodman represents a serious effort by the authors to communicate both the capabilities and promise of a remote sensing technology to archaeologists with little background in geophysics. For this effort, I believe they should be congratulated. Good introductory handbooks for archaeologists interested in geophysical remote sensing are rare, and this book constitutes an earnest effort to secure a wider acceptance of these techniques.
The book is organized into nine chapters. Chapter 1 introduces ground penetrating radar (GPR) methods and their history in archaeology. Chapters 2, 3 & 4 cover technical considerations of GPR surveys, equipment, and post-acquisition data processing. Chapter 5 briefly illustrates synthetic GPR modeling of archaeological features and chapter 6 describes the use of velocity analyses in GPR. Chapters 7 & 8 present the results of various GPR surveys from around the world emphasizing the use of amplitude time slices to present 3-dimensional data in a 2-dimensional format. Chapter 9 concludes with the authors stressing the integration of GPR into archaeological research, not simply as an anomaly finder, but as part of the overall interpretive framework for sites. Chapter 9 also provides a feasibility table assessing the appropriateness of a GPR survey for a variety of archaeological targets.
In chapter 1 the authors clearly identify the intended audience of the book, stating that most Aarchaeologists trained today have more than enough scientific background to allow them to understand and use this exciting and promising method of archaeological mapping (Conyers & Goodman 1997:17). The use of the term archaeological mapping versus archaeological prospecting is deliberate. The use of GPR as a means to more thoroughly investigate a site, rather than a simple prelude to excavation, is thus emphasized.
The specifics of the GPR method are presented in the next chapter (2) in a clear non-technical manner. The authors introduce many terms and concepts related to GPR survey yet they do not overwhelm the reader with excessive details. The physical specifications of various GPR receivers, general survey logistics and radar signal properties are addressed through the judicious use of illustrations. The use of simple equations aids in the understanding of the material.
The great advantage of GPR over other geophysical methods is its ability to determine the actual depth of archaeological features and model them in three dimensions. The key principle of relative dielectric permitivity (RDP), the property which determines the velocity at which radar waves travel through a substance, is introduced. A series of clear, well-executed illustrations does much to simplify some technical aspects of GPR. Variables critical to the design of a successful GPR survey such as the relationship of the receiver wavelength to the depth and dimensions of features sought are discussed in detail. The chapter gives the novice a good overview of GPR survey logistics, as well as the factors that will eventually determine the success of a GPR survey.
Chapter 3 focuses on GPR data gathering. Basic procedures are discussed, including surveying in grids, correcting for topography and the mapping of natural features. Specialized GPR survey parameters are also presented and explained. The time window (amount of time the receiving antenna will listen for the returning signal), samples per scan (especially the relationship between sample interval and possible feature resolution) and other pre-survey adjustments are described.
Post-acquisition data processing is described in the next chapter (4). A brief discussion of signal filtering techniques (which curiously includes filters which are not presently applicable to GPR) unfortunately does not address computer programs for the application of these filters. This problem recurs in the ensuing chapter on synthetic radargrams and continues throughout the book. Despite the fact that most GPR filtering software is custom-written by geophysicists, some suggestions as to where one could acquire signal processing software that could be adapted to the analysis of GPR would be quite helpful, especially considering the introductory nature of the book.
Chapter 6 presents a thorough discussion of the various velocity tests which can be employed at a site. These tests determine the radar wave velocity by measuring the amount of time it takes for a radar wave to travel a known distance. Once this velocity is known, time-depth conversions allow the display of GPR data as a series of time slices which represent real horizontal slices of the subsurface. The authors emphasize the fundamental role of the velocity test, through which one determines the RDP of the various soil strata, and the need to redetermine wave velocity with each new survey due to the effect of soil moisture content on RDP. Overall, Conyers and Goodman do an excellent job explaining time slices and the methods through which digital imaging algorithms can be applied to GPR data.
The authors complement their theoretical discussion with some very impressive examples of GPR surveys, mostly from Japan and El Salvador, that effectively illustrate radars great promise. Most of the surveys are presented as a series of time slices (well illustrated in a series of color plates), which allows the remote construction of horizontal sub surfaces. The arrangement of these amplitude slices in horizontal layers, representing real depths, is in a visual format familiar to archaeologists, analogous to that of excavation levels. By presenting GPR results in this manner they are made much more accessible to archaeologists.
A series of surveys in chapter 8 demonstrates the use and importance of GPR surveys in real archaeological situations. The surveys also provide excellent examples of the application of filters to correct for surface topography, uneven subsurface topography and the filtering of the near field zone to disclose shallow features. The use of GPR for feature interpretation is illustrated in the radar survey of the Nyutabaru burial mound group, which provides data that allow the formulation of probable scenarios of construction and abandonment. Moreover, successful surveys from North America, including Spiro and Shawnee Creek, acquaint the reader with the ability of GPR to work well on non-volcanic landscapes.
Despite the fact that the examples presented in chapter 8 show the exceptional promise of GPR, two critiques arise, one general and the other of a specific nature. At least five of the GPR surveys presented leave out the method by which the soil RDP was established. Having absorbed the directive of chapter 6, I was quite interested in the type of velocity test used. The bar method is the preferred velocity test but it requires an open excavation, a condition not always present on prospective survey sites. Since there are alternative velocity tests which do not require open excavations, I was interested to see if theoretically less effective ways of establishing soil velocity would still produce useful results.
My specific critique focuses on the comparison of geophysical techniques at the Matsuzaki site in chapter 8. The comparison of magnetic, resistivity and GPR surveys focused exclusively on the superiority of GPR over the other two techniques. Most of us who have incorporated geophysical techniques into our research are aware that cross-referencing geophysical surveys dramatically increases the ability to identify features. Rather than simply extolling the virtues of GPR, the integration of the different information sets would have set a better precedent. As in most archaeological endeavors, a single technique is unlikely to answer all our questions. The use of multiple geophysical survey methods is crucial to the success of such techniques in archaeology.
This book is a valuable guide to any archaeologist interested in the application of GPR surveys. The authors should be complimented on their efforts to write at a level approachable by the general archaeological public while at the same time providing sufficient information to get one started in GPR surveying.