Schmidt - Earth Resistance for Archaeologists

Acknowledgments

First of all, I would like to thank Prof. Arnold Aspinall, Dr. Chris Gaffney, and Rob Fry for reading the entire manuscript and giving excellent comments. It was Arnold who converted me from a pure magnetist to a multimethod enthusiastmany thanks! I also wish to thank my editor, Wendi Schnaufer, for her patience, encouragement, and interest in this book. The book is also dedicated to my students, whose enthusiasm for archaeological geophysics often matched my own. Thanks also to Prof. Robin Coningham, Dr. Jan Harding, and Dr. Rick Jones for leading the archaeology teams on the case studies presented here and for their permission to use the relevant data. Thanks also to Dr. Roger Walker from Geoscan Research for providing access to the MSP40, for permission to use and reproduce data, and for insightful general support. Stuart Fox, thanks a lot, for fabulous technical support, including on fieldwork spent more often than not in pouring rain, and to Jane Hammond for rallying the postgraduate students. Thanks to my father, Siegfried, for constantly reminding me that the book isnt finished yet, and to my wife for unwavering encouragement despite a complete lack of interest in archaeological geophysics. Large parts of the book were written during my time at Queens College, University of Melbournemany thanks for the excellent research environment and friendship! The book was finished at my parents-in-laws house; thanks to Albert and Mathilde for their hospitality. Last but not least, thanks to the University of Bradford, Department of Archaeological Sciences, for being an invigorating place for seventeen years of my academic career, and for providing the resources to collect and process the geophysical data.

Introduction

Trailing wiresthats probably the first thing that comes to mind when thinking of electrical methods in archaeology. But there is much more to it. While magnetometer surveys are the workhorse of archaeological geophysics (Clark 1996; Gaffney and Gater 2003; Aspinall et al. 2008), and ground penetrating radar (GPR) has become the favorite technique (Conyers 2004) of many archaeological geophysicists, earth resistance methods are key to detecting many other archaeological remains, yet they have not been given nearly enough attention.

This book will redress the balance, giving archaeologists the know-how required to exploit the potential of earth resistance methods. A wide variety of possible uses are explored, including cases in which earth resistance surveys succeeded in mapping buried archaeological remains that magnetometer surveys were unable to detect, or in which they provided a very different view of the ground than GPR surveys could. The examples include prehistoric henges, Buddhas palace at Tilaurakot, Roman forts, Scottish brochs, and many other sites where new archaeological insights were gained from carefully interpreted earth resistance data. The archaeological features that can be detected through earth resistance methods are varied, ranging from ditches, pits, and grave cuts to stone and brick foundations, and even including whole landscapes. While area surveys were traditionally the most common earth resistance method, depth profiling and vertical imaging have become well-developed tools that allow electrical depth investigations in three dimensions. Both measurement techniques will be described in detail.

The challenges faced when using earth resistance surveys are best highlighted in comparison to magnetometer investigations of archaeological remains.

• Magnetic properties of archaeological features are nearly independent of current environmental factors, and results of a magnetometer survey will look the same, whatever the weather. It is quite the opposite for electrical investigations. Earth resistance anomalies are created by moisture variations in the ground and can even depend on weather events that happened several weeks prior to a survey, similar to cropmarks in aerial archaeology. It is therefore crucial to understand how these anomalies are created so that realistic archaeological interpretations can be obtained.
• As with most other geophysical techniques, visualized data from earth resistance surveys are not direct images of the underlying archaeological features. Instead, they are created as a result of the measurement process, which needs to be understood to derive the outlines of the buried remains.
• Magnetic fields of archaeological features exist whether measured or not, and in its simplest form there is not much more to a magnetometer survey than to switch on the instrument and go. Earth resistance measurements are active (i.e., create their own electrical fields), and there are many survey parameters to adjust. For example, one needs to decide on the best electrode configuration for a site or the earth resistance meter has to be adjusted to allow measurements even in difficult conditions.

The knowledge required to interpret earth resistance anomalies and to select instrument configurations will be conveyed in this book. It provides a foundation from which to explore these issues without requiring a degree in physics or mathematics. A balance between the underlying principles and practical examples has been chosen in order to explain the relationship between parameters with as few mathematical expressions as possible. Instead, easy-to-understand analogies are used to illuminate important principles.