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This monograph is concerned with observations of earth resistance as observed at the surface of the Earth by measuring the potential field set up on passing an electric current between specific surface locations.

Such observations are normally referred to as apparent resistivity measurements, and a legitimate objective of conducting such observations is to gain an insight into the distribution of true electrical resistivity within the subsurface.
Since, owing to the complexity of the problem, it is not possible to proceed from surface observations to derive an unambiguous subsurface resistivity distribution, interpretation rests rather on recognising signatures in the surface observations and interpreting them in terms of probable simple structures. A danger, implicit in such an approach, is that an observer wishing to use a specific interpretation system may already be postulating the likely nature of the target and may, as a consequence, ignore important information (as noise) or, by his field practices, fail to gather such information.

This monograph is thus directed primarily to a consideration of the total observable information available to the observer and how it may be sampled and assembled. Insofar as the resistivity observations at a site endure, so the investigated field pattern will endure, regardless of the interpretation purposes to which it is subsequently put. Not surprisingly, revealed field patterns are often complex and substantial space is also given to subsequent interpretation. This part of the monograph is necessarily more restricted and condensed. For this reason, considerations of potential theory, which are given at length elsewhere, have largely been omitted and such solutions introduced where necessary without proofs.

1 The measurement of an earth resistance (Electrode locations, Electrode polarisation, Natural earth potentials, Current injection and contact resistance, Potential field in a uniform and isotropic medium, Stake resistance, Earth resistance measurement using four electrodes, Signal strength considerations, D.C. measurements, The induced polarisation effect, Commutated D.C. methods) 1

2 The nature of an earth resistance measurement (The "two electrode" configuration, Earth resistance space, "Three electrode" arrangements, Four electrode arrangements, The square array, Variable configurations, Multiple electrode configurations, Errors in earth resistance measurement, Topographic effects) 14

3 The concept of apparent resistivity (Apparent resistivity based on a two electrode configuration, Four electrode apparent resistivity, Orientational variation, Crossed observations and inhomogeneity ratios) 30

4 Basic exploration procedures (Resistivity mapping, Resistivity sounding, Wenner sounding, Square array sounding, Traverse of soundings, Saturation coverage) 46

5 Interpretation. Qualitative and "layered media" methods (Resistivity map interpretation, Sounding interpretation, A traverse of soundings) 67

6 Interpretation. Treatment of two and three dimensional structures (The single dipping interface, Parallel interfaces and the dyke problem, Other structures, Tank analogue solutions, Composition, Direct space calculations) 95

7 Orientational variation of apparent resistivity (Potential in a uniform anisotropic medium, Potential at the surface of an anisotropic half space, Prolate magnitudes of anisotropy (k), The effect of anisotropy on four electrode resistivity measures, Comparison of the performance of four electrode arrays, The anisotropic definition of apparent resistivity and anisotropic analyses of crossed square data, General analysis of crossed square data, Use of the apparent anisotropic parameters n and Q, Orientational variation of anisotropic parameters, Spatial representations) 118

8 Conclusion / References / Appendix: Tables of apparent resistivity (square array) over a dipping interface / Index 139-152