CHAPTER 3: PROFILING METHODS USING SMALL SOURCES
Frank C. Frischknecht, Victor F. Labson, Brian R. Spies, Walter L. Anderson, 1991. "PROFILING METHODS USING SMALL SOURCES", Electromagnetic Methods in Applied Geophysics: Volume 2, Application, Parts A and B, Misac N. Nabighian
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Chapter 3 treats electromagnetic (EM) profiling methods using small dipolaf sources. Profiling techniques are designed to detect changes in electrical conductivity laterally, along a traverse, in contrast to sounding techniques which are designed to determine variations in the conductivity of the earth with depth (see Spies and Frischknecht, this volume). The distinction between the two approaches is blurred when broadband or multispacing measurements are made at closely spaced stations. The source and receiver antennas are usually small loops which can be treated as magnetic dipoles, although, in practice, a grounded electric dipole could be used for either of the antennas. When dipolar sources are used in fixed positions, the methods have much in common with large source methods, so there is overlap between Chapter 3 and Chapter 4 by Parasnis (this volume). Most of the profiling techniques described operate in the frequency domain (which reflects their historical development), but many of the interpretation procedures are applicable in the time domain. A detailed discussion of time-domain methods is given in Nabighian and Macnae (this volume).
The first and most common use of dipolaf source EM profiling methods is detection and characterization of highly conductive bodies, and most instrumentation and interpretative techniques have been developed for this application. In direct exploration for conductive mineral deposits the geologic model is generally assumed to consist of a highly conductive target such as a massive sulfide ore body in a much less conductive host rock covered by an overburden of intermediate conductivity. For this model to be realistic, however, varying the composition and thickness of the overburden may be necessary.
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Electromagnetic Methods in Applied Geophysics, Volume I, Theory presented the mathematical and physical foundations common to all EM methods. The purpose of Volume I was to help facilitate the understanding of the theory involved and to provide a limited amount of interpretational aids. Volume II, Applications is devoted to a method-by-method treatment of the principal EM techniques in common use.