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Transient Em Induction

Authors:
  • Geomaxo Limited (UK)

Abstract

Landfill sites commonly use the space available in disused quarries or special purpose-built structures but not all past landfill operations were adequately controlled or documented such that the site boundaries, and the type and volume of fill are unknown in some old covered landfill sites. Even in controlled sites, the final form and depth extent of the landfill may not conform to those indicated in the original plan submitted to the regulatory authorities during the application for a site license. Thus, a significant amount of work is required in order to accurately define the relevant parameters of a covered landfill site. Our hydrogeophysical interest in landfill sites lies in assessing the pollution threat they pose since they may contain hazardous substances. In conventional geophysical investigation of landfill sites, the usual goals are to determine the geometrical characteristics (size and shape) of the repository and the physiochemical properties of the infill. Of the several non-invasive geophysical methods used in landfill studies, the electrical and electromagnetic (EM) methods are the most popular owing to their inherent ability to detect changes related to variations in fluid content, chemical composition and temperature in the subsurface, and the minimum capital and labor outlay required to use them in small-scale surveys (Whiteley and Jewell, 1992; Meju 2000). Since the presence of saline fluids in the ground enhances its ability to conduct electrical current, it is possible to locate a leachate plume by measuring the resistivity distribution in the subsurface. The main ground resistivity measurement techniques employed in landfill studies are the direct current (dc) resistivity and/or induced polarization (IP) methods (e.g.
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Article
The first four chapters provide an introduction to electrical techniques in geophysics, covering rock and mineral properties and EM field behaviour. Chapters 5-7 deal with the standard geoelectrical techniques of direct current (DC), magnetotelluric (MT), and controlled source electromagnetic (EM) methods. Chapters 8-11 provide details on aspects of geoelectrical theory common to all techniques, and are: modelling and simulation; insensitivity and simulation; practical aspects of data acquisition; and interpretation. Chapter 12 provides details of other methodologies, such as airborne EM surveys, borehole techniques, ground penetrating radar (GPR), and piezoelectric method (PEP). Chapter 13 gives details of 13 case studies using geoelectrical techniques from around the world, whilst the final chapter provides some generalisations for geoelectrical work. Two appendices provide details on mathematical conventions and FORTRAN programming. -R.Gower
Chapter
The aim of geophysical surveys is to obtain information on subsurface geology. While execution of surveys using specific techniques may differ in detail, it will almost invariably consist of three steps: surveying, data processing, and data interpretation. A successful survey will yield more information on the geological target—its existence, location, shape, size, etc. New information is obtained by interpreting geophysical data. The success of a survey depends to a large extent on decisions made before the survey initiation. An exploration geophysicist working for a mining company is often asked the following question: Can we use geophysics in prospecting for this particular commodity? If yes, what techniques should we use and how do we specify survey parameter. Decisions that are usually based on experience often cannot be justified scientifically. The proper approach would be to carry out test surveys to investigate the physical properties of the target and other bodies that might interfere with its response. In recent years, exploration geophysics has progressed beyond target finding to mapping subsurface geology. Analyzing the sequence of geophysical survey steps as shown in Figure 1, the main flow (surveying, processing, interpretation) and the associated areas of research can be identified. To make an intelligent decision on the use of a technique, the geophysicist should have at least a rudimentary knowledge of the physical properties of the target and the surrounding media the response of which might interfere with target identification. Most physical property studies have been done in the laboratory on samples collected in the field. While this approach may be satisfactory for some geophysical methods (gravity, magnetics), it is not for others. Electrical properties of earth materials vary substantially (by several orders of magnitude) depending on whether they are measured in situ or in a laboratory. It is virtually impossible to simulate real conditions in the laboratory. An attempt can be made to recompose the original water content, but microinhomogeneities typical of many geological environments (e.g., rock fractures and their frequency and variation with depth) cannot be duplicated.
Chapter
INTRODUCTION The Story of Airborne Electromagnetics After the end of World War II, the reconstruction of war-ravaged economies fueled a great demand for natural resources. The emerging Cold War caused explorationists to seek secure supplies in countries geographically and politically close to the United States. With vast areas that were then little explored, Canada was one obvious choice. These circumstances provided a great incentive to develop geophysical methods whereby a sparsely populated country, where the climate is often harsh and frigid for part of the year, could be scanned quickly and effectively for deposits of strategic base metals, such as copper, lead, zinc, and nickel. Airborne magnetometer systems that were developed from early war-time prototypes used in submarine detection became widely used in mineral exploration in Canada. However, it soon became obvious that the magnetic information was of more value indirectly in aiding geologic reconnaissance than it was directly in ore exploration. The abundance of magnetic bodies in deformed metamorphic terrains with base metal potential made it difficult to select specific targets for more detailed exploration on the ground. An alternative or additional technique was, therefore, required to carry out prospecting from the air.