Chapter 10: Electromagnetic Induction Methods for Environmental Problems
Introduction Electromagnetic methods have a special place in the arsenal of geophysical tools available for environmental investigations. They owe their status to five factors. First, many environmental investigations are concerned with detecting the location of contaminants in groundwater and identifying pathways for contaminant transport. These types of problems boil down to learning as much as possible about the fluids in the pore spaces. Contaminants can modify both the electrical conductivity and dielectric constant of the pore fluid, and electromagnetic and electrical methods are the only geophysical methods that are directly influenced by the electrical properties of pore fluids. Second, electromagnetic techniques are sensitive to changes in geology such as rock type, porosity, grain size, fractures, and clay content. This sensitivity allows electromagnetic methods to be used for geologic mapping, which is of great value in developing geological and hydrological models needed for environmental studies. Third, there is a wide range of electromagnetic equipment available for purchase or rental. Over the past 30 years companies such as Geonics, Zonge Engineering, GeoInstruments, Geophex, Iris Instruments, Geosystems, and others have developed modern, reliable instrumentation making electromagnetic surveys relatively easy to perform. Hopefully, this new found ease of data collection does not lull the naïve and untrained into thinking that these methods are without pitfalls. Fourth, electromagnetic techniques are noninvasive. While drilling is often necessary to unequivocally confirm an interpretation, drilling can worsen an environmental problem by puncturing an intact, buried container, or by producing a high permeability flow path for further contaminant migration. Finally, compared to other geophysical methods and to drilling, electromagnetic techniques are relatively inexpensive due to their ease of use and low labor requirements.
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Near-surface geophysics uses the investigational methods of geophysics to study the nature of the very outermost part of the earth’s crust. Man interacts with this part of the earth’s crust: he walks on it; he drills and excavates into it; he constructs structures on and in it; he utilizes its water and mineral resources; and his wastes are stored on and in it and seep into it. The very outermost part of the Earth’s crust is extremely dynamic-in both technical (physical properties) and nontechnical (political, social, legal) terms-which leads to both technical and nontechnical challenges that are much different than the challenges faced by “traditional” applications of geophysics for regional geologic mapping and for oil and gas exploration (see Chapter 2).