Electromagnetismo

Surveys in Geophysics, 23, 101-132, 2002

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Applications of Electrical and Electromagnetic Methods for Environmental and Geotechnical Investigations
Louise Pellerin University of Utah, Energy and Geoscience Institute Salt Lake City, Utah 84112 E-mail: lpellerin@egi.utah.edu

Abstract. Electrical andelectromagnetic methods are powerful tools in environmental and geotechnical investigations. Techniques developed for deeper applications, such as mining, geothermal and crustal studies, are scaled for shallow targets by moving to higher frequencies, earlier times and/or smaller array configurations. Another extremely important factor is dense station spacing, to reduce spatial aliasing, and high qualitydata to resolve small features. Hence new instruments are concerned with making continuous or dense measurement with high precision, and interpretational methods fast enough to handle large datasets quickly. Continuously measuring electrical and time-domain electromagnetic systems have been developed for geological mapping in hydrological investigations with one-dimensional inversion routines thatare rapid and robust. At a smaller scale an electrical system is used for archaeology studies with excellent results. Working to and above the upper limits of the quasistatic approximation, a very early time electromagnetic system is proving successful at mapping subsurface infrastructure in areas of conductive, clay cover, where ground penetrating radar is ineffective. Induced polarization andresistivity systems that employ multiplexing techniques, while not continuously measuring, allow for relatively rapid production rates and dense sampling for applications ranging from landfill and contaminant characterization studies, to verifying the integrity of engineered subsurface structures and monitoring infiltration of the vadose zone. Keywords: environmental, geotechnical, electrical,electromagnetic, IP, contamination, aquifer mapping, site characterization, hydrological investigation, archaeology

1. Introduction
Merely scaling a deep investigation tool with respect to frequency, time or array configuration to adapt it to near surface investigations is insufficient. It is also important to increase the spatial sampling density and thus reduce aliasing when looking for smalltarget or rapid changes in the near surface geology. As a result electrical resistivity (ER) and electromagnetic (EM) instruments have been developed that make continuous measurements, such as that used with ground penetrating radar (GPR) methods. Traditional discrete measurements systems have been modified with multiplexing large electrode arrays so that large, dense datasets can be collected morerapidly, thereby making them economic to use for routine investigations. The key aspects to densely sampled data include the obvious factors such as enhanced resolution of the subsurface in addition to the ability to identify noise and multi-dimensional effects, and reduce spatially aliasing of the data, which is important for inversion schemes. Some of the most significant achievements have beenaccomplished by focusing on a specific problem and letting the tools be used on different applications, instead of applying generic tools without thought to an optimal application. An excellent example of a transfer of technology from one application to another is the rapid TEM inversion by Christensen (2000) developed to interpret high-density ground data acquired for groundwater characterization.This one-dimensional (1D) inversion code was used on an airborne mining prospect where over 700 000 sounding were inverted in roughly 4 hours (Poulsen et al., 1999). Shown in many case histories reported in this review and in methodology studies (Supper et al., 1999; Vanhalla, 1999) Induced Polarization (IP) is one of the most powerful techniques for environmental application. In 1974 Angoran,...