When considered relative to the study of the atmosphere, rivers, lakes and oceans, the study of groundwater suffers from the disadvantage that groundwater is not visible. Boreholes are required to obtain direct measurements of groundwater systems, but boreholes are costly and disrupt subsurface conditions. Electrical imaging does not require drilling and provides insight about groundwater systems. This book describes that powerful geophysical method known as electrical imaging. It is founded on Ohm’s Law in much the same way that understanding of groundwater flow is based on Darcy’s Law. Darcy’s Law concerns the resistance to water flow through permeable media whereas Ohm’s Law involves the resistance of current flow through geological media. The aim of electrical imaging is to scan the subsurface by applying an electric current to the ground and monitoring voltage at many locations. Most often electrical imaging is employed to learn about the geology of a study area, although it can also be used to estimate depth to water or to find zones of saline water or oily industrial liquids. Learning about geology is essential to understanding a groundwater system. Drilled holes reveal the geology at each drill location but interpolation of the geology between holes is fraught with uncertainty. Electrical imaging can reduce the number of boreholes needed to characterize the subsurface or guide where boreholes can be most informative. Electrical imaging can also be used to monitor progress when liquids are injected into contaminated aquifers to remove or destroy the contaminants.
The ability of electrical imaging to provide valuable insight has increased markedly over the past few decades as technology for measurement of electrical responses and computing power for analyzing the responses have advanced with decreasing cost. Still, conversion of the data into meaningful information, requires careful design and analysis as well as consideration of multiple working hypotheses of subsurface conditions to best explain the electrical signals. This book enhances the reader’s ability to use electrical imaging in understanding groundwater systems.
The authors of this book: Kamini Singha, a professor of hydrogeologic science at Colorado School of Mines; Timothy Johnson, a research geophysicist with Pacific Northwest National Laboratory; Frederick Day-Lewis, a chief geophysicist with Pacific Northwest National Laboratory, and Lee Slater, a professor of geophysics at Rutgers University, have many decades of cumulative experience with research, teaching and practice of electrical imaging. Herein, they explain, demonstrate and document best practices for collecting and analyzing electrical imaging data. They provide guidelines for those who decide to use this powerful geophysical method to reveal essential hydrogeologic information that is not feasibly obtained by other means.
John Cherry, The Groundwater Project Leader
Guelph, Ontario, Canada, November 2021