Thursday, December 6, 2007 : 2:55 p.m.

Geophysical Technology for Groundwater Resource Evaluation: Electrical and Electromagnetic Methods

Louise Pellerin, Ph.D., Green Engineering Inc.

Electrical and electromagnetic (E&EM) geophysical methods are used to estimate the electrical resistivity (or inverse conductivity) of the subsurface, which is determined predominantly by the chemistry of the pore water (dissolved, dissociated ions) and clay content. In addition to geological mapping of stratigraphy and structure, they can be used in a large variety of applications including plume delineation and seawater incursion. E&EM methods can be cost effectively applied to guide future characterization efforts and optimize placement and number of boreholes. Active research is focused on relating subsurface resistivity values to hydrological parameters such as soil moisture and porosity.

With all E&EM methods the Earth is excited through the injection of current, either galvanically, capacitvely or inductively, and a resulting voltage response is measured using electrodes, magnetic sensors or a combination of these. The depth of investigation is controlled by the electrode array configuration for electrical methods, and time-varying fields for EM sounding methods. There are two basic EM sounding techniques: the transient electromagnetic (TEM), and the magnetotelluric (MT) methods, which include audiomagnetotelluric (AMT) and controlled-source AMT (CSAMT). Electrical resistivity surveys are often deployed on the surface and in boreholes. EM data can be acquired on the ground or from the air on a fix-wing or helicopter platform.   

Data can be used to create thematic maps in plan view or inverted to create a two-dimensional (2D) or quasi 2-D resistivity sections from several meters to tens of kilometers. Resistivity can be related to basic geological structure, such as depth to basement, stratigraphy, faults, fractures, paleochannels, and hydrogeological features such as depth to groundwater and aquifer characterization. Full three-dimensional modeling is available for electrical resistivity data and the MT method, but data acquisition can be expensive. The choice of method depends on the geophysical target, local geology and required resolution.

Louise Pellerin, Ph.D., Green Engineering Inc. Working with all aspects of electrical and electromagnetic methods, Louise is the geophysical manager for Green Engineering, Inc. After receiving her BS degree from the University of California, Berkeley she spent several years working in the field before returning to school to earn her MS and Ph.D. degrees from the University of Utah. She has nearly 30 years experience, including positions with the US Geological Survey, a visiting professor at the University of Aarhus, Denmark and a consulting professor at Stanford University. Since 1992 she has been working in near-surface geophysics as related to environmental and hydrological applications.


2007 NGWA Ground Water Expo and Annual Meeting