The Iowa Geological Survey (IGS) incorporates electrical geophysics into a host of geologic and hydrogeologic research projects. Geophysical surveys provide a continuous model of the shallow subsurface, complementing traditional point data.
Subsurface materials can be characterized by their ability to conduct or resist electrical current. Physical and chemical properties such as grain size, moisture content, mineralogy, and ion concentrations influence how geologic material responds to electrical charges. Geophysical models correlate to site geology, showing the orientation and distribution of subsurface materials.
Recent applications of geophysical investigations include:
- Characterization of alluvial and buried sand and gravel aquifers
- Determination of depth to bedrock
- Fate and transport studies
- Characterization of surficial materials
- Evaluation of earthen dam or levee integrity
- Identification of karst regions or fractures
- Subsurface fluid-flow direction
- Delineation of buried man-made objects
Electrical Resistivity Imaging
Electrical resistivity (ER) imaging is the geophysical method most often used by the IGS. ER measurements are obtained by introducing direct current to the subsurface and measuring resulting voltages. ER imaging assists in vertical and horizontal characterization of geologic materials. This method provides a high resolution model showing subsurface variability, penetrating depths of up to 90 meters (300 feet).
Modern geophysical technology opens doors to new and exciting research possibilities. In addition to two-dimensional geophysical site characterizations, the IGS is able to collect and interpret three-dimensional data. Modeling the subsurface in three dimensions provides a more complete understanding of site geology.
Electromagnetic Terrain Conductivity
Electromagnetic (EM) terrain conductivity assists with horizontal characterization of geologic materials. Ground conductivity data can also be useful in determining lateral variability or the extent of contaminant plumes. EM surveying shows average conductivity and in-phase data to approximately 6 meters (20 feet) below the ground surface. The figure above shows EM conductivity results from a recent investigation.
For more information about the Geophysics Program at the Iowa Geological Survey, please contact Jason Vogelgesang.