Iowa Geological & Water Survey Geophysics Program
Using Geophysics to Benefit Society
Knowledge about the shallow subsurface is crucial to understanding and finding solutions for a host of geologic and hydrogeologic investigations. The IGWS is utilizing geophysics as a tool for gathering subsurface information to answer basic geologic questions. Geophysical surveys are enabling IGWS to better target subsurface drilling and, in some instances, minimize drilling. To date, geophysical techniques have been used by the IGWS to:
Identify shallow bedrock and karst terrains.
Characterize fracture patterns in carbonate bedrock systems.
Assess aggregate resource potential.
Map alluvial aquifers.
Evaluate flood-control levee integrity.
Verify depth to shallow bedrock for geologic mapping projects.
In addition, the IGWS is using geophysical techniques to locate and identify buried tanks as well as other potential contaminant sources.
Electromagnetic Ground Conductivity (EM):
An EM raster model image showing conductivity values across a surveyed area.
Subsurface materials can be characterized by their ability to conduct (or resist) electrical currents, which is determined by various physical and chemical properties not limited to grain-size, moisture content, mineralogy, and ion concentrations. When an efficient, non-invasive characterization of lateral variations in the shallow subsurface is desired, EM is often the preferred geophysical method. Ground conductivity data are useful in differentiating average lithologic differences or determining the extent of contaminant plumes. IGWS uses a Geonics Limited EM-31 MK2 electromagnetic (EM) ground conductivity meter to conduct EM surveys. EM field measurements are obtained by walking transects with the EM-31 at hip level, while data are recorded onto a field computer. The unit is 12 feet in length with one end serving as a transmitter coil and the other as a receiver coil. The EM-31 produces average conductivity and/or resistivity (quadrature component, mS/m) and in-phase (metal content, ppt) to approximately 20 feet below ground surface. While walking in roughly straight lines, the unit is capable of recording multiple measurements per second, with each measurement associated with a precise GPS location. Alternatively, the unit can be mounted to an all terrain vehicle (ATV) to provide efficient means of gathering data over large areas. Once plotted using GIS software, measurements can be displayed in a plan view raster. Further investigation of areas of interest can be completed using ER.
An EM survey can be conducted on foot, and/or by utilizing an ATV.
Electrical Resistivity (ER):
When characterization of the vertical and horizontal dimensions of stratigraphic units or contaminated zones is required, ER profiling is often the preferred geophysical method. This method is more time consuming than performing an EM survey, but provides better resolution of subsurface variations and is able to penetrate greater depths. The IGWS uses an Advanced Geosciences Inc. (AGI) SuperSting R8 ER meter to conduct surveys. The field measurement of ER is obtained by injecting a known direct current via transmitter electrodes and measuring voltage differences between potential electrodes. An array of up to 56 stainless steel electrodes are spaced up to 20 feet apart, planted in the ground to a depth of approximately 1 foot and connected via electrode cables to a multi-channel resistivity meter. In areas where the surface is impenetrable, such as hard rock surface or concrete, electrodes can be coupled to the ground using sand piles. Measurements are collected and stored in a predetermined manner, then processed using AGI EarthImager inversion software. Final two-dimensional models are produced showing how subsurface materials respond to electrical properties. Model images seek to display their best indication of what the subsurface might look like, to estimated depths of up to 280 feet. The geometry of subsurface geologic materials can be determined when electrical resistivities are properly correlated with nearby borehole data. In addition to ER, the SuperSting can be used in Self Potential (SP) surveying, which detects natural currents in subsurface materials often associated with groundwater flow. Induced Polarization (IP) surveys can also be completed, which measure the ability of subsurface materials to hold a charge. IP is commonly used to determine the extent of ore-bodies in mining regions, but could be useful for groundwater studies in Iowa where ER is insufficient.
The ER meter/processing unit, switch box and 12 volt battery, as well as the electrodes and cables attached to the unit.
For more information about the Geophysics Program at the Iowa Geological & Water Survey, please contact Jason Vogelgesang.
Link to more information on Geophysics:
Society of Exploration Geophysisists