Characterization of a Complex Permafrost Aquifer System Using Airborne Electromagnetic Data
Monday, March 20, 2017: 1:30 p.m.
Steven Humphrey, PG
,
Geomega, Inc., Boulder, CO
David S. Lipson, Ph.D., PG
,
Hydro Science + Engineering, Arvada, CO
An airborne RESOLVE® electromagnetic geophysical survey was completed in a sub-arctic area near Fairbanks, Alaska, USA, containing a regional permafrost aquifer system in unconsolidated alluvium. The permafrost is discontinuous, highly heterogeneous, and separates a shallow unconfined supra-permafrost aquifer (i.e., above the permafrost) from a deeper sub-permafrost aquifer. The sub- and supra-permafrost aquifers have historically been used for water supply and are hydraulically connected by taliks, surface water features, and other discontinuities. Because of these complexities, characterizing the permafrost aquifer system has been challenging using conventional characterization techniques such as drilling, sampling, and hydraulic testing.
The purpose of the survey was to collect airborne electromagnetic data that would allow us to more precisely map the extent of discontinuities in the permafrost aquifer system and refine a numerical groundwater flow model. Permafrost has a higher electrical resistivity than thawed material, and its resistivity depends on the amount of frozen versus thawed material. Therefore, resistivity data can be used to differentiate zones of permafrost from thawed material.
The RESOLVE® system consists of a 9-meter “bird” containing electromagnetic transmitter and receiver coil pairs, five coplanar and one coaxial, attached to a 30-meter cable hung from a helicopter. As the helicopter is flown over an area, apparent resistivity is generated from the in-phase and quadrature electromagnetic components for multiple coplanar frequencies, using a pseudo-layer half-space model. The multiple frequencies of resistivity data are used to evaluate specified depths, with higher frequencies reflecting shallower depths. The survey area encompassed 12 square miles to a depth of approximately 400 feet below ground.
The data were used to create a geological model which differentiated permafrost from thawed zones in three dimensions and facilitated refinement of the groundwater flow model. Modeling results demonstrate that the model was more accurate after incorporating permafrost discontinuities mapped with the geophysical survey.
Steven Humphrey, PG, Geomega, Inc., Boulder, CO
Steven Humphrey is a hydrogeologist and groundwater modeler at Geomega, Inc. in Boulder, Colorado. He has been professionally active for more than 9 years supporting team-oriented regulatory and litigation projects involving groundwater flow and transport modeling (using MODFLOW and FEFLOW), collection, analysis, and interpretation of groundwater and environmental data, and technical writing and presentation. Steven graduated with a Master's degree in Hydrogeology from the University of Nevada, Reno, and a Bachelor's degree in Geology from California State University, Chico.
David S. Lipson, Ph.D., PG, Hydro Science + Engineering, Arvada, CO
David Lipson has more than 22 years of experience as a contaminant hydrogeologist with particular emphasis on chemical transport, subsurface remediation, and fractured bedrock hydrogeology. He provides technical support on a wide range of groundwater contamination and remediation projects. Lipson is well-versed at using mathematical models, engineering controls, and risk-based corrective action approaches at sites regulated under CERCLA, RCRA, and state-led regulatory programs. He earned a doctorate degree in Geological Engineering at Colorado School of Mines, a master’s degree in Hydrogeology at Syracuse University, and a bachelor’s degree in Geology at the State University of New York.
