Imaging at Depth: Applying Lessons Learned in Geothermal Exploration to General Ground Water Investigation

For the better part of a decade, researchers have understood the potential of Controlled Source-Audio Frequency Domain Magnetics (CS-AFDM) for use in ground water imaging.  The standard method for applying CS-AFDM entails the strategic placement of electrodes to induce a low voltage, low amperage, high frequency audio electrical current into the groundwater system.  This current naturally gathers in areas of highest conductivity.  Per the Biot-Savart law, this current emits a magnetic field that reveals vital information about the location, character and preferential flow paths of the groundwater system through which it passes.  When properly captured, measured, filtered, and reduced, the data derived from that magnetic field can be used to create both two-dimensional maps and three-dimensional models of the subsurface water network.

This process has proved effective in ground water characterizations at relatively shallow depths, usually for the purposes of diagnosing dam seepage or charting the paths of pollution plumes.  Yet, although actual applications had until recent years remained close to the surface, researchers have been aware that CS-AFDM methods hold the potential to go much deeper.  In contrast to resistivity surveys, which charge the surface of a project area, there seemed to be no reason why the CS-AFDM method would not be able to accurately image groundwater systems up to and beyond one kilometer.

That assumption was tested in a geothermal investigation that took place in central Utah.  The details of that project are the focus of this presentation.  In an effort to properly characterize the deep geothermal reservoir, investigators employed a CS-AFDM technique that indeed proved capable of properly characterizing the subsurface water at depths beyond 5,000 ft. This paper will discuss the lessons learned from that deep-water application and consider their implications for a wider range of ground water investigations.