SUBSURFACE INVESTIGATIONS FOR WATER USING CONTROLLED-SOURCE EM, DC RESISTIVITY AND POTENTIAL FIELDS

Groundwater aquifer systems are a viable target for a number of geophysical methods because of the unique physical properties of water. Compared with typical near-surface Earth materials, water has lower mass density, lower electrical resistivity, lower magnetic susceptibility, and a higher dielectric constant. For magnetic susceptibility and electrical resistivity, the property contrast between water and surrounding aquifer materials may span several orders of magnitude. Using field data examples, we demonstrate how we can exploit these strong property contrasts to better understand subsurface water distribution.

For the first example, controlled-source EM data were used to image the geological structure of a proposed aquifer storage and recovery site in Southern California. Because electromagnetic fields are sensitive to the distribution of subsurface fluids, these data were useful in imaging the preferred fluid migration pathways and the subsurface structure of the study area.

The DC resistivity data for the second example were collected at four distinct times over a period of one year to image the changing water saturation within a small watershed in Southwest Idaho. Most earth materials are poor conductors of electricity, especially when dry, thus water saturation is the dominant cause of subsurface variations in electrical resistivity. By imaging the geoelectrical structure of the subsurface during wet and dry seasons, we were able to relate the electrical property variations to saturation changes using petrophysical relationships.

In the third field study, total field magnetics data were used to image the fluid flow conduits within a hot spring system in Southeastern Oregon. The magnetic susceptibility model for the area agrees closely with the known hot spring locations at the site.