CONTROLLED-SOURCE ELECTROMAGNETIC MAPPING OF A FAULTED SANDSTONE AQUIFER IN CENTRAL TEXASE. Gorman and M.E. Everett
Dept. of Geology and Geophysics, Texas A&M University,
College Station, TX 77843
An understanding of the response of an aquifer to pumping is important for water resources management and conservation. The response of sandstone aquifers to pumping is often affected by the presence of faults, which can serve either as conduits or barriers to groundwater flow. For example, closely spaced monitoring wells can have greatly different water-level variations if they are separated by a fault. In a regional context, faults tend to compartmentalize aquifers. A knowledge of the subsurface structural geology is therefore essential to understand monitoring-well responses in a faulted aquifer.
We have used a standard controlled-source electromagnetic (CSEM) mapping technique to identify lateral variations in electrical conductivity, which are then interpreted as faults and local structural variations. The study area is the Hickory sandstone aquifer in Mason County, central Texas where the structural geology and role of the faults in affecting groundwater flow have been previously studied. Our research demonstrates that the CSEM technique provides useful structural information that can be applied to hydrological modeling. CSEM mapping techniques are inexpensive, noninvasive and time-efficient compared to conventional hydrologic monitoring. However, the electrical conductivity profiles are indirect hydrogeological indicators that must be interpreted using prior geological knowledge.
In the Hickory sandstone aquifer, for example, the distribution of clays affects both hydraulic and electrical conductivity. Previous geological studies have determined that different stratigraphic units contain characteristic amounts of clay. Therefore, we use electrical conductivity measurements to discern faults juxtaposing different units. The CSEM technique has much better fault-resolving power than other geophysical methods, including ground-penetrating radar and gravity, which were used earlier at this site.