Controlled-Source Electromagnetic Mapping Of A Faulted Sandstone Aquifer In Central Texas

We have used a standard controlled-source electromagnetic (CSEM) mapping technique<br>to identify lateral variations in electrical conductivity associated with faults and lithological<br>contacts in a shallow sandstone aquifer in Mason County, central Texas. Faulting<br>often plays a significant role in affecting groundwater flow. Faults can act as barriers or<br>conduits to fluid flow. The structural geology and role of the faults in affecting groundwater<br>flow in this aquifer, the Hickory Sandstone, have been previously studied. Our results<br>indicate that the CSEM technique provides useful complementary subsurface structural<br>information that can be used to refine hydrological models and their interpretation for<br>local water resources management and conservation.<br>Electrical conductivity profiles are inexpensive and convenient to obtain, compared to subsurface<br>information acquired by conventional invasive methods such as drilling. However,<br>electrical conductivity profiles are indirect hydrogeological indicators that require prior<br>geological knowledge for accurate interpretation. In the Hickory sandstone aquifer the<br>distribution of clays affects both hydraulic and electrical conductivity. Previous geological<br>studies have determined that different stratigraphic units within the Hickory aquifer<br>contain characteristic amounts of clay. We have interpreted lateral variations of apparent<br>electrical conductivity profiles in terms of contacts or faults juxtaposing different stratigraphic<br>units. CSEM measurements have much better shallow fault-resolving power than<br>the gravity data collected earlier at this site. The gravity data appear to respond primarily<br>to undulations of the crystalline basement at this site.