Paine, J. G., and Collins, E. W., 2003, Ground-based geophysical investigations in the Seco Creek area, Medina County, Texas: report prepared for the U.S. Geological Survey under order no. 02CRSA0768, 36 p.
[Abstract in Word format]

Ground-Based Geophysical Investigations in the Seco Creek Area, Medina County, Texas
Jeffrey G. Paine, principal investigator

In 2002, the U.S. Geological Survey (USGS) conducted an airborne geophysical survey of the Seco Creek area in Medina and Uvalde Counties, Texas, to better understand geological and hydrological issues relating to this important recharge area for the Edwards aquifer. We began working with the USGS in 2003 to conduct ground-based electromagnetic induction measurements in support of the airborne geophysical effort and to examine the influence of rock type and geological structure on the geophysical measurements.

We collected ground conductivity measurements along nine lines on and near Seco Creek, Medina County, Texas. These ground-based measurements demonstrate that (a) mapped geologic units consisting of Cretaceous limestones and dolomitized limestones, marls, mudstones, and shales and Quaternary alluvial deposits have mappable differences in apparent conductivity, (b) geologic features such as faults and karst can have detectable apparent conductivity signatures, and (c) conductivity measurements can be combined with geologic maps and outcrop studies to identify hidden contacts, covered strata, and unmapped structural features. Limited comparisons of measurements from ground and airborne instruments confirm that the instruments produce similar apparent conductivities at the same primary frequency and coil orientation. Ground instruments can be used to capture small-scale lateral conductivity change, complementing smoothed but spatially dense airborne measurements over large areas that are inaccessible or impractical to survey using ground-based instruments.

For more infomation, please contact Jeff Paine, principal investigator. Telephone 512-471-1260;
March 2004