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.
in Word format]
Geophysical Investigations in the Seco Creek Area, Medina County,
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.
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.
more infomation, please contact Jeff Paine, principal investigator.