The volcanic field, sometimes called the Uvalde igneous field, is centered in Uvalde County and extends west into Kinney County, south into Zavala County, and to the east as far as Bexar County. Isolated dikes and plugs also occur to the north in the Texas Hill Country near Austin. New occurrences of intrusive bodies at both surface and subsurface levels were unveiled by the FY 2001 aeromagnetic survey (http://pubs.usgs.gov/of/2002/ofr-02-0049/). A GIS map and in-depth interpretation of the aeromagnetic data is currently in review.
The Balcones fault zone (BFZ) of south-central Texas developed along the subsurface trend of the Quachita fold belt extends from Travis County south through Bexar County and west into eastern Kinney County (see section on Geologic Mapping). Miocene-age faults within the BFZ are en echelon, with primarily normal displacement, trending northeast, and are downthrown to the south and southeast. Numerous smaller cross-faults, trending to the north and northwest, connect the larger primary faults. Intrusions of the Balcones type are profuse in both Uvalde and Kinney Counties where the Ouachita belt trend changes sharply. This suggests a relation between the older Ouchita fold belt and younger Cretaceous-age intrusions (see section on the Argon40/Argon39 Geochronology of the southern Balcones volcanic intrusive rocks).
The rocks comprising the Edwards aquifer and the lower and upper confining units are essentially nonmagnetic, with magnetic susceptibilities on the order of 0.000010 emu (electromagnetic units). The igneous rocks, by contrast, have susceptibilities near 0.000400 emu. Follow-up ground geophysical surveys and further modeling of the magnetic data will give a better structural description of the intrusive igneous bodies. Increased knowledge about the complex hydrologic processes that control water movement and storage in the Edwards aquifer is imperative for optimal resource management. This information will be used to improve studies of the surface and subsurface geometry as shown by current 3-D EarthVision and MODFLOW models.
The San Antonio audio-magnetotelluric data sets are one part of a larger study of the Trinity and Edwards aquifers by the U.S. Geological Survey (http://pubs.usgs.gov/of/2002/of02-118/). The AMT line locations depicted in the 2002 USGS study were specifically selected to resolve a number of key geological issues including: (1) the fresh-/saline-water interface near the town of Kyle (see section on the noble gas isotopes of the fresh-/saline water interface) and the Tri-County area (intersection of Bexar, Comal, and Guadalupe counties), (2) a water-filled cave and fault structures along Honey Creek, and (3) a volcanic plug along the Frio River located south of U.S. Highway 90 near the town of Knippa in Uvalde County. The study summarized here concerns the near-surface volcanic plug along the Frio River.
AMT is an electrical technique that uses either natural signals or a controlled transmitter to measure earth conductivity as a function of depth, much like a conventional resistivity survey. All of the sounding sites in this survey were shot using the Geometric EH-4 receiver augmented by a transmitter to fill in the middle frequencies (450-4,500 Hz) where the natural signal strength is sometimes below the detection limit of the instrument. The Bostick Depth Section shows two prominent features with high resistivities assumed to be the subsurface expression of two igneous intrusions identified from the 2001 aeromagnetic survey.
A helicopter electromagnetic and magnetic (HEM) survey was completed in 2002 for a 209 square kilometer (81 square miles) area (http://pubs.usgs.gov/of/2003/ofr-03-226/) in the Seco Creek drainage area, Medina and Uvalde Counties. The survey area was centered on the Valdina Farms sinkhole, a significant karst feature in western Medina County. Flight lines were flown north to south with three east - west tie lines to aid in leveling the magnetic data. Additional lines were flown on each side of the Seco and Little Seco Creek drainages. A five kilometer (4 mile) extension of 15 lines was flown north of the main survey block centered on Seco Creek.
The Edwards aquifer in this area consists of three hydrologic zones: the catchment, recharge, and confined. The upper part of the Glen Rose Limestone containing the upper Trinity aquifer is exposed in the catchment area. The recharge zone is located in the Balcones fault zone where the Devils River Formation of the Edwards aquifer is exposed by normal faults. The magnetic data is not discussed in detail here but does render high-amplitude closed anomalies caused by shallow igneous intrusives. The Woodard Cave fault that separates the catchment and recharge zones is, in places, associated with a weak linear magnetic low.
The HEM data has been processed to produce apparent resistivities for each of the six EM coil pairs and frequencies. The higher frequencies have the least depth of penetration. Each part of the Edwards aquifer system is characterized by contrasting apparent resistivities. A map of the 100 kHz apparent resistivity shows that the catchment, recharge, and confined zones all have numerous linear features that are likely caused by tectonic structures, many of which are unmapped. The maximum depth of penetration for this band would be 3-5 meters. From this frequency, the higher-order colors (reds and purples) denote more resistive lithologies (limestone and dolomite) and the softer colors (blues) denote conductive lithologies (shales and mudstones).
The reds and purples are indicative of the Devils River Formation, Buda Limestone, etc. The blues denote the Del Rio Clay and the Eagle Ford Shale. The intermediate colors represent the Austin Chalk and the Glen Rose Limestone (greens). Linear apparent resistivity features correlate with known structures and possibly structures that have not been mapped. Below is an example of the structural data that can be obtained along just one HEM flight line (line 10370).
Apparent resistivity is the resistivity of a homogeneous isotropic volume that would give the same electromagnetic signal as measured by the HEM system. High-resistivity linear features in the Edwards recharge area suggest structures along which there may be increased permeability due to fracturing and/or karst development. The resistivity cross-section indicates the presence of faults and may possibly delineate the void space of the Valdina Farms sinkhole.
Apparent resistivity maps are shown with color scales that reflect maximum (red) and minimum (blue) values for the indicated frequency. Thus, particular color scales are unique to each map. Heavy, wavy blue lines represent the major drainages of the study area and black lines show boundaries of the military camps. Decreasing frequencies probe deeper, with specific depth of investigation dependant on the resistivity of the subsurface. At the highest frequency (115 kHz), the exploration depth is only a few meters at most. The maximum exploration depth at the lowest frequency (400 Hz) is on the order of 100 meters in areas of highest resistivity.
The multi-frequency resistivity data can be used to produce three-dimensional images of subsurface electrical properties similar to the way CAT scans are used to image the human body. Some challenges in doing this imaging are unique to airborne electromagnetic geophysics. As demonstrated for the Seco Creek HEM survey, structural and lithologic subsurface imaging can provide a valuable contribution to hydrogeologic studies.
Geologic mapping in the Camp Bullis area reflects the complexity of the Cretaceous Trinity (Glen Rose Limestone) and Edwards aquifers. The Edwards recharge zone at the southern end of the survey is characterized by resistive strata. The Glen Rose is composed of resistive limestones and more conductive silts and mudstones. The diagram below shows the geoelectric signatures and geologic map of the Camp Bullis area. More detailed distribution of limestones is shown in the upper part of the Glen Rose Limestone and reefal features are better defined in the lower part of the unit.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://esp.cr.usgs.gov/info/edwards/geophys.html
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Page Last Modified: Wed 12-Oct-2005 10:02:32 MDT
