References

Blome, C.D., Faith, J.R., Smith, B.D., Collins, E.W., Clark, A.R., and Ozuna, G.B [2005] Geologic mapping of the Edwards and Trinity aquifer systems and its synergy with hydrogeologic modeling. National Ground Water Association 2005 Summit, April 17-20, 2005, San Antonio, abstracts with program, 223-225.

Clark, A.K. [2003] Geologic Framework and Hydrogeologic Features of the Glen Rose Limestone, Camp Bullis Training Site, Bexar County, Texas. U.S. Geological Survey, Water-Resources Investigations Report 03-4081, 9 p.

Sengpiel, K.-P. and Siemon, B. [1998] Examples of 1D inversion of multifrequency AEM data from 3D resistivity distributions. Exploration Geophysics 29, 133-141.

Sengpiel, K.-P. and Siemon, B. [2000] Advanced inversion methods for airborne electromagnetic exploration. Geophysics 65, 1983-1992.

Siemon, B., Auken, E. and Christiansen, A.V. [2007] Laterally constrained inversion of HEM data, Journal of Applied Geophysics, submitted.

Smith, B.D., Faith, J., Cain, M., Clark, A.K., Moore, D.W. and Hill, P.L. [2005] Helicopter Electromagnetic and Magnetic Survey Data and Maps, Northern Bexar County, Texas. U.S. Geological Survey, Open-File Report, 05-1158, 48p.

Cuxhaven

An important reason to map and to understand the subsurface Edwards aquifer is that it is the sole-source water supply for the rapidly expanding city of San Antonio, immediately south of the Bexar HEM survey boundary.

Lithological variations in the Edwards Recharge were mapped in both the shallow and deep subsurface. Images of the subsurface in the Trinity aquifer demonstrated a structural complexity not previously appreciated.

Geophysical mapping in the Trinity aquifer also showed previously unmapped hydrostratigraphical units.

Helicopter-borne Electromagnetics (HEM)

The transmitter signals producing the primary magnetic fields are generated by sinusoidal current flow through the transmitter coils at discrete frequencies.

The oscillating primary magnetic fields induce eddy currents in the subsurface that are dependent on the electrical conductivity distribution.

These currents generate the secondary magnetic fields measured by the receiver coils.

Technical data

RESOLVE system (Fugro Airborne Surveys)

5 horizontal coplanar coils:

400 Hz, 1.5 kHz,

6.2 kHz, 25 kHz,

115 kHz

1 horizontal coaxial coil:

3.3 kHz

Coil separations: ~ 7.9 m

Bird altitude: 30 m

Flight speed: 110 km/h

Sampling distance: 3 m

Line spacing: 100 m

Resolution of hydrostratigraphic parameters from inversion of HEM measurements, Northern Bexar County, Texas

Annika Steuer1 ( annika.steuer@bgr.de ) , Maryla Deszcz-Pan2 and Bruce D. Smith2

1 German Federal Institute for Geosciences and Natural Resources (BGR), 2 U.S. Geological Survey (USGS)

The Project

Helicopter-borne electromagnetics (HEM) measurements were conducted in Bexar County, Texas, to map subsurface electrical resistivity distribution to better understand the geology and hydrology of the Edwards and Trinity (Glen Rose Limestone) aquifers (Smith et al., 2005).

This survey is part of an integrated geological, geophysical, and hydrological study of the Edwards aquifer (Blome et al., 2005).

Helicopter system of the BGR

1-D Inversions

Starting models from apparent resistivities vs. centroid depth values (Figure 4a).

Standard Marquardt-Levenberg 1-D inversions (Sengpiel and Siemon, 1998; Sengpiel and Siemon, 2000) fail to resolve the extension beneath the mesa of resistive fossiliferous limestone (Interval D) exposed in the drainage (Figure 4b).

Laterally constrained inversion (Auken and Christiansen, 2004) recovers a more realistic layered resistivity model (Figure 4c).

L10450

Interval A Alternating and interfingering medium-bedded mudstone, wackestone, and packstone with evaporites locally

Moderately high resistivity

Interval B Alternating and interfingering mudstones marls, wackestone, and packstones

Low resistivity (B1) to high resistivity (B2)

Interval C Yellow-to-white calcareous mud and vuggy mudstone

Low resistivity

Interval D Thin-bedded mudstone; thin-to-medium-bedded wackestone, packstone, and thick bedded rudist biostromes locally

High resistivity (D1) and low resistivity (D2)

Interval E Yellow calcareous mud and vuggy mudstone

Very low resistivity

Table 1: Lithologic units of the upper member of the Glen Rose Limestone (Trinity aquifer) modified after Smith (2003).

Figure 3: The apparent resistivity map for the highest frequency displays different lithologic units of the Trinity aquifer (Smith, 2003). At flight line L10450 different inversion techniques were carried out.

Figure 2: Hydrogeologic map of the northern Bexar County study area modified from Clark (2003). The upper member of the Glen Rose Limestone is divided into five hydrogeological subdivisions: A through E, youngest to oldest, respectively.

Figure 1: Location map of the survey boundaries in northern Bexar County, Texas.

Apparent resistivity map

The apparent resistivity map derived from the highest-frequency HEM measurements correlates with the known outcropping hydrostratigraphical intervals.

Figure 4: The vertical resistivity section at flight line L10450 crossing a mesa structure shows the results of: a) a(z*)-section, b) standard Marquardt-Levenberg 1-D inversion and c) lateral constrained inversion.

Geologic and Hydrologic Setting

One particular area has been chosen for a detailed study where the upper Glen Rose Limestone has been exposed in two drainages with a 30-m flat-top hill (mesa) between them.

The Cretaceous Glen Rose Limestone crops out over most of the study area. The Trinity aquifer (Glen Rose Limestone) consists of alternating mudstones and limestones divided into upper and lower zones (Clark, 2003).

The upper zone is subdivided into five hydrostrati-graphical intervals based on lithology and fossil content.

The lower zone consists of more massive limestone deposits.

The HEM apparent resistivity map (Figure 3) shows that the intervals can be further divided into resistive (limestone) and relatively more conductive (mudstone) units.