MODELING COMPLEX AQUIFER GEOMETRY WITH AIRBORNE, SURFACE, AND SUBSURFACE DATA, SECO CREEK AREA, SOUTH-CENTRAL TEXAS

Three-dimensional structural and stratigraphic modeling are critical to evaluating potential flow paths in complex carbonate aquifer systems of south-central Texas. The Edwards aquifer west of San Antonio consists of about 200 m of relatively uniform reefal limestone (Lower Cretaceous Devils River Formation) confined by low permeability strata of the micritic Glen Rose Limestone below and by the unconformable Upper Cretaceous Del Rio Clay and other units above. This straightforward layered stratigraphy is disrupted by numerous Miocene normal faults of the Balcones fault zone that cause parts of the aquifer to be juxtaposed against parts of the upper and (or) lower confining zones. The geometry of the aquifer in adjacent fault blocks controls the effective thickness of the most permeable zone and leads to irregular hydraulic head values and divergent ground-water flow paths.

We used a combination of airborne, surface, and subsurface data to construct a 3-D computer model of the Edwards aquifer system in a 13 x 15 mile area along Seco Creek, Uvalde and Medina Counties, Texas. Detailed electromagnetic (EM) data from a helicopter survey over about two-thirds of the model area were processed to identify conductive and resistive layers at depth, and were especially valuable for defining the unconformable Del Rio Clay and other conductive units above the aquifer. These interpretations were cross-checked against drillers' lithologic logs from scattered water-wells, against a very small number of borehole geophysical logs, and against limited surface geophysical soundings. High-quality geologic maps for the model area provided additional information on the altitudes of formation contacts, locations of faults, and local bedding attitudes. Each of the different data sources provided glimpses of different parts of the final 3-D model (based on scale, coverage, and sensitivity). By cross-checking results from various sources, we identified some original data errors and revised some prior interpretations in the drill-hole records and the geologic mapping. Our results are presented through an interactive 3-D viewer that allows the input data to be examined against the faulted-stratigraphic model, and allows for direct visualization of the cross-fault geometry of adjoining fault blocks.