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Aeromagnetic Survey of Medina and Uvalde Counties, Texas

By David V. Smith, Bruce D. Smith, and Pat L. Hill

Data Specifics

The aeromagnetic map was generated from digital data acquired from a high-resolution aeromagnetic survey flown in 2001. Flight-line elevation in this draped-mode survey was nominally 160 m (500 ft) above terrain; east-west flight line spacing was nominally 400 m (0.25 mi) with north-south tie lines at approximately 4000 m (2.5 mi). Survey coverage roughly spanned a 1.25 x 0.4 degree quadrangle, or approximately 5833 square km (2252 square mi). The data were projected onto a cartesian coordinate system using a WGS84 UTM Zone 14 projection with a central meridian of 99°W.

Processing Details

• After standard compensations and corrections were applied, the survey data were interpolated to a square grid using a bi-directional algorithm with a grid spacing of 80 m, which is equal to 1/5 the original flight-line spacing.
• The magnetic anomaly grid was calculated by subtracting the provisional International Geomagnetic Reference Field (IGRF2000) from the total field intensity, using a base level of 48,400 nT.
• The finished map displays the data after processing with a reduction-to-the-pole algorithm, which corrects for shifts in magnetic anomalies caused by the inclination of the Earth's local magnetic field.

Other Available Data

The study area was covered by an aeromagnetic survey in 1980 as part of the National Uranium Resource Evaluation (NURE) program of the U.S. Department of Energy. These data are currently available in digital form and provide a useful regional context for this high-resolution survey. However, because magnetic surveying was not the primary objective in the design of the NURE surveys, these data are subject to certain limitations. Although the NURE lines were flown at elevations close to 160 m (500 ft), comparable to the present survey, the spacing between flight lines was nominally 3.0 km. To remove artifacts generated by gridding data that is dense along lines but sparse between lines, the interpolated grid is usually smoothed, resulting in a significant loss of spatial resolution.

Grid Projection Specifications

• Projection = WGS84 UTM Zone 14
• Central Meridian = 99°W