Aeromagnetic anomalies are due to variations in the Earth's magnetic field caused by the uneven distribution of magnetic minerals (primarily magnetite) in the rocks that make up the upper part of the Earth's crust. The features and patterns of the aeromagnetic anomalies can be used to delineate details of subsurface geology including the locations of buried faults, magnetite-bearing rocks, and the thickness of surficial sedimentary rocks (which are generally nonmagnetic). This information is valuable for mineral exploration, geologic mapping, and environmental studies.

The Texas aeromagnetic map in this report is constructed from grids that combine aeromagnetic data (see data processing details) collected in many separate aeromagnetic surveys flown between 1967 and 2001. The data from these surveys are of varying quality.  The design and specifications (terrain clearance, flight line separation, flight direction, analog/digital recording, navigation, and reduction procedures) may vary between surveys depending on the purpose of the project and the technology of that time. All of the pre-1976 data are available only on hand-contoured analog maps and had to be digitized. These maps were digitized along flight-line/contour-line intersections, which is considered to be the most accurate method of recovering the original data. Non-proprietary digital data are available as USGS Open-File Report 02-361 (USGS/NGDC, 2002). All surveys have been continued to 304.8 m (1,000 ft) above ground and then blended or merged together.  Grids and images were created using a PC version of Geosoft/OASIS montaj software.  An index map and a data table give an overview of the original surveys and summarize the specifications of the surveys.  The resulting grid has a data interval of 1,000 m and can be downloaded.  A color-shaded relief image of the grid is shown on the opening page of this web report.

This grid is an interim product.  Where local high-resolution surveys were not available, in either digital or digitized format, we used aeromagnetic data collected by the National Uranium Resource Evaluation (NURE) program of the U.S. Department of Energy, which are available in digital format and cover the entire State.  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 surveys were flown at elevations close to the reduction datum level, the spacing between flight lines ranged from 4.8 to 9.6 km (3 to 6 mi).  In some areas of the U.S., detailed NURE surveys were flown with a finer line spacing, usually at a 0.4 km (0.25 mi) interval.  In Texas, the NURE program flew the Christmas Mountains (survey ref. no. 5025), El Solatario (survey ref. no. 5026), Green Valley (survey ref. no. 5027), Buckshot (survey ref. no. 5028), and Dryden (survey ref. no. 5030) areas at this interval.

This Texas aeromagnetic compilation is one part of a national digital compilation by the U.S. Geological Survey (USGS).  Certain characteristics are common to all of the State compilations.  Surveys are typically flown either at a constant elevation above sea level or draped to a constant mean terrain clearance.  The standard elevation datum selected for this national compilation is a survey elevation of 304.8 m (1,000 ft) above mean terrain.  To conform to the national standard, the entire State grid was analytically continued upward to 304.8 m (1,000 ft) above ground (Hildenbrand, 1983).

This project was supported by the Mineral Resource and Geologic Mapping Programs of the USGS. Thanks to USGS colleagues Robert Kucks, Pat Hill, and David Daniels for their assistance in preparing this report.

A special thanks also to G. Randy Keller, University of Texas at El Paso, for his years of geophysical studies in Texas (among other places) and congenial cooperation with the USGS over many years.  Much of the data herein originated with him and his students.