Wyoming Aeromagnetic Compilation

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 non-magnetic). This information is valuable for mineral exploration, geologic mapping, and environmental studies.

The Wyoming aeromagnetic map is constructed from grids that combine information (see data processing details) collected in 27 separate aeromagnetic surveys conducted between 1950 and 1997. The data from these surveys are of varying quality. The design and specifications (terrain clearance, sampling rates, line spacing, and reduction procedures) varied from survey to survey depending on the purpose of the project and the technology of that time. Every attempt was made to acquire the data in digital form. Most of the available digital data were obtained from aeromagnetic surveys flown by the U.S. Geological Survey (USGS), flown on contract with the USGS, or were obtained from other federal agencies and state universities. Much of the pre-1975 data are available only on hand-contoured 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. Digitized data are available as USGS Open File Report 99-557.  All surveys have been continued to 304.8 meters (1000 feet) above ground and the surveys are blended or merged together. An index plot gives an overview of the original surveys and a data table summarizes the detailed specifications of the surveys. The map can be downloaded as a grid with a 500 meter grid interval. 

Also included are maps and grids of some of the individual surveys at their original flight elevation that were generated with a finer grid spacing due to a finer flight line spacing. These were then regridded to the final grid spacing of 500 meters for use in the state merge. The National Uranium Resource Evaluation (NURE) grid is included at the optimum grid interval of 1000 meters and original flight elevation of 400 meters above ground.

The entire study area is covered by aeromagnetic data collected as part of the National Uranium Resource Evaluation (NURE) program of the U.S. Department of Energy. These data are available in digital form and provided the framework for the map compilation. 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  3200 meters to 9600 meters (2 to 6 mile), with the exception of part of the Rawlins 1° x 2° quadrangle, which was flown at 400 meters (0.25 mile) flight line spacing. The wide spacing between flight lines flown at low altitudes over surface rock units having high magnetizations causes anomalies with short spacial wavelengths to be elongated between flight lines, producing lineations perpendicular to the flight-line direction and 'pearl string' anomalies along the flight line. Problems related to the reduction of navigation control and inconsistent datum levels between surveys causes herringbone features which can be observed within and at the boundaries of  1° x 2° quadrangles. Consequently, data from surveys other than NURE were incorporated into the framework of NURE surveys wherever possible.

This grid is an interim product. Efforts to mathematically merge these 27 surveys were hindered by poor-quality data of some surveys and the limited USGS software available in past years. For example, minimal editing of digital flight-line data could be undertaken. Additionally, several aeromagnetic surveys could not be smoothly added to the merged data grid due poor quality and extreme boundary differences. Recent commercial software packages that merge geophysical data have demonstrated the ability to merge poor-quality data such as these in ways that were not available to the authors. We are aware of problems with our merging efforts; such as visible survey boundaries, poor anomaly resolution, and errors within surveys, and anticipate improving the results using new programs and techniques.

This project was supported by the Mineral Resource Program of the USGS. The authors wish to thank our USGS colleagues for their assistance in preparing this report, and Rob Bracken, Jeffrey Phillips, Ron Sweeney, and Mike Webring for the in-house software used to prepare the aeromagnetic data.

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