U.S. Geological Survey

Colorado 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 Colorado aeromagnetic map is constructed from grids that combine information (see data processing details) collected in 34 separate aeromagnetic surveys conducted between 1952 and 1986. 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. 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 half kilometer grid interval. All surveys have been continued to 1000 feet above ground and the surveys are blended or merged together.

This grid is an interim product. Efforts to mathematically merge these 34-plus 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 aermagnetic 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 and Geologic Mapping Programs of the USGS. The authors wish to thank our USGS colleagues Viki Bankey, Pat Hill, and Richard Saltus for their assistance in preparing this report, and Rob Bracken, Jeffrey Phillips, and Mike Webring for the in-house software used to prepare the aeromagnetic data.

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