USGS Data Series 355: Oregon Magnetic and Gravity Maps and Data: A Web Site for Distribution of Data

Magnetic 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 magnetic anomalies can be used to delineate details of subsurface geology, including the locations of buried faults and magnetite-bearing rocks and the depth to the base of sedimentary basins. This information is valuable for mineral exploration, geologic mapping, and environmental studies.

The Oregon magnetic map is constructed from grids that combine information (see data processing details) collected in 49 separate magnetic surveys conducted between 1950 and 2002. 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, released in USGS Open File Report 02-361, were obtained from magnetic surveys flown by or on contract with the U.S. Geological Survey (USGS) or were obtained from other federal agencies. Much of the pre-1975 data are available only on hand-contoured maps from the USGS and through contractors 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 in USGS Open File Report 99-0557. All surveys have been continued to 305 m (~1,000 ft) above ground and merged together to form the State compilation. An index map shows the location of the original surveys, and a data table summarizes the detailed specifications of the surveys. The final magnetic anomaly grid with a 500-m (~1,640-ft) interval can be downloaded from the data directory as magor.grd.

Also included are maps and grids of some individual surveys at their original flight elevation, which were generated with a finer grid spacing due to a 1,000-m (~3,280-ft) or less flight-line spacing. These were then regridded to the final grid spacing of 500 m (~1,640 ft) for use in the State compilation. Grids for all maps are in Geosoft Oasis montaj binary format (downloadable free software for conversions to other grid formats at www.geosoft.com) and available in the data directory. A 'README.txt' file located there further defines the format.

Our priority in the construction of the State of Oregon magnetic compilation was always to acquire the best resolution magnetic data sets for any given area (see the Oregon magnetic data index map). 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 together 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 was either 4,800 m (~3 mi) or 9,600 m (~6 mi). The wide spacing between flight lines flown at low altitudes over surface rock units having high magnetizations causes anomalies with short spatial wavelengths to be elongated between flight lines, producing lineations perpendicular to the flight-line direction and "pearl string" anomalies along the flight line.

This report is a minimal update of an earlier compilation for the State of Oregon by the USGS (Roberts, Blakely, and Finn, 1997) that was released as paper publication USGS Open-File Report 97-440. The update, other than producing digital results, consists of the addition of three surveys, and the inclusion of images and grids of higher resolution surveys produced at the nominal grid interval for flight-line spacings.

This project was supported by the Mineral Resource Program / Crustal Imaging & Characterization Team of the USGS.

Oregon Magnetic and Gravity Maps and Data: A Web Site for Distribution of Data

By Carter W. Roberts, Robert P. Kucks, and Patricia L. Hill