Data Series 355
Oregon Magnetic Data ProcessingThe following text, described here to define the processing steps, is the original wording taken from USGS Open-File Report 97-440. Although it differs slightly from the standard text used for State compilations, it was decided that it would be more appropriate to use for this description. The only variance of the final product is that a Lambert Conformal projection with a central meridian of 120.5 degrees west was the final choice because it is central to the State of Oregon. This is generally the standard used for the State compilations. PROCESSING STEPS: A digital synthesis of various aeromagnetic surveys was compiled to simulate, as best possible, a single survey flown at 305 m above terrain. This makes possible geologic interpretation over wide regions across boundaries. Surveys flown at a constant distance above ground and along closely spaced flight lines inherently show more detail than surveys flown far above ground level. Caution should thus be used when interpreting features seen at survey boundaries. Aeromagnetic data were obtained from more than 40 surveys collected at different times, elevations, orientations, and flight-line spacings. Most recent surveys were flown at a nominal 305 m above terrain and could be used with little processing. Older surveys often were flown at a constant barometric elevation and hence much of each survey was flown far above ground level. Many older surveys existed in non-digital form. These were manually digitized to facilitate computer processing. To produce a coherent magnetic database, the following procedures were applied to the individual surveys: (1) The International Geomagnetic Reference Field (IGRF), updated to the date that the survey was flown, was removed from each survey to generate a temporally consistent set of residual magnetic data. This subtraction had already been applied to most individual surveys. On some early surveys it was unclear whether a regional field had been removed from the published data. In some cases we removed the IGRF only if it provided a better fit with surrounding data sets. (2) All surveys were then gridded with a 500-m spacing, Lambert Conformal projection, central meridian of 123 degrees west, then placed on a common geographic datum located 305 m above the ground surface. Surveys not collected at this height were mathematically modified to approximate the magnetic field that would have been measured at a height of 305 m above the ground surface (Cordell, 1985). (3) The individual surveys were compared in areas of overlap, datum shifted as necessary to give the best fit, and finally merged into a single file. In areas where two or more surveys overlapped, the best available survey was used, and that was typically the one with the lowest flight height and closest line spacing. Almost the entire State was flown for the Department of Energy National Uranium Resource Evaluation (NURE) program. These surveys were typically flown about 120 m above ground with a flight-line spacing of 5 or 10 km. These surveys were used in areas where no other data were available or other data were of lower quality and resolution. Gridding such data at a fine interval can cause artifacts to appear along flight lines. Generally a somewhat larger grid spacing was used initially then that grid was regridded finer to minimize this problem. The magnetic declination ranges between 14 degrees east at the southeastern corner of the State to 19 degrees at the northwestern corner. GRID PROJECTION SPECIFICATIONS
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Version 1.0 Posted August 2008
Contains: gridded data. Refer to the Readme file for more information.
For additional information about this report, contact: Bob Kucks |
