DIGITAL AND DIGITIZED MAGNETIC ANOMALY DATA FOR THE UNITED STATES AND ACCESS TO OTHER USGS DIGITAL GEOPHYSICAL DATASETS

The USGS has acquired aeromagnetic data for essentially all of the U.S. in a piecemeal fashion. Surveys were designed for many purposes, so they varied widely in both coverage and anomaly resolution (determined primarily from flight height and line spacing). Data collection methods spanned changes in acquisition techniques, from analog-based before the 1970’s to current digital systems that use Global Positioning System (GPS) navigation. The newly updated North American magnetic anomaly map is a culmination of the compilation of these analog-converted (digitized) and digital magnetic data for the United States. The need to create individual U.S. state compilations generated a project designed to recover archived magnetic data from either analog map form or digital media form (mostly 9-track tape) that date back to 1944. More than 1,000 surveys and 5,800,000 line-km of data that would cost hundreds of millions of dollars to re-fly today have been retrieved in the process. Recovery of aeromagnetic survey data was initially attempted through “dumping” of the contractors’ data tapes that had been stored for many years, sometimes resulting in failed attempts. Tapes created before 1973 commonly were not recoverable. Failure to recover a digital file forced us to use the original contractor’s final map product and digitize the contour lines to produce a digital product. Care had to be taken to understand what processes had been used in originally creating the map. Much work has also been undertaken to create an ASCII data file, a flight-line image, and a metadata file of each dataset as well as a table of specifications and location map for all surveys within a state. These data, as well as grids of the North American magnetic anomaly compilation and a separate conterminous U.S. portion, are currently being released as publications (both website and DVD) through the U.S. Geological Survey. The Mineral Resources Program of the USGS also maintains a website page that accesses or locates a variety of digital geophysical information such as magnetic, gravity, radiometric, electrical, or telluric data as well as software and fact sheets relating to these disciplines (http://crustal.cr.usgs.gov/geophysics/index.html).