MINNESOTA GEOLOGICAL SURVEY 9/5/2000 GRAVITY AND MAGNETIC GRIDS OF THE DULUTH COMPLEX Val W. Chandler University of Minnesota Minnesota Geological Survey 2642 University Ave. St. Paul, Minnesota 55114-1057 Ph. 612-627-4780 ext. 203 email: chand004@tc.umn.edu In response to the recent increase in exploration activity in the Duluth Complex, the Minnesota Geological Survey (MGS) has released its preliminary gravity and magnetic grids for the Duluth Complex of northeastern Minnesota and adjacent areas. Besides MGS data, the grids incorporate data produced by the Wisconsin Geological and Natural History Survey, the U.S. Geological Survey, the National Geophysical Data Center, and the Geological Survey of Canada. The grids are being used as part of a ongoing study funded by the Minnesota Legislature through the Minnesota Minerals Coordinating Committee. Users are warned that the grids are subject to minor revision. All data files and associated metadata are available for download from the MGS ftp site. Zip and PDF files require binary transfer by an FTP program, the readme is an ASCII file. Seven documents are contained within a single WinZip file named dulgrmag.zip. Files can be extracted singly or as a group. Total file size is approximately 25 mb. See below for names and descriptions of the individual files. The metadata for all of the data sets is available in the pdf file named gmdcmeta.pdf. (Although readable with Acrobat Reader 3.0, version 4.0 is recommended for best results). All compilation and processing were accomplished using software available through the U.S. Geological Survey (http://minerals.usgs.gov/fact-sheets/#ofr-97-0725), and the grids use the standard binary-grid format and header structure established by that agency. The grids are UTM based (North American Datum 1927) and have 921 rows, 1123 columns, 213.36-meter grid spacing, and a southwest origin with an easting of 532,270.4 meters and a northing of 5,144,545 meters. The data are: 1. Dcmerge6.hdr--Grid of total intensity aeromagnetic data. 2. Dcmrtp.usg--Grid of aeromagnetic data reduced to the pole (assuming an Earth's field declination/inclination = 2 degrees/76 degrees, and a polarization declination/inclination = 295 degrees/50 degrees). 3. Dcmrp1vd.usg--Grid of the first vertical derivative of the reduced-to-pole aeromagnetic data. 4. Dcmrp2vd.usg--Grid of the second vertical derivative of the reduced-to-pole aeromagnetic data. 5. Dcallg.rgr--Grid of the Bouguer anomaly gravity data (data over gridded, actual station spacing=2- 10 km). 6. Dcallg2d.rgr--Grid of the second vertical derivative of the Bouguer anomaly gravity data continued upward to 2 km (data over gridded, actual station spacing=2-10 km). Also available: 7. Dcallg.hdr--Principal-fact gravity data used to make gravity grids described above (note: first line identifies the individual columns, ASCII format). Note: Flight profile Aeromagnetic data (Minnesota only) are available on-line from the Division of Minerals, Minnesota Department of Natural Resources. Select the Theme Catagory: Geology and Soils; then the layer: Minnesota Aeromagnetic Data. This document should be cited as follows: Chandler, V.W., and Lively, R.S., 2000, Gravity and magnetic grids for the Duluth Complex of northeastern Minnesota and adjacent areas: Minnesota Geological Survey Open-File Report 00-1, 6 grids and 1 data file.