Aeromagnetic Surveying in Wisconsin 1997-98: Digital Data Files By David L. Daniels, Suzanne W. Nicholson, and William F. Cannon U.S. Geological Survey Open-File Report 99-28 1999 Introduction Over the past 10 years the U.S. Geological Survey has conducted a series of aeromagnetic surveys in Wisconsin aimed at completing the coverage of high-quality, moderate-resolution aeromagnetic surveying in the State. The data from these surveys have proven to be an effective tool for delineating structures in the Precambrian basement in Wisconsin and have been useful in the study of the mineral resources in the region. Precambrian basement rocks rarely crop out in this region because of glacial deposits and Paleozoic sedimentary cover rocks. Surveys by the U.S. Geological Survey (USGS) in 1988 (Hittleman and others, 1992) and 1996 (Snyder, 1998) had closed gaps in the aeromagnetic coverage of the Midcontinent rift area, adjacent to Minnesota. The most recent survey flown in 1997-98 is the focus of this report. It extends aeromagnetic coverage southwards to the middle of the state (Daniels and others, 1998). Aeromagnetic contour maps (scales 1:100,000 and 1:50,000) for the 1997-98 survey were recently released as open-file paper maps (U.S. Geological Survey 1998a-i). This report releases the digital data for the same survey. Both digital point and gridded data are included as well as images of the aeromagnetic data for the five areas. Facts about this survey and parameters for the digital files are listed below. Disclaimer This Compact Disc-Read Only Memory (CD–ROM) publication was prepared by an agency of the U. S. Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed in this report, or represents that its use would not infringe privately owned rights. Reference therein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the Government or any agency thereof. Any views and opinions of the authors expressed herein do not necessarily state or reflect those of the Government or any agency thereof. This report has not been reviewed for conformity with the U.S. Geological Survey editorial standards. Although all data published on this CD–ROM have been used by the USGS, no warranty, expressed or implied, is made by that agency as to the accuracy of the data and related materials and/(or) the functioning of the software. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of these data, software, or related materials. Survey Facts The 1997-98 Wisconsin aeromagnetic survey was flown between October 1997 and March 1998 in five separate areas. These blocks are outlined on the index map of Wisconsin and are named: areas1_2, area3E, area3W, areas4_5, and area6 (click on the colored text to see these images or go to the images at the end of this report). The index map shows the distribution of aeromagnetic coverage in Wisconsin prior to the current survey and the 5 areas where new data were collected. The color-shaded-relief images of the aeromagnetic maps for the 5 areas show the magnetic field as if illuminated from the north. South-directed illumination is parallel to the flightline direction, and minimizes enhancement of “flightline noise.” Flightlines were in a North-South direction, spaced ½-mile (805m) apart, draped to topography at 1000 feet (305m) above ground. The survey was flown on contract by Aero Surveys, Inc. of Golden, Colorado. Data processing and production of the contour maps was carried out by Pearson, deRidder, and Johnson of Lakewood, Colorado. The five blocks of the survey required a total of about 21,000 line-miles of flying. Disc Organization This CD-ROM disc is structured with 5 directories/folders as follows: \DATA The basic survey measurements are in five ASCII files (file extension .dat) derived from the original contractor files. Each record of each file contains 15 fields of data per measurement, at a sampling interval of one second. USGS style DOS binary point files (.pos) with 9 fields are also given. See below for the explanation of each of these fields. \GRIDS The point data have been interpolated into binary raster USGS style grids (file extension .grd) by using a minimum curvature gridding algorithm (program MINC, Phillips, 1997). ASCII versions of the USGS grids are also given (.agd). USGS grids and binary point files can be manipulated using USGS (DOS) software for the PC (Phillips, 1997). This is freeware that is included in a separate directory on this disc, or may be downloaded from ftp://greenwood.cr.usgs.gov/pub/open-file-reports/ofr-97- 0725/pfofr.htm. Two alternate ASCII file formats are also included. These ASCII grids permit importation into other software packages. Files with extension .arc can be imported into Arcview 3 - Spatial Analyst, and file extension .gxf can be imported into several programs including ER Mapper and Geosoft Oasis Montaj. \IMAGES Images of the aeromagnetic data for each of the five areas covered by the survey are included in this directory and are given in 4 graphic formats (file extensions .ps, .pdf, .bmp, .jpg), the latter three derived (using Adobe Illustrator) from an initial Postscript image file. The postscript images were created using GCLR, and associated UNIX programs written by Robert W. Simpson, Jr. of the USGS. \ACROBAT Adobe Acrobat software for viewing PDF files. The latest version can be downloaded from the web at URL http://www.adobe.com/prodindex/acrobat/readstep.html. \USGS_PF The USGS potential field software package (Phillips, 1997). Installation is described in the readme files. File Facts Point Files File Name Bytes Records Areas1_2.dat 37,788,679 271,861 Areas1_2.pos 11,418,164 271,861 Area3E.dat 1,682,873 12,107 Area3E.pos 508,496 12,107 Area3W.dat 2,666,298 19,182 Area3W.pos 805,646 19,182 Areas4_5.dat 27,176,307 195,513 Areas4_5.pos 8,211,548 195,513 Area6.dat 21,605,743 155,437 Area6.pos 6,528,356 155,437 ASCII Point Data File Format: Each record represents one measurement recorded at one-second flight intervals. The record length is 140 bytes. The “Residual Magnetic Intensity (IGRF removed)” field is the residual total magnetic intensity data with the International Geomagnetic Reference Field (IGRF) removed. The grid files are constructed from this field. Format Field Units ------ ----------- ----- A8 Flight-Line number, Direction F11.5 Longitude Degrees F11.5 Latitude Degrees F13.4 UTM - X Meters F14.4 UTM - Y Meters F9.1 Fiducial I7 Date YYJDD I11 Time HHMMSS I4 Radar Altimeter Meters I5 Barometric Altimeter Meters I5 GPS Elevation (WGS 84) F10.1 Diurnal Base Magnetic Intensity Nanoteslas F10.2 Raw Flight Magnetic Intensity Nanoteslas F10.2 Diurnally-Corrected Magnetic intensity Nanoteslas F10.2 Residual Magnetic Intensity (IGRF removed) Nanoteslas USGS Binary Post File (.POS) Data Fields: Field Description Units ------ --------------- ------- 1 Flight-line number, Direction 2 Longitude Degrees 3 Latitude Degrees 4 Residual Magnetic Intensity Nanoteslas 5 Diurnally-corrected Magnetic Intensity Nanoteslas 6 Radar Altimeter Meters 7 Barometric Altimeter Meters 8 Fiducial 9 Year-Julian day YYYY.DDD Grid Files Residual Total Magnetic Intensity Binary Grid Files, USGS Grid Format Grid File Rows Columns Areas1_2.grd 789 925 Area3E.grd 230 85 Area3W.grd 204 203 Areas4_5.grd 615 392 Area6.grd 338 543 Grid Parameters (.grd, .agd, .gxf, .arc) X, Y units: Kilometers Grid node interval: 0.20 kilometers No-Data Value (.grd, .agd): 10E+38 No-Data Value (.gxf): 10E+30 No-Data Value (.arc): -99999.99 Spheroid: Clark 1866 Horizontal Datum: 1927 North American Datum Projection: Transverse Mercator Central Meridian: 90 degrees West Base Latitude: 0 degrees References Cited Daniels, D.L., Snyder, S.L., Nicholson, S.W., and Cannon, W.F., 1998, New aeromagnetic surveys in Wisconsin by the U.S. Geological Survey: Institute on Lake Superior Geology, 44th Annual Meeting, May 6-10, 1998, Minneapolis, Minnesota, pp. 62-63. Hittleman, A.M., Buhmann, R.W., Racey, S.D., Chandler, V.W., 1992, Aeromagnetics Earth System Data, Minnesota Region: National Geophysical Data Center, CD-ROM, diskettes, and User’s Manual. Phillips, J.D., 1997, Potential-Field Geophysical Software for the PC, version 2.2: US Geological Survey Open-File Report 97-725 34 p. Snyder, S.L., 1998, Aeromagnetic map of part of northwestern Wisconsin and adjacent areas: U.S. Geological Survey Open-File Report 98-228, Scale 1:125,000, 2 sheets. U.S. Geological Survey, 1998a, Aeromagnetic survey of parts of the Black River Lake, Eau Claire, Hastings, Stillwater, and Winona 1:100,000 quadrangles in Wisconsin, north- west sheet: U.S. Geological Survey Open-File Report 98-431, scale 1:100,000. [AREAS 1 & 2] U.S. Geological Survey, 1998b, Aeromagnetic survey of parts of the Black River Lake, Eau Claire, Hastings, Stillwater, and Winona 1:100,000 quadrangles in Wisconsin, north- east sheet: U.S. Geological Survey Open-File Report 98-432, scale 1:100,000. [AREAS 1 & 2] U.S. Geological Survey, 1998c, Aeromagnetic survey of parts of the Black River Lake, Eau Claire, Hastings, Stillwater, and Winona 1:100,000 quadrangles in Wisconsin, south- east sheet: U.S. Geological Survey Open-File Report 98-433, scale 1:100,000. [AREAS 1 & 2] U.S. Geological Survey, 1998d, Aeromagnetic survey of parts of the Black River Lake, Eau Claire, Hastings, Stillwater, and Winona 1:100,000 quadrangles in Wisconsin, south- west sheet. U.S. Geological Survey Open-File Report 98-434, scale 1:100,000. [AREAS 1 & 2] U.S. Geological Survey, 1998e, Aeromagnetic survey of part of the Iron Mountain 1:100,000 quadrangle in Wisconsin: U.S. Geological Survey Open-File Report 98-436, scale 1:50,000. [AREA 3 EAST] U.S. Geological Survey, 1998f, Aeromagnetic survey of parts of the Ironwood and Wakefield 1:100,000 quadrangles in Wisconsin: U.S. Geological Survey Open-File Report 98-435, scale 1:50,000. [AREA 3 WEST] U.S. Geological Survey, 1998g, Aeromagnetic survey of parts of the Marinette, Shawano, Sturgeon Bay and Wabeno 1:100,000 quadrangles in Wisconsin, north sheet: U.S. Geological Survey Open-File Report 98-437, scale 1:100,000. [AREAS 4 & 5] U.S. Geological Survey, 1998h, Aeromagnetic survey of parts of the Marinette, Shawano, Sturgeon Bay and Wabeno 1:100,000 quadrangles in Wisconsin, south sheet: U.S. Geological Survey Open-File Report 98-438, scale 1:100,000. [AREAS 4 & 5] U.S. Geological Survey, 1998i, Aeromagnetic survey of the Wisconsin Rapids and part of the Appleton 1:100,000 quadrangles in Wisconsin: U.S. Geological Survey Open-File Report 98-439, scale 1:100,000. [AREA 6] 6 6