Every reasonable effort has been made to ensure the accuracy of the factual data on which these grids are based; however, the Minnesota Geological Survey does not warrant or guarantee that there are no errors. Users may wish to verify critical information; sources include both the references listed here and information on file at the offices of the Minnesota Geological Survey in St. Paul. No claim is made that the data as shown are rigorously correct, however, and it should not be used to guide engineering-scale decisions without site-specific verification.
Sponsored Project A78649 Minnesota Minerals Coordinating Committee
Line and grid data released in Geosoft formats and can be viewed and exported into other formats using the free Oasis montaj Viewer available from the GEOSOFT web site. In addition, GEOSOFT provides a free plug-in for ArcGis that allows ArcMap to view and work with the Geosoft grids. Alternatively type in www.geosoft.com to your browser and follow links to the downloads.
Final Report -- Revised Aeromagnetic Data
The data can also be obtained from MGS Map Sales on CD or DVD. Request all or parts of the revised aeromagnetic magnetic data.
(Fill out the Order Form or call 612-627-4782)
logs from individual surveys
(Does not include the Central survey which was only partially recovered)
Download Individual Blocks of Line Data by clicking within a block
A zip file will start to immediately download
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DESCRIPTION OF DOWNLOADABLE DATA
Data is available from the FTP site above. This site is comprised of a folder containing grid files, a folder of line data, and a readme.txt file. The located line database files are provided in 23 blocks, WinZipped and labeled with area names. Some of these data blocks are composited from multiple blocks of the NOAA data to reduce the overall number. Further information, as well as a description of the variables that are in each of the new data blocks are included (as an ASCII text file) within the zipped file. The databases (gdb) and grid files (grd) are provided in standard GEOSOFT format, and can be viewed and exported into other formats using the free Oasis montaj Viewer available from the GEOSOFT web site. In addition, GEOSOFT provides a free plug-in for ArcGis that allows ArcMap to view and work with the Geosoft grids. Alternatively type in www.geosoft.com to your browser and follow links to the downloads.
The materials that are presented in this report are the result of a two-year program to upgrade the state-wide aeromagnetic database at the Minnesota Geological Survey (MGS). These data were originally acquired during a 1979-1991 aeromagnetic survey program, and they were compiled and processed using the more limited computing and data storage capabilities that were available at that time. Since then, significant improvements have been made to computers and software that massive database operations, such as line leveling and gridding can now be efficiently handled with desktop systems. Thus the MGS decided to upgrade the existing aeromagnetic database using these newer capabilities. The upgrade had three major objectives. One was to recover line data that turned up missing from the primary archive, a data CD that was produced in 1992 by the National Geophysical Data Center of the National Oceanic and Atmospheric Administration (NOAA). Another objective was to improve on some line-leveling errors in the data, which caused striping artifacts in some areas. The third objective was to take the revised line data and produce an improved aeromagnetic grid for Minnesota. Preliminary testing indicated that a higher resolution grid was possible from the revised line data, improving on the previous statewide aeromagnetic grid node interval of 213.36 meters. SOFTWARE The software selected for this project was the OAISIS/MONTAJ system produced by the Canadian company GEOSOFT. This system, which offered the options needed for the current project, has a wide network of users, including the U. S. Geological Survey (USGS). The OAISIS/MONTAJ software represents a significant upgrade to the processing capabilities at MGS for working with aeromagnetic data. Older compilation and processing software available at MGS consisted was a mixture of DOS-based programs from various sources. These were increasingly cumbersome to use and were not capable of producing the large, high-resolution data sets that are more in demand. In fact, some crucial operations, such as line leveling, were not possible for the MGS prior to the purchase of the OAISIS/MONTAJ software.
DATA USED IN THIS STUDY
The primary source of data was line data on the NOAA CD, which was compiled from multiple sets of 9-track tapes that were delivered to MGS by the original contractors. The line data consists of individual magnetometer readings with pertinent locational, temporal and fiduciary information, as well as magnetic anomaly values corrected for temporal and spatial variations in the geomagnetic field. In recent years David Dahl of the Division of Minerals of the Minnesota Department of Natural Resources (MNDNR) converted the line data from the NOAA CD into an ArcGIS format of shape files for use in programs such as ESRI's ArcView. As part of that process the data were also transformed from the NAD27 to the NAD83 datum, and considerable effort was made to eliminate spurious points and repeated line segments that plagued several of the data blocks. This edited data saved considerable time at the beginning of this project because the shape files from MNDNR could be readily imported into the Oaisis/Montaj software whenever possible. Data excluded from the MNDNR compilation consisted chiefly of tie-lines and overlapping segments between individual data blocks. These were separately imported into the Oaisis/Montaj software, edited for spurious values and repeats, and merged into the compilation database.
RECOVERY OF LOST DATA
Sporadic omissions in line coverage have long been known to exist on the NOAA CD and efforts were made to fill some of the larger gaps, particularly if they were longer than 5 km. The initial approach was to search for the missing data in digital media archived with the MGS and the USGS. One of the largest coverage gaps in the North-Central Survey were eliminated in this way. However, many of the other omissions appear to have originated with the contractors and to have been part of the tapes that were delivered to the MGS. Inspection of detailed (1:24,000 scale) aeromagnetic flight data maps indicated that many of the missing segments were most likely present during the original map compilation, but were subsequently lost. One possibility was that "working files" used by the contractors during map compilation contained edits and additions that simply were not included when the "final" data tapes were generated, usually near the end of a project. Whatever the cause, recovery of these omissions required use of analog materials stored at the MGS. Some of the missing line segments were recovered using analog paper plots recorded by the in-flight magnetometer. However, many of the analog plots turned out to be poorly annotated and determination of fiducial numbers along the length of the plots could not be accurately made. Furthermore, the scaling of the magnetometer was usually set a low value causing plots of high-magnetic-gradient areas to have multiple ranging steps that were difficult to reliably track and digitize. Consequently, only partial segments of a line could be reliably recovered by digitization of these records. After the recovery of the missing line segments in the Northeastern Survey by digitizing the paper plots, reformatting the data and merging it into the compilation, the overall procedure was deemed to be too time-consuming and error-prone for the relatively minor improvements in resolution that resulted. Therefore the remaining gap recovery of missing data was based the grid interpolation procedure described below. The most satisfactory method of filling line gaps involved interpolating magnetic values from the preexisting state grid. In this procedure locations along the missing line segment were obtained by digitizing photo-spotted points from the original flight path recovery maps, usually 1:24,000 scale topographic maps. The residual magnetic anomaly values along the recovered line segment were interpolated at a 50 meter interval from the original state aeromagnetic grid using the "sample a grid" option from the OAISIS/MONTAJ software. Some reduction of along-line resolution occurs with this approach, because the original state aeromagnetic grid has a spacing of 213.36 meters, while the original data acquisition interval along line was 45-75 meters. However, lines recovered in this way sampled the most reliable parts of the state grid, i.e. nodes that were closest to the original line. Inspection of test grids that included some in-filled segments using the interpolation procedure showed no significant degradation of grid resolution. The interpolation procedure was used for subsequent infill because it provided the best compromise between the time constraints of the project and the need to fill the data gaps for the new state grid. Several scattered and irregular areas of missing line data were the result of drop-outs by the magnetometer and are consequently are not recoverable because there is no line data. These dropouts often occur in areas of extremely high magnetic gradients, such as that near Tower-Soudan in northeastern Minnesota and near some radio transmitters in populated areas. At the gridding stage of the recompilation, these areas were filled by grid interpolation methods.
RE-LEVELING OF THE LINE DATA
The line data were re-leveled in two stages. During the first stage specific lines were selected from areas where striping artifacts were particularly noticeable in the original gridded data. Much of this selective re-leveling was focused in the areas of the Northeastern and East-Central Surveys. The selective leveling started with the original residual anomaly values in the RESIDMAG channel of the databases and used either the "statistical leveling" option (least-squares removal of 0 to 3rd order curves) or the special leveling (curves created by hand) option of the OAISIS/MONTAJ software to create a new value. The re-leveled magnetic data are given in the RELEV_MAG channel of the databases. In the case of lines that were not selected for leveling, the magnetic value in the RELEV_MAG channel is equal to the RESIDMAG value channel. During the second stage of leveling, all line data were re-leveled using the micro-leveling option of the OAISIS/MONTAJ software. Because this leveling is filter-based, care must be taken that the procedure does not introduce distortions in the data, especially along the original direction of flight lines, which in our case is primarily north-south. To reduce the potential for filter-related distortions, high-amplitude anomalies were windowed out, as were north-striking anomalies that were not believed to represent artifacts. Usually after several trial and error cycles, suitable micro-leveled values were determined for each data block, and they are given in the MICROLEV_MAG channel of the databases. Because they are not used in the procedure, tie lines do not have a MICROLEV_MAG value. The micro-leveling procedure resulted in dramatic improvement in many areas. Improvements were especially noticeable in the North-Central Survey, where striping artifacts were very abundant and a lack of tie-lines precluded selective leveling. Consequently the micro-leveled values in the databases, and the grids generated from these values, will probably be of the greatest use to potential users. However, if users suspect that micro-leveling is introducing distortion in a given area, they can check their results against the re-leveled values or the original residual values.
CREATION OF STATEWIDE MINNESOTA GRIDS
The gridding procedure was selected after a series of trial-and error runs on several data blocks. The Geosoft/Montaj software provides several options for creating grids, including minimum curvature, bi-directional gridding, tinning, and kriging. The integrity of the gridding was evaluated in each test case by considering its stability and general appearance of the data after second vertical derivative enhancement. In general, the grids that were produced by either minimum-curvature or bi-directional gridding gave the most favorable-appearing results, and following some adjustments of a tolerance parameter, the minimum curvature procedure was selected as the most suitable option. A grid interval of 100 meters was selected as the best compromise between improving grid resolution and the introduction of between-line noise. Magnetic anomaly grids were generated for each data block using the re-leveled and micro-leveled magnetic anomaly data, respectively. Following continuation to a common level of 150 meters above surface, the individual grid blocks from the two data sets were composited into separate statewide grids. Gridded data from areas surrounding Minnesota were also composited, including data from the North American Magnetic Anomaly Map (North American Magnetic Anomaly Group, 2002) and the aeromagnetic map of Wisconsin, Daniels and Sneider, 2002). The composited grids of the re-leveled and micro-leveled data were reduced to vertical polarization using the variable reduction to pole option of the OAISIS/MONTAJ software and the resulting grids were enhanced by first and second vertical derivative filtering. These statewide grids are for general use, although some users may wish to tailor their own grids for investigation of certain features or areas.
This study was supported by a grant form the Minnesota Legislature as Recommended by the Minnesota Minerals Coordinating Committee. The original data acquisition was supported by the Minnesota Legislature as recommended by the Legislative Commission on Minnesota Resources. David Dahl of the Division of Minerals of the Minnesota Department of Natural Resources generously donated his shape file versions of the aeromagnetic data, which through his diligent editing saved the current investigators a considerable amount of work. The authors also wish to thank Rob Bracken, Patricia Hill, and Robert Kuchs of the USGS in Denver for their help and advice in retrieving materials for the North-Central Survey. Robert Kuchs was especially helpful in tracking down some missing data and in providing advice on gridding with the minimum curvature option of the OAISIS/MONTAJ software.
North American Magnetic Anomaly Group, 2002, Magnetic anomaly map of North America: U. S. Geological Survey Special Map, Scale 1:10,000,000 with accompanying text.
Daniels, D. L., and Sneider, S. L., 2002, Wisconsin gravity and aeromagnetic maps and data: A web site for distribution of data: U. S. Geological Survey Open File Report 02-493 (On line only).
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Aeromagnetic data were acquired in Minnesota by the Minnesota Geological Survey (MGS) and other organizations from 1979 to 1991 (Chandler, 1991). The impetus for a new statewide survey came from the late Matt Walton, who was director of the MGS from 1973 to 1986. Primary support for the survey was provided by the Minnesota Legislature as recommended by the Legislative Commission on Minnesota Resources (LCMR). Directors of LCMR during the time of the surveying were Robert Hanson (1979-1986) and John Velin (since 1988). Initially the chief application for the data collected by the program was its use for mineral exploration. However, it quickly became obvious that the program would provide information for variety of geological information requirements in both the public and private sectors. It would also greatly assist the MGS in its own mission of geologic mapping and development of a geological framework for the state. Because the Precambrian rocks in Minnesota are almost completely covered by Pleistocene glacial deposits and Phanerozoic sedimentary rocks, studies of the Precambrian relies significantly on information provided by aeromagnetic and other geophysical surveys.
Organizations other than the MGS that have made significant contributions to collection of aeromagnetic data are shown in Fig. 1 and Tables 1-3. Data covering north-central Minnesota were acquired and compiled between 1984 and 1991 by the U.S. Geological Survey (USGS), in cooperation with the MGS as part of the Conterminous United States Mineral Assessment Program (CUSMAP). The Geological Survey of Canada conducted an aeromagnetic survey over the Lake of the Woods in 1985 and over Lake Superior in 1987. The Minnesota part of this survey was incorporated into the present data grids and appears on the State wide map published by the MGS in 1991(Chandler, 1991). However, when the grids were released for distribution on the Web in 1994, they were trimmed to the shoreline of northeastern Minnesota rather than the state outline. Hence that gridded data did not include Lake Superior. Figure 1 and Tables 1-3 referenced above do not include the Lake Superior data either. USX (U. S. Steel) Corporation donated aeromagnetic data acquired in 1979, for part of southwestern Minnesota. The metropolitan areas of Minneapolis and St. Paul were not included in the 1979-1991 data collection because of, flight restrictions over the cities and excessive amounts of cultural noise. Data digitized from a 1961 map published by the USGS were used instead (Sims and Zietz, 1967).
The data for the various surveys differ somewhat in how they were acquired and compile, but several generalizations are possible. Most of the surveys that took place over land flew north-south lines with average terrain clearances of 91-213 meters (m). Flight-line spacing depended primarily on depth to magnetic basement and ranged between 380 to 1000 m apart. The Lake of the Woods and Lake Superior data were collected from flights 300 m above the lake surface and line spacings of 926 and 1900 meters respectively. Most of the surveys also flew tie-lines, in east-west directions at spacings generally 5 to 10 times wider than flightline spacing. In the MGS-LCMR surveys, data were not accepted if the diurnal variations exceeded 3 or 4 nanoTeslas (nT) across any 5-minute chord. Aircraft compensation, (for MGS_LCMR surveys), checked at the start, middle and end of each field season, was within 0.5 NT for all flightline directions, and for 10° of pitch, yaw and roll.
In general, flight-path recovery was accomplished photographically with spotted points every 2 to 5 kilometers (km) along line. Points were picked to achieve a horizontal accuracy plus or minus 50 m. Flight path recovery was also accomplished by electronic methods, including Global Positioning System in parts of Blocks 5, 6A and 7A, and ™Loran-C in parts of Block 8, and a portable radio positioning system in parts of Block 8 (Fig. 1). Corrections for diurnal variations were made by removal of a generalized or smoothed form of the diurnal monitor record, tie leveling, or a combination of the two. Correction for the regional (core-derived) field was based on the American World Charts-1975 model (Peddie and Fabiano, 1976), or the International Geomagnetic Reference Field and the Definitive Geomagnetic Reference Field models (Peddie, 1983; Inter. Assoc. of Geomagnetism and Aeronomy, 1988), updated to the approximate times of acquisition. In general, no attempts were made to remove cultural features. All data within each survey block were gridded using minimum curvature (Briggs, 1974) or a similar method.
Most of the aeromagnetic surveying and the publication of related maps has been funded by the Minnesota Legislature as recommended by the Legislative Commission on Minnesota Resources. W. J. Hinze of Purdue University provided valuable advice during the early stages of the surveying program. Gratitude is expressed to the US Geological Survey (USGS) and the Geological Survey of Canada for allowing their data to be included in this compilation. Robert Bracken and Robert Kuchs of the USGS helped prepare the original data grids for Lake of the Woods. The efforts of Mr. Cedric Iverson, who helped in the release of the USX data in southwestern Minnesota, are also greatly appreciated. The aeromagnetic data from the Twin Cities metropolitan area was digitized by Bryan D. Schaap of the MGS, using stable base copies of the original aeromagnetic map that were provided by Richard Godson of the USGS.
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Briggs, I.C., 1974, Machine contouring using minimum curvature: Geophysics, v. 39, p. 39-48.
Chandler, V. W., 1991, Aeromagnetic anomaly map of Minnesota: Minnesota Geological Survey State Map Series S-17, scale 1:500,000.
International Association of Geomagnetism and Aeronomy, Division I, Working Group 1, 1988, International Geomagnetic Reference Field revision 1987: Geophysics v. 53, p. 576-578.
National Oceanic and Atmospheric Administration, 1995, Geomagnetic field models and synthesis software, Version 2.1, The National Geophysical Data Center, Boulder, Colorado.
Peddie, N. W., 1983, International Geomagnetic Reference Field- Its evolution and the difference in total field intensity between new and old models for 1965-1980: Geophysics, v. 48, p. 1691-1696.
Peddie, N. W., and Fabiano, E. B., 1976, A model of the Geomagnetic Filed for 1975: Journal of Geophysical Research, v. 81, p. 2539-2547.
Phillips, J. D. 1997 Phillips, J. D., 1997, U. S. Geological Survey Potential-Field Software Package, version 2.2 for the PC: United States Geological Survey Open File Report 97-725.
Sims, P. K., and Zietz, I., 1967,
Aeromagnetic and inferred Precambrian paleogeological map of east-central Minnesota
and part of Wisconsin: U. S. Geological Survey Geophysical Investigations Map
GP-563, scale 1:250,000, section, text.
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