Identification_Information: Citation: Citation_Information: Originator: Ronald E. Sweeney Originator: Patricia L. Hill Publication_Date: 2003 Title: North Dakota Aeromagnetic and Gravity Maps and Data: A Web Site for Distribution of Data Geospatial_Data_Presentation_Form: raster digital data Series_Information: Series_Name: U.S. Geological Survey Open-File Report Issue_Identification: Open-File Report 03-249 Publication_Information: Publication_Place: Denver, Colorado, USA Publisher: U.S. Geological Survey Online_Linkage: http://pubs.usgs.gov/of/2003/ofr-03-249 Description: Abstract: The North Dakota aeromagnetic grid is constructed from grids that combine information collected in 13 separate aeromagnetic surveys conducted between 1978 and 2001. The data from these surveys are of varying quality. The design and specifications (terrain clearance, sampling rates, line spacing, and reduction procedures) varied from survey to survey depending on the purpose of the project and the technology of that time. Every attempt was made to acquire the data in digital form. Most of the available digital data were obtained from aeromagnetic surveys flown by the U.S. Geological Survey (USGS), flown on contract with the USGS, or were obtained from other federal agencies and state universities. Some of the 1980 data are available only on hand-contoured maps and had to be digitized. These maps were digitized along flight-line/contour-line intersections, which is considered to be the most accurate method of recovering the original data. Digitized data are available as USGS Open File Report 99-557. All surveys have been continued to 304.8 meters (1000 feet) above ground and then blended or merged together. Purpose: Presentation of gridded aeromagnetic data values display aeromagnetic anomalies used in geological and geophysical studies. Aeromagnetic anomalies are due to variations in the Earth's magnetic field caused by the uneven distribution of magnetic minerals (primarily magnetite) in the rocks that make up the upper part of the Earth's crust. The features and patterns of the aeromagnetic anomalies can be used to delineate details of subsurface geology including the locations of buried faults, magnetite-bearing rocks, and the thickness of surficial sedimentary rocks (which are generally non-magnetic). This information is valuable for mineral exploration, geologic mapping, and environmental studies. Supplemental_Information: Also included are maps and grids of some of the individual surveys at their original flight elevation that were generated with a finer grid spacing due to a finer flight line spacing. These were then regridded to the final grid spacing of 1000 meters for use in the state merge. Time_Period_of_Content: Time_Period_Information: Range_of_Dates/Times: Beginning_Date: 1978 Ending_Date: 2001 Currentness_Reference: survey dates Status: Progress: complete Maintenance_and_Update_Frequency: none planned Spatial_Domain: Bounding_Coordinates: West_Bounding_Coordinate: -104.2 degrees West longitude East_Bounding_Coordinate: -96.5 degrees West longitude North_Bounding_Coordinate: 45.8 degrees North latitude South_Bounding_Coordinate: 49.1 degrees North latitude Keywords: Place: Place_Keyword_Thesaurus: none Place_Keyword: North Dakota Theme: Theme_Keyword_Thesaurus: none Theme_Keyword: geophysical data Theme_Keyword: aeromagnetic data Theme_Keyword: aeromagnetic grid Access_Constraints: none Use_Constraints: none Point_of_Contact: Contact_Information: Contact_Person_Primary: Contact_Person: Ronald Sweeney Contact_Organization: U.S. Geological Survey Contact_Address: Address_Type: mailing address Address: Box 25046, MS964 Denver Federal Center City: Denver State_or_Province: CO Postal_Code: 80225-0046 Country: USA Contact_Voice_Telephone: 303-236-1389 Contact_Electronic_Mail_Address: rsweeney@usgs.gov Native_Data_Set_Environment: These grids were created using USGS processing programs on a UNIX system and on a PC using Geosoft/OASIS montaj application software. Data_Quality_Information: Attribute_Accuracy: Attribute_Accuracy_Report: The digital data are grids, which were created from aircraft magnetometer observations with a typical accuracy of 1 to 10 nanotesla (nT). In addition to corrections for the IGRF, the original individual magnetic surveys typically contain corrections for the diurnal variations of the Earth's magnetic field that occurred during the flight (as recorded by a base station magnetometer) and leveling adjustments made at flightline intersections. The absolute accuracy of the gridded data are probably better than 5 nanotesla, with accuracy increasing with increasing wavelength. A final grid interval of 1 km was chosen as representative. The data in the original grids have been processed using formulas and methods that are not usually documented but that represent industry standard practices for airborne data reduction. Logical_Consistency_Report: Our priority in the construction of the state of North Dakota aeromagnetic compilation was always to acquire the best resolution aeromagnetic data sets for any given area. Where local high- resolution surveys were not available, in either digital or digitized format, we used aeromagnetic data collected by the National Uranium Resource Evaluation (NURE) program of the U.S. Department of Energy, which is available in digital format and covers the entire state. However, because magnetic surveying was not the primary objective in the design of the NURE surveys, these data are subject to certain limitations. Although the NURE surveys were flown at elevations close to the reduction datum level, the spacing between flight lines ranged from 4800 meters to 9600 meters (3 to 6 miles). The wide spacing between flight lines flown at low altitudes over surface rock units having high magnetizations causes anomalies with short spacial wavelengths to be elongated between flight lines, producing lineations perpendicular to the flight-line direction and 'pearl string' anomalies along the flight line. Also, problems related to the reduction of navigation control and inconsistent datum levels between NURE surveys can cause herringbone features which are sometimes observed within and at the boundaries of these surveys. Standard industry practices of the time were followed in data collection and processing. More details are found in the report describing the compilation of the original grids: http://pubs.usgs.gov/of/2003/ofr-03-249 Completeness_Report: Areas with no aeromagnetic data are flagged with dummy values, which are described for the grids in a readme.txt file. Positional_Accuracy: Horizontal_Positional_Accuracy: Horizontal_Positional_Accuracy_Report: Horizontal position of the survey aircraft used to collect data being integrated into the composite grid were determined by varying standard aircraft navigation aids. The latest surveys determined the high-resolution horizontal position from differential positioning techniques based on Global Positioning System (GPS) satellites. Vertical_Positional_Accuracy: Vertical_Positional_Accuracy_Report: The individual aeromagnetic surveys that make up this data set were flown at various fixed barometric altitudes and as draped surveys with constant terrain clearance specifications. All surveys were adjusted to simulate a flight altitude of 1000 feet above ground. Lineage: Process_Step: Process_Description: 13 individual aeromagnetic surveys and grids were used to create the North Dakota state-wide compilation. Descriptions of the data acquisition procedures are contained in the data references in open-file report http://pubs.usgs.gov/of/2003/ofr-03-249 Process_Date: 1978-2001 Process_Step: Process_Description: Grids were constructed from the original aeromagnetic survey data with a cell size of between 1/3 and 1/5 of the flightline spacing of the survey, using a minimum curvature gridding algorithm. For digitized contour line data, the initial grid was constructed using a minimum curvature algorithm and a spacing appropriate for the scale of the digitized map. The Definitive Geomagnetic Reference Field (DGRF calculated for the date of the original survey was removed (in some cases this required the determination and addition of the original reference field removed before removal of the DGRF). Much of the data required microlevelling, using OASIS montaj, before continuation to our reference surface. This procedure uses decorrugation, and attempts to remove any artificial trends or anomalies due to residual flight line noise remaining in the data. The original survey grids were continued to 304.8 meters (1000 feet) above ground by draping as necessary. Upward continuation of the NURE and digitized draped surveys was by standard 2D FFT filtering techniques. Level-to-drape continuation, and surface-to-drape continuation where the true measurement elevations were known, were performed using the chessboard method. The survey grids were regridded, as necessary, to the final grid cell size of 1000 meters using a minimum curvature algorithm. The datum levels of the converted grids were adjusted (by addition or subtraction of a constant value) to minimize differences at the boundaries. These adjusted grids were combined into a single merged grid that shows the magnetic field measured or calculated at 304.8 meters (1000 feet) above ground. Process_Date: 2001-2003 Spatial_Data_Organization_Information: Direct_Spatial_Reference_Method: raster Spatial_Reference_Information: Horizontal_Coordinate_System_Definition: Planar: Map_Projection: Lambert_Conformal_Conic: Standard_Parallel: 33.0 Standard_Parallel: 45.0 Longitude_of_Central_Meridian: -100.0 Latitude_of_Projection_Origin: 45.0 False_Easting: 0.0 False_Northing: 0.0 Map_Projection_Name: Lambert Conformal Conic Planar_Coordinate_Information: Planar_Coordinate_Encoding_Method: row and column Coordinate_Representation: Abscissa_Resolution: 1000.0 Ordinate_Resolution: 1000.0 Planar_Distance_Units: meters Geodetic_Model: Horizontal_Datum_Name: North American Datum of 1927 Ellipsoid_Name: Clarke 1866 Semi-major_Axis: 6378206.4 Denominator_of_Flattening_Ratio: 1/294.98 Entity_and_Attribute_Information: Overview_Description: Entity_and_Attribute_Overview: Three primary grids are included in this report: nd1000ag.gxf - Grid exchange format Final North Dakota composite magnetic grid continued to observation surface 1,000 ft above terrain. ndcba.gxf - Grid exchange format Final North Dakota complete-Bouguer gravity anomaly grid. ndisoanom.gxf - Grid exchange format Final North Dakota isostatic gravity anomaly grid. The 3 grids above are also provided in the report in 1 additional format: Geosoft binary. (See the open-file report for details.) There are also subsets of these main grids included in the data download area of this report. These also are explained in detail in the open-file report. Entity_and_Attribute_Detail_Citation: Open-File Report 03-249 Detailed_Description: Entity_Type: Entity_Type_Label: gridded aeromagnetic Entity_Type_Definition: Ascii real number value Entity_Type_Definition_Source: well-established geophysical data quantities Attribute: Attribute_Label: Residual total intensity of Earth's magnetic field Attribute_Definition: Gridded values found in file 'nd1000ag.gxf' The total magnetic value minus a geomagnetic reference field, which is a long-wavelength regional magnetic field. The most commonly used reference field is calculated from the theoretical International Geomagnetic Reference Field (IGRF). The individual magnetic surveys were continued from their original flight elevation surfaces to a draped surface 1,000 ft above the terrain using the chessboard method of Cordell and others (1992), and then merged into the final North Dakota composite aeromagnetic grid. Attribute_Definition_Source: see: Nettleton, L.L., 1971, Elementary Gravity and Magnetics for Geologists and Seismologists: Society of Exploration Geophysicists Monograph Series No. 1, p. 83-87. see: Cordell, L., Phillips, J.D., and R.H. Godson, 1992, U.S. Geological Survey potential-field geophysical software, version 2.0: U.S. Geological Survey Open-File Report 92-18A-G, 16 p. Attribute_Domain_Values: Range_Domain: Range_Domain_Minimum: -948.352 Range_Domain_Maximum: 5569.460 Attribute_Units_of_Measure: nanoteslas Enumerated_Domain: Enumerated_Domain_Value: -1e32 Enumerated_Domain_Value_Definition: dummy value; area of no data Enumerated_Domain_Value_Definition_Source: common out-of-range value used for areas of no data Detailed_Description: Entity_Type: Entity_Type_Label: gridded gravity Entity_Type_Definition: Ascii real number value Entity_Type_Definition_Source: well-established geophysical data quantities Attribute: Attribute_Label: Complete Bouguer gravity anomaly Attribute_Definition: Gridded values found in file 'ndcba.gxf' The complete-Bouguer gravity anomaly grid of North Dakota. Attribute_Definition_Source: see: Nettleton, L.L., 1971, Elementary Gravity and Magnetics for Geologists and Seismologists: Society of Exploration Geophysicists Monograph Series No. 1. see: http://pubs.usgs.gov/fs/fs-0239-95/fs-0239-95.pdf Attribute_Domain_Values: Range_Domain: Range_Domain_Minimum: -106.534 Range_Domain_Maximum: -3.6244 Attribute_Units_of_Measure: miliGals Enumerated_Domain: Enumerated_Domain_Value: -1e32 Enumerated_Domain_Value_Definition: dummy value; area of no data Enumerated_Domain_Value_Definition_Source: common out-of-range value used for areas of no data Attribute: Attribute_Label: Isostatic gravity anomaly Attribute_Definition: Gridded values found in file 'ndisoanom.gxf' The isostatic residual gravity anomaly grid of North Dakota. Attribute_Definition_Source: see: Nettleton, L.L., 1971, Elementary Gravity and Magnetics for Geologists and Seismologists: Society of Exploration Geophysicists Monograph Series No. 1. see: http://pubs.usgs.gov/fs/fs-0239-95/fs-0239-95.pdf Attribute_Domain_Values: Range_Domain: Range_Domain_Minimum: -28.426 Range_Domain_Maximum: 49.2125 Attribute_Units_of_Measure: miliGals Enumerated_Domain: Enumerated_Domain_Value: -1e32 Enumerated_Domain_Value_Definition: dummy value; area of no data Enumerated_Domain_Value_Definition_Source: common out-of-range value used for areas of no data Distribution_Information: Distributor: Contact_Information: Contact_Person_Primary: Contact_Person: Ron Sweeney Contact_Organization: U. S. Geological Survey Contact_Address: Address_Type: mailing address Address: Box 25046, MS 964 Denver Federal Center City: Denver State_or_Province: CO Postal_Code: 80225 Country: USA Contact_Voice_Telephone: 303-236-1389 Contact_Electronic_Mail_Address: rsweeney@usgs.gov Resource_Description: USGS Open-File Report OFR 03-249 Distribution_Liability: Although all data published in the grids have been used by the USGS, no warranty, expressed or implied, is made by the USGS as to the accuracy of the data and related materials. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of these data or related materials. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Standard_Order_Process: Digital_Form: Digital_Transfer_Information: Format_Name: gxf - gridded data files txt - ASCII description files Format_Information_Content: gxf: grid exchange format Transfer_Size: 3.4 M is the largest file size Digital_Transfer_Option: Online_Option: Computer_Contact_Information: Network_Address: Network_Resource_Name: http://pubs.usgs.gov/of/2003/ofr-03-249 Fees: no charge for online downloads Metadata_Reference_Information: Metadata_Date: 20030519 Metadata_Contact: Contact_Information: Contact_Person_Primary: Contact_Person: Ron Sweeney Contact_Organization: U.S. Geological Survey Contact_Address: Address_Type: mailing address Address: U.S. Geological Survey Box 25046 Mail Stop 964 Denver Federal Center City: Denver State_or_Province: CO Postal_Code: 80225 Country: USA Contact_Voice_Telephone: 303-236-1389 Contact_Electronic_Mail_Address: rsweeney@usgs.gov Metadata_Standard_Name: FGDC Content Standards for Digital Geospatial Metadata Metadata_Standard_Version: FGDC-STD-001-1998