Identification_Information: Citation: Citation_Information: Originator: David L. Daniels Publication_Date: 2008 Title: Michigan Magnetic 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 Data Series Issue_Identification: Data Series 3?? Publication_Information: Publication_Place: Reston, Virginia USA Publisher: U.S. Geological Survey Online_Linkage: http://pubs.usgs.gov/ds/411/ Description: Abstract: The Michigan aeromagnetic grid is constructed from grids that combine information collected in 25 separate aeromagnetic surveys conducted between 1947 and 1999. 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. All of the available data were obtained from aeromagnetic surveys flown by the U.S. Geological Survey (USGS), or flown on contract with the USGS. The data for surveys flown before 1973 were 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 305 meters (1,000 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 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 nonmagnetic). This information is valuable for mineral exploration, geologic mapping, and environmental studies. Time_Period_of_Content: Time_Period_Information: Range_of_Dates/Times: Beginning_Date: 1947 Ending_Date: 1999 Currentness_Reference: survey dates Status: Progress: complete Maintenance_and_Update_Frequency: none planned Spatial_Domain: Bounding_Coordinates: West_Bounding_Coordinate: -98.8892 degrees West longitude East_Bounding_Coordinate: -80.5189 degrees West longitude North_Bounding_Coordinate: 48.3062 degrees North latitude South_Bounding_Coordinate: 37.7717 degrees North latitude Keywords: Place: Place_Keyword_Thesaurus: none Place_Keyword: Michigan 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: Stephen Snyder Contact_Organization: U.S. Geological Survey Contact_Address: Address_Type: mailing address Address: MS964 National Center 12201 Sunrise Valley Drive City: Reston State_or_Province: VA Postal_Code: 20192 Country: USA Contact_Voice_Telephone: 703-648-6384 Contact_Electronic_Mail_Address: ssnyder@usgs.gov Native_Data_Set_Environment: These grids were created on a PC (Windows XP) using the proprietary software Oasis Montaj, from Geosoft, Inc. 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 International Geomagnetic Reference Field (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 flight-line intersections. The absolute accuracy of the gridded data is probably better than 5 nT, with accuracy increasing with increasing wavelength. A final grid interval of 400 m 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 Michigan aeromagnetic compilation was to acquire the best resolution aeromagnetic datasets for any given area. For some surveys, the spacing between flight lines was not optimum, especially surveys flown at low altitudes over magnetized rocks at the surface. This spacing causes anomalies with short spatial wavelengths to display a "string of pearls" effect along flight lines. 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/ds/411/ Completeness_Report: Areas with no aeromagnetic data are flagged with dummy values. 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 was determined by varying standard aircraft navigation aids. Vertical_Positional_Accuracy: Vertical_Positional_Accuracy_Report: The individual aeromagnetic surveys that make up this dataset 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 1,000 feet (305 m) above ground. Lineage: Process_Step: Process_Description: 24 individual aeromagnetic surveys and grids were used to create the Michigan state-wide compilation. Descriptions of the data acquisition procedures are contained in the data references in open-file report http://pubs.usgs.gov/ds/411/ Process_Date: 1947-1999 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 flight-line 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 removal required the determination and addition of the original reference field data. Many of the data required microlevelling, using Oasis montaj, before continuation to the reference surface. This procedure uses decorrugation, and attempted to remove any artificial trends or anomalies due to residual flight-line noise remaining in the data. The original survey grids were continued to 305 meters (1,000 feet) above ground by draping as necessary. Upward continuation of the NURE and digitized draped surveys was performed by standard two-dimensional Fast Fourier Transform 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 400 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. The adjusted grids were combined into a single merged grid that shows the magnetic field measured or calculated at 305 meters (1,000 feet) above ground. Process_Date: 2008 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: -84.0 Latitude_of_Projection_Origin: 0.0 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: 400.0 Ordinate_Resolution: 400.0 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: Two primary grids are included in this report: mi_mag_400m.grd - Geosoft Oasis montaj binary grid format with 400-m grid interval, Michigan composite residual aeromagnetic anomaly grid continued to observation surface 305 m above terrain. mi_mag_400m.grd.gi - Grid information file for above Geosoft binary grid. mi_grav_2500m.grd - Geosoft Oasis montaj binary grid format with 2,500-m grid interval, Michigan complete Bouguer anomaly. mi_grav_2500m.grd.gi - Grid information file for above Geosoft binary grid. Related files: mi_mag_1.0M_Oasis.map - Geosoft Oasis montaj binary map file - Michigan magnetic map, scale 1:1,000,000 viewable with the Geosoft free viewing software (http://www.geosoft.com/). mi_mag_survey_index.map - Geosoft Oasis montaj binary map file - Michigan magnetic survey index map, viewable with the Geosoft free viewing software. mi_grav_1.0M_Oasis.map - Geosoft Oasis montaj binary map file - Michigan complete Bouguer gravity anomaly map, scale 1:1,000,000 viewable with the Geosoft free viewing software. mi_grav_stations_1.0M_Oasis.map - Geosoft Oasis montaj binary map file - Michigan gravity map showing distribution of gravity stations, scale 1:1,000,000, viewable with the Geosoft free viewing software. Entity_and_Attribute_Detail_Citation: Data Series Report 3?? Detailed_Description: Entity_Type: Entity_Type_Label: gridded aeromagnetic Entity_Type_Definition: Ascii or binary 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 aeromagnetic file 'mi_mag_400m.grd'. 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 Definitive International Geomagnetic Reference Field (DGRF). The individual magnetic surveys were continued from their original flight elevation surfaces to a draped surface 1,000 ft above the terrain using FFT routines and a chessboard method called compudrape in Geosoft's Oasis montaj, and then merged into the final Michigan 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: http://pubs.usgs.gov/fs/fs-0236-95/fs-0236-95.pdf Attribute_Domain_Values: Range_Domain: Range_Domain_Minimum: -2295.06 Range_Domain_Maximum: 14,669.69 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 or binary 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 files 'mi_grav_2500m.grd' The complete-Bouguer gravity anomaly grid of Michigan. 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: -91.79 Range_Domain_Maximum: -29.94 Attribute_Units_of_Measure: milliGals 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: Stephen Snyder Contact_Organization: U. S. Geological Survey Contact_Address: Address_Type: mailing address Address: MS 954 National Center 12201 Sunrise Valley Drive City: Reston State_or_Province: VA Postal_Code: 20192 Country: USA Contact_Voice_Telephone: 703-648-6384 Contact_Electronic_Mail_Address: ssnyder@usgs.gov Resource_Description: USGS Data Series 3?? 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: grd - Oasis montaj binary gridded data grd.gi - Oasis montaj binary grid projection information map - Oasis montaj binary map file txt - ASCII description files Transfer_Size: 13,886 KB is the largest file size. Digital_Transfer_Option: Online_Option: Computer_Contact_Information: Network_Address: Network_Resource_Name: http://pubs.usgs.gov/ds/411/ Fees: No charge for online downloads Metadata_Reference_Information: Metadata_Date: 20080726 Metadata_Contact: Contact_Information: Contact_Person_Primary: Contact_Person: Stephen Snyder Contact_Organization: U.S. Geological Survey Contact_Address: Address_Type: mailing address Address: MS 954 National Center 12201 Sunrise Valley Drive City: Reston State_or_Province: VA Postal_Code: 20192 Country: USA Contact_Voice_Telephone: 703-648-6384 Contact_Electronic_Mail_Address: ssnyder@usgs.gov Metadata_Standard_Name: FGDC Content Standards for Digital Geospatial Metadata Metadata_Standard_Version: FGDC-STD-001-1998