Line, grid, and vector data, plot files, and descriptive project report for the airborne geophysical survey of part of the western Fortymile mining district, east-central Alaska

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Frequently-anticipated questions:

What does this data set describe?

Title:
Line, grid, and vector data, plot files, and descriptive project report for the airborne geophysical survey of part of the western Fortymile mining district, east-central Alaska
Abstract:
This digital publication, GPR 2008-1, contains geophysical data and a digital elevation model that were produced from airborne geophysical surveys conducted in 2007 for part of the western Fortymile mining district, east-central Alaska. Aeromagnetic and electromagnetic data were acquired for 250 sq miles during the helicopter-based survey. Data provided in GPR 2008-1 include processed (1) linedata ASCII database, (2) gridded files of magnetic data, a calculated vertical magnetic gradient (first vertical derivative), apparent resistivity data, and a digital elevation model, (3) vector files of data contours and flight lines, and (4) the Contractor's descriptive project report. Data are described in more detail in the "GPR2008-1Readme.pdf" and "linedata/GPR2008-1-Linedata.txt" files included on the DVD.
Supplemental_Information:
The maps were compiled and drawn under contract between the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (DGGS), and Stevens Exploration Management Corp. Airborne geophysical data for the area were acquired and processed by Fugro Airborne Surveys in 2007. This publication also contains HPGL2 and Adobe Acrobat files for all maps produced to date for this project. All maps included on this publication are full color. Authors for the individual maps are the same as for this publication. The maps on GPR 2008-1 include: GPR 2008-1-1a - Total magnetic field of part of the western Fortymile mining district, east-central Alaska; topography included; scale 1:63,360, 1 sheet. GPR 2008-1-1b - Total magnetic field of part of the western Fortymile mining district, east-central Alaska; magnetic data contours included; scale 1:63,360, 1 sheet. GPR 2008-1-1c - First vertical derivative of the total magnetic field of part of the western Fortymile mining district, east-central Alaska; topography included; scale 1:63,360, 1 sheet. GPR 2008-1-2a - 56,000 Hz coplanar apparent resistivity of part of the western Fortymile mining district, east-central Alaska; topography included; scale 1:63,360, 1 sheet. GPR 2008-1-2b - 56,000 Hz coplanar apparent resistivity of part of the western Fortymile mining district, east-central Alaska; 56,000 Hz resistivity data contours included; scale 1:63,360, 1 sheet. GPR 2008-1-3a - 7200 Hz coplanar apparent resistivity of part of the western Fortymile mining district, east-central Alaska; topography included; scale 1:63,360, 1 sheet. GPR 2008-1-3b - 7200 Hz coplanar apparent resistivity of part of the western Fortymile mining district, east-central Alaska; 7200 Hz resistivity data contours included; scale 1:63,360, 1 sheet. GPR 2008-1-4a - 900 Hz coplanar apparent resistivity of part of the western Fortymile mining district, east-central Alaska; topography included; scale 1:63,360, 1 sheet. GPR 2008-1-4b - 900 Hz coplanar apparent resistivity of part of the western Fortymile mining district, east-central Alaska; 900 Hz resistivity data contours included; scale 1:63,360, 1 sheet. An index map showing the locations of the 1:63,360-scale map sheets are included on this publication. Bounding coordinates for the sheets are given immediately below. The bounding coordinates of the gridded data are given in the "Bounding_Coordinates" section below. 1:63,360-scale maps, sheet 1 West, -143.42 1:63,360-scale maps, sheet 1 East, -142.58 1:63,360-scale maps, sheet 1 North, 64.38 1:63,360-scale maps, sheet 1 South, 63.92
  1. How should this data set be cited?

    Burns, L.E., U.S. Bureau of Land Management, Fugro Airborne Surveys Corp., and Stevens Exploration Management Corp., 2008, Line, grid, and vector data, plot files, and descriptive project report for the airborne geophysical survey of part of the western Fortymile mining district, east-central Alaska: Geophysical Report GPR 2008-1, State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (DGGS), Fairbanks, AK, USA.

    Online Links:

    Other_Citation_Details: 1 DVD

  2. What geographic area does the data set cover?

    West_Bounding_Coordinate: -143.39
    East_Bounding_Coordinate: -142.61
    North_Bounding_Coordinate: 64.36
    South_Bounding_Coordinate: 63.95

  3. What does it look like?

  4. Does the data set describe conditions during a particular time period?

    Beginning_Date: Sep-2007
    Ending_Date: Dec-2007
    Currentness_Reference: publication date

  5. What is the general form of this data set?

    Geospatial_Data_Presentation_Form:
    raster digital data, tabular digital data, vector digital data, and atlas

  6. How does the data set represent geographic features?

    1. How are geographic features stored in the data set?

    2. What coordinate system is used to represent geographic features?

      Grid_Coordinate_System_Name: Universal Transverse Mercator
      Universal_Transverse_Mercator:
      UTM_Zone_Number: 7
      Transverse_Mercator:
      Scale_Factor_at_Central_Meridian: 0.9996
      Longitude_of_Central_Meridian: -141
      Latitude_of_Projection_Origin: 0
      False_Easting: 500000
      False_Northing: 0

      Planar coordinates are encoded using row and column
      Abscissae (x-coordinates) are specified to the nearest 25
      Ordinates (y-coordinates) are specified to the nearest 25
      Planar coordinates are specified in meters

      The horizontal datum used is North American Datum of 1927.
      The ellipsoid used is Clarke 1866.
      The semi-major axis of the ellipsoid used is 6378206.4.
      The flattening of the ellipsoid used is 1/294.978698.

  7. How does the data set describe geographic features?

    Entity_and_Attribute_Overview:
    Entities (file names) are listed in the "GPR2008-1Readme.pdf" file included on this DVD. Attributes recorded during the geophysical survey are listed with the units in the text files "linedata/Fortymile-Linedata.txt". Further description is available in the descriptive project report included in this publication.
    Entity_and_Attribute_Detail_Citation:
    L.E. Burns, Division of Geological & Geophysical Surveys and Fugro Airborne Surveys

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)

  2. Who also contributed to the data set?

    The U.S. Bureau of Land Management provided funding for the 2007 geophysical data and project management.

  3. To whom should users address questions about the data?

Why was the data set created?

The survey was funded by the U.S. Department of Interior, Bureau of Land Management in support of the State of Alaska's Airborne Geophysical/Geological Mineral Inventory project. The data were produced to provide information for mineral resource evaluation and land management decisions.

How was the data set created?

  1. From what previous works were the data drawn?

    Akima, 1970 (source 1 of 1)
    Akima, H., 1970, A new method of interpolation and smooth curve fitting based on local procedures: Journal of the Association of Computing Machinery v. 7, no. 4.

    Online Links:

    • None

    Type_of_Source_Media: paper
    Source_Contribution:
    Fugro Airborne Surveys used a modification of this method while making grids.

  2. How were the data generated, processed, and modified?

    Date: 2007 (process 1 of 6)
    The Alaskan section grid, taken from the DGGS network, was modified by DGGS to align some of the section lines with the section lines on the U.S. Geological Survey 1:63,360-scale topographic maps of the area. Modification involved moving about an eighth or less of the east-west and north-south lines, particularly in the western portions of the quadrangle sheets. Movement was generally on the order of a tenth of a mile. The section grid file contained here may not be identical to the section grid available through Alaska Department of Natural Resources, Land Record Information Section (LRIS).

    Date: 2007 (process 2 of 6)
    The airborne geophysical data were acquired with a DIGHEM(V) Electromagnetic (EM) system and a Fugro D1344 cesium magnetometer with a Scintrex CS3 cesium censor. The EM and magnetic sensors were flown at a height of 100 feet. In addition, the survey recorded data from a radar altimeter, GPS navigation system, 50/60 Hz monitors, and video camera. Flights were performed with an AS350B-3 Squirrel helicopter at a mean terrain clearance of 200 feet along N-S survey flight lines with one-quarter mile line spacing. Tie lines were flown perpendicular to the flight lines at intervals of approximately 3 miles. A Novatel OEM4 Global Positioning System was used for navigation and flight path recovery. The helicopter position was derived every 0.5 seconds using post-flight differential positioning to a relative accuracy of better than 5 m. Flight path positions were projected onto the Clarke 1866 (UTM zone 7) spheroid, 1927 North American datum using a central meridian (CM) of 141 degrees, a north constant of 0 and an east constant of 500,000.

    Date: 2007 (process 3 of 6)
    The total magnetic field data were acquired with a sampling interval of 0.1 seconds, and were (1) corrected for measured system lag, (2) corrected for diurnal variations by subtraction of the digitally recorded base station magnetic data (saved as tfmag in Fortymile-Linedata.xyz), (3) adjusted for regional variations (or IGRF gradient, 2005, updated to September 2007) using altimeter adjusted IGRF, (4) leveled to the tie line data (saved as magigrf in Fortymile-Linedata.xyz), and (5) interpolated onto a regular 80-m grid using a modified Akima (1970) technique. The total magnetic field data were subjected to a processing algorithm that enhances the response of magnetic bodies in the upper 500 m and attenuates the response of deeper bodies. The resulting vertical gradient grid (Fortymile-cvg) provides better definition and resolution of near-surface magnetic units. It also identifies weak magnetic features that may not be evident on the total field data. All magnetic grids were then resampled from the 80-m cell size down to a 25-m cell size using a modified Akima (1970) technique to produce the maps and final grids contained on this publication.

    Data sources used in this process:

    • Akima, 1970

    Date: 2007 (process 4 of 6)
    The DIGHEM V EM system measured inphase and quadrature components at five frequencies. Two vertical coaxial-coil pairs operated at 1125 (1000) and 5454 (5500) Hz while three horizontal coplanar-coil pairs operated at 875 (900), 7153 (7200), and 55,400 (56,000) Hz. The EM data were sampled at 0.1 second intervals. The EM system responds to bedrock conductors, conductive overburden, and cultural sources. The EM inphase and quadrature data were drift corrected using base level data collected at high altitude (areas of no signal). Along-line filters are applied to the data to remove spheric spikes. The data were inspected for variations in phase, and a phase correction was applied to the data if necessary. Apparent resistivities were then calculated from the inphase and quadrature data for all frequencies based on a pseudo-layer half-space model. Manual leveling of the inphase and quadrature of each coil pair, based on the resistivity data and comparisons to the data from the other frequencies, was performed. Automated micro-leveling is carried out in areas of low signal. The EM data were interpolated onto a regular 80-m grid using a modified Akima (1970) technique. The resulting grids were subjected to a 3x3 hanning filter and resampled to a 25-m cell size before contouring and map production.

    Data sources used in this process:

    • Akima, 1970

    Date: 2007 (process 5 of 6)
    To produce the digital elevation model, the GPS-Z data were differentially corrected and transformed into the local datum. The ALTBIRD data were filtered using a 13, 13 filter. Both the Z-COR (differentially corrected GPS-Z) was checked for spikes, which were removed manually. The corrected altimeter was then subtracted from the Z-COR data to produce profiles of the height above mean sea level along the survey lines. The data were manually leveled to remove any errors between lines. After all leveling, the data were DC shifted to match the local maps, in this case, NAD27. The final Z value is recalculated as final Z = DEM + altbird. The 80-m DEM grid was then resampled to a 25-m cell size to produce the DEM grid contained on this publication.

    Date: 2007 (process 6 of 6)
    The included plot files of the maps were produced by Fugro Airborne Surveys. The HPGL2 files were created with HP Designjet 5000 printer driver v5.32 and will not work with all plotters, but do plot on the DGGS HP Design Jet 5000. The HPGL2 files have brighter colors and sharper topography than the Adobe Acrobat files. The Adobe Acrobat format files were created with Adobe Acrobat Distiller v7.0 (PDF 1.3) from Postscript files. The Postscript files were produced by Hewlett Packard Designjet 5000 Postscript 3 printer driver v4.293 and are not included on this publication.

  3. What similar or related data should the user be aware of?

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?

    Geophysical data were acquired during an airborne survey. The magnetometer used had a sensitivity of 0.01 nT. Resistivity sensitivity varies among the different frequencies from 0.06 ppm to 0.30 ppm.

  2. How accurate are the geographic locations?

    The helicopter position was derived every 0.5 seconds using post-flight differential positioning to a relative accuracy of better than 5 m. Positional accuracy of the presented data is better than 10 m with respect to the UTM grid. Further inaccuracies may be introduced by the interpolation and gridding process.

  3. How accurate are the heights or depths?

    The accuracy of the elevation calculation used for the digital elevation model is directly dependent on the accuracy of the two input parameters, ALTR and HMSL. The ALTR value may be erroneous in areas of heavy tree cover, where the altimeter reflects the distance to the tree canopy rather than the ground. The HMSL (or GPS-Z) value is primarily dependent on the number of available satellites. Although post-processing of GPS data will yield X and Y accuracies in the order of 5 meters, the accuracy of the Z value is usually much less, sometimes in the +/-20 meter range.

  4. Where are the gaps in the data? What is missing?

    This dataset includes files that contain processed aeromagnetic measurements, electromagnetic measurements, altimetry measurements, and locational data for the survey area.

  5. How consistent are the relationships among the observations, including topology?

    No topological relationships are included.

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?

Access_Constraints:
This report, map and/or dataset are available directly from the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (see contact information below).
Use_Constraints:
Any hard copies or published datasets utilizing these datasets shall clearly indicate their source. If the user has modified the data in any way the user is obligated to describe the types of modifications the user has made. User specifically agrees not to misrepresent these datasets, nor to imply that changes made by the user were approved by the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (DGGS).

  1. Who distributes the data set? (Distributor 1 of 1)

    State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (DGGS)
    Natural Resource Technician
    3354 College Road
    Fairbanks, AK 99709-3707
    USA

    907-451-5020 (voice)
    907-451-5050 (FAX)
    dggspubs@alaska.gov

    Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays
    Contact_Instructions:
    Please view our Web site (<http://www.dggs.alaska.gov>) for the latest information on available data. Please contact us using the e-mail address above whenever possible.
  2. What's the catalog number I need to order this data set?

    Geophysical Report 2008-1

  3. What legal disclaimers am I supposed to read?

    The State of Alaska makes no express or implied warranties (including warranties of merchantability and fitness) with respect to the character, function, or capabilities of the electronic services or products or their appropriateness for any user's purposes. In no event will the State of Alaska be liable for any incidental, indirect, special, consequential, or other damages suffered by the user or any other person or entity whether from the use of the electronic services or products, any failure thereof, or otherwise, and in no event will the State of Alaska's liability to the requester or anyone else exceed the fee paid for the electronic service or product.

  4. How can I download or order the data?

  5. Is there some other way to get the data?

    Custom views or processing may be requested. Please contact Laurel Burns by phone (907-451-5021), e-mail (laurel.burns@alaska.gov), or fax (907-451-5050) to discuss custom processing availability, fees, and turnaround time.

  6. What hardware or software do I need in order to use the data set?

    Software with ability to use, import, or convert Geosoft .grd or ER Mapper .ers files, Geosoft .xyz (ASCII) files, Autocad .dxf files, Adobe Acrobat files, and text files. Free downloadable interfaces for accessing the gridded and dxf files are available at the Geosoft Web site (<http://www.geosoft.com>; Oasis Montaj viewer), and the ER Mapper Web site (<http://www.ermapper.com>). Maps are accessed digitally using Adobe Acrobat. Maps are printed from the HPGL/2 files using HP 5000 series plotters or the Adobe Acrobat files. The HPGL/2 files will provide the clearest hard copy versions. "Printfile" (<http://www.lerup.com/printfile/>) is a freeware MS Windows utility program that will enable you to print HPGL/2 files; the plotter will still need to be compatible with files produced for the HP 5000 series.

Who wrote the metadata?

Dates:
Last modified: 06-May-2008
Last Reviewed: 06-May-2008
To be reviewed: 06-Nov-2011
Metadata author:
State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (DGGS)
Geophysicist
3354 College Road
Fairbanks, AK 99709-3707
USA

907-451-5021 (voice)
907-451-5050 (FAX)
dggspubs@alaska.gov

Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays.
Contact_Instructions:
Please contact us through the e-mail address above whenever possible.
Metadata standard:
FGDC Content Standards for Digital Geospatial Metadata (FGDC-STD-001-1998)

Generated by mp version 2.9.6 on Tue May 06 14:29:34 2008