Line, grid, and vector data, and maps for the airborne geophysical survey data of the Styx River Survey, parts of the Lime Hills and Tyonek quadrangles, southcentral Alaska

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Frequently-anticipated questions:


What does this data set describe?

Title:
Line, grid, and vector data, and maps for the airborne geophysical survey data of the Styx River Survey, parts of the Lime Hills and Tyonek quadrangles, southcentral Alaska
Abstract:
This digital publication, GPR 2008-3, contains data produced from airborne geophysical surveys conducted in 2007 and 2008 for the Styx River Survey area, southcentral Alaska. Aeromagnetic and electromagnetic data were acquired for about 715 sq miles. Data provided in GPR 2008-3 include (1) raw and processed linedata in Geosoft ASCII XYZ files, (2) gridded files of magnetic data, a calculated vertical magnetic gradient (first vertical derivative), apparent resistivity data, and a digital elevation model, (3) flight lines in vector format, and maps of the data in PDF format. Data are described in more detail in the 'gpr2008_003_readme.pdf' and 'Styx-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 and 2008. A contribution from Anglo American Exploration (USA) Inc. allowed DGGS to acquire data for 180 of the 715 sq miles included in this DVD.
  1. How should this data set be cited?

    Burns, L.E., Fugro Airborne Surveys Corp., and Stevens Exploration Management Corp., 2008, Line, grid, and vector data, and maps for the airborne geophysical survey data of the Styx River Survey, parts of the Lime Hills and Tyonek quadrangles, southcentral Alaska: Geophysical Report GPR 2008-3, 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: -153.77
    East_Bounding_Coordinate: -152.48
    North_Bounding_Coordinate: 62.09
    South_Bounding_Coordinate: 61.52

  3. What does it look like?

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

    Beginning_Date: Oct-2007
    Ending_Date: Jun-2008
    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: 5
      Transverse_Mercator:
      Scale_Factor_at_Central_Meridian: 0.9996
      Longitude_of_Central_Meridian: -153
      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:
    Folders named 'LINEDATA', 'GRIDS', 'VECTORS', and 'PLOT_FILES' are the main DVD structure. The file 'gpr2008_003_readme.pdf' is included in the ROOT directory and lists file names. Channels recorded or calculated during the geophysical survey are listed with units in 'LINEDATA/Styx-Linedata.txt'. Further description is available in the 'Detailed_Description' below for each folder.
    Entity_and_Attribute_Detail_Citation:
    L.E. Burns, Division of Geological & Geophysical Surveys and Fugro Airborne Surveys
    Styx_LinedataAsXYZ.zip
    The file 'Styx-Linedata' contains processed linedata and related calculated fields for locational, magnetic, and electromagnetic data. The file is provided in a Geosoft ASCII XYZ format. Channel names and more information are provided in 'Styx-Linedata'.TXT. (Source: Fugro Airborne Surveys)

    GPR2008-3GridsGRD.zip and GPR2008-3GridsERS.zip
    The gridded files are provided in ER Mapper (ERS) and Geosoft binary float (GRD) format. The Geosoft grid consists of one file (.GRD); an ER Mapper grid consists of two files, which are a header (projection) file (.ERS) and a data file (no extension). Both ER Mapper files are necessary to view a grid or to convert it to another software format. A Geosoft projection (.GI) file is included for both the ER-Mapper grids (.ERS.GI) and the Geosoft grids (.GRD.GI). When the projection files are placed in the same directory as the main grid file, ER Mapper and Geosoft automatically register the grids correctly. Setting the projection can also be done manually. All grids are in NAD27 datum, UTM Zone 5N, and have a 25-m cell size with x and y in meters. Styx-magigrf - Total Magnetic Field (TMF)-IGRF removed (nT) Styx-cvg - Calculated Vertical Gradient of TMF- IGRF removed (nT/m) (First vertical derivative) Styx-res56k - 56000 Hz Apparent Resistivity (ohm?m) Styx-res7200 - 7200 Hz Apparent Resistivity (ohm?m) Styx-res900 - 900 Hz Apparent Resistivity (ohm?m) Styx-dtm - Digital Elevation Model (m) (Source: Fugro Airborne Surveys)

    GPR2008-3Vectors.zip
    The digital versions of the vectors published within the map boundaries are included in AutoCAD DXF v.13 format. The digital vectors include data contours, section grid, and flight path files. (Source: Fugro Airborne Surveys)

    GPR2008-3_MapsAsPDF.zip and GPR2008-3_MapsAsHPGL2.zip
    The 'Plot_Files' are the published maps, which are provided in HPGL2 (.PRN) and Adobe Acrobat (.PDF) formats. Three sheets are needed to cover the area at 1:63,360-scale (refer to GPR2008-3LocMaps63360.jpg location index). The main map number, e.g. GPR 2008-3-1, refers to a particular type of map; the subscript 'A', 'B', and 'C' refers to the western, southern, and eastern sheet respectively. (Source: 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?

    Funding was provided by the Alaska State Legislature as part of the DGGS Airborne Geophysical/Geological Mineral Inventory (AGGMI) program and by a contribution by Anglo America Exploration (USA) Inc. The contribution from Anglo American allowed us to acquire data for about 180 square miles of the 715 sq mile survey, all included in this DVD.

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

    Alaska Division of Geological & Geophysical Surveys
    Geophysicist
    3354 College Rd
    Fairbanks, AK 99709-3707
    USA

    (907) 451-5020 (voice)


Why was the data set created?

The survey was part of the Alaska Airborne Geophysical/Geological Mineral Inventory project funded by the Alaska State Legislature and managed by State of Alaska, Department of Natural Resources (DNR), Division of Geological & Geophysical Surveys (DGGS). The project seeks to catalyze private-sector mineral development investment. The project delineates mineral zones on Alaska state lands that: 1) have major economic value; 2) can be developed in the short term to provide high quality jobs for Alaska; and 3) will provide economic diversification to help offset the loss of Prudhoe Bay oil revenue.


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: 2008 (process 1 of 5)
    The airborne geophysical data were acquired with a DIGHEM(V) Electromagnetic (EM) system and a Fugro D1344 cesium magnetometer with a Scintrex CS3 cesium sensor. Data were acquired during two separate periods, Oct. 16 to Nov 12, 2007 and March 21 to April 22, 2008. See 'Logical_Consistency_Report' section. The same EM bird, frequencies, magnetometer, and GPS system were used for both periods of data acquisition. 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 a video camera. Flights were performed with an AS350B-3 Squirrel helicopter at a mean terrain clearance of 200 feet along N70E 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 5) spheroid, 1927 North American datum using a central meridian (CM) of 153 degrees, a north constant of 0 and an east constant of 500,000.

    Date: 2008 (process 2 of 5)
    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 (mag_diu in Styx-Linedata.xyz), (3) adjusted for regional variations using date of flight and altimeter-adjusted IGRF, (4) leveled to the tie line data resulting in the final residual magnetic intensity (mag_rmi), (5) increased by a constant of 55,624 to provide an IGRF-corrected total magnetic field column (mag_igrf) and (6) interpolated onto a regular 80-m grid using a modified Akima (1970) technique. Both data acquisition and magnetic processing occurred in 2007 and 2008. 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 (Styx-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: 2008 (process 3 of 5)
    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 56,400 (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. Both data acquisition and electromagnetic processing occurred in 2007 and 2008.

    Data sources used in this process:

    • Akima, 1970

    Date: 2008 (process 4 of 5)
    To produce the digital elevation model, the GPS-Z data were differentially corrected (GPSZ). The ALTRAD_BIRD data were filtered using a 13 median, followed by a 13 hanning filter. Both the GPSZ (differentially corrected GPS-Z) and ALTRAD_BIRD were checked for spikes, which were removed manually. The corrected altimeter was then subtracted from the GPSZ 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 80-m DEM grid was then resampled to a 25-m cell size to produce the DEM grid contained on this publication.

    Date: 2008 (process 5 of 5)
    The Adobe Acrobat format files were created with Adobe Acrobat Distiller v7.0 (PDF 1.3) from Postscript files.

  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. Survey contracts specified the conditions and specifications under which these data were collected. Altimeter, heading, lag, and frequent EM calibrations were done. More information will be available in the project report to be published in the future.

  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. Areas with no data for a given channel are flagged with the dummy variable (*). These may be representing 1) spikes and dropouts in the magnetic reading, due to various reasons; 2) resistivity and depth values that are not calculated because flying height was too high to produce meaningful calculations, 3) sampling differences, e.g., most sampling was 10 samples per second, but the diurnal_filt was 1 sample per second, and 4) lag shift at the end of a line. The Res900 has more dummy variables than the Res7200, which has more dummy variables than the Res56K.

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

    Data for this survey were collected by a single subcontractor (Fugro Airborne Surveys) who was responsible for collecting and processing the data. At the request of DGGS, the data were acquired in two different time periods, October-November 2007 and March-April 2008. A late fall contribution of funds from Anglo American Exploration (USA) Inc. allowed us to acquire more data for the public, but logistics then required a delay for part of the survey. The data from both parts of this survey were collected with the same instruments (magnetometers, electromagnetic bird and sensors, altimeters, and navigational system). All channels of the electromagnetic system functioned with very little noise through both parts of the survey. See Process_steps for information on the IGRF calculations.


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)
    Geophysicist
    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 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 2008-3

  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 the geophysicist 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 binary float .grd, Geosoft ASCII .xyz, Er Mapper .ers, Autocad .dxf, Adobe Acrobat files and text files. A free downloadable interface (currently called Oasis Montaj Viewer) is available at the Geosoft website (<http://www.geosoft.com>) and will open Geosoft files and Autocad dxf files. For magnetic files, the default color scheme (colour.tbl) is appropriate for viewing. In order to view the resistivity files in the standard color scheme, change the color table to reflect resistivity colors (resis.tbl). Freeware software 'printfile' (<http://www.lerup.com/printfile/>) prints HPGL/2 files easily on compatible printers. The HPGL/2 files have brighter colors and sharper topography than the PDF maps and should be used for printing when possible. The PDF format maps are the only maps digitally viewable in this publication.


Who wrote the metadata?

Dates:
Last modified: 09-Jun-2008
Last Reviewed: 03-Nov-2011
To be reviewed: 03-Nov-2031
Metadata author:
State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (DGGS)
Geophysicist
3354 College Road
Fairbanks, AK 99709-3707
USA

907-451-5020 (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 contact us through the e-mail address above whenever possible.
Metadata standard:
FGDC Content Standards for Digital Geospatial Metadata (FGDC-STD-001-1998)
Metadata extensions used:


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