Preliminary final version of part of the airborne geophysical data from the Styx River survey, southcentral Alaska; Parts of the Lime Hills and Tyonek quadrangles

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Frequently-anticipated questions:


What does this data set describe?

Title:
Preliminary final version of part of the airborne geophysical data from the Styx River survey, southcentral Alaska; Parts of the Lime Hills and Tyonek quadrangles
Abstract:
This digital publication, GPR 2008-2, contains 'preliminary final' geophysical data that were produced from airborne geophysical surveys conducted in 2007 for the eastern part of the Styx River survey area, southcentral Alaska. Aeromagnetic and electromagnetic data were acquired for about 180-sq miles of a 708-sq mile survey in 2007. The remaining survey data will be acquired in early spring 2008, processed, and released to the public later in 2008 as GPR 2008-3. Upon its completion, the new data will be merged with this preliminary final dataset. 'Preliminary final' denotes that the data has been processed to the final stage but the merging process may require minor changes to the maps and data. Slightly revised maps will replace these maps when the data for the entire survey is released as GPR 2008-3. Similarly, the database and grids will be expanded, any may contain slightly different data near the merging edges of this data contained on this CD-ROM. Data provided in GPR 2008-2 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) flight lines in vector format, and maps of the five geophysical grids in PDF format. Data are described in more detail in the "GPR2008-2Readme.pdf" and "PF_EastStyx-Linedata.txt" files included on the CD-ROM.
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. Acquisition of survey data was temporarily suspended for several months at the request of DGGS when the opportunity to acquire additional data due to the monetary contribution by Anglo American Exploration (USA), Inc arose. It was deemed that acquiring significantly more data was worth delaying release of the larger dataset for several months. This publication contains 'Preliminary Final' full color maps of the geophysical data in Adobe Acrobat PDF format. Authors for the individual maps are L.E. Burns, Fugro Airborne Surveys Corp., and Stevens Exploration Management Corp. All the maps are in full color, include topography, and are at a scale of 1:63,360. One sheet is needed per map. Maps include total magnetic field, first vertical derivative of the magnetic field, 56,000 Hz coplanar apparent resistivity, 7200 Hz coplanar apparent resistivity, and 900 Hz coplanar apparent resistivity. Each map contains an index map showing the locations of these preliminary final data and the entire Styx River survey area. 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, -153.08 1:63,360-scale maps, sheet 1 East, -152.42 1:63,360-scale maps, sheet 1 North, 62.00 1:63,360-scale maps, sheet 1 South, 61.75
  1. How should this data set be cited?

    Burns, L.E., Anglo American Exploration (USA), Inc., Fugro Airborne Surveys Corp., and Stevens Exploration Management Corp., 2008, Preliminary final version of part of the airborne geophysical data from the Styx River survey, southcentral Alaska; Parts of the Lime Hills and Tyonek quadrangles: Geophysical Report GPR 2008-2, State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (DGGS), Fairbanks, AK, USA.

    Online Links:

    Other_Citation_Details: 1 CD-ROM

  2. What geographic area does the data set cover?

    West_Bounding_Coordinate: -153.07
    East_Bounding_Coordinate: -152.48
    North_Bounding_Coordinate: 62.00
    South_Bounding_Coordinate: 61.81

  3. What does it look like?

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

    Beginning_Date: Oct-2007
    Ending_Date: Jan-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:
    Entities (file names) are listed in the "GPR2008-2Readme.pdf" file included on this CD-ROM. Attributes recorded during the geophysical survey are listed with the units in the text files "PF_eastStyx-Linedata.txt".
    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 Alaska State Legislature provided funding for the geophysical data and project management through the Alaska Geophysical/Geological Mineral Inventory Project, a Capital Improvement Project. A contribution from Anglo American Exploration (USA), Inc. is permitting DGGS to acquire data for an additional 190-sq miles beyond the funding provided by the Alaska State Legislature.

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


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: 2007 (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 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 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: 2007 (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 (saved as tfmag in PF_eastStyx-Linedata.xyz), (3) adjusted for regional variations (or IGRF gradient, 2005, updated to November 2007) using altimeter adjusted IGRF, (4) leveled to the tie line data (saved as magigrf in PF_eastStyx-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 (PF_eastStyx-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 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 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 4 of 5)
    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 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.

  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 2008-2

  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, 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 Adobe Acrobat files.


Who wrote the metadata?

Dates:
Last modified: 06-May-2008
Last Reviewed: 06-May-2008
To be reviewed: 07-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)


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