Candle Hills airborne electromagnetic survey, Kuskokwim region, Alaska

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Frequently anticipated questions:


What does this data set describe?

Title:
Candle Hills airborne electromagnetic survey, Kuskokwim region, Alaska
Abstract:
Candle Hills airborne electromagnetic survey, Kuskokwim region, Alaska, Geophysical Report 2026-3, covers a portion of the McGrath and Iditarod quadrangles, southwest of McGrath, across part of the Kuskokwim Mountains in western interior Alaska. The McGrath and Iditarod quadrangles are prime exploration targets because they lie within historically productive mining districts and contain geologic structures and rock units known to host gold and other metallic mineral deposits (Bundtzen and Miller, 1996; Bundtzen and others, 2004; Miller and others, 2005). The survey was conducted to characterize subsurface electrical resistivity variations and to support geologic mapping and mineral resource evaluation. These data provide a framework for identifying concealed lithologic contacts, structural features, and alteration zones that may indicate areas of elevated mineral potential. Time-domain electromagnetic data were acquired using the helicopter-mounted VTEM Terrain system on August 5-12, 2025, by Geotech Ltd. A total of 1060 line-kilometers were flown over a single survey block measuring 369.5 square kilometers. The block was flown with northwest-southeast line orientation and a nominal spacing of 400 meters, along with northeast-southwest oriented tie lines with a nominal spacing of 4000 meters. Mean ground clearance was 44 meters, with a mean survey speed of 86 km/hr. Geotech Ltd. provided initial resistivity models, produced via simple inversion using a technique termed Resistivity Depth Imaging (RDI). These models are referred to in the provided data files as "geotech_rdi." Aqua Geo Frameworks then produced a spatially constrained inversion model (referred to in the data files as "agf_sci"), which incorporates both along-track and cross-track constraints. The Aqua Geo Frameworks model is recommended for end-users as it resolves finer-scale structures, including faults. We also provide a 3D voxel dataset registered to elevation rather than raw depth for the agf_sci model. These data are provided as a Geophysical Report under an open end-user license and are available on the DGGS website https://doi.org/10.14509/31963.
Supplemental_Information:
ascii_data:    ASCII format line data, other ASCII data	
databases_geosoft:    Geosoft format database of final line data, other Geosoft format databases	
documents:    Project report, calibration reports	
grids_geosoft:    Oasis montaj Geosoft GRD format gridded data	
grids_tif:    Geographically registered data value rasters of gridded data, GeoTiff format	
maps_pdf_format:    Printable and geographically registered maps in pdf format.	
models_resistivity:    Models of subsurface resistivity inverted from measured EM response. agf_sci is the recommended model for end-users.	
vector_data:    Line path and survey boundary in Esri shapefile (shp) format
  1. How might this data set be cited?
    Petersen, E.I., Fusso, L.A., Ltd., Geotech, and Frameworks, Aqua Geo LLC, 2026, Candle Hills airborne electromagnetic survey, Kuskokwim region, Alaska: Geophysical Report GPR 2026-3, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 9 p
  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -156.315876
    East_Bounding_Coordinate: -155.554223
    North_Bounding_Coordinate: 62.953699
    South_Bounding_Coordinate: 62.664598
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
    Calendar_Date: 2025
    Currentness_Reference:
    ground condition
  5. What is the general form of this data set?
    Geospatial_Data_Presentation_Form: digital, data
  6. How does the data set represent geographic features?
    1. How are geographic features stored in the data set?
      This is a raster 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: 4
      Transverse_Mercator:
      Scale_Factor_at_Central_Meridian: 0.999600
      Longitude_of_Central_Meridian: -159
      Latitude_of_Projection_Origin: 0
      False_Easting: 500000.000000
      False_Northing: 0
      Planar coordinates are encoded using coordinate pair
      Abscissae (x-coordinates) are specified to the nearest .00000001
      Ordinates (y-coordinates) are specified to the nearest .00000001
      Planar coordinates are specified in Meters
      The horizontal datum used is WGS84.
      The ellipsoid used is WGS 84.
      The semi-major axis of the ellipsoid used is 6378137.
      The flattening of the ellipsoid used is 1/298.257223563.
  7. How does the data set describe geographic features?
    ascii_data
    ASCII format line data, other ASCII data (Source: Geotech Ltd. and Aqua Geo Frameworks LLC)
    databases_geosoft
    Geosoft format database of final line data, other Geosoft format databases (Source: Geotech Ltd. and Aqua Geo Frameworks LLC)
    documents
    Project report, calibration reports (Source: Geotech Ltd. and Aqua Geo Frameworks LLC and Alaska Division of Geological & Geophysical Surveys (DGGS))
    grids_geosoft
    Oasis montaj Geosoft GRD format gridded data (Source: Geotech Ltd. and Aqua Geo Frameworks LLC and Alaska Division of Geological & Geophysical Surveys (DGGS))
    grids_tif
    Geographically registered data value rasters of gridded data, GeoTiff format (Source: Alaska Division of Geological & Geophysical Surveys (DGGS))
    maps_pdf_format
    Printable and geographically registered maps in pdf format. (Source: Geotech Ltd. and Aqua Geo Frameworks LLC)
    models_resistivity
    Models of subsurface resistivity inverted from measured EM response. agf_sci is the recommended model for end-users. (Source: Geotech Ltd. and Aqua Geo Frameworks LLC)
    vector_data
    Line path and survey boundary in Esri shapefile (shp) format (Source: Alaska Division of Geological & Geophysical Surveys (DGGS))

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?
    This work was supported by State of Alaska capital improvement project funds. Doyon, Limited funded survey liness acquired over their land holdings.
  3. To whom should users address questions about the data?
    Alaska Division of Geological & Geophysical Surveys
    GIS Manager
    3354 College Rd
    Fairbanks, AK
    USA

    907-451-5020 (voice)
    dggsgis@alaska.gov
    Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays

Why was the data set created?

The purpose of the Candle Hills airborne electromagnetic survey is to characterize subsurface electrical resistivity across a portion of the McGrath and Iditarod quadrangles in western interior Alaska to support geologic mapping and mineral resource evaluation. This survey provides high‑resolution geophysical data that help identify concealed lithologic contacts, structural features, and alteration zones associated with gold and other metallic mineral deposits within the Kuskokwim Mountains. The resulting resistivity models and derived 3D datasets offer a framework for assessing mineral potential in an area of historically productive mining districts and geologically favorable host rocks.

How was the data set created?

  1. From what previous works were the data drawn?
  2. How were the data generated, processed, and modified?
    Date: 2025 (process 1 of 2)
    Data acquisition - Geotech Ltd. conducted the Candle Hills airborne electromagnetic survey from August 5-12, 2025, acquiring 1,060 line-kilometers of VTEM Terrain time-domain electromagnetic data over a 369.5 km2 block in the McGrath and Iditarod quadrangles of western interior Alaska. The survey, designed to map subsurface resistivity and support geologic mapping and mineral resource assessment, used northwest-southeast flight lines spaced 400 m apart with 4,000 m tie lines, maintaining an average 44 m ground clearance and 86 km/hr speed. Geotech produced initial Resistivity Depth Imaging models (geotech_rdi), after which Aqua Geo Frameworks generated a more detailed spatially constrained inversion (agf_sci) incorporating along-track and cross-track constraints. A 3D voxel dataset registered to elevation was also created for the agf_sci model to support interpretation.
    Date: 2026 (process 2 of 2)
    Data cataloged for archival and publication - Final data were published and archived by DGGS.
  3. What similar or related data should the user be aware of?
    Fusso, L.A., Petersen, E.I., and Emond, A.M., 2026, Kuskokwim airborne magnetics and radiometric geophysical survey, McGrath, Alaska: Geophysical Report GPR 2026-2, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Fusso, L.A., Petersen, E.I., and Inc., EON Geosciences, 2025, Kuskokwim airborne magnetic and radiometric survey, Ganes Creek, Alaska: Geophysical Report GPR 2026-1, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Petersen, E.I., Fusso, L.A., Ltd., Geotech, and Frameworks, Aqua Geo LLC, 2026, Granite Mountain airborne electromagnetic survey, Kuskokwim Region, Alaska: Geophysical Report GPR 2026-4, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:


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

  1. How well have the observations been checked?
    Geophysical data were acquired according to the contractor, contract, and request-for-proposals specifications. For additional detail about survey parameters, see the accompanying project report available in the documents directory of this data distribution package.
  2. How accurate are the geographic locations?
    Station coordinates were positioned using a real-time or post-processed corrected differential GPS.
  3. How accurate are the heights or depths?
    Not applicable
  4. Where are the gaps in the data? What is missing?
    The dataset provides processed geophysical and positional data for the entire survey extent.
  5. How consistent are the relationships among the observations, including topology?
    Data were supplied by providers in accordance with DGGS contract requirements (see accompanying project report). The originating coordinate system was retained for all publication products.

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 is 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. The 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. The State of Alaska makes no express or implied warranties (including warranties for merchantability and fitness) with respect to the character, functions, or capabilities of the electronic data 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 or any failure thereof or otherwise. In no event will the State of Alaska's liability to the Requestor or anyone else exceed the fee paid for the electronic service or product.
  1. Who distributes the data set? (Distributor 1 of 1)
    State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys
    3354 College Road
    Fairbanks, AK
    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 website (http://www.dggs.alaska.gov) for the latest information on available data. Please contact us using the e-mail address provided above when possible.
  2. What's the catalog number I need to order this data set? GPR 2026-3
  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 requestor or anyone else exceed the fee paid for the electronic service or product.
  4. How can I download or order the data?

Who wrote the metadata?

Dates:
Last modified: 12-Feb-2026
Metadata author:
Simone Montayne
3354 College Road
Fairbanks, AK
USA

907-451-5020 (voice)
Metadata standard:
FGDC Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)
Metadata extensions used:

Generated by mp version 2.9.50 on Fri Feb 13 13:14:35 2026