Tonsina electromagnetic and magnetic airborne geophysical survey data compilation

Frequently anticipated questions:

• What does this data set describe?
  1. How should this data set be cited?
  2. What geographic area does the data set cover?
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
  5. What is the general form of this data set?
  6. How does the data set represent geographic features?
  7. How does the data set describe geographic features?
• Who produced the data set?
  1. Who are the originators of the data set?
  2. Who also contributed to the data set?
  3. To whom should users address questions about the data?
• Why was the data set created?
• How was the data set created?
  1. From what previous works were the data drawn?
  2. How were the data generated, processed, and modified?
  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?
  2. How accurate are the geographic locations?
  3. How accurate are the heights or depths?
  4. Where are the gaps in the data? What is missing?
  5. How consistent are the relationships among the data, including topology?
• How can someone get a copy of the data set?
  1. Are there legal restrictions on access or use of the data?
  2. Who distributes the data?
  3. What's the catalog number I need to order this data set?
  4. What legal disclaimers am I supposed to read?
  5. How can I download or order the data?
• Who wrote the metadata?

What does this data set describe?

1. How should this data set be cited?

   Emond, A.M., CGG, Burns, L.E., Graham, G.R.C., and (US), CGG Land Inc., 2014, Tonsina electromagnetic and magnetic airborne geophysical survey data compilation: Geophysical Report GPR 2015-1, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

   Online Links:

   • <http://dx.doi.org/10.14509/29169>

2. What geographic area does the data set cover?

   West_Bounding_Coordinate: -145.4467763

   East_Bounding_Coordinate: -144.7425039

   North_Bounding_Coordinate: 61.7144202

   South_Bounding_Coordinate: 61.4790324

3. What does it look like?

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

   Calendar_Date: 2014

   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?

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

      Grid_Coordinate_System_Name: Universal Transverse Mercator

      Universal_Transverse_Mercator:

      UTM_Zone_Number: 6

      Transverse_Mercator:

      Scale_Factor_at_Central_Meridian: 0.999600

      Longitude_of_Central_Meridian: -147

      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 0.01
      Ordinates (y-coordinates) are specified to the nearest 0.01
      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 1/294.9786982.
      

7. How does the data set describe geographic features?

   ascii_data

   ASCII format final line data with readme files; note: these data are provided in both UTM NAD27 and Geopgraphic WGS84 projections. (Source: CGG)

   databases_geosoft

   Geosoft format database of final line data with readme files; note: these data are provided in both UTM NAD27 and Geopgraphic WGS84 projections. (Source: CGG)

   documents

   Field operations report, gridded data explanations, survey background information, processor notes. (Source: CGG)

   grids_ermapper

   Geographically registered gridded data, ErMapper ERS format. (Source: CGG)

   grids_geosoft

   Geosoft-format binary grids, these grids can be viewed in ESRI ArcMap using a free plugin from Geosoft. (Source: CGG)

   kmz

   Google Earth kmz files of of all gridded data; note: these files use the WGS84 datum. (Source: CGG)

   images_registered

   GeoTiff format images of all gridded data. (Source: CGG)

   maps_pdf_format

   Print format maps in pdf format. (Source: CGG)

   maps_prn_format

   Print format maps in HPGL/2 printer file format with extension .prn. (Source: CGG)

   vector_data

   Line path, data contours, and survey boundary in ESRI shape file (SHP) and Autocad (2000) DXF formats. (Source: CGG)

   video_flightpath

   Downward looking video to verify position and record ground conditions. Video synchronized with data. (Source: CGG)

   profiles_stacked

   Electromagnetic and magnetic data profiles with EM anomalies. (Source: CGG)

Who produced the data set?

1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
   • Emond, A.M.
   • CGG
   • Burns, L.E.
   • Graham, G.R.C.
   • CGG Land (US) Inc.

2. Who also contributed to the data set?

   Funding for this survey was provided by the Alaska Division of Geological & Geophysical Surveys' (DGGS) Airborne Geophysical/Geological Mineral Inventory (AGGMI) and Strategic and Critical Minerals Assessment Capital Improvement programs and managed by State of Alaska, Department of Natural Resources (DNR), Division of Geological & Geophysical Surveys (DGGS). The survey data was collected and processed by CGG.

3. To whom should users address questions about the data?
   Alaska Division of Geological & Geophysical Surveys
   GIS Manager
   3354 College Road
   Fairbanks, AK 99709-3707
   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 Tonsina electromagnetic and magnetic airborne geophysical survey data compilation is a product of the Alaska Division of Geological & Geophysical Surveys' (DGGS) Airborne Geophysical/Geological Mineral Inventory (AGGMI) program and the Alaska Strategic and Critical Minerals Assessment Capital Improvement Project. The goal of the Alaska Division of Geological & Geophysical Surveys' (DGGS) Airborne Geophysical/Geological Mineral Inventory (AGGMI) program is to enhance the understanding of Alaska's mineral resources and stimulate private-sector mineral development. The program seeks to delineate 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 Alaskans; and (3) will provide economic diversification to help offset the loss of Prudhoe Bay oil revenue. Candidate lands for this program are identified on the basis of existing geologic knowledge, land ownership, and nominations from Alaska's geologic community. The AGGMI program and resulting new geologic knowledge are recognized worldwide and have encouraged millions of dollars of venture capital expenditures in the local economies of the surveyed mining districts. These venture capital expenditures have led to discovery and delineation of new mineral resources. The multi-year Strategic and Critical Minerals Assessment Capital Improvement Project (SCMA) was established to determine Alaska's geologic potential for rare-earth elements and other minerals that are essential for our modern, technology-based society. This program began in FY2012 with a data survey and compilation of existing information on rare-earth-element occurrences in Alaska, and expanded significantly in FY2013 to include additional critical minerals, re-analyses of existing samples, and obtaining new field and analytical data, including airborne geophysics.

How was the data set created?

1. From what previous works were the data drawn?

   Akima, H., 1970 (source 1 of 2)

   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, Association for Computing Machinery, New York, NY, USA.

   Type_of_Source_Media: paper

   Source_Contribution: magnetic and derivative magnetic grids

   Fraser, D.C., 1978 (source 2 of 2)

   Fraser, D.C., 1978, Resistivity mapping with an airborne multicoil electromagnetic system: Geophysics v. 43, Society of Exploration Geophysicists, Tulsa, OK, United States.

   Type_of_Source_Media: paper

   Source_Contribution: apparent resistivity

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

   Date: 2014 (process 1 of 10)

   Geophysical data - The airborne geophysical data were acquired between July 25, 2014 through August 20, 2014 with DIGHEM (V) Electromagnetic (EM) systems, Scintrex CS3 cesium magnetometer sensors. The EM and magnetic sensors were flown at a height of 30 meters (m), with the magnetic sensor installed in the EM sensor. In addition, the survey recorded data from radar and laser altimeters, GPS navigation system, 50/60 Hz monitors, thermometer, barometric pressure instrument, and video camera. Flights were performed with one Aerospatiale AS-350-B3 helicopters. All were flown at a nominal mean terrain clearance of 60 m. Survey flight lines were flown at a heading of 345 degrees (NW-SE) with one-quarter mile (402.3 m) line spacing). Tie lines were flown at a heading of 75 degrees (NE-SW), perpendicular to the flight lines, and were spaced at intervals of approximately 4,800 m. Novatel OEM4-G2L Global Positioning Systems were used for navigation and flight path recovery. The helicopter positions were derived every 0.5 seconds (2 Hz); the ground GPS base station data were collected at 1.0 second (1 Hz) intervals. The positional xy data are interpolated from 2 Hz to 10 Hz. The use of the differentially-corrected base station data results in a positional accuracy of better than five meters. Flight path positions were projected onto the Clarke 1866 (UTM zone 6N) spheroid, 1927 North American datum using a central meridian (CM) of 147 degrees, a north constant of 0, and an east constant of 500,000.

   Date: 2014 (process 2 of 10)

   Total magnetic field data - The total magnetic field data were acquired with a sampling interval of 0.1 seconds. The raw magnetic data (channel 'mag_raw') were (1) corrected for measured system lag (resulting in channel 'mag_lag'), (2) corrected for diurnal variations by subtraction of the digitally recorded base station magnetic data (resulting in channel 'mag_diu'), (3) adjusted for regional variations (by subtracting IGRF model 2010, updated for date of flight and elevation variations), (4) levelled to the tie line data resulting in the final residual magnetic intensity, (5) manually levelled with final small microleveling (resulting in channel 'mag_rmi'), and (6) increased by a constant IGRF average value to restore the mag_rmi values to a total magnetic field channel (resulting in channel 'magigrf'). 'Mag_RMI' and 'MagIGRF' were then interpolated onto a regular 80-m grid using a modified Akima (1970) technique.

   Date: 2014 (process 3 of 10)

   Magnetic and derivative magnetic grids - Three different algorithms were applied to the total magnetic field 80 m grid, resulting in three magnetic derivative grids. The analytic signal grid is the total amplitude of all directions of magnetic gradient calculated from the sum of the squares of the three orthogonal gradients. Mapped highs in the calculated analytic signal of the magnetic parameter locate the anomalous source body edges and corners (e.g., contacts, fault/shear zones, basement fault block boundaries or lithologic contacts, etc.). Analytic signal maxima are located directly over faults and contacts, regardless of structural dip, and independently of the direction of the induced and/or remnant body magnetizations. The calculated magnetic tilt grid is the angle between the horizontal gradient and the total vertical gradient, and is useful for identifying the depth and type of magnetic source. The tilt angle is positive over the source, crosses through zero at, or near, the edge of a vertical sided source, and is negative outside the source zone. It has the added advantage of responding equally well to shallow and deep sources and is able to resolve deeper sources that may be masked by larger responses caused by shallower sources. The first vertical derivative grid was calculated using a fast Fourier transform (FFT)-based frequency-domain filtering algorithm. The vertical gradient algorithm enhances the response of magnetic bodies in the upper 500 m and attenuates the response of deeper bodies. The resulting (calculated) vertical gradient grid provides better definition and resolution of near-surface magnetic units and helps to identify weak magnetic features that may not be evident in the total field data. All magnetic and derivative 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 in this publication. When resampling the grids to a 25-m cell size, the original grids are scanned to determine the minimum and maximum values which the new grids are then limited to, avoiding extreme extrapolation errors between lines.

   Data sources used in this process:

   • Akima, H., 1970

   Date: 2014 (process 4 of 10)

   Inphase and quadrature components - The DIGHEM (V) EM systems measured inphase and quadrature components at five frequencies. Two vertical coaxial-coil pairs operated at nominal frequencies of 1000 and 5500 Hz while three horizontal coplanar-coil pairs operated at 900, 7200, and 56,000 Hz. The actual recorded frequencies and coil separations are listed by flight and date in these metadata.

   Date: 2014 (process 5 of 10)

   Apparent resistivity - 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. The apparent resistivity and depth were then calculated from the inphase and quadrature data for all coplanar frequencies. The calculation used the pseudo-layer (or buried) half-space model defined by Fraser (1978). This model consists of a resistive layer overlying a conductive half-space. The apparent depth is defined as the sensor-source distance minus the measured altitude of the sensor above the ground. The apparent depth channels estimate the depth below surface of a conductive half-space and are the apparent thickness of the overlying resistive layer. The apparent depth (or thickness) parameter will be positive when the upper layer is more resistive than the underlying material, in which case the apparent depth may be quite close to the true depth assuming it is a buried halfspace and altimeter is correct. The apparent depth will be negative when the upper layer is more conductive than the underlying material, and will be zero when a homogeneous half-space exists. The apparent depth parameter must be interpreted cautiously because it will contain any errors that might exist in the measured altitude of the EM bird (e.g., as caused by a dense tree cover). Manual levelling 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-levelling 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. When re-sampling the grids to a 25-m cell size, the original grids are scanned to determine the minimum and maximum values which the new grids are then limited to, avoiding extreme extrapolation errors between lines.

   Data sources used in this process:

   • Fraser, D.C., 1978

   Date: 2014 (process 6 of 10)

   Digital elevation/terrain model - The digital elevation/terrain model was produced from the differentially corrected GPS-Z data (channel 'GPSZ' in line data) and the laser altimeter data measured in the bird (channel 'ALTLAS_BIRD' included in the same files). Both the GPSZ and ALTLAS_BIRD data were checked for spikes, which were removed manually. The ALTLAS_BIRD data were despiked and then filtered using a 13 median filter, followed by a 13 Hanning filter. The corrected altimeter data were then subtracted from the GPSZ data to produce profiles of the height above mean sea level along the survey lines. The data were manually levelled to remove any errors between lines. After all levelling, the data were DC shifted to match the local maps, in this case, NAD27. The 80-m DTM grid was then resampled to a 25-m cell size to produce the DTM grid contained in this publication.

   Date: 2014 (process 7 of 10)

   Geographically registered gridded data - All grids with 25 m cell size were converted to GeoTIFFs and KMZs using CGG's specialized software.

   Date: 2014 (process 8 of 10)

   Print files - The HPGL/2 files were created with HP Designjet T1300ps HPGL driver, and plot on some plotters, but not all plotters correctly. The Adobe Acrobat format files were created with Adobe Acrobat Distiller v9.0 from Postscript files.

   Date: 2015 (process 9 of 10)

   Boundary polygon - The boundary polygon was generated by loading the final line data in Geosoft Oasis montaj v8.4. No tie (control) line data were used in this process. The final magnetic data was gridded using the Minimum Curvature option with the following settings: grid cell size = 1/5 of line spacing, blanking distance = double the line spacing, cells to extend beyond data = 0. The polygon file was created using Grid Outline tool with edge resolution = 1. The polygon file was exported to ERSI shapefile.

   Date: 2015 (process 10 of 10)

   Discrete anomalous EM responses (EM anomalies) were computer picked from the electromagnetic data set and classified by type and grade. Line and polygon features of the electromagnetic and magnetic data were identified by contractor staff. EM anomalies are presented on map and profile data and are available in ASCII file format. These features are named and stored in geographically registered formats. More information about these products is found in the project report.

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?

    Electromagnetic system contract specification:
   
    900 Hz coplanar apparent resistivity cutoff 1325 Ohm-m
    1000 Hz coaxial apparent resistivity cutoff 1501 Ohm-m
    5500 Hz coaxial apparent resistivity cutoff 8001 Ohm-m
    7200 Hz coplanar apparent resistivity cutoff 10750 Ohm-m
    56000 Hz coplanar apparent resistivity cutoff 60000 Ohm-m
   
    Frequency; Coil Orientation; Peak-to-Peak Noise Envelope (ppm)
    900 Hz      horizontal coplanar    10
    1000 Hz     vertical coaxial       10
    5500 Hz     vertical coaxial       10
    7200 Hz     horizontal coplanar    20
    56,000 Hz   horizontal coplanar    40
   
    Magnetic data contract specification:
    The in-flight noise envelope will not exceed 0.1 nT in straight and level flight. In rugged areas, the noise level will not exceed 0.5 nT.
   
   

2. How accurate are the geographic locations?

   A Novatel OEM5-G2L Global Positioning System was used for navigation. The helicopter position was derived every 0.5 seconds using post-flight differential positioning to a relative accuracy of better than 5 m.

3. How accurate are the heights or depths?

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

    CHANNEL; Valid Data Points; Missing Data Points (dummy value = *)
     res900; 950,442; 6,944
     res7200; 950,442; 6,944
     res56k;  950,442; 6,944
     dep900;  565,245; 392,141
     dep7200; 876,697; 80,689
     dep56k; 900,248; 57,138
   
    Missing values in the calculated 'resistivity' (res56k, res7200, and res900) and 'depth' (dep56k, dep7200, and dep900) are due to calculations being meaningless, generally caused by increase in altitude. See supporting documentation for more information.
   

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

   Data originate from one company using the same contract specifications for all data. The originating data were collected using the NAD27 UTM Zone 6N coordinate system. The originating coordinate system was retained for all publication products except the KMZ files, which utilize WGS84 geographic coordinates.

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)
   Alaska Division of Geological & Geophysical Surveys
   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 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 2015-1

3. What legal disclaimers am I supposed to read?

   The State of Alaska makes no expressed 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.

4. How can I download or order the data?
   • Availability in non-digital form:

     DGGS publications are available as free online downloads or you may purchase paper hard-copies or digital files on CD/DVD or other digital storage media by mail, phone, fax, or email from the DGGS Fairbanks office. To purchase this or other printed reports and maps, contact DGGS by phone (907-451-5020), e-mail (dggspubs@alaska.gov), or fax (907-451-5050). Payment accepted: Cash, check, money order, VISA, or MasterCard. Turnaround time is 1-2 weeks unless special arrangements are made and an express fee is paid. Shipping charge will be the actual cost of postage and will be added to the total amount due. Contact us for the exact shipping amount.

   • Cost to order the data: Contact DGGS for current pricing

   • Availability in digital form:

     Data format:	GeoTiff, ESRI shapefile, Geosoft binary grid, Microsoft Office documents, PDF, ER Mapper ERS, ASCII text files (including CSV), Geosoft ascii grid (GXF), Geosoft database, AutoCAD DXF, Geosoft PLT, Printer file type HPP, Geosoft ASCII XYZ, and KMZ
     Network links:	<http://dx.doi.org/10.14509/29169>

   • Cost to order the data: Free download

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

   Technical requirements for use of all of the data on this publication includes software with ability to use, import, or convert Geosoft float GRD, Geosoft binary GDB,, ESRI Shape files or Autocad DXF, Adobe Acrobat PDF, Google Earth files, and text files. Free downloadable interfaces to view or convert the gridded and shape files are available at the Geosoft Web site (<http://www.geosoft.com>; Oasis Montaj viewer). The KMZ files can be dragged and dropped into the 'My Places' folder of the free downloadable 'Google Earth' software. 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: 29-Dec-2015

Metadata author:

Alaska Division of Geological & Geophysical Surveys
Metadata Manager
3354 College Road
Fairbanks, AK 99709-3707
USA

(907)451-5020 (voice)

Metadata standard:

FGDC Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)

Metadata extensions used:

• <http://www.dggs.alaska.gov/metadata/dggs.ext>