Airborne magnetic geophysical survey of the Tanacross region, Alaska

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Frequently anticipated questions:


What does this data set describe?

Title:
Airborne magnetic geophysical survey of the Tanacross region, Alaska
Abstract:
Airborne geophysical data for the Tanacross area were acquired and processed by Goldak Airborne Surveys under contract with the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (DGGS). Project funded by the US Geological Survey, Mineral Resources Program. This high sensitivity aeromagnetic survey was carried out by Goldak Airborne Surveys (Goldak) on behalf of the Alaska Department of Geological and Geophysical Surveys (DGGS) between June 17th and July 31st, 2015. Aircraft equipment operated included three cesium vapor magnetometers, a GPS/GLONASS real-time and post-corrected differential positioning system, a flight path recovery camera, a digital video titling and recording system, as well as radar and barometric altimeters. All data were recorded digitally in GEDAS binary file format. Reference ground equipment included three GEM Systems GSM-19W Overhauser magnetometers and a Novatel 12 channel GPS/GLONASS base station which was set up at the base of operations for differential post-flight corrections. Forty-eight flights (including test and calibration sorties) were required to complete the survey block. A total of 26,533 line kilometers of high resolution magnetic data were collected, processed and plotted. The traverse lines were flown at a spacing of 500 meters with control lines flown at a separation of 5000 meters. Nominal terrain clearance was specified at 100 meters above ground. Tok, Alaska was used as the base of operations throughout the entire survey.
Supplemental_Information:
Project Name:	Tanacross
Contracting Agency:	State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (DGGS)
DGGS Section:	Mineral Resources section
Program:	Funding Source:	The USGS Mineral Resources Program
Contractor:	Goldak Airborne Surveys
DGGS Contract Manager:	Abraham Emond
Line km:	26,533
Data Acquisition Start Date (YYYY-MM-DD):	2015-06-17
Data Acquisition End Date   (YYYY-MM-DD):	2015-07-31
Platform:	Fixed Wing Aircraft
Aircraft Model:	Piper PA-31 Navajo
Survey Altitude Model:	Smooth Drape
Target Ground Clearance:	100 meters
Mean Ground Clearance:	290 meters
Traverse: Line Azimuth:    	10 degrees / 190 degrees
Traverse: Line Spacing:	500 meters
Tie: Line Azimuth:	100 degrees / 280 degrees
Tie: Line Spacing:	5000 m
Border lines:	present around all non-parallel and non-perpendicular edges
Data Type: Magnetic
Magnetometer:   	three cesium vapor magnetometers mounted to aircraft
Positioning:	GPS/GLONASS real-time and post-corrected differential
Additional equipment: 	flight path recovery camera, 3 external Garmin VIRB Elite action cameras, radar and barometric altimeters

FLIGHT AND LINE NAMING

Example: L11020:29082

L = L for transverse (production)
1 = Line number digit 1
6 = Line number digit 2
0 = Line number digit 3
0 = Reflight number
:
1 = Flight number digit 1
4 = Flight number digit 2

T = T for tie (cross)
8 = Line number digit 1
6 = Line number digit 2
0 = Line number digit 3
0 = Line number digit 3
0 = Reflight number
:
1 = Flight number digit 1
4 = Flight number digit 2
  1. How should this data set be cited?

    Emond, A.M., Saltus, R.W., Graham, Gina, and Surveys, Goldak Airborne, 2015, Airborne magnetic geophysical survey of the Tanacross region, Alaska: Geophysical Report GPR 2015-6, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

  2. What geographic area does the data set cover?

    West_Bounding_Coordinate: -144.533
    East_Bounding_Coordinate: -140.9851
    North_Bounding_Coordinate: 64.370627
    South_Bounding_Coordinate: 62.804075

  3. What does it look like?

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

    Calendar_Date: 2015
    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: 7
      Transverse_Mercator:
      Scale_Factor_at_Central_Meridian: 0.999600
      Longitude_of_Central_Meridian: -141
      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 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, Geosoft GXF ascii format grids; note: these data are provided in both UTM NAD27 and Geographic WGS84 projections (Source: Goldak)

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

    documents
    Final report, gridded data explanations, survey background information (Source: Goldak)

    grids_ermapper
    Geographically registered gridded data, ErMapper ERS format (Source: Goldak)

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

    images_registered
    GeoTiff format images of all gridded data (Source: Goldak)

    kmz
    Google Earth kmz file(s); note: these files use the WGS84 datum (Source: Goldak)

    maps_geosoft_format
    maps as Geosoft packed map files (Source: Goldak)

    maps_63k_pdf_format
    Print format maps in pdf format. 1:63,360 Scale (Source: Goldak)

    maps_250k_pdf_format
    Print format maps in pdf format. 1:250,000 Scale (Source: Goldak)

    maps_ps_format
    Print format maps in ps (postscript) format (Source: Goldak)

    vector_data
    line path, data contours, and survey boundary in ESRI shapefile (SHP) format (Source: Goldak and DGGS)

    video_fligthpath
    video recorded with geophysical data system to verify position and record cultural items. Video synchronized with data (Source: Goldak)

    video_aerial
    video footage from three externally mounted Garmin VIRB Elite action cameras (Source: DGGS and Goldak)


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 USGS Mineral Resources Program provided funding for this survey in support of the Alaska/Yukon Geologic and Geophysical Reconciliation Project; Cooperative Agreement Number: G15AC00033 Supplement Number: 0001 Effective Date: 2015-06-09 Completion Date: 2016-09-30 CFDA Number: 15.808 DUNS Number: 111188079+0000

  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 Tanacross geophysical survey was implemented by DGGS and funded by the U.S. Geological Survey (USGS) Mineral Resources Program. The U.S. Geological Survey (USGS) Mineral Resources Program supports data collection and research on a wide variety of nonfuel mineral resources that are important to the Nation's economic and national security. The Program's science portfolio encompasses the full spectrum of mineral resource science, allowing for a comprehensive understanding of the complete life cycle of nonfuel mineral resources and materials; this includes resource formation, discovery, production, consumption, use, recycling, and reuse, as well as an understanding of environmental issues of concern throughout the life cycle. The Mineral Resources Program is part of the USGS Energy and Minerals Mission Area.


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: 2015 (process 1 of 11)
    Positioning Data and Altitude Data - Processing of the positioning data takes place in the field and is performed on a post-flight basis. The raw airborne GPS data are corrected using the corresponding GPS base station data and NovAtel Inc.'s Waypoint GrafNavGNSS Post-Processing software suite. The corrected GPS World Geodetic System 1984 (WGS84) longitude, latitude and altitude are merged into a Geosoftdatabase with aircraft flight data and re-projected to the local UTM Zone 7 NAD27 datum. Velocity is then calculated from the corrected positions. Corrected UTM co-ordinates are trimmed to online. The primary radar altimeter data is lagged by 0.9 seconds and the secondary radar altimeter data is lagged by 3.0 seconds. The digital elevation model is calculated by subtracting the radar altimeter data from the GPS altitude data. Attitude information is derived from 3 GPS receivers mounted on the tail, cabin and right wing-tip. Moving baseline software by Waypoint is used to compute the relative positions of the antennas. By determining the relative apparent positions of the front-right and front-tail antenna pairs and comparing to the known reference geometry of the aircraft, the pitch, roll, azimuth and yaw of the aircraft are calculated to better than 0.5 degrees precision.

    Date: 2015 (process 2 of 11)
    Initial Field Processing - The total magnetic field data were acquired with a sampling interval of 0.1 seconds, and were (1) corrected for diurnal variations by subtraction of the digitally recorded base station magnetic data, (2) leveled to the tie line data, and (3) interpolated onto a regular 100 m grid using a modified Akima (1970) technique. The regional variation (or IGRF gradient, 1985, updated to October, 1996) was removed from the leveled magnetic data for the Petersville survey. The aeromagnetic data were interpolated onto a regular 100 m grid using a modified Akima (1970) technique. The magnetic base station was a GSM-19T digital recording proton precession machine with a sensitivity of 0.20 nT and was sampled once every 3 seconds.

    Date: 2015 (process 3 of 11)
    Control-Line Levelling - The intention of control-line levelling is to apply a smoothly-varying function to the measured data, which results in nearly identical values at the intersections of traverse and control lines. The most significant component of the correction is to accommodate the diurnal variation of the magnetic field. Other sources of error are altitude errors, GPS positioning errors and system drift.Levelling of the total field data consists of the following steps. Calculation of the positions of the survey-line-control-line intersection points and the extraction of mismatch values of the magnetic data between the line and control lines at these points. An iterative application of corrections, based of best-fit, first-order linear trends of mismatch values (with outliers removed), on the traverse and control lines until the resulting corrections approach zero. An iterative application of long-wavelength corrections on traverse and control lines determined by applying median and low-pass filters to the remaining intersection mismatches (with outliers removed) and then using Akima spline interpolation between the now-filtered intersection mismatch values. This enhances and isolates correction 'features' that span several intersections. The lengths of the filters are based on the traverse-line-control-line intersection separations. In this case, the initial filter lengths spanned 10 control-line intersections on survey lines and 50 survey-line intersections on control lines. The number of intersections spanned is reduced in increments to an appropriate minimum until the correction approaches zero. Calculation of the first vertical derivative from the gridded data of the intermediate levelled total field using a 2-D fast Fourier transform (FFT) operator. An altitude correction derived by multiplying the calculated vertical gradient by the aircraft's deviation from the planned surface height is then applied to the original unleveled magnetic data. Steps 1 to 3 are then repeated using the altitude-corrected magnetic data. Manual inspection of the remaining intersection mismatches and reducing it to zero (where appropriate) by applying the necessary corrections to either the survey or tie lines. Special attention is paid to ensuring that the overall correction profiles are as smooth as possible and that there is no line-to-line correlation in the correction profiles, which implies a misapplied correction. The second vertical derivative of the total field grid is analyzed to ensure that the corrections are sufficient and appropriate. Features that appear along the survey lines in the second vertical derivative may be the result of over-correction or under-correction. In either case, the solution is to revise the correction profile at those intersections.

    Date: 2015 (process 4 of 11)
    Calculation and Removal of the International Geomagnetic Reference Field - The International Geomagnetic Reference Field (IGRF) was calculated using the 2015 model year with a constant date of July 9, 2015 (roughly the mid-point of the survey) as the reference date. A constant altitude of 1048.5 m, the mean altitude over the course of the survey, was specified as the elevation. This value was subtracted from the tie-line levelled data to obtain the residual magnetic field data.

    Date: 2015 (process 5 of 11)
    Microleveling of the Magnetic Data - After control line levelling, any residual flight line noise or 'corrugation' in the magnetic field data was further reduced using Paterson, Grant & Watson's microleveling technique. This technique first involves the generation of line-to-line noise profiles by applying frequency domain sixth-order, high-pass Butterworth filter and a directional cosine filter perpendicular to the flight-line direction to the gridded data. This 'decorrugation' grid is then sampled back into the database. The initial noise profile data is then limited to a user-defined maximum amplitude and then filtered using a Naudy-Dreyer non-linear filter to obtain the microleveling correction. Finally, the correction and gridded microleveled data are inspected to ensure no geological signal was removed and an overall improvement in the gridded data was achieved. The following parameters in Paterson, Grant & Watson's 'Miclev' routine were used: Decorrugation wavelength cutoff: 2000 m; Decorrugation grid cell size: 100 m; Naudy filter length: 1000 m; Naudy filter tolerance: 0.0001; Amplitude limit: 1.0 nT.

    Date: 2015 (process 6 of 11)
    Processing of Measured Magnetic Gradients - Processing of the magnetic gradient data consists of the following steps: Attitude correction is performed before levelling the gradient information. The effect of attitude on a particular measurement is dependent on the magnitude of the local gradient and the degree of deviation from straight and level flight. This compound effect cannot be accounted for by tie-line levelling of the data, as it is both non-systematic and at a much shorter wavelength than the tie-line separation. Correcting the data for attitude before levelling insures that levelling corrections are kept to a minimum. Horizontal gradients are calculated from the gridded total field data, sampled into the database and subtracted from the measured, rotated gradients. A 31 second median filter followed by a 31 second low-pass filter is applied to the difference and then added back to the measured gradient. This ensures that the lower wavelengths accurately represent the regional field, which is otherwise difficult to achieve.

    Date: 2015 (process 7 of 11)
    Gradient-Enhanced Gridding of the Residual Magnetic Intensity - Gradient enhancement of the residual magnetic intensity was achieved using Goldak's Gradient Variable Trend (GVT) gridding algorithm, which utilizes the horizontal gradients to guide the between-line interpolation of the data to generate a more realistic image free of artifacts and irregularities present grids generated from minimum curvature algorithms.

    Date: 2015 (process 8 of 11)
    Calculation of the Vertical Derivative, Tilt Derivative and Analytic Signal Grids - The first and second vertical derivative, tilt derivative and analytic signal grids were calculated directly from the gradient-enhanced residual magnetic intensity grid using the Geosoft MagMap grid processing suite.

    Date: 2015 (process 9 of 11)
    Gridding of the Measured Magnetic Gradients - The final measured lateral and longitudinal horizontal gradient data was gridded using minimum curvature methods.

    Date: 2015 (process 10 of 11)
    Boundary polygon - The boundary polygon was generated by loading the final line data in Geosoft Oasis montaj v8. Next all control/tie lines were deselected (unused). 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 11 of 11)
    Aerial Video - From June 17 to July 31, three Garmin VIRB Elites were externally mounted on the aircraft. Data were recorded during the entire data acquisition. Video (MP4), Garmin Activity files, and Garmin GPX were recorded and stored. Geographic location for each video is stored in the Activity and GPX files. No processing has been performed on these data.

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

    Burns, L.E., Fugro Airborne Surveys Corp., and Fugro GeoServices, Inc., 2011, Ladue survey area: Magnetic and electromagnetic line, grid, and vector data and Maps, Fortymile mining district, Tanacross Quadrangle, eastern Alaska: Geophysical Report GPR 2011-1, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 26 sheets, 1 DVD, scale 1:63,360
    Burns, L.E., Fugro Airborne Surveys Corp., and Stevens Exploration Management Corp., 2006, Line, grid, and vector data, and plot files for the airborne geophysical survey of the Alaska Highway corridor, east-central Alaska: Geophysical Report GPR 2006-6, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 1 DVD
    Burns, L.E., Geoterrex-Dighem, Stevens Exploration Management Corp., Emond, A.M., and Graham, G.R.C., 2015, Fortymile mining district electromagnetic and magnetic airborne geophysical survey, data compilation: Geophysical Report GPR 2015-4, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    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, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 9 sheets, 1 DVD


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

  1. How well have the observations been checked?

    June 17 to July 27 - figure of merit data for each magnetometer and measured gradient
    Starboard Wing Tip: 0.74
    Port Wing Tip: 1.31
    Tail: 1.44
    Transverse Gradient: 1.91
    Lateral Gradient: 1.19
    
    July 27 to July 31 - figure of merit data for each magnetometer and measured gradient
    Starboard Wing Tip: 1.3
    Port Wing Tip: 1.25
    Tail: 0.59
    Transverse Gradient: 2.1
    Lateral Gradient: 1.19
    

  2. How accurate are the geographic locations?

    Positional accuracy of the presented data is better than 10 m with respect to the UTM grid.

  3. How accurate are the heights or depths?

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

    This publication release provides the entire dataset generated by this project.

  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 coordinate system was retained for all publication products except the KMZ files, which utilize WGS84 geographic coordinates. In some cases we provide values in both UTM NAD27 and geographic WGS84 projections.


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 2015-6

  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?


Who wrote the metadata?

Dates:
Last modified: 14-Oct-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:


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