Aeromagnetic data were collected along flight lines by instruments in an aircraft that recorded magnetic-field intensity values and location. In surveys such as this one where the data were originally collected in digital form and not digitized from contour maps, the information we provide typically includes latitude, longitude, magnetic anomaly in nanoTeslas, and intermediate values used to derive the magnetic anomaly such as total magnetic field.
Aeromagnetic surveys are used for geophysical prospecting. Some variations in magnetic measurements are caused by rocks that contain significant amounts of magnetic minerals (magnetite being the most common). These anomalies reflect variations in the amount and type of magnetic material and the shape and depth of the body of rock. The features and patterns of aeromagnetic anomalies can also be used to delineate details of subsurface geology including the locations of buried faults and the thickness of surficial sedimentary rocks.
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Recent aeromagnetic surveys such as this one have generally been carried out on contract by private-sector companies that specialize in these surveys. In some cases we will retain a report from the contractor detailing methods and survey parameters, which could be consulted in order to understand the likely accuracy of the measurements.
In general each original survey was collected by a single contractor, who would be responsible for processing the field observations, and reflect industry standard practices and controls. Variation among surveys is possible, especially if they were carried out during very different time periods.
Observations were made along flight lines, so these data may not extend into all parts of the geographic bounding extents.
Detailed positional information is determined by the contractor using navigational controls to specifications given in the contractor reports. Generally these are more precise and accurate for newer surveys than for older ones.
The aircraft vertical position was determined using the navigational positioning equipment on the aircraft, which were radar altimeter and barometric altimeter.
Connard, G.G., Saltus, R.W., Hill, P.L., Carlson, L., Milicevic, B.
Publication_Date:1999 Title:Alaska digital aeromagnetic database description Series_Information:
Northwest Geophysical Associates, Inc. (NGA) was contracted by the U.S. Geological Survey (USGS) to construct a database containing original aeromagnetic data (in digital form) from surveys, maps and grids for the State of Alaska from existing public-domain magnetic data. This database facilitates the detailed study and interpretation of aeromagnetic data along flightline profiles and allows construction of custom grids for selected regions of Alaska. The database is linked to and reflects the work from the statewide gridded compilation completed under a prior contract. The statewide gridded compilation is also described in Saltus and Simmons (1997) and in Saltus and others (1999a, 1999b). NGA subcontracted a significant portion of the work described in this report to Paterson, Grant, and Watson Limited (PGW).
Processed data values were tied to the two statewide aeromagnetic grids reported in Saltus and Simmons (1997). The two grids are the Alaska Composite Grid (AKC) and the Alaska Merged Grid (AKM). The AKC is a one-kilometer grid consisting of all the individual surveys at original altitudes with datum shifts to minimize differences at the boundaries. For the AKM, all of the individual surveys were draped at a level of 1,000 feet above the terrain and fit together with seamless joins along the survey boundaries. Saltus and others (1999) presented updated versions of the AKC and the AKM with additional long-wave components removed. For this project, we used the updated versions.
In addition, a new Digital Elevation Model (DEM) for the State of Alaska with a 300-meter cell spacing (Riehle and others, 1997) was used to calculate terrain separation or construct a plausible flight surface when measured values for terrain separation or flight surface were not available in the original survey data.
Contact_Organization:U.S. Geological Survey, Mineral Resources Program Contact_Person:Richard W. Saltus
Address_Type:mailing address Address:
Mail Stop 964 Building 20 Denver Federal Center, West 6th Avenue and Kipling Streets
Horizontal_Datum_Name:North American Datum of 1927 Ellipsoid_Name:Clarke 1866 Semi-major_Axis:6378206.4 Denominator_of_Flattening_Ratio:294.98
The majority of these surveys are available as digital flightline data (data for which digital records of original flightline data exist). We attempted to recover or construct the standard channels (fields) to be included in the final database for all digital flightline data. For some surveys, some of the standard channels (e.g., MAG_BASE ) were not available and were removed from the individual final database file. The standard channels are:
Standard Database Channels For Digital Flightline Surveys
Channel Name Contents
Line Line number
X X-coordinate (meters) in an Alaska Albers projection
Y Y-coordinate (meters) in an Alaska Albers projection
HEIGHT Elevation above ground (meters) if available
FLTSURF Flight surface (meters above sea level) consistent with AKC
FLTSURF_M Flight surface (meters above sea level) consistent with AKM
MAG_RAW Raw magnetics (if available)
MAG_BASE Basemag-corrected magnetics (if available)
MAG_DGRF DGRF-corrected magnetics (if available)
MAG_COMP Composite magnetic value (consistent with AKC)
MAG_MERGE Merged magnetic value (consistent with AKM)
USE_FLAG Used/not-used flag to signal use in AKC
Two additional channels were included in the database files for a few of the INTAK surveys:
MAG_IGRF IGRF-corrected magnetics where DGRF-corrected magnetic were not available
MAG_DRAPE 1,000 ft. draped version of MAG_DGRF or MAG_IGRF
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