Richard W. Saltus

US Geological Survey



13 July 1995



CENTRAL ALASKA MERGED AND COMPOSITE AEROMAGNETIC DATA GRIDS



These grids were used to produce the color maps in USGS Map GP-1014,

at a scale of 1:500,000.  The text for GP 1014 is appended to the end of

this file - please read it for more information on the processing.



GRIDS



1. AKmag_composite.asc = A compilation of individual aeromagnetic surveys 

                         without adjustment of survey level (1 row & column

                         of "no data" separates individual surveys).



2. AKmag_merge.asc = A compilation of individual aeromagnetic surveys

                     with adjustment of survey levels to a common specification

                     (1,000 ft drape).  Surveys were upward or downward continued

                     to create a "seamless" map (see detailed discussion in the

                     map text below).



GRID SPECIFICATIONS



GRID NAME: AKmag_merge



Grid interval: 1 km

Projection: Albers Conical Equal-Area Projection



            Central Meridian = -151.

            Base Latitude = 55.

            Standard Parallels = 55 and 65



            X (east-west) origin = -429 km

            Y (north-south) origin = 669 km



            Number of columns = 850

            Number of rows    = 790



Grid layout:



             +---------------------------------+

             |                                 |  

             |                                 |  

             |                                 |  

             |.                                |  

             |.                                |  

             |.                                |  

             |9                                |  

             |8  rows (790)                    |  

             |7                                |  

             |6                                |  

             |5                                |  

             |4                                |  

             |3                                |  

             |2                                |  

             |123456789... => columns (850)    |  

             +---------------------------------+

          origin (-429, 669)



Grid format: ASCII



The grids are in ascii format, 1 numeric value per 20-character line.

There are two lines of header information at the beginning of the grid.

There are 790 rows and 850 columns, so the ascii

grid file contains 790*850 + 2 = 671502 records.



Any grid value greater than 1.0e30 indicates no data at that grid location.



GRID NAME: AKmag_composite



Grid interval: 1 km

Projection: Albers Conical Equal-Area Projection



            Central Meridian = -151.

            Base Latitude = 55.

            Standard Parallels = 55 and 65



            X (east-west) origin = -429 km

            Y (north-south) origin = 667 km



            Number of columns = 850

            Number of rows    = 792



Grid layout:



             +---------------------------------+

             |                                 |  

             |                                 |  

             |                                 |  

             |.                                |  

             |.                                |  

             |.                                |  

             |9                                |  

             |8  rows (792)                    |  

             |7                                |  

             |6                                |  

             |5                                |  

             |4                                |  

             |3                                |  

             |2                                |  

             |123456789... => columns (850)    |  

             +---------------------------------+

          origin (-429, 667)



Grid format: ASCII



The grids are in ascii format, 1 numeric value per 20-character line.

There are two lines of header information at the beginning of the grid.

There are 792 rows and 850 columns, so the ascii

grid file contains 792*850 + 2 = 673202 records.



Any grid value greater than 1.0e30 indicates no data at that grid location.



-----------------------------------------------------

Text for US Geological Survey Map, GP-1014

-----------------------------------------------------



Meyer and Saltus, Merged Aeromagnetic Map of Interior Alaska		1





Merged Aeromagnetic Map of Interior Alaska



By

John F. Meyer, Jr., and R. W. Saltus







Introduction



	As part of a cooperative effort between the State of Alaska and the U.S. 

Geological Survey (USGS), a set of total-intensity magnetic-anomaly maps for the 

interior of Alaska were prepared at a scale of 1:500,000.  The purpose of this 

study was to compile and merge all of the publicly available magnetic data into a 

single compatible data set covering the major interior basins of Alaska.  The data 

processing was performed by Paterson, Grant & Watson Limited of Toronto, 

Ontario, Canada, under contract to the State of Alaska, and the maps were 

produced jointly by the State of Alaska, Division of Oil and Gas, and the U.S. 

Geological Survey.  These maps are presented here in four sheets using the 

Albers Conical Equal-Area projection.  The digital data grids are available from 

the National Geophysical Data Center in Boulder, Colo.



Data Compilation



	These maps of interior Alaska were compiled from 23 different surveys 

that were flown between 1954 and 1982 (figure 1).  The surveys have varied flight 

specifications which are detailed in table 1.  The raw digital data were obtained 

as ASCII files in one of four formats:  (1) flight-line records, (2) digitized pseudo-

flight-line records from published contour maps, (3) digitized contour lines from 

published contour maps, or (4) digital grid files.  The locations were projected 

from their latitude-longitude coordinates to an Albers Conical Equal-Area  

projection (Snyder, 1983) defined as follows:



Northern Standard Parallel	65 N

Southern Standard Parallel	55 N

Origin Latitude			55 N

Central Meridian			151 W

False Easting			0 m

False Northing			0 m

Spheroid				Clarke 1866



	Once the data were projected, the individual surveys were gridded using 

a bi-directional gridding algorithm  (Patterson and others, 1994) with a cell size 

of approximately one-fifth the flight-line spacing.  This initial cell size ranged 

from 2 km for the NURE (National Uranium Resource Evaluation) data to 200 m 

for the high resolution surveys.  Bi-directional gridding can create  unrealistic 

features for regions with wide line spacing.  For this reason, the NURE data, 

which have a line spacing of six miles, were gridded using a minimum-curvature 

gridding algorithm (Briggs, 1974) to minimize this effect and preserve the 

anomalies.  For the digitized data sets, a minimum-curvature gridding algorithm 

was also used to better reproduce the original maps.

	These initial data sets were then inspected for any obvious errors.  To do 

this, a set of histograms of the data ranges and coordinates were created to isolate 

any spurious data points which were then corrected or removed.  The data were 

then viewed interactively using shading techniques from different angles and 

elevations.  This allows identification of digitizing errors, poorly leveled flight 

lines and flight-line noise.  Most of these errors were removed using a 2-D Fast 

Fourier Transform flight-line decorrugation technique (Patterson, et.al., 1994).  

This technique employs a high-pass Butterworth filter in conjunction with a 

directional cosine filter to remove magnetic signals that have a direction and 

wavelength matched to the flight-line spacing and direction. Since this reduces 

the real as well as erroneous magnetic signals parallel to the line direction, the 

amount of decorrugation applied was kept to a minimum, resulting in improved 

data sets with some flight line noise. To do this, the Butterworth filter was set to 

four times the line spacing to only pass frequencies on the order of the flight-line 

spacing, and the directional cosine filter was set to pass wavelengths only in the 

direction of the flight-lines.

	Most of these surveys were previously processed to remove the 

International Geomagnetic Reference Field (IGRF), although specific information 

on what was removed was not available for all of the surveys.  When possible, 

the original datum was recovered.  When the information was available, both the 

total field and residual magnetic data were gridded and the difference between 

them was calculated.  This field represented the regional datum that was 

removed as well as any error corrections that were originally applied to the data.  

To determine the regional field, a first-order approximation of this field was 

derived; this regional field was then added to the new grids.  Finally, the 

definitive  International Geomagnetic Reference Field (DGRF; Peddie, 1983) was 

calculated for the survey altitude and year of flight.  It was removed from each 

grid to yield DGRF corrected grids.  For data digitized from maps, the original 

regional corrections were used.

	Two distinct data compilations were produced from these data.  The first 

grid, which we refer to as the composite grid, was produced by adjusting each of 

the surveys (by addition or subtraction of a constant) to a common datum level, 

otherwise preserving the original data.  This grid retains the highest resolution 

for each survey and should be used for detailed modeling and local depth 

determinations.  The second grid, which we refer to as the merged grid, was 

produced by adjusting the gridded data using upward and downward 

continuation to produce a grid at a constant height of 1,000 feet above ground.  

This grid is designed for regional analysis of the magnetic field.

	To produce the composite grid and combine these different surveys into 

one data set, they were all regridded to a 1 kilometer cell size using a bicubic 

splining algorithm and compared along their edges.  Since the datum levels of 

the various surveys were not the same, datum shifts had to be applied.  The 

largest and most coherent data set was the series of surveys comissioned by the 

State of Alaska (these surveys are indicated by `AK  as a prefix to the survey 

number in figure 1 and table 1), so these were used as the reference grid to which 

all the others were leveled.  The differences along the survey boundaries were 

used to determine the datum shifts required for each survey grid and were 

added to the grids prior to combining them.  Once the complete grid was in 

place, a subtraction of 4,960 nT was applied to all of the data so that the mean 

value of the area matched that for the NURE data.  This allows the maps to 

match previously published maps more closely.  The composite grid contains 

discrepancies at some survey boundaries that are caused by differences in data 

density, survey specifications, regional field datum, and data quality.  

	To produce the merged grid, the low-level survey grids were upward 

continued to a constant height of 1,000 feet above the ground.  This was done 

using standard 2-D FFT filtering techniques applied to surveys AK23, 5006 and 

the five NURE data sets (6016, 6023, 6035, 6036 and 6121; table 1).  The two 

barometrically flown surveys (surveys 173 and 193) were flown at 4,000 feet and 

were downward continued to a drape of 1000 feet above ground using the 

Compudrape algorithm (Paterson and others, 1990).  In this technique (Cordell, 

1985) the magnetic grid is downward continued to ten parallel survey datums 

that bracket the topographic relief.  The final draped grid was computed by 

interpolating the magnetic value at each grid cell to the 1,000 foot terrain 

clearance level as determined using a digital terrain model.

	The final merged grid was created by combining the  adjusted data sets, 

then further adjusting the data sets to minimize offsets along the survey borders 

starting with a base grid of the AK surveys (AK08, AK11, AK12 and AK13).  A 

coincident profile from each adjacent data set was extracted and the difference 

was calculated between the profiles.  A correction surface was then generated 

which was added to the grid being joined to the merged grid using a weighted-

averaging technique.  Because this alters the data near the survey boundaries, the 

corrections were applied to the lowest resolution surveys, keeping the higher 

resolution data intact.  The merged grids were then inspected for seamless joins 

along the survey boundaries using interactive shading programs, and some fine 

tuning of the correction grids was applied to yield a smooth fit.  Once the final 

grid was completed, a single pass of a Hanning smoothing filter (Patterson and 

others, 1994) was used to eliminate the high-frequency errors along the boundary 

edges.  The final result is a grid that is suitable for regional analysis but still 

contains variable data resolution because of the variable survey specifications.



Map Production



	The merged grid was split into four subgrids and was interpolated to a 

0.25 km grid using a minimum-curvature algorithm (Webring, 1981).  These 

grids were converted to PostScript images using the color scale shown on the 

maps.  Reds are residual magnetic highs, blues are lows.  Computer-generated 

contour lines are superimposed on the color image.  Rivers and town locations 

are from the USGS DLG 1:2,000,000-scale data set (USGS, 1992).  The color image 

and line-work for the map were combined into a single encapsulated PostScript 

file for each map sheet.  These files were imported into Adobe Illustrator and the 

map annotations were added.  The files were then converted to the Scitex format 

which was used to create the color-separation negatives.

	The aeromagnetic data used to produce this set of maps, gridded at an 

interval of 1 km, are available from the National Geophysical Data Center, 325 

Broadway, Boulder, Colorado 80303.  Both the composite grid and the merged 

grid are available.





Acknowledgments



	Esther Castellanos, USGS, digitized all of the analog aeromagnetic maps 

used in this compilation.  Pat Hill, USGS, provided original data and reference 

material on the aeromagnetic surveys and assisted with data quality assurance.  

Stewart Racey, National Geophysical Data Center, provided original data for 

several of the aeromagnetic surveys.  Louis Racic of Paterson, Grant & Watson 

Limited, directed the editing and merging of the data into the grids that were 

used to produce the maps.  The final maps were produced using Adobe 

Illustrator.



References Cited



Aero Service Corporation, 1980a, National Uranium Resource Evaluation 

airborne gamma-ray spectrometer and magnetometer survey, Four Corners 

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quadrangles, Alaska: U.S. Department of Energy Grand Junction Office Report 

GJBX 116(80), scale 1:125,000.



____1980b, National Uranium Resource Evaluation airborne gamma-ray 

spectrometer and magnetometer survey, Iditarod Quadrangle, Alaska: U.S. 

Department of Energy Grand Junction Office Report GJBX 80(80), scale 1:500,000.



_____1980c, National Uranium Resource Evaluation airborne gamma-ray 

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1:500,000.



_____1980d, National Uranium Resource Evaluation airborne gamma-ray 

spectrometer and magnetometer survey, McGrath Quadrangle, Alaska: U.S. 

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_____1980e, National Uranium Resource Evaluation airborne gamma-ray 

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_____1980f, National Uranium Resource Evaluation airborne gamma-ray 

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_____1980g, National Uranium Resource Evaluation airborne gamma-ray 

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_____1980h, National Uranium Resource Evaluation airborne gamma-ray 

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_____1973b (revised, 1977), Aeromagnetic map, eastern two-thirds of Healy 

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_____1973c (revised, 1977), Aeromagnetic map, Fairbanks Quadrangle, Alaska: 

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1:250,000.



_____1973d (revised, 1977), Aeromagnetic map, Mt. Hayes Quadrangle, Alaska: 

Alaska Division of Geological and Geophysical Surveys Open File Report 10, 

scale 1:250,000.



_____1973e, Aeromagnetic map, northeastern part of Gulkana Quadrangle, 

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12, scale 1:250,000.



_____1973f, Aeromagnetic map, northern part of Anchorage Quadrangle, Alaska: 

Alaska Division of Geological and Geophysical Surveys Open File Report 21, 

scale 1:250,000.



_____1973g, Aeromagnetic map, Talkeetna Mountains Quadrangle, Alaska: 

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scale 1:250,000.



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_____1978, National Uranium Resource Evaluation aerial radiometric and 

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_____1973c, Aeromagnetic survey, western half of Livengood Quadrangle, 

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_____1974a, Aeromagnetic map of the Circle Quadrangle, northeastern Alaska: 

U.S. Geological Survey Open-File Report 74-1101, scale 1:250,000.



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northeastern Alaska: U.S. Geological Survey Open-File Report 74-1104, scale 

1:250,000.



_____1976, Aeromagnetic survey of the west one-half of the Ruby 1:250,000 

Quadrangle, Alaska: U.S. Geological Survey Open-File Report 76-188, scale 

1:250,000.



_____1979a, Aeromagnetic map of the Medfra 1 x 3 Quadrangle, Alaska:  U.S. 

Geological Survey Open-File Report 79-380, scale 1:250,000.



_____1979b, Aeromagnetic map of part of the Valdez 1 x 3 Quadrangle, Alaska: 

U.S. Geological Survey Open-File Report 79-381, scale 1:250,000.



____1980, Aeromagnetic map of the Chugach area, Alaska: U.S. Geological 

Survey Open-File Report 80-058, scale 1:250,000.



____1984a, Aeromagnetic map of the western part of the Healy 1 by 3 

Quadrangle, Alaska: U.S. Geological Survey Open-File Report 84-295, scale 

1:250,000.



____1984b, Aeromagnetic map of part of the Anchorage 1 by 2 Quadrangle, 

Alaska: U.S. Geological Survey Open-File Report 84-352, scale 1:250,000.



_____1985, Aeromagnetic map of part of the Wrangell Mountains, Alaska: U.S. 

Geological Survey Open-File Report 85-605, scale 1:250,000.



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in the eastern Copper River Basin, Alaska: University of Alaska, Geophysics 

Institute Report UAG R-302, 158 p., 8 pls., scale 1:63,360.



Figure 1.  Index map showing the boundaries of the individual surveys used in 

this report.