U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Composite and Merged Aeromagnetic Data for Alaska: A Web Site for Distribution of Gridded Data and Plot Files

by

R.W. Saltus and G.C. Simmons U.S. Geological Survey, Denver, Colorado

Open-File report 97-520

Thie report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Use of brand names is for descriptive purposes and does not constitute endorsement by the U.S. Geological Survey.

7997

Saltus and Simmons: Alaska Aeromagnetic Compilation Web Site

Abstract

This report is a listing of the primary information pages in the following web site:

http://minerals.cr.usgs.gov/publications/ofr/97-520/alaskamag.html

This web site describes the results of a USGS project to merge the best available aeromagnetic data into consistent 1-km grids spanning the state of Alaska. The website allows users to download (via FTP) data files (in several formats) and plot files. The anonymous FTP sites are:

ftp://minerals.cr.usgs.gov/minerals/ofr/97-520/data

and

ftp://minerals.cr.usgs.gov/minerals/ofr/97-520/plots

Table of Contents

Pages Content

1...............Home page - color figure

2...............Map description - text page

3-4............Project description - text page

5...............Data density - color figure

6...............Data index map - black and white figure

7-8............Data sources - table

9...............FTP available plot files - readme text

10-13........ FTP available data files - readme text

14-15........ Annotated reference list

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Alaska Aeromagnetic Data Index

Alaska aeromagneti data index map

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T*SFlightline spacing

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Digital nightline lines indicate

The Alaska aeromagnetic compilation maps contain data from 85 separate surveys, digitized maps, and previous gridded compilations. This map is an index to the flightline spacing of the original surveys. Move the mouse around the map and click on the individual surveys for more information. A tabular index (see index) provides a summary of the data sources. The raw data for all these surveys are available from the National Geophysical Data Center (NGDC), Boulder, Colorado.

Top II ALASKA MAG II Minerals II Geology II USGS

URL: http://minerals, usgs.gov/publications/ofr/97-520/\veb_index. html Maintainer: Rick Saltus Last modified: 6 Oct 1997

science fora changing world

Alaska Aeromagnetic Compilation

These maps show variations in the Earth's magnetic field caused primarily by the uneven distribution of the mineral magnetite in the rocks that make up the upper part of the Earth's crust. The features and patterns of the maps reveal details of subsurface geology including the locations of buried faults, the location of magnetite-bearing rocks (which include many kinds of rocks of interest to mineral exploration), and the thickness of surficial sedimentary rocks.

The maps are constructed from grids that combine information (data processing details), collected in 85 separate aeromagnetic surveys conducted between 1945 and 1982. The data from these surveys are of varying quality; large regions of Alaska are covered only by very coarse surveys. An index plot (see plot) gives an overview of the flightline spacing of the original surveys. A data table (see table) summarizes the detailed specifications of the surveys.

The images displayed here were prepared by Northwest Geophysical Associates under contract to the U.S. Geological Survey. In addition to an overview image of the entire state, 19 separate detailed images display more of the details in the magnetic field. HPGL plot files for these images can be downloaded (download plots) for plotting at true scale (1:2,500,000 for the overview map, 1:500,000 for the detailed maps).

This project was supported by the Mineral Resources Program of the USGS in cooperation with the National Geophysical Data Center (NGDC), The State of Alaska, Department of Natural Resources, Division of Oil and Gas (DOG), and the State of Alaska, Department of Natural Resources, Division of Geological and Geophysical Surveys (DGGS),

Top II ALASKA MAG II Minerals II Geology II USGS

URL: http.'//minerals, usgs. gov/publications/ofr/97-520/map_desc_text. html Maintainer: Rick Saltus Last modified: 6 Oct 1997

USGSscience fora changing wortd

Alaska Aeromagnetic Data Processing

The assembly of 85 individual aeromagnetic surveys and grids to build the Alaska state-wide compilation was carried out in two stages. (1) A compilation for interior Alaska (spanning from 61 to 66 N latitude and 144 to 159 W longitude) was completed in 1994, (2) the rest of the state-wide compilation was completed in August 1997.

Data processing for the interior Alaska aeromagnetic data compilation was done by Paterson, Grant and Watson, Ltd, under contract to the State of Alaska, Division of Oil and Gas with cooperation from the USGS. Data from 25 aeromagnetic surveys were compiled. Paper maps at a scale of 1:500,000 were published by the USGS (USGS Map GP-1014; reference list). These maps are available from USGS Map Sales (goto USGS Map Sales).

Data processing for the state-wide compilation was done by Northwest Geophysical Associates under contract to the USGS. Data from an additional 60 surveys and previously processed grids were added to the interior Alaska compilation. In addition, some modification of the interior Alaska compilation was done to improve the merge to adjacent surveys in the northwest and southeast comers.

DATA PROCESSING STEPS

1. Grids were constructed from the original aeromagnetic survey data with a cell size of between 1/3 and 1/5 of the flightline spacing of the survey, using a bidirectional gridding algorithm when necessary due to wide flightline spacing. For digitized contour line data, the initial grid was constructed using a minimum curvature algorithm and a spacing appropriate for the scale of the digitized map.

2. Data quality problems were addressed in consultation with the USGS.3. The Definitive Geomagnetic Reference Field (DGRF) was applied for the date of the

original survey (in some cases this required the determination and removal of the original reference field applied).

4. The survey grids were regridded, as necessary, to the final grid cell size of 1 km using a minimum curvature algorithm.

5. The datum levels of adjacent surveys were adjusted (by addition or subtraction of a constant value) to minimize differences at the boundaries.

6. These adjusted grids were combined, leaving a 1 cell gap between surveys, into a composite grid (one of the final products).

7. The original survey grids were upward or downward continued and converted from level to drape as necessary to produce a consistent survey specification of 1000ft above ground. Upward continuation of the NURE surveys was by standard 2D FFT filtering techniques. Downward continuation and level-to-drape was performed using the COMPUDRAPE technique (as implemented in the GEOSOFT software package).

8. The datum levels of the converted grids were then adjusted to minimize differences at the boundaries.

9. These adjusted grids were combined into a single merged grid (with no data gaps inserted between surveys).

GRID PROJECTION SPECIFICATIONS

Projection = Albers conical equal-areaCentral meridian = 151 WBase latitude = 55 NStandard parallels = 55 and 65 NSemi-major ellipsoid axis = 6378206.4 mEccentricity squared = 0.0067686579973

Top II ALASKA MAG II Minerals II Geology II USGS

URL: http://minerals, usgs. gov/publications/ofr/97-520/proj_desc. html Maintainer: Rick Saltus Last modified: 6 Oct 1997

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Alaska Aeromagnetic Maps and Data

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GetSura Click here

to view detailed state map

_l_ \ Click in a box to view one of the 19 detailed regional maps About this

Top II Minerals II Geology II USGS

URL: http://minerals.usgs.gov/publications/ofr/97-520/aloskamag.html Maintainer: Rick Saltus Last modified: 6 Oct 1997

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ted

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XX

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aps

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XX

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JGR

is J

ourn

al o

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phys

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63,3

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aps

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d in

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197

0's

by th

e A

lask

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tate

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t. of

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ural

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ourc

es,

Div

isio

n of

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logi

cal a

nd G

eoph

ysic

al S

urve

ys.

For

mor

e co

mpl

ete

refe

renc

es s

ee:

Hill

, P

.L.,

1991

, B

iblio

grap

hies

and

loc

atio

n m

aps

of p

ublic

atio

ns o

n ae

rom

agne

tic a

nd a

eror

adio

met

ric s

urve

ys f

or H

awai

i an

d A

lask

a: U

SG

S O

pen-

file

91-3

70-E

, 36

pp.

10/8

/97

Rick Saltus18 September 1997

README.TXT FILE FOR PLOT TRANSFER DIRECTORY

This directory contains binary HPGL format plot files for aeromagnetic maps that span the state of Alaska. See the web site for index maps and more information. These plot files will plot on plotters such as the HP750. The sizes of the resulting maps vary, but are mostly about 2 ft x 3 ft. The state maps are at 1:2,500,000 scale, the regional maps are at 1:500,000 scale.

Listing of plot files:

Whole State Maps (scale 1:2,500,000)

akmerge.bin - color contour map of merged aeromagnetic data for Alaska akcomp.bin - color contour map of composite aeromagnetic data for Alaska

Regional Maps (scale 1:500,000)

mapt_m.bin (19 maps) - color contour maps of merged aeromagnetic data mapt_c.bin (19 maps) - color contour maps of composite aeromagnetic data

Regional Map Index

71 ________________________(18) Barrow

(19) Barter Island

(15) Kotzebue

(16) Selawik

(17) Yukon

(11)Nome

(12) Galena

(13) Fairbanks

(14) Eagle

(6) Bethel

(7) Sleetmute

(8) Anchorage

(9) Yakutat

(10) Haines

56-If

(1) Chignik

55

(2) Kodiak

(3) W. Gulf of Alaska

(4) E. Gulf of Alaska

(5) Juneau

13

Rick Saltus18 September 1997

README.TXT FILE FOR GRIDDED DATA TRANSFER DIRECTORY

This directory contains ascii and binary gridded files for the Alaska composite and merged aeromagnetic data. Each data grid is available in four different data formats:

1. GeoSoft GXF format (an ascii grid transfer format)2. ER Mapper format (a two-part transfer format)3. ARC/INFO grid export format (a binary format)4. USGS ODDF grid format (a binary format)

Each of these formats is discussed briefly below.If none of these formats are directly readable by software on your system, then your best bet is probably to write a program to input and translate the GXF format.

BASIC GRID STATISTICS (Merged Grids!

Cell Size =Number of Rows =Number of Columns =

1000.000 meters 1716 2216

Data Type: Floating Point

BOUNDARY

Xmin = Xmax = Ymin = Ymax =

-902500.000 meters1313500.000 meters109500.000 meters

1825500.000 meters

STATISTICS

Minimum Value = -1867.067Maximum Value = 4640.109Mean = 52.478Standard Deviation = 170.791

COORDINATE SYSTEM DESCRIPTION

Projection Units Parameters : 1st standard parallel 2nd standard parallel central meridian latitude of projection's false easting (meters) false northing (meters)

ALBERS METERS

origin

Spheroid

55 65

-151 55

0 0 0

0 0 0

CLARKE1866

0.000 0.000 0.00

0.000 .00000 .00000

1. Grid exchange Format (AKCOMP.GXF, AKMERGE.GXF)

GXF (Grid exchange File) is a standard ASCII file format for exchanging gridded data among different software systems. Software that supports the GXF standard will be able to import properly formatted GXF files and export grids in GXF format.

Grid Description:A grid is a rectangular array of points at which single data values define a two dimensional function. Grid point locations are related to a Grid Coordinate System (GCS), which is a right handed Cartesian system with X and Y axis defined by the bottom and left sides of a grid array. The grid point at the bottom, left corner of the array is the origin of the GCS. All distances are in meters.

10

GCS coordinates are related to a Base Coordinate System (BCS) through a plane translation and rotation. (For these Alaska grids, the base coordinate system is the Albers conical equal-area projection with standard parallels of 55 and 65 degrees, a base latitude of 55 degrees north, a central meridian of 151 degrees west, a semi-major axis of ellipsoid of 6378206.400 meters, an eccentricity of 0.0067686579973.) The origin of the GCS is located at point (xO,yO) in the BCS, and the X and Y grid indices are related to BCS units through the separation between points in the GCS X and Y directions.

Labeled Data Objects and Comments

A GXF file is an ASCII file made up of a number of labeled data objects and comments. Each labeled data object has a label line followed by one or more data lines. A label line is identified by a '#' character in the first column followed immediately by an upper-case label. The data associated with that label are found on one or more lines that follow the label.

Lines

All lines in a GXF file must be less than or equal to 80 characters in length. Any lines that are not part of a labeled data object are ignored and can be used to place comments within a GXF file. Programs that read GXF files will skip such comment lines while they search for the next GXF data object.

GXF Object Definitions

#TITLEA one line descriptive title of the grid. Some grid formats include textualdescriptions of the grid, and this information can be placed in a #TITLEobject.Default: blank title

#POINTSThe number of points in each grid row (horizontal or vertical as defined bythe #SENSE object).Default: no default - this object is required.

#ROWSThe number of rows in the grid. A grid row (or vector) is a collection ofconsecutive grid points that represent the grid values along a horizontal orvertical line in the grid. The complete grid is then defined by a consecutivesequence of grid rows.Default: no default - this object is required.

#PTSEPARATIONThe separation between points in the grid. This should be in Base CoordinateSystem units (ground units for geographically based grids).Default: 1.0

#RWSEPARATIONThe separation between rows in the grid. These should be in Base CoordinateSystem units (ground units for geographically based grids).Default: 1.0

#XORIGINThe X location of the bottom left corner of the grid in the Base CoordinateSystem.Default: 0.0

#YORIGINThe Y location of the bottom left corner of the grid in the Base Coordinate

11

System.Default: 0.0

#ROTATIONThe rotation angle of the grid. This is the counter-clockwise angle of thebottom edge of the grid with respect to the Base Coordinate System X axis.Rotation only has meaning for Base Coordinate Systems that use the same unitson the X and Y axis.Default: 0.0

#SENSEThe first point of the first row of the stored grid can be at any corner of the grid rectangle, and the grid rows can be run vertically or horizontally. The SENSE object defines this storage sense as follows:

±1 first point at bottom left of grid±2 first point at upper left of grid±3 first point at upper right of grid±4 first point at bottom right of grid

A positive SENSE stores rows in a right-handed sense; a negative SENSE stores rows in a left-handed sense. This means that if you were standing at the first grid point and looking into the grid, the first grid row would extend to your right for a right handed grid (positive sense), or to your left for a left handed sense (left-handed grid): (All grids on this CD have SENSE=+1.) Default: 1 (first point at bottom left, rows left to right)

#TRANSFORMThis keyword is followed by two numbers on the same line: SCALE and OFFSET,which are used to transform the grid data to desired units:Z = G * SCALE + OFFSETwhere

Z grid value in the desired unitG are grid values as specified in the #GRID object

Default: SCALE =1.0, OFFSET =0.0

#DUMMYThe grid must be rectangular (every row must have the same number of points). The dummy value defined by this object is used to define blank areas of the grid. Any grids that include blank areas must define a dummy value. Default: no dummy value.

#GRIDThe grid data is listed point by point and row by row. The #GRID object anddata is always the last object in a GXF file.The first data point is at the location indicated by #SENSE, and is followedby successive points in that row of points (either horizontal or vertical),then the points in the next row, and so on. The points in a row can follow onto the next data line, although each new row must start on a new data line. AGXF reading program can expect #ROWS of #POINTS for a total of #ROWS times

#POINTS data values.Default: none, must be included as the last object in a GXF file.

2. ER Mapper grid format (akmerge & akmerge.ers, akcomp & akcomp.ers)

The ER Mapper grid format consists of two files: a binary data file (no file suffix) and a ascii header file (*.ers). These files can be read directly by the ER Mapper software or by other packages such as ERDAS IMAGINE. Note that the header files refer to the projection

12

called "ALINTAK1". This is not a standard ER Mapper projection. To register the grid properly within ER Mapper you must create a projection entry that corresponds to the parameters listed above for the projection of these data.

3. ARC/INFO Grid Export format (akc.eOO, akm.eOO)

This format is readable by ARC/INFO, ERDAS IMAGINE, and other packages When imported into ARC/INFO, this file will unpack into a directory containing all the components of the registered grid coverage.

4. USGS ODDF binary grid format (akcomp.gd, akmerge.gd)

This is a binary format with an included ascii header. This format is used by the USGS Geophysics Group within the Minerals Program of the Geologic Division.

13

USGSscience fora changing world

Alaska Aeromagnetic References

Godson, R.H., 1984, compiler, Composite magnetic anomaly map of the United States, Part B - Alaska and Hawaii: Geophysical Investigations Map GP-954-B, U.S. Geological Survey, scale 1:2,500,000, 2 sheets.

Previous (analog) aeromagnetic compilation for Alaska. Although the final compilation was digitized, the resolution of the resulting grid was poor.

Hill, P.L., 1991, Bibliographies and location maps of publications on aeromagnetic and aeroradiometric surveys for Hawaii and Alaska: U.S. Geological Survey Open-file report 91-370-E, 36 pp.

Full index maps and complete bibliographic data references for public domain aeromagnetic and aeroradiometric surveys known to the USGS.

Meyer, J.F., and Saltus, R.W., 1995, Merged Aeromagnetic Map of Interior Alaska: U.S. Geological Survey Map GP-1014, scale 1:500,000.

Color contour maps of the merged magnetic data for interior Alaska (spans from 61 to 66 N latitude and from 144 to 159 W longitude).

Meyer, J.F., Saltus, R.W., Barnes, D.F., and Morin, R.L., 1996, Bouguer Gravity Map of Interior Alaska: U.S. Geological Survey Map GP-1016, scale 1:500,000.

Color contour maps of gravity for the same region as the interior Alaska magnetic maps.

Saltus, R.W., Meyer, J.F., Barnes, D.F., and Morin, R.L., 1997, Tectono-Geophysical Domains of Interior Alaska as Interpreted From New Gravity and Aeromagnetic Data Compilations: in Geologic Studies in Alaska by the U.S. Geological Survey, 1995, Dumoulin, J.A., and Gray, I.E., eds., U.S. Geological Survey Professional Paper 1574, pp. 157-171.

A preliminary tectonic interpretation of the interior Alaska gravity and magnetic

14

compilations.

Saltus, R.W., and Simmons, G.C., 1997, Composite and Merged Aeromagnetic Data for Alaska: A Web Site for Distribution of Gridded Data and Plot Files: U.S. Geological Survey Open-file report 97-520,14 pp.

Paper report describing this web site. This report can be used as a bibliographic reference for these data grids in formal publications.

Top II ALASKA MAG II Minerals II Geology II USGS

URL: http://minerah. usgs.gov/publications/ofr/97-520Aveb_refs. html Maintainer: Rick Saltus Last modified: 6 Oct 1997

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