Richard W. Saltus

US Geological Survey



25 June 1996



CENTRAL ALASKA BOUGUER GRAVITY DATA GRIDS



These grids were used to produce the color maps in USGS Map GP-1016,

at a scale of 1:500,000.  The text for GP 1016 is appended to the end of

this file - please read it for more information on data sources and

processing.



GRIDS



1. AKgrv_int.asc = Bouguer gravity grid of interior Alaska.  All grid

                   cells within the rectangular data area (from 61 to

                   66 degrees N lat and from 144 to 159 degrees W lon)

                   have interpolated data values.



2. AKgrv_10km.asc = Bouguer gravity grid of interior Alaska.  Only

                    those grid cells within 10 km of a gravity data

                    point have gravity values



GRID SPECIFICATIONS



GRID NAME: AKgrv_int.gd



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    = 600



Grid layout:



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

             |                                 |  

             |                                 |  

             |                                 |  

             |.                                |  

             |.                                |  

             |.                                |  

             |9                                |  

             |8  rows (600)                    |  

             |7                                |  

             |6                                |  

             |5                                |  

             |4                                |  

             |3                                |  

             |2                                |  

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

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

          origin (-429, 669)



Grid format: ASCII



The grids are in ascii format, 5 numeric values per 80-character line.

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

A new data row always begins at the beginning of a line.  An extra (dummy)

value appears at the beginning of every row.  So, a single row (850+1 values)

occupies 851/5 = 171 lines in the file.  There are 600 rows, so the ascii

grid file contains 600*171 + 2 = 102602 records.



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



GRID NAME: AKgrv_10km.asc



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    = 600



Grid layout:



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

             |                                 |  

             |                                 |  

             |                                 |  

             |.                                |  

             |.                                |  

             |.                                |  

             |9                                |  

             |8  rows (600)                    |  

             |7                                |  

             |6                                |  

             |5                                |  

             |4                                |  

             |3                                |  

             |2                                |  

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

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

          origin (-429, 669)



Grid format: ASCII



The grids are in ascii format, 5 numeric values per 80-character line.

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

A new data row always begins at the beginning of a line.  An extra (dummy)

value appears at the beginning of every row.  So, a single row (850+1 values)

occupies 851/5 = 171 lines in the file.  There are 600 rows, so the ascii

grid file contains 600*171 + 2 = 102602 records.



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



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

Text for US Geological Survey Map, GP-1016

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



Bouguer Gravity Map of Interior Alaska



by John F. Meyer, Jr., Richard  W. Saltus, David F. Barnes, and Robert L. Morin



Introduction

	These complete Bouguer gravity maps of interior Alaska are part of a 

cooperative effort by the State of Alaska Division of Oil and Gas and the 

U. S. Geological Survey (USGS) to provide background information for future 

Alaskan petroleum and mineral exploration.  Along with an accompanying set of 

aeromagnetic maps (Meyer and Saltus, 1995) they provide a compilation of all 

publicly available potential-field geophysical data covering the primary 

Cenozoic structural basins within the state-controlled lands of the Alaskan 

interior.  The maps are 1:500,000-scale and are prepared from digital 

data sets using an Albers equal-area projection. 



Gravity data



	The gravity maps summarize about 9,800 measurements made by the 

U. S. Geological Survey, the State of Alaska, and other groups between 1958

and 1992.  Most of the data are contained in previous compilations, but this 

report contains the first digitally compiled maps that have terrain 

corrections on all measurements.  Previous map compilations covering the same 

area and using much of the same data are a simple Bouguer gravity map of 

Alaska (Barnes, 1977) and the state isostatic gravity map (Barnes and others, 

1994).  Previous larger scale compilations of specific sedimentary basins 

covered the Minto Flats (Barnes, 1962), the Copper River Basin (Andreasen and 

others, 1964), the Medfra-McGrath Lowlands (Barnes, 1973), and the Beluga Basin

(Hackett, 1977).  These early maps used the 1930 international ellipsoid and 

the Potsdam gravity datum and have a resulting anomaly difference of about 

+8 mGal from the present maps.  More recent, detailed compilations using the 

present IGSN-71 datum and 1967 ellipsoid cover the Holitna Basin and the 

Minchumina Basin (Meyer and Krouskop, 1984, 1986) and the Nenana Basin 

(Valin and others, 1991; Frost and others, in press).  Sources of data used 

for more mountainous parts of the maps are summarized by Barnes (1977).  

Those sources are now supplemented by the surveys of Hackett (1981 a, b), 

Burns (1982), Burns and others (1985), Cady and Weber (1983), Cady and Morin 

(1988), and Wescott and others (1983).  The maps also include unreduced data 

contributions received as unpublished written communications from 

U. S. Geological Survey investigators William F. Isherwood (1976), David L. 

Campbell (1982, 1983, 1984), and Michael A. Fisher (1984-92)  and the 

University of Alaska's Ronald G. Cothren (November 1993).  Unpublished data 

from the U. S. Department of Defense Gravity Library (Kenneth E. Burke, 

written commun., 1994) provide additional control.



Data compilation and Reduction



	The gravity values are complete Bouguer anomalies, which show the 

variation of gravitational attraction after the removal of a theoretical 

value calculated for the effects of latitude and elevation differences on 

the earth surface (Heiskanen and Vening Meinesz, 1958).  The latitude effects 

were calculated by the 1967 Geodetic Reference System (International 

Association of Geodesy, 1971), and the elevation effects were derived from 

the standard free-air gradient of 0.3086 mGal/m (neglecting second order terms)

and a standard density of 2.67 g/cm3.  The gravity datum was established from 

the IGSN- 71 network values at Fairbanks and Anchorage (Morelli and others, 

1974).  Other data reduction steps followed the procedures for Alaskan gravity 

data as outlined by Barnes (1972).  Terrain corrections between an inner radius

of 0.39 km (zone E of Hammer, 1934) through 166.7 km (Hayford's zone O from 

Swick, 1942) were calculated by the terrain correction program of Plouff (1977)

using digitized mean elevations.  The primary sources of the terrain data were 

the USGS digital-elevation model (DEM) tapes (Elassal and Caruso, 1983),  

which were read and gridded into three files of 1/4  x 1/2-minute, 

1 x 2-minute, and 3 x 6-minute mean elevations for the terrain correction 

program (Barnes, 1984). The accuracy of these terrain files has not been 

carefully evaluated, but small errors are known to exist for data in the 

Anchorage, Russian Mission, Bethel, Goodnews, Seward and possibly other 

quadrangles.  The DEM data do not include bathymetric or ice-thickness data 

for lake, ocean  and glacier areas;  only the elevations of the upper surfaces 

of most such areas were used for the terrain corrections.  However, 

the 3 x 6-minute mean-elevation file, which was used for outer-zone 

corrections, included oceanic bathymetric data derived by regridding 

the 5 x 5-minute topographic data set of the world (National Geophysical 

Data Center, 1986).  First-order level lines and triangulation networks cover 

only small parts of the Alaskan interior, so the vertical control is very 

limited.  Photogrammetric mapping at 1:63,360 scale now covers most of the 

state, but primarily because of poor vertical control this mapping does not 

conform to national map-accuracy standards.  The local relief is well 

portrayed, but broad areas (covering thousands of square kilometers) that 

have absolute elevation errors exceeding 30 m have been found on some maps.  

Thus, altimetry measurements have supported almost all Alaskan gravity 

measurements. The data files for most stations also contain a second 

elevation, if possible a surveyed elevation, perhaps a spot elevation, 

and otherwise a contour-interpolation elevation.  Initially, comparison 

between the altimetry and the second elevation was used primarily as a check 

on the accuracy of both the altimetry and the station location.  After the 

digital-elevation-model (DEM) data made routine terrain corrections possible, 

the second elevation proved useful because it almost always agrees with the 

relative terrain data.  In Alaskan gravity data reduction, all surveyed 

elevations and many river-gradient elevations have been used as station 

elevations for the entire Bouguer correction.  Otherwise, the altimeter 

elevation (usually considered preferable) is used for the infinite-slab part 

of the Bouguer correction, and the second (map) elevation is used for the 

terrain correction.  In limited tests this system seems to produce 

the most consistent results.  Inner-zone corrections (within 0.39 km) were 

not made, because a large sampling (Barnes, 1981, and unpublished data) shows 

that these inner-zone corrections seldom exceed 1.0 mGal.  One milligal is 

also approximately equivalent to a  5-m elevation error, the estimated 

accuracy of most altimetry and one half the smallest contour interval on 

these maps.  

	The bulk of the gravity data are probably accurate to about 

1.0 mGal, but a few larger errors are undoubtedly still present in the data.  

Possible sources of these larger errors include extreme altimetry 

errors caused by misread scales or abnormal weather, inaccurate field 

locations, and data processing mistakes.  Poor data coverage over much of the 

study area makes it difficult to evaluate accuracy by comparison to nearby 

measurements.  As highlighted by the uncolored areas of these maps, there is 

a clear need for additional gravity data in interior Alaska.

	The  1-km gravity data grids for these maps are available from the 

National Geophysical Data Center in Boulder, Colorado; most of the principal 

facts are available on a CD-ROM (Hittleman and others, 1994).



Map production



	The locations of all the gravity measurements were projected from 

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

projection defined by a base latitude of 55o and central meridian of 151o 

(Snyder, 1983).  Once the data were projected, the complete Bouguer anomalies 

were gridded using a proprietary gridding code (Surface Gridding Library by 

Dynamic Graphics, Inc., Berkeley, CA.).  Two grids with row and column spacing 

of 1 km were constructed: a complete grid contains interpolated and 

extrapolated values for the entire study area, and a limited grid contains 

interpolated and extrapolated values for regions of the study area within 

10 km of an observation point.  The two grids were then split into four 

subgrids. These subgrids were converted to postscript images as follows: 

they were interpolated to 0.25-km row and column spacing.  Color images 

(10 mGal color interval; reds are gravity highs and blues are gravity lows) 

and 2-mGal contour lines were generated from the limited grid for regions 

within 10 km of an observation point.  Ten-mGal computer generated contour 

lines, generated from the complete grid were then superimposed on the color 

image.  River and town locations are from the digital line graph (DLG) data 

set (Domeratz and others, 1983, and U.S. Geological Survey, 1992).  The color 

images and linework for the maps were combined into single PostScript files 

for each map sheet, and the files were imported into Adobe Illustrator for 

map annotation.  The files were then converted to the Scitex format, which 

was used to create the color-separation negatives.



Map Usage



	The maps were planned as an attempt to show the size of the interior 

basins and the availability of pertinent data, but they should be used with 

caution.  The many colorless areas clearly show where additional data are 

needed, but even the colored areas may give a deceptive indication of basin 

size.  The gridding routine employed attempts to fit the smoothest possible 

surface between the data points.  In some cases this technique may produce 

circular contours around widely spaced measurements so that linear anomalies 

may appear as chains of circles instead of continuous features.  Hand 

contouring aided by knowledge of available geologic and aeromagnetic data 

was used to draw the earlier state gravity maps (e.g., Barnes, 1962; Barnes, 

1973; Barnes, 1977).  The initial Bouguer map of Alaska (Barnes, 1977) 

contained an interpretive index that outlined the principle Cenozoic-

basin anomalies.  In areas of low topography the simple Bouguer anomalies 

should be similar to the complete Bouguer anomalies on this map.  The 

isostatic anomaly map (Barnes and others, 1994) shows the extensions of some 

of these anomalies into adjacent mountainous areas, and its interpretive 

inset and table references other sources of geophysical data.  For example, 

interpretive contouring of the 1977 map used evidence of a southward decrease 

in gravity across the meander belt of the Yukon River west of the town of 

Tanana.  This gradient was combined with geologic knowledge of Tertiary 

outcrops along the Yukon River Palisades to suggest a narrow Tertiary trough 

south of the river, which was interpreted as evidence for the Lower Tanana 

Basin proposed by Miller and others (1959).  The present map shows a 

smaller, more discontinuous trend for this feature and thus indicates the need 

for much additional data to clearly define the anomaly and size of this 

possible petroleum basin.



References cited



Andreasen, G. E.,Grantz, Arthur, Zietz, Isidore, and Barnes, D. F., 1964, 

Geologic interpretation of magnetic and gravity data in the Copper River Basin,

Alaska: U. S. Geological Survey Professional Paper 

316-H, 153 p, 2 plates



Barnes, D. F., 1962, Gravity low at Minto Flats, Alaska, in Geological Survey 

Research 1961: U. S. Geological Survey Professional Paper 424-D, p. 254-257.



__________ 1972, Notes on processing and presentation of U. S. Geological 

Survey Alaskan gravity data: U. S. Geological Survey Open-File Report 72-16, 

25 p.



__________ 1973, U. S. G. S. gravity data maps of Medfra and McGrath 

quadrangles:  U. S.  Geological Survey Open-File Report,  2 sheets, 

scale 1:250,000.



__________ 1977, Bouguer gravity map of Alaska: U. S. Geological Survey 

Geophysical Investigations Map  GP-913,  scale 1:2,500,000.



__________ 1981, Supplementary terrain corrections for gravity data from the 

National Petroleum Reserve in Alaska: National Geophysical Data Center NPRA 

data set TGR-0230, 10 p.



__________ 1984, Digital elevation models improve processing of Alaskan 

gravity data, in Coonrad, W. L. and Elliott, R. L. eds., U. S. Geological 

Survey in Alaska: Accomplishments during 1981: U. S. Geological Survey 

Circular 868, p. 5-7.



Barnes, D. F., Mariano, John, Morin, R. L., Roberts, C. W., and 

Jachens, R. C., 1994, Incomplete isostatic gravity map of Alaska, in 

Plafker, George, and Berg, H. C., eds., The geology of Alaska: Boulder,

Colorado, Geological Society of America, The Geology of North America v. G1, 

plate 9, scale 1:2,500,000.



Burns, L. E., 1982, Gravity and aeromagnetic modelling of a large gabbroic 

body near the Border Ranges fault, southern Alaska: U. S. Geological Survey 

Open-File Report 82-466, 59 p.



Burns, L. E., Pessel, G. H., and Szumigala, G. J., 1985, Simple Bouguer 

gravity map of part of the northwest Chugach Mountains, Anchorage quadrangle, 

south-central Alaska:  Alaska Division of Geological and Geophysical Surveys 

Public Data File 85-44.



Cady, J. W. and Weber, F. R., 1983, Aeromagnetic map and interpretation of 

aeromagnetic and gravity data, Circle quadrangle, Alaska: U. S. Geological 

Survey Open-File Report 83-170-C, 38 p.



Cady, J.W. and Morin. R. L., 1988, Aeromagnetic and gravity data, in  

Weber, F. R., McCammon, R. B., Rinehart, C. R., Light, T, D., and 

Wheeler, K. L., eds. Geology and mineral resources of the White 

Mountains National Recreation Area, east central Alaska: 

U. S. Geological Survey Open-File Report 88-284, p. 59-77.



Domeratz, M. A., Hallam, C. A., Schmidt, W. E., and Calkins, H. W., 1983, 

Digital line graphs from 1:2,000,000-scale maps: U. S. Geological Survey 

Circular 895-D, 38 p.



Elassal, A. A..,and Caruso, V. M., 1983, Digital elevation models: 

U. S. Geological Survey Circular 895-B, 40 p.



Frost, G. M., Barnes, D. F., and Stanley, R. G., (in press) Geologic and 

isostatic gravity map of the Nenana basin area, Alaska: U. S. Geological 

Survey Geologic Investigations Map I-2543, 3 sheets, scale 1:250,000.



Hackett, S. W., 1977, Gravity survey of Beluga basin and adjacent area, 

Cook Inlet region, south central Alaska: Alaska Division of Geological and 

Geophysical Surveys Geologic Report 49, 26 p., 1 plate.



__________, 1981a, Tabulated gravity field data, north flank of the 

Alaska Range, Alaska:  Alaska Division of Geological and Geophysical 

Surveys Open File Report 138, 6 p.



__________, 1981b, Tabulated gravity field data Cook Inlet, south central 

Alaska: Alaska Division of Geological and Geophysical Surveys Open-File 

report 139, 15 p.



Hammer, Sigmund, 1934, Terrain corrections for gravimeter stations: 

Geophysics, v 4, p. 184-194.



Heiskanen, W. K. and Vening Meinesz, F. A., 1958, The earth and its gravity 

field: New York City, N. Y., McGraw Hill, 470 p.



Hittleman, A. M., Dater, D. T., Bohman, R. W.,  and Racey, S. D., 1994, 

Gravity - 1994 edition CD ROM and users' Manual: National Geophysical 

Data Center, 29 p.



International Association of Geodesy, 1971, Geodetic Reference System 1967:  

International Association of Geodesy Special Publication 3, 116 p.



Meyer, J. F. and Krouskop, D. L., 1984,  Preliminary gravity data, Holitna 

basin, south central Alaska: Alaska Division of Geological and Geophysical 

Surveys Report of Investigations 84-25, 6 p, 2 plates, scale 1:500,000.



__________ 1986, Preliminary gravity data of the Minchumina basin, south 

central Alaska: Alaska Division of Geological and Geophysical Surveys Report 

of Investigations 86-1, 14 p., 2 plates, scale 1:500,000.



Meyer, J. F. and Saltus, R. W., 1995, Merged aeromagnetic maps of interior 

Alaska: U. S. Geological Survey Geophysical Investigations Map GP-1014, 

2 sheets, scale 1:500,000.



Morelli, Carlo, Gantar, C., Honkasalo, Tauno, McConnell, R. K., Tanner, J. G., 

Szabo, Bela, Uotila, U. A. and Whalen, C. T., 1974, The international gravity 

standardization net 1971 (I.G.S.N  71): Paris Bureau Central de l'Association 

International de Geodesie Special Publication 4, 174 p.



National Geophysical Data Center, 1986, Digital relief of the surface of the 

earth, 5-min x 5-min gridded data:  National Geophysical Data Center 

magnetic tape ETOPO-5, 1 reel.



Plouff, Donald, 1977, Preliminary documentation for a FORTRAN program program 

to compute gravity terrain corrections based on topography digitized on a 

geographic grid: U. S. Geological Survey Open-File Report 77-535, 44 p.



Snyder, J. P., 1983, Map projections used by the U. S. Geological Survey:  

U. S. Geological Survey Bulletin B 1532, 313 p.



Swain, C. J., 1976, A FORTRAN IV program for interpolating irregularly 

spaced data using the difference equations for minimum curvature: 

Computers and Geosciences, v 1, p 231-240.



Swick C. H., 1942, Pendulum gravity measurements and isostatic reductions: 

U. S. Coast and Geodetic Survey Special Publication No. 232, 82 p.



U. S. Geological Survey, 1992, 1:2,000,000-scale digital line graph (DLG) 

data CD-ROM: U. S.Geological Survey.



Valin, Z. C., Bader, J. W., Barnes, D. F., Fisher, M. A., and Stanley, R. G., 

1991, Simple Bouguer anomaly map of the Nenana basin area, Alaska: 

U. S. Geological Survey Open-File Report 91-33, 5 sheets, scale 1:250,000.



Westcott, E. M., Petzinger, Betty, Witte, William, Turner, R. L. and Bender, 

Gary, 1983, A gravity survey of part of the eastern Copper River Basin, Alaska,

in Wescott, E. M., and Turner, D. L. eds. Final report on the investigation of 

the geothermal energy resources potential of the eastern Copper River 

Basin, Alaska: University of Alaska report RSA82-5X-670, 127p, 2 plates.