Sunday, March 29, 2009

USGS Digital Data Series DDS-9

National Geophysical Data Grids:
Gamma-Ray, Gravity, Magnetic, and Topographic
Data for the Conterminous United States


The data published in the \ASCII directory on this CD-ROM consist of regular grids of ASCII values.Each grid has a different origin and sample spacing as defined in its first record, the header.The grid format is explained in detail below.Each grid represents data that have been projected from latitude and longitude coordinates into map coordinates of kilometers.Thus, the x-origin and y-origin values given in the header represent the distance in kilometers from the central meridian and base latitude of the geographic projection (discussed below) to the lower left corner of the grid.
Binary versions of these grid files, suitable for processing on IBM or compatible personal computers using the potential-field software contained on this CD-ROM, are located in the \DOSBIN directory on this CD-ROM.Use the EXTRACT BINARY GRID FILES submenu to access these grid files.For more information on the binary grid file format and the potential-field software, access the POTENTIAL-FIELD SOFTWARE submenu from the MAIN menu.
Projection information

All grids on this CD-ROM have been projected by using an Albers equal-area conic projection with standard parallels of 29.5 degrees and 45.5 degrees north.A central meridian of 96 degrees west and a base latitude of 0 degrees were used.The projection is referenced to the Clarke 1866 ellipsoid, which has an equatorial radius of 6378.2064 km and a polar radius of 6356.5838 km.
Grid format

Each ASCII grid consists of two header records followed by a series of data records.The first header record contains 56 alphanumeric characters of data identification, 8 alphanumeric characters containing the name of the computer program that created the grid, the integer number of columns in the grid, and the integer number of rows in the grid.The second header record contains a dummy integer value of 1, the x-coordinate in kilometers of the first (leftmost) column, the distance in kilometers between columns of the grid, the y-coordinate in kilometers of the first (bottommost) row of the grid, and the distance in kilometers between rows of the grid.

Each data record contains five values in scientific notation.The bottommost row is presented first, starting from the leftmost column.The first value in each row is a dummy value, usually zero (0.000000000E+00). If a row ends in the middle of a record, the record is padded with zeros. Areas of the grid containing no data are represented by a special value, 0.999999968E+38.

The gamma-ray data grids

Aerial gamma-ray surveys measure the gamma-ray flux produced by the radioactive decay of the naturally occurring elements K-40, U-238, and Th-232 in the top few centimeters of rock or soil (Duval, Cook, and Adams, 1971).If the gamma-ray system is properly calibrated (for example, see Grasty and Darnley, 1971), the data can be expressed in terms of the estimated concentrations of the radioactive elements.The potassium concentration data are usually expressed in units of percent potassium (percent K), uranium as parts per million equivalent uranium (ppm eU), and the thorium as parts per million equivalent thorium(ppm eTh).The term equivalent is used because the technique actually measures the gamma-ray flux from the decay of bismuth (Bi-214), which is a decay product of U-238, and from the decay of thallium (Tl-208), which is a decay product of Th-232.Radioactive disequilibrium in the thorium decay series may cause the measured equivalent uranium and equivalent thorium to differ from the actual uranium andthorium present in the surface rocks and soils. Because Rn-222 is a daughter product of the U-238 decay series, the U-238 concentrations can also be used to estimate the amounts of Rn-222 in the near-surface soil gas.

During the period 1975-83, the U.S. Department of Energy carried out the National Uranium Resource Evaluation (NURE) Program, which included aerial gamma-ray surveys of most of the conterminous United States.Although many of the airborne gamma-ray systems used to make these surveys were calibrated, many of the earlier surveys were done without calibration and conversion to the concentrations of the radioactive elements.Detailed examinations of the digital data available on magnetic tape also showed that many of the "calibrated" surveys do not match the data from other "calibrated" surveys of adjacent areas.For these reasons, the data must be corrected to obtain a consistent data base for the conterminous United States.
Because uranium, thorium, and potassium concentration data are useful in geologic studies and because the NURE data are the only nationwide data base on the natural radiation environment, the U.S. Geological Survey (USGS) reprocessed the aerial gamma-ray data to produce maps showing surface concentrations of potassium, uranium, and thorium for the conterminous United States.These maps have been released as USGS Open- File Reports (Duval and others, 1989, 1990).Some of the reprocessed data have also been released in profile form (Duval, 1995).

The magnetic anomaly data grid

Magnetic anomalies are produced by variations in the distribution of iron minerals, usually magnetite, in the rocks of the Earth's crust.Igneous and metamorphic rocks can be very magnetic.By comparison, sedimentary rocks are usually nonmagnetic.Magnetic anomalies therefore provide a way of mapping exposed and buried crystalline rocks.
The grid of magnetic anomaly data for the conterminous United States and adjacent marine areas (Godson, 1986) was created from digitized contours of the east half of the Composite Magnetic Anomaly Map of the United States, Part A (U.S. Geological Survey, 1982), and the Composite Magnetic Anomaly Map of the Conterminous United States West of 96 Degrees Longitude (Bond and Zietz, 1987),with additional data used in the compilation of the Magnetic Anomaly Map of North America (Geological Society of America, Committee for the Magnetic Anomaly Map of North America, 1987).A regional gradient present in the 1982 map was removed by using a corrected geomagnetic reference field (Godson, 1986).The data, originally gridded on a 2-km interval using the spherical Transverse Mercator projection of the Magnetic Anomaly Map of North America, were reprojected to the Albers projection used on this CD-ROM and regridded on a 2-km interval using a minimum curvature gridding program (Webring, 1981).An interpretation of the 1982 anomaly map was presented by Hinze and Zietz (1985).

The Bouguer gravity anomaly data grid

Gravity anomalies are produced by density variations within the rocks of the Earth's crust and upper mantle.Mapping of these density variations is the primary use of gravity anomalies.
Gravity measurements made on the surface of the Earth must be corrected in various ways before they can be made into an anomaly map.The free-air correction reduces the measurement to sea level by assuming that there is no intervening mass.The simple Bouguer correction accounts for the intervening mass as a uniform slab of constant density, and the complete Bouguer correction includes the effects of constant density topography within 166.7 km of the measurement location.A gravity reference field is subtracted from the corrected measurements to produce the free-air, simple Bouguer, or complete Bouguer anomaly.
The grid of gravity data for the conterminous United States and adjacent marine areas (Godson and Scheibe, 1982; Godson, 1985) was constructed from Defense Mapping Agency gravity data files.The onshore data consisted of nearly one million Bouguer gravity anomaly values computed by using a reduction density of 2.67 grams per cubic centimeter.The offshore data consisted of approximately 800,000 free-air gravity anomaly values. Because the Bouguer anomaly equals the free-air anomaly at sea level, there is no discontinuity in the gridded data at the shoreline.All computations were performed by using the International Gravity Standardization Net of 1971 (International Association of Geodesy, 1974) and the 1967 Geodetic Reference System formula for theoretical gravity (International Association of Geodesy, 1971).
In areas of substantial relief, terrain corrections were computed about each station location at radial distances of 0.895 km to 166.7 km by using a density of 2.67 grams per cubic centimeter.The data were projected and gridded on a 4-km interval using minimum curvature (Webring, 1981).The gridding procedure resulted in the extrapolation of grid values up to 40 km beyond the limits of the data; therefore, values around the edges should be viewed with caution.These gridded data were published in map form as the Gravity Anomaly Map of the United States (Society of Exploration Geophysicists, 1982).This map was further discussed by O'Hara and Lyons (1985) and Kane and Godson (1985, 1989).

The isostatic residual gravity anomaly data grid

Isostatic residual gravity anomaly maps are produced by subtracting long- wavelength anomalies produced by masses deep within the crust or mantle from the Bouguer anomalymap.The long-wavelength anomalies are assumed to result from isostatic compensation of topographic loads.Isostatic residual gravity anomaly maps therefore reveal more clearly than Bouguer anomaly maps the density distributions within the upper crust that are of interest in many geologic and tectonic studies.
The grid of isostatic residual gravity anomaly data (Simpson and others, 1986) was produced from the grid of Bouguer gravity anomaly data (Godson and Scheibe, 1982) by using an Airy-Heiskanen compensation model (Heiskanen and Moritz, 1967) with three parameters.The depth to the compensating root at sea level was chosen to be 30 km.The density contrast across the root was chosen to be 0.35 grams per cubic centimeter, and the density of the topography was chosen to be 2.67 grams per cubic centimeter.Other reasonable choices of these parameters would produce similar-looking residual maps.
The computer program and topographic data sets used to produce the data
grid were described by Simpson and others (1983a,b). The data were
published in map form by Jachens and others (1985).Interpretations of the
isostatic residual gravity anomaly map were presented by Simpson and others
(1986) and by Jachens and others (1989).

The topographic data grid

The topographic data grid for the conterminous United States and adjacent areas was constructed from 30x30 second digital terrain files used by the U.S. Geological Survey for the reduction of gravity data.Elevations are in meters; sea level elevations are listed as 1 meter.

The topographic-bathymetric data grid

The topographic and bathymetric data grid for the conterminous United States and adjacent areas was constructed from 5x5 minute North American topographic data and 5x5 minute Synthetic Bathymetric Profiling System data available from the National Oceanic and Atmospheric Administration.The data were interpolated onto an 8x8 km grid in the Albers projection (Simpson and others, 1983b).Elevation units are in meters relative to sea level.

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