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Directory Identifier: GRAVCD-seggrav
Directory Title: SEG US Bouguer Gravity Anomaly Grid
Principal Investigators: Paul L. Lyons and Norbert W. O'Hara

Contributing Organization: Mr. Richard H. Godson
U.S. Geological Survey
Denver, Colorado 80225

Summary: The SEG gravity data are the product of the ad hoc Gravity Anomaly Map (GAM) Committee, sponsored by the Society of Exploration Geophysicists (SEG) and the U.S. Geological Survey (USGS). The committee co-chairmen were Paul L. Lyons and Norbert W. O'Hara.

For a more detailed list of contributors see the references on the map “Gravity Anomaly Map of the United States (exclusive of Alaska and Hawaii), 1982” by the Society of Exploration Geophysicists.

The compilation was developed primarily from the digital files of the Defense Mapping Agency (DMA). Solicitation of data resulted in the release of about 1 million land and 0.8 million marine gravity values previously contributed to DMA and members of the GAM Committee.

The data were screened to eliminate redundant and/or spurious data points. Onshore data consist of Bouguer gravity anomaly values computed using a rock density of 2.67 g/cc. Offshore, where data are less adequately corrected for topography, free air gravity anomaly values were incorporated. Datum control for the gravity computations is based on the International Gravity Standardization Net of 1971 and the 1967 Geodetic Reference System formula for theoretical gravity (International Association of Geodesy, 1971).

Data from areas with substantial relief were terrain-corrected by the USGS. Terrain corrections were calculated at radial distances from station locations ranging from 0.895 km (Hammer zone F) to 166.7 km (Hayford zone O) using a rock density of 2.67 g/cc. The National Geodetic Survey (NGS) of the National Oceanic and Atmospheric Administration (NOAA) made similar terrain-corrected calculations for editorial comparison. The corrected data were then transformed from geodetic coordinates to map coordinates.

A gridding program based on a minimum curvature procedure produced values at 4-km intervals, where non-proprietary data exists, to develop a digital data set suitable for contouring and data manipulation. This data base, covering 95% of the conterminous United States, provides a station distribution of at least 5 minutes. The program, applied through the USGS facilities, used a search radius of 40 km to determine values at grid positions in areas of sparse data. This procedure extrapolates the data out to the search radius around the irregular boundaries of the data. Therefore, contours based on extrapolated values should be used with caution. Station values included in the digital data set were evaluated and computer- edited by DMA, USGS, and NOAA.

Additional information is available.

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