USGS Data Series 352: Arkansas and Louisiana Aeromagnetic and Gravity Maps and Data

Arkansas and Louisiana Aeromagnetic Compilation

Aeromagnetic anomalies are caused by variations in the Earth's magnetic field that result from the uneven distribution of magnetic minerals (primarily magnetite) in rocks in the upper part of the Earth's crust. The features and patterns of the aeromagnetic anomalies can be used to delineate details of subsurface geology such as the locations of buried faults, the distribution of magnetite-bearing rocks, and the thickness of surficial sedimentary rocks (which are generally nonmagnetic). This information is valuable for mineral exploration, geologic mapping, and environmental studies.

This Arkansas and Louisiana aeromagnetic compilation is one part of a national digital compilation by the U.S. Geological Survey (USGS). Certain characteristics are common to all of the State compilations. Individual surveys that make up this compilation were typically flown either at a constant elevation above sea level or draped to a constant mean terrain clearance. The standard elevation datum selected for this national compilation is a survey elevation of 304.8 m (1,000 ft) above mean terrain. To conform with the national standard, the entire State grid was analytically continued upward to 305 m (1,000 ft) above ground.

The Arkansas and Louisiana aeromagnetic map in this report is constructed from grids that combine aeromagnetic data (see data processing details) collected in 14 separate aeromagnetic surveys flown between 1948 and 1980. The data from these surveys are of varying quality. The design and specifications (terrain clearance, flight line separation, flight direction, analog or digital recording, navigation, and reduction procedures) may differ between surveys depending on the purpose of the project and the technology of that time. Some of the survey data are available only on hand-contoured analog maps and were digitized. These maps were digitized along intersections of flight lines and contour lines, which is considered to be the most accurate method of recovering the original data. Nonproprietary digital data are available as USGS Open-File Report 02–361 (USGS and National Geophysical Data Center (NGDC)), 2002). All surveys have been continued to 305 m (1,000 ft) above ground and then blended or merged together. Grids and images were created using a personal-computer version of Geosoft/OASIS montaj software. An index map for Arkansas and Louisiana and a data table give an overview of the original surveys and summarize the specifications of the surveys. The resulting grid has a data interval of 1,000 m (3,300 ft) and can be downloaded. A color-shaded relief image of the grid is shown on the opening page of this web report.

This grid is an interim product. Where local high-resolution surveys were not available in either digital or digitized format, we used aeromagnetic data collected by the National Uranium Resource Evaluation (NURE) program of the U.S. Department of Energy, which are in digital format and cover the entire State. However, because magnetic surveying was not the primary objective in the design of the NURE surveys, these data are subject to certain limitations. Although the NURE surveys were flown at elevations close to the reduction datum level, the spacing between flight lines ranged from 4.8 to 9.6 km (3 to 6 mi). In some areas of the United States, detailed NURE surveys were flown with a finer line spacing, usually at a 0.4 km (0.25 mi) interval.

The northeastern part of Arkansas contains the geologically important area of New Madrid and the Mississippi Embayment, and many aeromagnetic compilations and interpretive maps of this area have been published during the past decades. Most of these compilations are reworked versions of the five earliest surveys. These surveys are numbered 16, 1021, 2223, 6164, and AR02 and serve as the primary data sets used in this report.

The earliest survey, 16, was flown in 1948 but was unpublished until 1964 (Jesperson, 1964). Even though the survey is quite old and used a first-generation magnetometer, the detailed flight-line spacing (0.25–0.5 mile) makes this survey the most accurate over this area.

Survey 1021 (U.S. Geological Survey, 1974) is the Arkansas portion of a larger 1973 survey that extends into Missouri, Kentucky, and Tennessee. Survey 2223 (U.S. Geological Survey, 1976) was a companion survey to 1021. The original digital data were not available, and these data were digitized from the published map. Digitized data for survey 1021 are available online (U.S. Geological Survey, 1999).

Survey 6164 is part of the Blytheville NURE quadrangle survey (EG&G GeoMetrics, 1980g). Other NURE surveys in northeastern Arkansas that cover the Memphis and Poplar Bluff quadrangles were superseded in this compilation by the better data of the surveys discussed above.

Survey AR02 was flown in 1977 by Applied Geophysics, Inc., for the Arkansas Geological Commission and the U.S. Nuclear Regulatory Commission. This survey was never formally published; it appears in a 1979 compilation by Hendrix and Hildenbrand.

USGS Open-File Report 77–229 (Hildenbrand and Johnson, 1977) contains an early compilation of surveys 16, 1021, and 2223. An interpretive map and discussion of this compilation was published as USGS Miscellaneous Field Studies MF–914 (Hildenbrand and Kane, 1977). USGS Open-File Report 79–1208 (Hendrix and Hildenbrand, 1979) added survey AR02 to surveys 16, 1021, and 2223. These four surveys make up the Arkansas portion of USGS Miscellaneous Field Studies MF–1158 (Hildenbrand, 1979) and were interpreted in USGS Geophysical Map GP–948 (Hildenbrand, Kucks, and Johnson, 1981). Areas AR01 and AR03 were added to USGS Open-File Report 79–1208 in the larger compilation of USGS Open-File Report 81–758 by Hildenbrand, Hendrix, and Kucks (1981).

A detailed 1992 survey (U.S. Geological Survey and National Geophysical Data Center, 2002; available digitally in that online report) duplicates part of survey 2223. Although the flight-line spacing of the 1992 survey was a relatively detailed 0.25 mi, a visual comparison of survey 2223 and the 1992 survey determined that no new information was expressed at the scale and grid spacing of this report’s compilation. The methodology currently used to merge aeromagnetic data causes erroneous boundary effects that are magnified where survey edges are not north-south or east-west. Because the detailed survey covers a diagonal swath and has only two short east-west edges, and because we would have to degrade the data to a coarser resolution and a higher flight altitude for merging, we did not include it here.

This project was supported by the Mineral Resource and National Cooperative Geologic Mapping Programs of the U.S. Geological Survey. Thanks to colleagues Robert Kucks and Pat Hill for their assistance in preparing this report.