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Open-File Report 2009-1129

Aeromagnetic and Aeroradiometric Data for the Conterminous United States and Alaska from the National Uranium Resource Evaluation (NURE) Program of the U.S. Department of Energy

By Patricia L. Hill¹, Robert P. Kucks¹, and Dhananjay Ravat²

¹U.S. Geological Survey
²University of Kentucky, Lexington, KY

Abstract

The National Uranium Resource Evaluation (NURE) program was initiated in 1973 with a primary goal of identifying uranium resources in the United States. The airborne program’s main purpose was to collect radiometric data of the conterminous United States and Alaska. Magnetic data were also collected. After the program ended, most of the data were given to the U.S. Geological Survey (USGS).

All areas were flown at about 400 feet above ground, the optimum height for collecting radiometric data, and the line spacing varied from 3 to 6 mile intervals. A few selected quadrangles or parts of quadrangles were flown at 1- or 2-mile line spacing. About forty smaller areas were targeted and flown at 0.25-mile to 1 mile line spacing.

Brief History of the NURE Airborne Program

The National Uranium Resource Evaluation (NURE) program was initiated by the Atomic Energy Commission (AEC) in 1973, with a primary goal of identifying uranium resources in the United States. When the AEC was abolished by an act of Congress on Oct. 11, 1974, the NURE program transferred to the newly created Energy Research and Development Administration (ERDA). On Aug. 4, 1977, Congress terminated ERDA. All its functions, including the NURE program, were transferred to the new Cabinet-level Department of Energy (DOE). Most of the program functions ended in the early 1980s, and most of the data were given to the U.S. Geological Survey (USGS).

The airborne program’s main purpose was to collect radiometric data over the conterminous United States and Alaska, although not all of Alaska was surveyed by the program's end. The data were used to locate potential radioactive mineral deposits. Magnetic data were also collected. In the first years of the program, seven large areas were flown, and then the program began to systematically cover each 1° by 2° topographic quadrangle. The data for several early surveys are not available. In a few cases, some data from the early surveys have been merged with quadrangles that were flown later.

All areas were flown with either an airplane or helicopter at about 400 feet above ground, the optimum height for measuring radiometric data. The flight-line spacing usually varied from 3 to 6 mile intervals. A few selected quadrangles or parts of quadrangles were flown at 1- or 2-mile line spacing. Later, about forty smaller areas were targeted as promising for radioactive deposits and reflown at 0.25-mile line spacing. These surveys are labeled as detail areas. A few of these early detail areas were flown at 1-mile line spacing.

Original Aeromagnetic Data Processing by the NURE Contractors

Projects flown 1974–1975

Magnetic data were of little interest in the beginning of the program, and the earliest projects flown in 1974 and 1975 had little, if any, discussion on the aeromagnetic data processing. In general, for the first 13 projects, no corrections were applied to the magnetic data other than eliminating obvious data spikes. Diurnal magnetic information was not collected. When Bendix Corp., the DOE contractor for the NURE Program, began converting data to their standard format, they calculated the missing residual-field magnetic values. These calculations were done at a later date and are not documented.

Texas Instruments (T.I.) flew some of the early Alaska surveys. They reported using the ground base-station magnetometer data from College Station in Alaska simply to check for magnetic storms. For one Alaska area, they set up a ground magnetometer station and, again, only used it to check for unusual magnetic activity. They state they made no diurnal corrections for any of these Alaskan surveys.

In the early T.I. radiometric data files for Alaska, values labeled 'resmag' (residual field magnetic data) are most likely 'totmag' (total field magnetic data). In two cases where only residual field magnetic values ('resmag') were given, the USGS changed this to total field magnetic values ('totmag') because it was apparent that the International Geomagnetic Reference Field (IGRF) had not been removed. For some other early surveys flown by T.I., we are fairly sure the IGRF was removed at a later date. For example, in data from southeastern Arizona, the IGRF was removed when the data flown in late 1974 were merged with surveys flown in late 1977.

Geodata International (Geodata) flew several surveys in 1975, and their reports had no information about their magnetic processing. Both 'totmag' and 'resmag' were present in the data and, therefore, we assume an unspecified IGRF was removed. Most likely, the magnetic data were first corrected by line leveling before the IGRF was removed. If Geodata collected ground magnetic data for these surveys, they only used it to monitor any unusual magnetic activity that would require reflights. In addition, the data for some of their regional surveys are missing.

Projects flown 1976–1981

T.I., LK&B Resources, Carson Helicopters, and sometimes Aero Service Corp. did not collect ground magnetometer data. We speculate that they may have checked one of the nearby magnetic monitoring centers for any significant magnetic activity. Geodata reported collecting ground magnetometer data but did not include diurnal values in their magnetic data sets. In this case, we speculate that their ground system may have used a chart recorder rather than a digital one and that they chose not to digitize that data.

The most common method reported for correcting the raw magnetic data was to select one tie line and level all the flight lines to it. Those surveys that didn’t collect digital diurnal data used only that correction method. Those that had available digital diurnal data used a combination of removing the diurnal and line leveling. Often this is not clearly stated in the reports. Frequently the magnetic data exhibit poor leveling.

Only a few companies specified which IGRF model they used to calculate the residual magnetic values. Many years ago, USGS scientists compared similar surveys flown for the USGS with those flown for the NURE program in the same time frame by the same contractor. From this, we made a few intelligent guesses as to which model year was most likely used by that company. When the specific IGRF model used was in doubt, we used that information in this report. If we actually calculated a new residual magnetic data column for this report, we used the 1975 IGRF model. By 1978, most contractors had switched to the 1975 model.

Cross-Country Tie Lines flown 1980–1981

Near the end of the project, two cross-country tie lines were flown to identify leveling issues for some of the individual quadrangle surveys. Only magnetic data are included in the files for this report; the radiometric data are not available.

The north-south line was flown by Aero Services, Inc., in 1980 using a fixed-wing airplane flying 400 ft above terrain. The line was flown at –99º longitude from latitude 26.3º to 49º in three days and covered 1,555 mi. A ground base magnetometer was not used to record diurnal changes.

The east-west line was flown by High Life Helicopters in 1981 using a helicopter flying 300 ft above terrain. The line was flown at 39.5º latitude from longitude –123.8º to –74.3º over 8 days and covered 2,626 mi. A ground base magnetometer was not used to record diurnal changes.

The overall quality of these tie lines is only fair and no further work was done on these data.

Subsequent File Format Development and Data Manipulation by the USGS

The airborne data collected from the NURE program were originally delivered in a variety of formats on magnetic tapes. These data were given a unique USGS number assigned to either a single quadrangle or to a group of quadrangles flown at the same time with the same specifications by the same contractor. Later, these numbers were further augmented using A, B, C, and so forth, so that each quadrangle has a unique designation. In addition, if the flight-line spacing varied within a quadrangle, a separate number or letter was assigned to each part.

Two separate sets of tapes were obtained: one set was called 'magnetic files' and the other 'reduced files.' The magnetic files contain both the total field magnetic data and the residual field magnetic data, which is the total field data with the proper IGRF removed. The reduced files contain the residual field magnetic data and all radiometric element data. Both sets include positional information.

Some magnetic files contained only total field magnetic data or residual field magnetic data, but not both. The missing field was calculated from the given field using the appropriate IGRF. If the residual field magnetic data were recalculated in the magnetic file, they were not corrected or inputted into the reduced file.

Over the years, various members of the USGS have worked with these data, especially the magnetic data. Significant effort had been put in by the primary DOE contractor, Bendix Corp., to convert all of the data into their standard format, but about 25 percent were still in the original format when the USGS acquired the tapes. One USGS standard format was developed for the magnetic data and another for the radiometric data, and the data were converted and exported using the proprietary software Geosoft Oasis Montaj. As a rule, the earlier the data were collected, the more difficult it was to download from the original tapes.

If certain recoverable fields such as year and day were missing, they were added using information obtained from the final DOE reports housed in the Denver USGS Library. If the actual day was unknown, the day that flying began or the first day of the month was chosen. Time of day was supposed to be recorded as Greenwich Mean Time, but it is unclear whether or not all the contractors conformed to this requirement. For other missing, unrecoverable data, dummy values of –9999.9,–999.9, –99, and so forth, were added. Records containing no data in all fields were deleted.

Other than to convert the data to a standard format and fill in dummy values where needed, very little value-added work was done on the radiometric files. The magnetic and radiometric files were compared only on a cursory basis.

The 'geology' field was retained in both data sets. In most cases, a copy of the final DOE report is needed to make sense of the geology codes.

In recent years, these magnetic data have been merged into larger regional and continental compilations (for example the North American Magnetic Map). The residual magnetic data that used IGRF/DGRF values do not mesh well at the survey boundaries because of leveling artifacts and, at small scales, display long wavelength errors caused by warping of hundreds of state and local aeromagnetic surveys during the merging process. To minimize these problems, a temporally continuous main field called the Comprehensive Model (CM4) was developed. For the conterminous U.S. quadrangles (but not the Alaska quadrangles, detail areas, or miscellaneous areas), residual magnetic values were recalculated using CM4 and are included in the magnetic files in an additional field labeled 'resmagCM4.' See the metadata files for references on this topic.

Metadata files were prepared for each quadrangle and detail area, and for five miscellaneous areas. The latter includes the north-south and east-west tie lines and some of the earliest, larger area surveys. The metadata files contain descriptions of the standard formats for the magnetic and radiometric data sets. The link in each state page called "Files and Formats" also contain the formats for the data sets.


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