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|Title:||Investigations of magnetic field disturbances at Little Rock Air Force Base compass calibration hardstand|
|Authors:||United States. Army. Corps of Engineers. Little Rock District.|
Butler, Dwain K.
Kean, Thomas B.
Air Force Base
Little Rock, Arkansas
|Publisher:||Geotechnical Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Technical report (U.S. Army Engineer Waterways Experiment Station) ; GL-90-16.|
Abstract: Investigations were conducted to determine the nature and cause of magnetic field disturbances at the calibration hardstand (compass rose) at Little Rock Air Force Base (LRAFB), Arkansas. The investigations included the characterization of the total magnetic field strength horizontally and vertically both on and off the hardstand with a proton precession survey magnetometer. Also, the variation of the magnetic field strength with time was determined at selected locations on and off the hardstand with a proton precession recording magnetometer. On the hardstand, relative to a nominal earth's magnetic field strength of 53,000 nanoTeslas (nT) at the site, the field is found to vary by as much as 1,000 nT over tens of feet horizontally and by as much as 600 nT over 8 ft vertically. Also, on the hardstand, the magnetic field is found to vary extremely erratically with time by 50-60 nT over periods of a few seconds. Off the hardstand, the magnetic field is found to be extremely stable, and varies by less than 10 nT over tens of feet horizontally, 8 ft vertically, and over periods of 10-15 min. The magnetic field "stabilizes" at distances less than 25 ft horizontally from the edge of the hardstand. Both on and off the hardstand, the magnetic field variations are independent of the status (on or off) of the nearby ILS System (approach radar). The results of the magnetic field characterization on and off the hardstand, indicate conclusively that the problem is with the hardstand itself, and not subsurface geologic structure, buried utilities or metal debris, or nearby electromagnetic sources as the cause of the magnetic field disturbance. Examination of a piece of the hardstand concrete reveals that the aggregate is igneous (nepheline syenite) with magnetite as an accessory mineral. The permanent magnetization of the aggregate is sufficient to visibly deflect the needle of a compass. Also, the bulk magnetic susceptibility of the concrete is quite large. The permanent magnetization as well as the large magnetic susceptibility suffice to explain the magnetic disturbances at the hardstand. The time variations can be attributed to induced fields, via the large susceptibility, caused by the combined effects of all nearby manmade electromagnetic sources and distant natural electromagnetic disturbances, including magnetic field variations caused by solar flares and sunspots.
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