Investigations of Magnetic Field Disturbances at Little Rock Air Force BaseCompass Calibration Hardstand

摘要

Investigations conducted to determine the nature and cause of magnetic fielddisturbances at the calibration hardstand (compass rose) at LRAFB included the characterization of the total magnetic field strength horizontally and vertically both on and off the hardstand with a proton precession survey magnetometer. The variation of the magnetic field strength with time was also determined at selected locations on and off the hardstand with a proton precession recording magnetometer. On the hardstand, the field is found to vary by as much as 1,000 is found to vary extremely erratically with time by 50-60 nT over a few seconds. 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 with magnetite as an accessory mineral. Permanent magnetization of the aggregate is sufficient to visibly deflect the needle of a compass. The permanent magnetization as well as the large magnetic susceptibility suffice to explain the magnetic disturbances at the hardstand. 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.