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Distributions of Magnetic Field Variations, Differences and Residuals

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Temporal and spatial variations of the geomagnetic field were recorded in different geographic areas, using arrays of seven tri-axial magnetometers which resided on the sea bottom in 50-150 m depth for periods up to 16 days. The different geographic areas are characterized by different levels of the mean ambient magnetic noise. We discuss briefly data from a magnetically quiet area and then focus on a coastal zone highly disturbed by anthropogenic magnetic fields, in particular by noise from a busy coastal electrified railway line. In the latter area, the spatial uniformity of the ambient magnetic field is rather poor and the correlation between adjacent sites lower than in the former one. Simultaneous magnetic field observations from pairs of neighbouring magnetometers are analysed and compared in detail. We compute first vector differences of the magnetic variations measured at neighbouring sites, and subsequently vector residuals. The residuals are those contributions to the magnetic field variations which are not correlated between adjacent sites and which can not be represented through a linear trivariate model. In the magnetically quiet area, we find that the magnetic field residuals are of the order of the system noise for magnetometers with about 1 km spacing and slightly higher for those with 12 km spacing. The area leads support to the application of a Remote Reference Technique, i.e., a technique in which the ambient magnetic field is compensated for by using measurements from a remotely operated reference sensor. Remote means that the reference sensor is sufficiently far away so that it is not influenced by the magnetic field of a ship navigating in the vicinity of the surveillance magnetometers. Although the magnetic field variations in the highly disturbed coastal zone are several times larger than typical magnetic field variations of natural origin, and also spatially non-uniform, we find good linear correlation between adjacent sites.

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  • Magnetic and Electric Field Detection and Detectors
  • Geomagnetism
  • Undersea and Antisubmarine Warfare

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