Geomagnetic Field Variations Caused by the Processes Occurring at Different Depths in the Earth’s Crust and Upper Mantle

Abstract—The results of geomagnetic studies in the Tashkent and Fergana geodynamic test sites, Uzbekistan, are presented. Detail analysis of anomalous geomagnetic variations caused by the processes occurring at different depths in the Earth’s crust and upper mantle is conducted. By their linear size, variations are divided into regional (100 km and larger) and local (1 to 5 km and larger). Regional anomalous variations manifest themselves by sinusoidal, bay-like, and trend forms and last one to seven years. In the trend-type variations, the gradient is 0.5–2.0 nT/yr. Local anomalous variations typically have sinusoidal and bay-like shape. Bay anomalies have both positive and negative sign, and the change is reversible. The intensity of the anomalies ranges from 2 to 25 nT and the duration is from 10–15 days to three years or even longer. The local anomalous variations are neither precursory nor technogenic. Numerous anomalies of this type were for the first time revealed at the geodynamic test sites in Uzbekistan. The analysis of the data for the past 40–50 years has shown the absence of information on this type of anomalous variations. They can neither be explained by conventional mechanisms responsible for secular behavior of the geomagnetic field, nor by lithologic composition of the rocks, tectonics fetures, hydrogeological conditions, and geophysical fields of the studied territories. One of the peculiarities of local anomalous variations of this type is that they can change in time, in shape, in size of the area, in intensity, and in sign. The published data relevant to the anomalies in the secular variation of the geomagnetic field have been analyzed. However, unfortunately, these data provide almost no information on the origin of local anomalous variations discussed in our work.

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