Magnetovariation studies

Abstract

At the Earth’s surface, magnetometers measure the composite of external (from the external source currents) and internal (from the induced currents) field components. The observed magnetic field can be considered as the sum of a normal plus an anomalous induced field. Induced currents in the finitely conducting earth manifest as gradients in magnetic variations over spatial scales of ~10–1,000 km. The interpretation of electrical conductivity anomalies in the crust, provide additional insights into the nature of the Earth’s crust. In the upper crust (<15 km), most conductivity anomalies were correlated with deeper extensions of tectonic elements (faults, sutures) and compositional changes. Simultaneously, the phenomenon of seismic reflectors within the lower crust were found and combined with this, the significant role of fluids in the lower crust/upper mantle came to be recognized. A number of Geotransects across continents were carried out in the 1990s and the complexities of evolutionary history on the lower crust were revealed. The relationship between electrical conductivity, high heat flow, seismic parameters, and tectonic history were formulated (Jones, 1992). Induction studies and later magnetotelluric soundings have contributed to an understanding of processes in the Earth’s crust and upper mantle. This was also possible because of the refinements in instrumentation, data processing, and computational approaches in magnetovariation studies.