e based on electrical methods, magnetotellurics, transient electromagnetics, and other electromagnetic methods. The source of the electromagnetic field in these methods is located either immediately at the Earth surface or above it. Methods using external sources to monitor tectonic processes are classified as active methods (Svetov 1992). Alternatively, passive methods of monitoring rely on measurement of electric and magnetic fields caused by volcanic or tectonic processes inside the Earth. A number of electrokinetic, piezoelectric, thermoelectric, and other physical and chemical phenomena can be responsible for generating electrical currents in the interior of a volcano. Electric and magnetic fields induced by these currents are known to be measurable. Electrical structure of the Earth near a volcano is always laterally heterogeneous. Interpretation of measurements made in such conditions is a challenging task. On the other hand, as shown by Fitterman (1979) and will be further discussed in this publication, the magnetic field of the internal origin is often observed at the Earth surface only because the Earth is electrically heterogeneous. The magnetotelluric field caused by ionospheric and magnetospheric currents obscures the internally generated field. A significant suppression of the magnetotelluric noise can be achieved by using linear relationships between components of the electromagnetic field at different locations on the Earth surface. As shown by Svetov (1992), a reduction of the magnetotelluric noise by the orders of magnitude can be accomplished by finding the transfer functions that relate the electromagnetic field at an observation site to the magnetic field at the reference site (base), and using these transfer functions to forecast the magnetotelluric noise at the observation site from the magnetic field measured at the base during the monitoring periods. In this approach, the transfer functions are determined during "seismically quiet" periods. During the “monitoring” periods, the transfer functions are used to eliminate most of the magnetotelluric noise from the signal recorded at the monitoring site. The sites must satisfy rather different conditions. The monitoring site must be located in the area where the signal of the anticipated internal source is sufficiently strong to be registered. The reference site should satisfy the exactly opposite requirement, i.e. the field of the internal source should remain at a negligible level even during periods of seismic activity. On the other hand, the monitoring and reference sites must remain within the coherency distance of the magnetotelluric field. Both the internal and magnetotelluric fields are scattered by geoelectric heterogeneities. It can be expected that features of the subsurface conductivity distribution, relief, proximity to the seashore, as well as presence of active faults in the Earth crust may have a direct effect on the level and structure of the internal and magnetotelluric fields. The main purpose of this publication is to demonstrate on the example of volcano Etna that modern approaches to 3D modeling, including the so-called generalized thin sheet modeling, allow for a reliable simulation of effects of lateral heterogeneities on the field of the internal source, thus allowing for developing recommendations for the optimal allocations of the observational and reference sites.


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