Effects of cultural noise in natural magnetotelluric signals

Magnetotelluric method is an electromagnetic induction method which studies the dis- tribution of the electrical conductivity in the Earth by analyzing natural variations of the Earth's electromagnetic field of the external origin. A wide frequency range of the natural sources of the magnetotelluric field makes it possible to study the Earth's conductivity from near-surface structures down to depths of the upper mantle. Dependence on weak natural fields, however, causes the method to fail in case of data contaminated by electro- magnetic disturbances of cultural origin. This thesis summarizes basic principles of the statistical magnetotelluric data processing in the frequency domain, and shows the recent progress of the processing due to the application of robust statistical methods as well as due to employing reference data from remote stations. In some cases, the measurements are disturbed excessively, and even advanced statistics method fail in processing the data. In some of these cases a more thorough analysis of the noise field in terms of its directional and source characteristics may be useful. We present an example of simultaneous recor- dings of two magnetotelluric stations from the West Bohemian seismo-active region and show that the noise field is dominated by two types of strong electric..