1. Method:
Method applied in this research was “The magnetotelluric” (MT). This is an electromagnetic method that measures variations in the earth’s surface electromagnetic fields at different frequencies.
MT method samples the electric and magnetic fields in the audio-frequency range, about 1 Hz to 27 kHz. This method is discussed in detail by Strangway and others [1]. Sources of the AMT signals may be either artificial or natural. Equipment utilized in this investigation is designed for use with natural sources.
Solar energy and lightning cause natural variations in the earth’s magnetic field, inducing electric currents (known as telluric currents) under the Earth’s surface. Earth currents arise from both natural and man-made sources, including the magma in the earth’s core and the normal distribution and use of electricity. Combined, these phenomena create strong MT source signals over the ULF/LF frequency spectrum. The multidirectional MT sounding, performed on different locations can clearly indicate a different ratio of the electric to magnetic field and theirs phase shifting caused by the different speeds of propagation, pointing in that way to different subsurface areas conductivity, i.e. its anisotropy and in fact its structure. Different rocks, sediments, ground fluids and geological structures have a wide range of different electrical and magnetic parameters. Measuring of the resultant fields, and analyzing of theirs ratio allows different materials and structures to be distinguished from one another. In addition, electric and magnetic component phase shifting data allows us to assume a geometry of the detected phenomenon what can improve knowledge of tectonic processes and geologic structures.
Magnetic fields in the frequency range of 1 Hz to approximately 100 kHz, in which we performed our measurements, include the audio-magnetotelluric (AMT) range. These are parallel to the Earth surface and move towards the Earth’s centre. This large frequency band allows for a range of depth penetration from several meters to several kilometers below the Earth’s surface.
Processed AMT data is modeled using various techniques to create a subsurface map, with lower frequencies generally corresponding to greater depth below ground. Anomalies such as faults, hydrocarbons, and conductive mineralization appear as areas of higher or lower resistivity from surrounding structures.
