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MT & AMT

Overview

Magnetotellurics (MT) and Audio-frequency Magnetotellurics (AMT) are electromagnetic survey techniques that deploy (as passive energy sources) electromagnetic field of naturally-occurring ionospheric current and lightning storms. MT and AMT methods allow mapping geologic structures at depths of several km or more.

MT technique combines measurements of the Earth’s electric and magnetic fields over a wide range of frequencies. Low frequencies “go deep” into the Earth and high frequencies correspond to shallow samples.

Principles

Natural source signals are generated in the atmosphere and magnetosphere. MT signals (with frequencies that are less than 1 Hz) are generated by the interaction between the Earth’s magnetosphere and the solar wind, sunspot activity, and auroras. High-frequency sources (> 1 Hz) in the audio range (AMT) are generated by thunderstorms and lightning worldwide.

These time-varying electric and magnetic fields induce currents into the Earth and oceans. These currents produce magnetotelluric signals that are measured over a range of frequencies using grounded dipoles and magnetic field antennas. Ground resistivity values are then calculated from the MT measurements.

ABOUT THE UGC

We specialize in 1D / 2D / 3D / 4D land, marine and borehole geophysical surveys. We support our clients on all stages of research – from reconnaissance to high-resolution and from near surface to deep.

Applications

MT surveys are typically deployed for investigating deep structures. For example, geothermal systems and traps of hydrocarbons deposits typically exhibit electrical resistivity contrasts at depths of several kilometers i.e. depths that are accessible for MT but not for other electromagnetic methods. The ability to identify geologic features using this method varies with depth, target size, the resulting resistivity contrasts, and their geometry.

Equipment

Among the numerous types of magnetic field sensors that are used for geophysical applications, induction sensors of VEGA Geophysics are distinguished by their simplicity of design, manufacturability, availability of materials and operational reliability.

The VMTU-10 station is designed for MT and AMT soundings. The station registers in digital form three mutually orthogonal components of the alternating magnetic field and two components of the electric (telluric) field in a wide frequency range: from 0.0005 to 10 000 Hz.

The station is used for MT sounding, as well as for MT and AMT profiling during geological exploration at depths ranging from the first hundreds of meters to several kilometers. The station can also be utilized for exploration with artificial sources of electromagnetic fields.