The magnetotelluric (MT) method and its high-frequency version, audio-magnetotelluric (AMT), is a natural source electromagnetic (EM) technique that involves measuring fluctuations in the natural electric and magnetic fields in orthogonal directions at the surface of the Earth as a means of determining the conductivity structure of the Earth at depths ranging from a few tens of metres to several hundreds of kilometres.
The method uses the natural time variation of the Earth’s magnetic field as a source of signals. Electromagnetic fields that are naturally induced in the Earth and are exploitable for MT studies have wave periods ranging from 10-3 to 105 seconds. Short-period signals are generated by interactions in the Earth–ionosphere waveguide, whereas longperiod signals are generated by solar wind–magnetosphere interactions.
The MT method does not pose any damage to the environment since it does not generate any type of artificial signal and the stations can be installed without clearing of vegetation.
Time Domain Transient Electromagnetic Method (TDEM) uses a man-made electric current as source. This electric current is in the form of square intermittent pulses and it is applied on the surface of the earth, where the induced magnetic field is measured.
The simulation of an electromagnetic wave propagating through an oil(resistive)/water(conductive) contact model in the movie shows how the EM method can help in the exploration and characterization of reservoirs (Kriegshäuser, 1997)
The gravimetric method aims to measure variations in the terrestrial gravitational field, caused by the variation of subsurface densities. It is based on Newton’s law of gravitational attraction between masses. In oil prospecting it is used to delimit lateral variations of density that can favor oil and gas accumulations, such as structures and salt domes. This method can also be used for basement depth estimates and associated structures.
Magnetic prospecting is a geophysical method based on the examination of magnetic field anomalies of the Earth caused by non-similar magnetization intensity of rocks.
Physical and geological prerequisites of use of this method are based on magnetic dissimilarities of rocks, which form geological structures in the Earth crust. The magnetic field of the Earth magnetizes rocks to a variable degree which is determined by their magnetic susceptibility, intensity and magnetizing force.
The radiometric, or gamma-ray spectrometric method is a geophysical process used to estimate concentrations of the radioelements potassium, uranium and thorium by measuring the gamma-rays which the radioactive isotopes of these elements emit during radioactive decay.