explain magneto-telluric method

telluric current method

magnetstriction effect

The direction of magnetic field at any point can be found by using

The physical principle behind the micromagnetic method based on the Barkhausen noise is that every ferromagnetic material being magnetised will consist of small magnetic domains. Because of an external magnetic field, movement of Bloch walls of magnetic, i.e. Weiss, domains occurs, which produces changes in their size. The changes of size of the magnetic domains having the same orientation produces abrupt increase in magnetic flux density, which is reflected in induced voltage shocks in the measuring coil as a time-varying voltage signal. Ferromagnetic materials may show different microstructure states or may be subjected to various external mechanical stresses, which is reflected in the process of directing of magnetic-domain growth, i.e., in the voltage signal. The effects of complementary influences in the material such as microstructure, microhardness, residual stresses and dislocation density, produce the differences in the captured Barkhausen-noise (BN) voltage signals. Various methods of further processing of the captured BN voltage signals permit us to establish relations between the micromagnetic parameters obtained with the state of the analysed ferromagnetic material. Individual characteristics are determined on the basis of calibration curves which indicate a relationship between the known state of the material and the selected characteristic of the captured voltage signal.The calibration curves are determined by changing external mechanical loads applied to the samples with known properties, i.e., etalons. The external loads applied to the etalons, which may be uniaxial or biaxial, produce various stress conditions in the elastic domain of the material. Uniaxial calibration curves are those which are obtained by taking into account uniaxial stress conditions and mostly used for characterisation of residual stresses [1,2]. Lately some authors proposed a biaxial calibration curve elaborated on the basis of biaxially stressed etalons [3]. A comparison of experimentally obtained biaxial mechanical loads, i.e., stresses, which were determined on the basis of the captured magnetic BN signals, and of the calculated stress conditions with different external biaxial loads applied to the etalon showed that deviations are very small. These small differences between the calculated mechanical stresses and the measured ones, i.e., those determined from the BN voltage signal, indicate good reliability of the micromagnetic method when applied to determination of the size and variation of load stresses applied to etalons. The latter include also residual stresses in the unstressed condition and exist also when external mechanical loads applied to the etalon vary. That is to say that the micromagnetic method permits a similar determination of the size and variation of residual stresses in the thin surface layer of the material which have a decisive influence on the life of machine parts and products. Numerous researchers studied the influence of dislocation density on the changes of amplitude values of the captured voltage signals. They found that an increase in dislocation intensity produces an increase in the number of outbursts due to orientation of magnetic domains, which affects the size and variation of the amplitude values of the BN voltage signal [7]

explain where you will need to position the magnet to observe the first and second harmonics.