Quality control of the initial magnetotelluric data: analysis of calibration curves using a fitting function represented by the ratio of 4th-order polynomials

The research is based on the typical measurement calibration procedure of magnetotelluric soundings (MTS) prior to deep Earth exploration. We suggest the testing and new possibilities of the using MTS equipment calibration. We used two Phoenix MTS station boxes with different magnetic coil sets and mixed sets of coils for verification (check-up) stations calibration and the identity sessions. As a result of the experiments carried out and careful running over sorting of standard functions (exponential, power-law, polynomial), the verification of the proposed method was performed. Eventually this study proposes a “universal” fitting curve after one-station calibration curves to be normalized and are compared to another (etalon) station calibration curves (written in the form of two polynomials ratio). This procedure can be generalized for the case x → f(x), where f(x) is known function. It enables to fit all these 4 typical normalized curves (two for amplitudes (mV) and two for phases (degrees)) by the ratio of fourth degree function containing 9 fitting parameters C0-7 and B0, simply calculated as numeric constants through linear least squares method (LLSM).