The Study Of The Impact On The Magnetotelluric Data From The Static Shift And The Correction Method

The MT is a kind of electromagnetic sounding methods used for the study the deep geological structures in the frequency domain, of which the field resource is the natural changeable electromagnetical field which is widely distributed in the nature and has the very broad frequency range (10^(-4)-10^(4) H z).Like the other frequency domain electromagnetic sounding methods, the MT measured data are always subject to the static shift distortion. The static shift effect will make negative impact on the data in the whole frequency range and then the subsequent inversion results and the geological interpretation will have a large deviation with the actual situation.The researchers consider that the main mechanism of the generation of the static shift effect is the distortion of the normal electric field and it is made by the accumulated charge on the boundary between the mediums which have different conductivity. Under the action of the external electric field, the accumulated charge will be accumulated on the boundary between the shallow three-dimensional anomalies and the regional structural medium while the affect to the induced magnetic field is very weak. If the values of apparent resistivity are calculated by the values of the distorted electric field, the curves of the apparent resistivity will translate in the direction of the modulus value, comparing with the curves of the apparent resistivity calculated by the normal value, while the shape of the curves and the curves of the phases will not change.The correction of the static shift effect is a problem which must be solved in the MT data processing, but it is also hard to solve the problem completely. For the practical applications in different situations, researchers have made a lot of useful methods, such as:the curve translation method, the spatial filtering methods and jointing the TEM method into the correction, etc.Although the correction methods above are useful in the certain ranges, making the correction by only using the data from the measuring pot itself is still a problem which is unsolved.At present, the study of the MT3D/2D inversion model is a hot spot. In this paper, the new method of the correction is designed for the3D/2D inversion model.In the regional tectonic axis coordinate system, the impedance modulus tensor will be marked as:Za and the distortion of the measured data caused by the static shift can be expressed in the form of the static shift tensor C multiplying the normal impedance modulus tensor Zoa and the mathematical form of the calculation is showed below:The combinations of the static shift effect elements which are made by the brute-force method will be filtered by combining the new method with the appropriate restrictions and finally the unique combination of the approximations on the target digits will be got.The filtering process consists of the four steps.Step1:we can get the relationships below by formula(1): Z1/Z3=C2/C4, Z2/Z4=C1/C3formula(2)In the1D structure, the forms of formula(2) can be deformed into: Z2/Z3=-C1/C4, Z1/Z4=-C2/C3formula(3)Based on the limit relations above, the combinations of the estimates will be filtered by1D and2D restrictions.Step2:to calculate the remaining combinations by the formula below: Z0=C-1Za formula (4)In this step, the estimates of the impedance modulus tensors of the normal fields will be calculated at first and then the curves of Zte and Ztm will be observed. If the forms of the curves do not fit the characteristics of the regional tectonic structure, the relative combinations of the estimates will be filtered.Step3:according to16approximating situations of the4elements of the static shift tensor,16related M-D boards will be made and the estimates will be filtered by watching the effects whether the estimates will coincide with the points on the M-D board or not.Step4:by observing the effects of the diagonal elements and vice diagonal elements of the impedance modulus tensors respectively fitting regional structural characteristics or by learning from the existing geological data, the unique combination of the estimates will be got at last.By processing and analyzing the relative models and the actual measured data, the ideal effect of the correction has been got, however, this new method is still not perfect and it is necessary to do a lot study to improve the method.