Study Of Software For Processing And Analysis Of Magnetotelluric Data

Calculating of transfer function and analysis of dimensionality and distortion are two mainlyparts of Magnetotelluric (MT) data processing. The basic theory and key points of MT dataprocessing were studied in this paper. Experiments of data processing for synthetic and fielddata showed that the software which was developed independently could get ideal transferfunctions and electrical structure information. And the reliability of our software wasdemonstrated by comparing with other commercial software which was widely used. Thesoftware could satisfy the necessary of data processing and analysis of mass MT field data.To consider the methods in frequency domain only, there were two processes to gettransfer function from MT time series:1) the preprocessing;2) estimation research. The stepssuch as time window choosing, fourier coefficients calculating and correcting, evaluationfrequencies choosing were included in preprocessing. While estimation methods wereresearched mainly for stacking of spectral matrix (or partial transfer functions) from differenttime windows.The MT data analysis was mainly for the following purposes:1) analysis of subsurfaceelectrical dimensionality;2) determination of regional strike;3) removing of distortionresulted from local anomaly body under the2D/3D electrical model. In the paper, we extendedGroom-Bailey (GB) decomposition method under the constraints of phase tensor which wasindependent of local distortion form ‘galvanic effect’, and also we took anisotropy factor intothe objective function and solved the optimization problem with Conjugate Gradient (CG)strategy. The GB decomposition under the constraints of phase tensor was demonstrated toimprove stability of algorithm a lot. Moreover, the study showed GB algorithm withanisotropy factor performed well than the traditional way in terms of accuracy andcomputational cost.MT data were collected in Qaidam basin (QB) which is north of Tibetan plateau along thesouth-north profile from Golmud cement plant to Lvcaoshan. The data processing and analysisof MT data along the profile was done using our software. A minimum structure, two-dimensional resistivity model was got using MT non-linear conjugate gradient (NLCG)inversion method. The crustal and upper mantle south of Golmud was resistive, although aconductor existed in middle and upper crustal beneath the most south of profile. The surface ofQB was filled by conductive layer, and there was discontinuous resistive bodies in upper andmiddle crustal, however, it seemed that material in lower crustal and upper mantle present tobe conductive. We analyzed the main faults along our MT profile with the final2D resistivitymodel and these faults were as the following: The north thrust (NKT), The Qaibei fault zoneand The Qaibeiyuan thrust. The sedimentary fill which was indicated by conductive layerreached a maximum thickness of8km. Finally in the paper, we discussed the mechanism ofgrowth of the plateau to the north from the aspect of electrical structure.