Perpustakaan Digital ITB

3-D inversion of geophysical data has become a practical, commonly used tool in the interpretation of magnetotelluric (MT) data. However, 3-D inversion is a non-linear and non-unique task and requires adopting additional assumptions. In this paper, we analyze the influence of selection of inversion parameters in the case of previously published, real model for Pomerania region (Poland). This zone is a regional scale 2-D structure, which locally becomes 3-D. The analyzed model was accomplished by inversion of full impedance and tippers from 31 MT stations with the period range from 10 s to 10,000 s. The inversion parameters were chosen to provide the best possible agreement of the model with well-known geophysical and geophysical data. The purpose of the presented work is to determine how the change of some inversion parameters influences previously obtained model, and therefore we check which structure features remain in spite of the change of the implied parameters. A number of tests were performed employing an implementation of the freely available ModEM inversion package. These included the choice of an appropriate initial model and a model covariances evaluation as a means to control smoothness between adjoining model areas. Additionally, we compared the models calculated from all transfer functions (impedances and tippers) and from subsets (only tippers and off-diagonal components of the impedance tensor and tippers). Moreover, the tests with the model coordinate system rotation were performed. Our analysis has shown that the values of covariance matrix coefficients have an evident effect on the results. However, the multiple use of the covariance matrix with small coefficients gives a similar result as its single use with higher coefficients. The initial model and the selection of the transfer function for inversion can significantly change the model. However, changing the orientation of the coordinate system from geographic to the one that is aligned with a regional strike direction does not cause noticeable changes in the solution obtained. It can be only noted that for the rotated coordinate system the obtained model becomes closer to the model of a 2-D regional structure.