Three-dimensional inversion of magnetotelluric data in complex geological structures

Abstract

Interpretation of magnetotelluric data over inhomogeneous geologicalstructures is still a challenging problem in geophysical exploration.We have developed a new 3-D MT inversion method and a computer codebased on full nonlinear conjugate gradient inversion and quasianalyticalapproximation for forward modeling solution. Application of the QAapproximation to forward modeling and Frechet derivative computationsspeeds up the calculation dramatically. However, in order to controlthe accuracy of the inversion, our method allows application of therigorous forward modeling in the intermediate steps of the inversionprocedure and for the final inverse model. The 3-D magnetotelluricinversion code QAINV3D based on QA approximation, has been testedon synthetic models and applied to the practical MT data collectedin an area with complex geology. The inversion of 3-D MT data canbe done within a few minutes on a PC to generate a 3-D image of subsurfaceformations on a large grid with tens of thousands of cells.