Three-dimensional inversion of magnetotelluric data in complex geological structures

Abstract

Interpretation of magnetotelluric data over inhomogeneous geological structures is still a challenging problem in geophysical exploration. We have developed a new 3-D MT inversion method and a computer code based on full nonlinear conjugate gradient inversion and quasi-analytical approximation for forward modeling solution. Application of the QA approximation to forward modeling and Frechet derivative computations speeds up the calculation dramatically. However, in order to control the accuracy of the inversion, our method allows application of the rigorous forward modeling in the intermediate steps of the inversion procedure and for the final inverse model. The 3-D magnetotelluric inversion code QAINV3D based on QA approximation, has been tested on synthetic models and applied to the practical MT data collected in an area with complex geology. The inversion of 3-D MT data can be done within a few minutes on a PC to generate a 3-D image of subsurface formations on a large grid with tens of thousands of cells.