Three-dimensional inversion of magnetotelluric data in complex geological structures

Int erp retation of magneto telluri c data over inhomogeneous geolog­ ical st ructures is st ill 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 analyt ical approximation for forward modeling solut ion. Appli cat ion of the QA approximation to forward model ing and Frechet deriva­ t ive computat ions speeds up the calcu lati on dr amatically. However , in order to cont rol the accuracy of the invers ion, our method allows applicat ion of th e rigorous forward mod eling in the intermediate steps of the inversion procedure and for the final inverse model. The 3-D magneto telluri c inversion code QAI NV3D based on QA approxima­ t ion, has been tested on synthet ic models and applied to the practical MT dat a collecte d in an area with complex geology. T he inversion of 3-D MT data can be done within a few minutes on a PC to gener­ ate a 3-D image of subsur face formations on a large grid with tens of t housands of cells.