Abstract：2.5-D finite-element CSAMT numerical inversion is an inversion method based on 2.5-D finiter-element forward modeling and CSAMT-RRI sensitivity matrix approximative computation as well as pseudo-1-D inversion method.The method is CSAMT numeric inversion method characterized by fast and relax algorithm, which adopts 2.5-D forward modeling and uses sensitivity matrix calculated approximately by CSAMT-RRI method that both meets a demand for precision and improves computational speed, meanwhile, the method uses pseudo-1-D inversion that reduces a lot of algebraic operation of sensibility matrix and further accelerate the computational speed.The numeric inversion has been carried out in the model characterized by two low resistivity zones overburdened by high conductivity formation in the paper,the results of inversion showed that the tilted low resistivity zone can be better positioned but the anomalous scope of inverted model structure is bigger that the real model and pull-down phenomenon appears in the depth direction for tilted anomalous body.Nevertheless, the mean-square-root errors of fitting curves between model inversion and real structure are less than 2 except at individual point, which achieves better fit.
