The necessity of establishing a 3d/2d model is discussed. Owing to introducing the model, the full electric and magnetic field galvanic distortion tensor is simplified. The effects of a surface 3 d conductivity anomaly on a regional 2 d structure is derived by comparing data through Swift rotation with data through Bahr decomposition, then is expressed by a distortion matrix. The effect of a local anomaly can be separated from a regional structure by researching and decomposing the distortion tensor matrix. So it is available to find out the regional structure dimensionality, classification of the local structure and the effect of it on the regional structure. Eventually, the simple distortion decomposition method is tested through three sites of practical data.