The geoelectrical fields induced by oscillating dipole source are numerically simulated in different heterogeneous models. The results show that for the model of ordinary layered media it is impossible to observe such electrical signals on earth surface. It is found that the surface field can be enhanced 2 or 3 orders if these is high conductive fault in the layered model. The surface abnormal electrical signals of ordinary electric structure is about several mV/km or less, however it could reach hundreds mV/km in special electrical structure.It is deserved to point out that the area of strongest abnormal signals is not over the epicenter, but is in some points of fault.This means the heterogeneity of electric structure is advantageous to detect abnormal electrical signals, on the other hand, it leads to the complex phenomena of abnormal electrical signals distribution on surface.