Abstract:  Compared with the frequency domain electromagnetic method, the transient electromagnetic method can effectively distinguish between  air waves and seafloor electromagnetic response, which has a good application prospect in the shallow offshore oil and gas reservoir detection. The geological conditions of offshore area are generally more complicated, and there are strong cut terrain and complex structures,  making the electromagnetic field response characteristics become extremely complex. These problems will bring great difficulties to the data interpretation work. In this paper, based on the timedomain finite element method (FEM), the unstructured mesh is used to dissect the complex geological model. By constructing the timedomain finite element equations and combining the long wire source approximation technique with dipole discretization and the backward Euler discretization technique, the threedimensional forward modeling  simulation of   electromagnetic transient with  electrical source in shallow sea  area under  complex geological conditions is realized. After verifying the accuracy of the algorithm, the influence of different seawater depths on  air waves and the target body of  seafloor  oil and gas reservoir is analyzed through the threedimensional forward modeling  of the complex geological model. Based on these results, the influence of different surrounding rock resistivity and seafloor  topography on the resolution of the target body of  oil and gas reservoir is further analyzed.  The results show that under shallow sea conditions, the pulse response is greatly affected by air waves, while the step response is less affected by air waves. As the depth increases, the influence of air waves  decreases, and the resolution of oil and gas reservoirs also decreases. The electrical resistivity of surrounding rocks and the complex seafloor topopraphy  have a serious impact on the transient electromagnetic properties of electrical sources.

Key words:  , transient electromagnetic method, shallow sea, unstructured grid, 3D forward modeling, time domain finite element