Three-dimensional Vector Finite Element Forward Modeling Of Marine Controlled Source Electromagnetic Method Based On Unstructured Grid

With the acceleration of the pace of China's economic construction,the demand for minerals and energy has gradually increased,and the excessive exploitation of land resources has directly led to the lack of resources.Therefore,the focus of resource exploration must be shifted to the ocean.The ocean is rich in minerals,natural gas and other resources,but its detection rate is extremely low.Geophysical exploration methods such as seismic methods have higher survey accuracy,but their cost is too high.It is an inevitable trend to develop some low-cost,high-precision geophysical methods to improve drilling success rate.The marine controlled source electromagnetic method has attracted widespread attention due to its low exploration cost and high exploration accuracy.Three-dimensional data interpretation is more comprehensive than one or two dimensions,so it has become the focus of current research.Forwarding is the basis of inversion.Efficient and high-precision forwarding can provide a strong guarantee for subsequent inversions.To this end,this paper conducts a three-dimensional finite element forward modeling study of a marine controllable source vector under an unstructured grid.The research work firstly deduced the analytical expression of the electromagnetic field in any horizontal layered medium in detail.The complex Bessel function in the expression was solved by a filtering algorithm.The electromagnetic field components in space are compared to verify the effectiveness of the algorithm.Furthermore,the filtering algorithm in this paper is used to study the influence of related parameters of one-dimensional ocean model.A finite element simulation of a three-dimensional ocean controllable source vector based on an unstructured grid was realized.First,the complete integral expression of the electric field double-rotation equation based on the vector finite element in any unit was derived,and the unstructured grid versus space was used.Discrete to accommodate any complex terrain.Local refinement of the grid effectively reduces the singularity of the field source points.The vector finite element equation formed by forward modeling is solved by a more robust direct solution method.The effectiveness of the algorithm is verified by comparison with analytical solutions and published results.The influence of different parameters on the amplitude response was analyzed by setting up the classic disc model,complex sphere model and trapezoidal mountain model.The results show that:(1)the amplitude is inversely proportional to the frequency,so high frequency is not used in the actual exploration process.(2)Compared with conductivity,MCSEM responds more obviously to resistive anomalies,and the resistive anomalies body can be qualitatively and quantitatively explained by the curve of amplitude versus offset.(3)Under the condition of ensuring safety,the location of the emission source as close to the sea floor as possible is conducive to observation.(4)The terrain will cause distortion of the curve of the amplitude versus the offset.Therefore,the influence of the terrain cannot be ignored in the interpretation of actual data.