Research On 3D Inversion Of Marine Controlled-Source Electromagnetic Method

Nowadays,the demand for oil and gas resources becomes more and more serious,which makes the marine oil and gas resources become to the hot issue.Marin Controlled Electromagnetic Method(MCSEM)is an important offshore exploration method,which has been used in commercial exploration for more than 10 years.Its basic principle is to excite the electromagnetic field in the sea by the artificial source,to receive the reflected electromagnetic wave on the seabed.After processing the received information,we can obtain the electric distribution characteristic of the Submarine strata.Due to the stronger penetration of electromagnetic wave,it will be more effective for some areas without good seismic exploration,such as volcano rock covered area,coral reef area and carbonate rock area.The MCSEM method can determine the distribution area of high resistivity reservoirs better,which can improve the success rate of drilling.At present,the processing and interpretation methods of MCSEM data are mainly based on 1-D and 2-D simulation.However,the actual seabed geological conditions are complex,the results obtained by 1-D or 2-D processing will not be comprehensive,which requires the MCSEM three-dimensional inversion study.In this paper,the nonlinear conjugate gradient(NLCG)algorithm based on the staggered-grid finite difference technique is used to realize the three dimensional inversion of the marine controlled source electromagnetic data.The main research contents and results of this paper are as follows.1.Starting from the Maxwell equations,the electromagnetic field in the MCSEM exploration is divided into a primary field and a second field.Then deduced the first and second field solutions respectively.2.Different models are established to conduct 3-D forward study.By changing the depth of seawater and other important factors,we want to explore its impact of the MCSEM exploration method.3.Nonlinear conjugate gradient(NLCG)algorithm is used to study the 3D inversion.Based on the objective function,the calculation formula is deduced and the algorithm flow is established.On this basis,combined with the three-dimensional forward program,we realized the 3-D inversion of MCSEM.4.Validated the correctness of 3-D inversion program by establishment of models,and applied the program to the actual data processing.The results show that the program can performed the underground resistivity distribution better.