Frequency-domain 3D Marine Controlled-Source Electromagnetic Inversion Using Non-linear Conjugate Gradient Method

Marine controllable source electromagnetic method(marine CSEM)is an important method to distinguish oil and water from seabed tectonic reservoirs by using resistivity difference,and has become an important tool in offshore oil exploration in recent years.With the rapid economic development,people will focus more on marine energy.The ocean CSEM plays an important role in the interpretation of data as an important supplement to the marine seismic data.At present,method technology or instrument development of marine CSEM has not been well studied in China.This paper studied both 3D forward modeling and inversion method,using the finite volume method to analyze3 D forward response.And the nonlinear conjugate gradient method to realize the 3D inversion of marine CSEMThis paper aims to lay the foundation for the marine electromagnetic data processing and interpretationfor the future.In this paper,the three-dimensional control equation of marine CSEM is deserved from the Maxwell equation.The finite volume method based on the staggered grid is used to discretize the equations.After the discrete equations are combined,a large linear system is formed and solved by Krylov subspace method–Quasi-minimal Residual Method.In order to solve the singularity problem at the source point of the marine CSEM,the total field is decomposed into primary and secondary fields,and a full space or layered reference model is introduced to eliminate the singular current source term.In the process of solving the equation to add divergence correction,improve the calculation speed.The experimental results show that the response of the electric field decreases with the increase of the frequency,but the normalized electric field response is opposite.The location of the source and the size of the reservoir also affect the characteristics of the electric field response,the increase in the transmission and reception distance of the electric field response,reservoir thickness and length of the increase in electric field response is also enhanced.From the normalization curve,the amplitude normalization curve of the electric field and the magnetic field can indicate the reservoir boundary position,but the results of the electric field are better.On the other hand,the calculation process of sensitivity matrix is deduced in detail,and the sensitivity distribution of horizontal electric dipole source is discussed.The distribution of sensitivity in uniform half space and high impedance anomaly is analyzed.The sensitivity of this paper is based on the nonlinear conjugate gradient algorithm,the first derivative of the objective function is calculated,and only one row of the sensitivity matrix can be obtained by two forwards.At the same time,this paper adopts the nonlinear conjugate gradient algorithm to realize the three-dimensional inversion of marine CSEM.The factors related to the selection of different conditionsin the line search,model parameter transformation and iterative conditions are discussed.Through the inversion of several typical theoretical models,the inversion results of different frequencies and different measurements are analyzed.We demonstrate the feasibility and effectiveness of this approach in marine CSEM exploration.