3-D Marine CSEM Modeling in General Anisotropic Media by Using an Adaptive Finite Element Approach Based on the Vector-Scalar Potential

We present three-dimensional(3-D)modeling method of marine controlled-source electromagnetic(CSEM)fields in general anisotropic media using an adaptive finite element approach based on the vector-scalar potential.The modeling is based on the governing Helmholtz equations in the vector-scalar potential system.Unstructured tetrahedral grids are employed,which can exactly simulate the terrain relief and complex electrical structures.Moreover,based on the gradient recovery technology,the adaptive finite element approach is used to drive the mesh refinement,and make the finite element solutions converge gradually to the exact solutions.The primary-secondary field approach is used to improve the numerical accuracy of CSEM fields near the source point,where the primary field is calculated by using the quasi-analytical formula.The accuracy of this approach is verified by a one-dimensional model.Two 3-D models are used to demonstrate the effectiveness of the adaptive mesh refinement and the influences of dipping anisotropy layer on the marine CSEM responses for both inline and broadside geometries.The complex synthetic model is simulated to show the capability and flexibility of the approach for geometrically complex situations.

Research on 3D marine electromagnetic interferometry with synthetic sources for suppressing the airwave interference

In order to suppress the airwave noise in marine controlled-source electromagnetic （CSEM） data, we propose a 3D deconvolution （3DD） interferometry method with a synthetic aperture source and obtain the relative anomaly coefficient （RAC） of the EM field reflection responses to show the degree for suppressing the airwave. We analyze the potential of the proposed method for suppressing the airwave, and compare the proposed method with traditional methods in their effectiveness. A method to select synthetic source length is derived and the effect of the water depth on RAC is examined via numerical simulations. The results suggest that 3DD interferometry method with a synthetic source can effectively suppress the airwave and enhance the potential of marine CSEM to hydrocarbon exploration.

Adaptive Finite Element Modeling of Marine Controlled-Source Electromagnetic Fields in Two-Dimensional General Anisotropic Media

In this paper, we extend the scope of numerical simulations of marine controlled-source electromagnetic (CSEM) fields in a particular case of anisotropy (dipping anisotropy) to the general case of anisotropy by using an adaptive finite element approach. In comparison to a dipping anisotropy case, the first order spatial derivatives of the strike-parallel components arise in the partial differential equations for generally anisotropic media, which cause a non-symmetric linear system of equations for finite element modeling. The adaptive finite element method is employed to obtain numerical solutions on a sequence of refined unstructured triangular meshes, which allows for arbitrary model geometries including bathymetry and dipping layers. Numerical results of a 2D anisotropic model show both anisotropy strike and dipping angles have great influence on the marine CSEM responses.

Effects of Uncertainties in the Position and Orientation of Both the Transmitter and Receivers on Marine Controlled-Source Electromagnetic Data

Simulation and interpretation of marine controlled-source electromagnetic(CSEM) data often approximate the transmitter source as an ideal horizontal electric dipole(HED) and assume that the receivers are located on a flat seabed.Actually,however,the transmitter dipole source will be rotated,tilted and deviated from the survey profile due to ocean currents.And free-fall receivers may be also rotated to some arbitrary horizontal orientation and located on sloping seafloor.In this paper,we investigate the effects of uncertainties in the transmitter tilt,transmitter rotation and transmitter deviation from the survey profile as well as in the receiver's location and orientation on marine CSEM data.The model study shows that the uncertainties of all position and orientation parameters of both the transmitter and receivers can propagate into observed data uncertainties,but to a different extent.In interpreting marine data,field data uncertainties caused by the position and orientation uncertainties of both the transmitter and receivers need to be taken into account.

Large-Scale Electromagnetic Modeling for Multiple Inhomogeneous Domains

We develop a new formulation of the integral equation(IE)method for three-dimensional(3D)electromagnetic(EM)field computation in large-scale models with multiple inhomogeneous domains.This problem arises in many practical applications including modeling the EM fields within the complex geoelectrical structures in geophysical exploration.In geophysical applications,it is difficult to describe an earth structure using the horizontally layered background conductivity model,which is required for the efficient implementation of the conventional IE approach.As a result,a large domain of interest with anomalous conductivity distribution needs to be discretized,which complicates the computations.The new method allows us to consider multiple inhomogeneous domains,where the conductivity distribution is different from that of the background,and to use independent discretizations for different domains.This reduces dramatically the computational resources required for largescale modeling.In addition,using this method,we can analyze the response of each domain separately without an inappropriate use of the superposition principle for the EM field calculations.The method was carefully tested for the modeling the marine controlled-source electromagnetic(MCSEM)fields for complex geoelectric structures with multiple inhomogeneous domains,such as a seafloor with the rough bathymetry,salt domes,and reservoirs.We have also used this technique to investigate the return induction effects from regional geoelectrical structures,e.g.,seafloor bathymetry and salt domes,which can distort the EM response from the geophysical exploration target.