Ken Yoshioka

Integral equation method for 3D modeling of electromagnetic fields in complex structures with inhomogeneous background conductivity

We present a new formulation of the integral equation (IE) method for three-dimensional (3D) electromagnetic (EM) modeling in complex structures with inhomogeneous background conductivity (IBC). This method overcomes the standard limitation of the conventional IE method related to the use of a horizontally layered background only. The new 3D IE EM modeling method still employs the Greens functions for a horizontally layered 1D model. However, the new method allows us to use an inhomogeneous background with the IE method. We also introduce an approach for accuracy control of the IBC IE method. This new approach provides us with the ability to improve the accuracy of computations by applying the IBC technique iteratively. This approach seems to be extremely useful in computing EM data for multiple geologic models with some common geoelectrical features, like terrain, bathymetry, or other known structures. It may find wide application in an inverse problem solution, where we have to keep some known geologic structures unchanged during the iterative inversion. The method was carefully tested for modeling the EM field for complex structures with a known variable background conductivity. The effectiveness of this approach is illustrated by modeling marine controlled-source electromagnetic (MCSEM) data in the area of Gemini Prospect, Gulf of Mexico.

Electromagnetic Forward Modeling Based On the Integral Equation Method Using Parallel Computers

In this paper we discuss the parallel implementation of the 3-D electromagnetic forward modeling algorithm, based on the contraction integral equation (CIE) method. We use a Linux Beowulf Class Cluster available at the Center for High Performance Computing (CHPC) at the University of Utah. We have developed a working version of the 3-D CIE code based on the Complex Generalized Minimal Residual (CGMRM) method of solving the system of linear equations. This new parallel implementation of the CGMRM algorithm ensures the convergence of the corresponding matrix inversion and allows us to consider large-scale forward modeling problems. We have investigated the solution of a large-scale forward problem with different discretizations on a PC cluster of up to 32 nodes.

Three‐dimensional iterative inversion of the marine controlled‐source electromagnetic data

We developed a novel method of full 3-D inversion of the marine controlled-source electromagnetic (MCSEM) data. Our approach employs the localized quasi-linear (LQL) approximation in the initial stage of inversion process, and integral equation (IE) based method in the final stage of the inversion. To ensure the convergence of the iterative process for complex system of equations for electric field in the inversion domain, we use the Complex Generalized Minimal Residual Method (CGMRM) (Zhdanov, 2002). The numerical modeling results demonstrate the effectiveness of this approach for inversion of the synthetic MCSEM data simulated for the models of a sea-bottom petroleum reservoir.

Modeling large‐scale geoelectrical structures with inhomogeneous backgrounds using the integral equation method: applications to the bathymetry effect study in marine CSEM data

We apply a new formulation of the integral equation (IE) method with inhomogeneous background conductivity (IBC) for modeling marine controlled source electromagnetic (MCSEM) data in areas with significant bathymetric inhomogeneities. We also introduce an approach for accuracy control of the IBC IE method. This new approach provides us with the ability to improve the accuracy of computations by applying the IBC technique iteratively. This approach seems to be extremely useful in computing EM data for multiple geological models with some common geoelectrical features, like terrain, bathymetry, or other known structures. It may find wide application in inverse problem solution, where we have to keep some known geological structures unchanged during the iterative inversion. The effectiveness of this approach is illustrated by modeling marine controlled source electromagnetic (MCSEM) data in model with significant bathymetric inhomogeneities.

Interpretation of Sea Bed Logging Data Using the LQL Method

Summary In this paper we develop and study a new technique for 3-D interpretation of sea-bottom electrical profiling data, collected with a mobile horizontal electric dipole (HED) transmitter and an array of seafloor electric receivers (The Sea Bed Logging Method). Our method is based on the localized quasi-linear approximation, which allows modeling and inversion of seafloor electrical data simultaneously for all positions of the transmitters and receivers. This opens a way for a full 3-D inversion of the SBL data for petroleum exploration. We apply this method to interpretation of the synthetic EM data simulated for the model of a seabottom petroleum reservoir. Our results demonstrate that the SBL method with an LQL-based 3-D interpretation can be used for seabottom reservoir detection in the presence of a salt dome structure.

Three-dimensional Cross-well Electromagnetic Tomography

In this paper we develop a new technique for 3-D crosswell electromagnetic tomography, based on EM borehole survey consisting of a moving vertical magnetic dipole transmitter, located in one or several boreholes, and a tri-axial induction receiver, located in the other boreholes. The method is based on the LQL approximation for forward modeling, which results in a fast inversion scheme. The method incorporates both a smooth regularized inversion, which generates a smooth image of the inverted resistivity, and a focusing regularized inversion, producing a sharp focused image of the geoelectrical target. The practical application of the method to synthetic data demonstrates its ability to recover the resistivity, location, and shape of resistive and conductive rock formations.

2D Inversion of CSAMT Data For Exploring the Epithermal Gold Deposit In the Itaya Prospect, Northern Hottshu, Japan

In the exploration for epithermal gold deposit, subsurface resistivity structure is very important. Both low and high resistivities represent a kind of the alteration which is closely associated with the gold mineralization. The Metal Mining Agency of Japan conducted a CSAMT survey in tbe Itaya prospect, Northern Honshu, Japan to explore the epithermal gold deposit. The Itaya prospect consists of Neogene acidic tuff and tuff breccia with thin Quatanary volcanic cover. An open pit which has produced keolinite is included in the area, and goldbearing hydrothermal breccia which was expected to display high resistivity occurs in the pit. Eleven reading lines of ca 2 km long were set up in the north-south direction with 200 m separation. Total 207 reading stations speed 100m apart on the lines ware completed in 35 days in 1995. The Zonge system was employed to obtain the data.