Masashi Endo

Anisotropic 3D inversion of towed-streamer electromagnetic data: Case study from the Troll West Oil Province

One of the critical problems in the interpretation of marine controlled-source electromagnetic geophysical data is taking into account the anisotropy of the rock formations. We evaluated a 3D anisotropic inversion method based on the integral equation method. We applied this method to the full 3D anisotropic inversion of towed-streamer electromagnetic (EM) data. The towed-streamer EM system makes it possible to collect EM data with a high production rate and over very large survey areas. At the same time, 3D inversion of towed-streamer EM data has become a very challenging problem because of the huge number of transmitter positions of the moving towed-streamer EM system, and, correspondingly, the huge number of forward and inverse problems needed to be solved for every transmitter position over the large areas of the survey. We overcame this problem by exploiting the fact that a towed-streamer EM system’s sensitivity domain is significantly smaller than the area of the towed-streamer EM survey. This approach makes it possible to invert entire towed-streamer EM surveys with no approximations into high-resolution 3D geoelectrical sea-bottom models. We present an actual case study for the 3D anisotropic inversion of towed-streamer EM data from the Troll field in the North Sea.

A multigrid integral equation method for large-scale models with inhomogeneous backgrounds

We present a multigrid integral equation (IE) method for three-dimensional (3D) electromagnetic (EM) field computations in large-scale models with inhomogeneous background conductivity (IBC). This method combines the advantages of the iterative IBC IE method and the multigrid quasi-linear (MGQL) approximation. The new EM modelling method solves the corresponding systems of linear equations within the domains of anomalous conductivity, Da, and inhomogeneous background conductivity, Db, separately on coarse grids. The observed EM fields in the receivers are computed using grids with fine discretization. The developed MGQL IBC IE method can also be applied iteratively by taking into account the return effect of the anomalous field inside the domain of the background inhomogeneity Db, and vice versa. The iterative process described above is continued until we reach the required accuracy of the EM field calculations in both domains, Da and Db. The method was tested for modelling the marine controlled-source electromagnetic field for complex geoelectrical structures with hydrocarbon petroleum reservoirs and a rough sea-bottom bathymetry.

Three-dimensional inversion of towed streamer electromagnetic data

ABSTRACT A towed streamer electromagnetic system capable of simultaneous seismic and electromagnetic data acquisition has recently been developed and tested in the North Sea. We introduce a 3D inversion methodology for towed streamer electromagnetic data that includes a moving sensitivity domain. Our implementation is based on the 3D integral equation method for computing responses and Frechet derivatives and uses the re‐weighted regularized conjugate gradient method for minimizing the objective functional with focusing regularization. We present two model studies relevant to hydrocarbon exploration in the North Sea. First, we demonstrate the ability of a towed electromagnetic system to detect and characterize the Harding field, a medium‐sized North Sea hydrocarbon target. We compare our 3D inversion of towed streamer electromagnetic data with 3D inversion of conventional marine controlled‐source electromagnetic data and observe few differences between the recovered models. Second, we demonstrate the ability of a towed streamer electromagnetic system to detect and characterize the Peon discovery, which is representative of an infrastructure‐led shallow gas play in the North Sea. We also present an actual case study for the 3D inversion of towed streamer electromagnetic data from the Troll field in the North Sea and demonstrate our ability to image all the Troll West Oil and Gas Provinces and the Troll East Gas Province. We conclude that 3D inversion of data from the current generation of towed streamer electromagnetic systems can adequately recover hydrocarbon‐bearing formations to depths of approximately 2 km. We note that by obviating the need for ocean‐bottom receivers, the towed streamer electromagnetic system enables electromagnetic data to be acquired over very large areas in frontier and mature basins for higher acquisition rates and relatively lower cost than conventional marine controlled‐source electromagnetic methods.

High-frequency induced polarization measurements of hydrocarbon-bearing rocks

Summary We have investigated induced polarization (IP) effects in hydrocarbon-bearing artificial rocks at frequencies greater than 100 Hz. We have examined the instrumental and electrode phase responses of Zonge International’s complex resistivity (CR) system, and optimized the performance of the Zonge system for IP measurements over the 1 mHz to 10 kHz frequency range. The reliability of the highfrequency IP measurements were confirmed by independent measurement of the same samples using the Novocontrol BDS 80 system. Our results confirm the presence of IP effects in hydrocarbon-bearing rocks, and suggest the necessity to account for IP effects in the interpretations of electromagnetic data, particularly in induction logging data.

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 appli- cations 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 re- sult, a large domain of interest with anomalous conductivity distribution needs to be discretized, which complicates the computations. The new method allows us to con- sider multiple inhomogeneous domains, where the conductivity distribution is differ- ent from that of the background, and to use independent discretizations for different domains. This reduces dramatically the computational resources required for large- scale 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.

3D inversion of towed streamer EM data — A model study of the Harding field and comparison to 3D CSEM inversion

A towed streamer electromagnetic (EM) system capable of simultaneous seismic and CSEM data acquisition has been developed and tested in the North Sea. The towed EM data are processed and delivered as a time-domain impulse response. In this paper, we use 3D modeling and inversion to investigate the ability of the towed EM system to detect and characterize the Harding field, a typical North Sea-type target. The 3D model of the Harding field itself was constructed from dynamic reservoir simulations. We have compared our 3D inversion of time-domain towed streamer EM data with 3D inversion of conventional frequencydomain CSEM data. We observe similarities in the recovered models. Obviating the need for ocean bottom receivers, the towed-streamer EM system enables CSEM data to be acquired simultaneously with seismic over very large areas in frontier and mature basins for higher production rates and relatively lower cost than conventional CSEM.

Complex resistivity of mineral rocks in the context of the generalised effective-medium theory of the induced polarisation effect

This paper develops the generalised effective-medium theory of induced polarisation for rock models with elliptical grains and applies this theory to studying the complex resistivity of typical mineral rocks. We first demonstrate that the developed generalised effective-medium theory of induced polarisation model can correctly represent the induced polarisation phenomenon in multiphase artificial rock samples manufactured using pyrite and magnetite particles. We have also collected representative rock samples from the Cu–Au deposit in Mongolia and subjected them to mineralogical analysis using Quantitative Evaluation of Minerals by Scanning Electron Microscopy technology. The electrical properties of the same samples were determined using laboratory complex resistivity measurements. As a result, we have established relationships between the mineral composition of the rocks, determined using Quantitative Evaluation of Minerals by Scanning Electron Microscopy analysis, and the parameters of the generalised effective-medium theory of induced polarisation model defined from the laboratory measurements of the electrical properties of the rocks. These relationships open the possibility for remote estimation of types of mineralisation and for mineral discrimination using spectral induced polarization data.

Multi-grid IE Method For Large-scale Models With Inhomogeneous Background

We present a multigrid integral equation (IE) method for threedimensional (3D) electromagnetic (EM) field computations in largescale models with inhomogeneous background conductivity (IBC). This method combines the advantages of the iterative IBC IE method and the multigrid quasi-linear (MGQL) approximation. The new EM modeling method solves the corresponding systems of linear equations within the domains of anomalous conductivity, Da and inhomogeneous background conductivity, Db, separately on coarse grids. The observed EM fields in the receivers are computed using grids with fine discretization. The developed MGQL IBC IE method can be also applied iteratively by taking into account the return effect of the anomalous field inside the domain of the background inhomogeneity Db, and vice versa. The iterative process described above is continued until we reach the required accuracy of the EM field calculations in both domains, Da and Db. The method was tested for modeling the marine CSEM field for complex geoelectrical structures with hydrocarbon petroleum reservoirs and a rough sea bottom bathymetry.

3D Inversion of AEM Data Based on a Hybrid IE-FE Method and the Moving Sensitivity Domain Approach with a Direct Solver

The moving sensitivity domain method has been very successful in 3D inversion of the data from large airborne EM surveys. The modeling method was based on the integral equation (IE) method. However, in survey areas with rough topography and very high contrast, modelling based on the finite element (FE) method has advantages. We use the combined advantages of the IE and the FE methods in a hybrid scheme with the moving sensitivity domain to create a stable and efficient modeling method. To increase computational efficiency, we have reformulated the moving sensitivity domain so that multiple transmitters are contained in each sensitivity domain. This allows the economical use of a direct solver, because the sensitivity domain’s system matrix can be directly decomposed and then used for multiple transmitter positions. We show that there are an optimal number of transmitters to place in each subdomain: too many and the domain size become unwieldy, too few and the advantages of the direct solver are lost. We apply this technique to data from Germany’s Federal Institute of Geosciences and Natural Resources frequency domain AEM system. We validate the method by comparison with previously published results and our own 1D and 3D inversion results.

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

SUMMARY We present a new formulation of the hybrid method for threedimensional (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 method allows us to compute the effect of IBC structures by using any appropriate numerical method which may be able to build a model with a flexible grid. 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 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 known variable background conductivity. The effectiveness of this approach is illustrated by modeling marine magnetotelluric (MT) data.