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The 15th International Symposium on Recent Advances in Exploration Geophysics (RAEG 2011)
- Conference date: 24 Nov 2011 - 25 Nov 2011
- Location: Kyoto, Japan
- Published: 24 November 2011
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Numerical Studies on Relationship between Coda-Q and Stress Field, and Prospects for Application to Real Data
More LessAuthors K. Okamoto, H. Mikada, T. Goto and J. TakekawaCoda-Q is a stochastic parameter which reflects heterogeneity under the ground. In the past, it is said that the coda-Q has a certain relationship with the occurrence of earthquakes or with the change in a stress field. From the perspective that the coda-Q has quantitative relationship with the stress field, this study attempts to reveal that relationship using the numerical calculations. A 2-D Finite Difference Method (FDM) and a 2-D Boundary Integral Equation Method (BIEM) are employed to consider the effects of the loaded stress in the crust for two kinds of possible causes. One would be the occurrence of anisotropy in the velocity field, and the other the distribution of crack orientations. The former could be dealt with in FDM, while the other with BIEM. The results suggested that the variation in the coda-Q has the relationship with the magnitude of the confining pressure and the angle of principal stress. It is also suggested that both the magnitude of the variation in the coda-Q against the confining pressure and against the angle of principal stress shows higher values when the velocity anisotropy is considered. These results imply the possibility to obtain the change in the stress parameters, i.e., both the magnitude and the orientation, using the variation in the coda-Q. Our further work will be conducted to apply the present methodology and to confirm the stability of coda-Q estimation to real data.
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Prediction of Gas-Oil Contact from Surface Seismic Exploration
More LessAuthors Y. Iwaki, H. Mikada, T. Goto and J. TakekawaThe fluid behavior in the reservoir of hydrocarbon resources greatly affects the oil production. Quantifying the amount and the distribution of hydrocarbon in the subsurface offers useful information to modify the future planning. The gas-oil contact (GOC) is expected as an indicator of oil observed by surface seismic exploration at low frequency. In this study, we attempt to grasp the movement of GOC in a simulation based on the Biot equations. The Biot equations consider the poroelastic media composed by solid and fluid phase, which allow us to handle the fluid alteration directly. We simulated with a second-order, finite-difference Biot scheme and investigated the effect of GOC and oil-water contact (OWC) under some assumptions in fluid alteration. The results show the possibility of surface seismic to be used to estimate the behavior of GOC in the subsurface. We also introduce the concept of active/inactive pores to derive the property of solid matrix phase from solid grains and including fluid, which had little influence on the wave data received.
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Marine Controlled-source Electromagnetic Sounding on Submarine Massive Sulphides Using Numerical Simulation
More LessAuthors N. Imamura, T. Goto, J. Takekawa and H. MikadaRecently, controlled-source electromagnetic methods are widely used for shallow sub-seafloor explorations. In this paper, we propose a new controlled-source electromagnetic method using an autonomous underwater vehicle (AUV) for the exploration of submarine massive sulphides (SMS). A numerical simulation code for 2.5-D or 3-D electromagnetic field in the frequency domain is employed in order to estimate electromagnetic responses from possible conductivity structures. As a result, we confirmed that we could detect the area of SMS effectively using Ty-Hx and Tz-Ez for transmitter and receiver. This is because the electric field is attenuated and the electric flux is concentrated to the interior of conductive zones. We also confirmed that the effects of topography were relaxed using normalized electromagnetic fields in the simulation. Based on our results, we think that the new controlled-source electromagnetic method is realizable.
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Applications of the Full-Wave Inversion Techniques to Ocean Acoustic Tomography of the Synthetic Data
More LessAuthors Y. Kida, H. Mikada, T. Goto and J. TakekawaThe problem of Ocean Acoustic Tomography (OAT) is to estimate the state of ocean in temperature, salinity, etc. that are related to sound velocity structure from the travel-time or other properties in acoustic wave propagation. The ocean is nearly transparent for low frequency acoustic waves so that the acoustic wave could propagate for thousands of kms. OAT was first introduced by Munk et al. (1995) and based on a ray theoretical approach. A travel-time inversion method has been developed using a ray-tracing scheme in the Munk’s method. There is some similarity with seismic exploration both in theory and in data processing methods. However the waveform analysis is not common in OAT although its importance is widely recognized in seismic explorations. Actually there is hardly any precedent studies on waveform inversion in the application of OAT. In this study, a full-wave inversion technique is applied to OAT in the 2-D acoustic FDTD model in order to investigate the effectiveness of the method through the comparison of the results with that of the ray-tracing inversion approach. The result shows that full-wave inversion provides higher image construction than in travel-times inversions.
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