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The 5th International Symposium on Recent Advances in Exploration Geophysics (RAEG 2001)
- Conference date: 22 Jan 2001 - 22 Jan 2001
- Location: Kyoto, Japan
- Published: 22 January 2001
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Laboratory Studies of Surface Waves Using a Laser Doppler Vibrometer
More LessAuthors K. Hayashi and O. NishizawaWe have performed physical modeling of surface waves using a laser Doppler vibrometer (LDV). Surface wave methods have been applied to both engineering and earth science problems to image S-wave velocity of the Earth. In order to develop the surface wave methods, numerical and physical modelings of surface waves are important. The LDV converts a particle velocity of media to the Doppler shift frequency，and enables us to measure very precise ultrasonic waves. A piezoelectric transducer (PZT) was used as a source of elastic waves. 50KHz to 500KHz elastic waves were observed by using the LDV and PZT. We have measured surface waves propagating a homogeneous half space models and horizontal two-layer models using the LDV and PZT. Theoretical characters of surface waves，such as Rayleigh wave dispersion, have been clearly observed by the physical modeling.
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Advanced Wave-Equation Migration
More LessWave-equation migration methods can more accurately account for complex wave phenomena than ray-tracing-based Kirchhoff methods that are based on the high-frequency asymptotic approximation of waves. With steadily increasing speed of massively parallel computers, wave-equation migration methods are becoming more and more feasible and attractive for imaging complex 3D structures. We present an overview of several efficient and accurate wave-equation-based migration methods that we have recently developed. The methods are implemented in the frequency-space and frequency-wavenumber domains and hence they are called dual –domain methods. In the methods, we make use of different approximate solutions of the scalar-wave equation in heterogeneous media to recursively downward continue wavefields. The approximations used within each extrapolation interval include the Born, quasi-Born, and Rytov approximations. In one of our dual-domain methods, we use an optimized expansion of the square-root operation in the one-way wave equation to minimize the phase error for a given model. This leads to a globally optimized Fourier finite difference method that is a hybrid split-step Fourier and finite-difference scheme. Migration examples demonstrate that our dual-domain migration methods provide more accurate images than those obtained using the split-step Fourier scheme. The Born-based, quasi-Born-based, and Rytov-based methods are suitable for imaging complex structures whose lateral variations are moderate, such as the Marmousi model. For this model, the computational cost of the Born-based method is almost the same as the split-step Fourier scheme, while other method is almost the same as the split-step Fourier scheme, while other methods takes approximately 15-50 per cent more computational time. The globally optimized Fourier finite-difference method significantly improves the accuracy of the split-step Fourier method for imaging structures having strong lateral velocity variations, such as the SEG/EAGe salt model, at an approximately 30 per cent greater computational cost than the split- step Fourier method.
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