The 9th International Symposium on Recent Advances in Exploration Geophysics (RAEG 2005)

Wave equation method is one of the fundamental methods for seismic modeling and imaging. In this paper a meshless numerical method - the Element-Free Method (EFM) was applied to solve seismic wave equation and handle modeling and imaging problems. The theory of EFM consisted of two parts -the Moving Least Squares (MLS) criterion and the variational principle, in contrast with the Lagrange interpolation and the variational principle for the theory of the Finite Element Method (FEM). In EFM, it was necessary to calculate the Gauss quadrature on each Gauss point. Only the numbered nodes near to the Gauss point needed to be considered for the quadrature. These nodes were determined by the so-called influence domain of each node. The influence domain was a significant feature of EFM because it effected on the accuracy and cost of the method. At the same time, the absence of elements was also a key point of EFM, which yielded easier preprocessing and lower cost than those of FEM. In this paper, the scheme of EFM would be shown for full scalar or elastic wave equation. Based on the theory, a simple example was discussed in details to indicate the good effectivity of EFM. Furthermore, some synthetic examples will be shown to discover the good performance of EFM in seismic modeling and imaging problems. It is clear that complex structures can be modeled and imaged very well such as high-angle dip and high-velocity anoma1y even under complex surface conditions.

To reduce the earthquake disaster, it is very important to evaluate strong ground motions as soon as possible after an earthquake occurs. In Japan, high-density seismograph networks were installed after the Hanshin-Awaji earthquake. The typical spacing between each seismograph is approximately 20 km. The seismograph density in Japan is enough to evaluate the seismic intensity distribution throughout Japan. However it is not enough to evaluate ground motions in local area that is important not only for the government of local area but also for the civilian self-disaster prevention. In this article, therefore, we have developed a new method to estimate high-resolution PGA (Peak Ground Acceleration) distribution from earthquake data observed at only two seismic stations using neural network techniques. The neural network learns relationship between the PGA of two seismic stations, H/V resonant frequency data and the soil zoning data. After the training period the neural network becomes to be able to estimate PGA at any given points. The new method has been applied to estimate PGA distribution in Zushi City, Kanagawa, Japan. As the results, high-resolution PGA distribution map has been estimated immediately with good accuracy.

The ground vibration generated by the Shinkansen, Japanese super express train，can be problematic for people and structures nearby. The purpose of this study is to make clear the mechanism of the ground vibration generated by a train and to develop a numerical simulation tool which can quantitatively evaluate the effect of wave filtering. This study is consist with 2 parts: First, the Doppler effect in the field vibration data from train motion is shown to be found via FK spectrum. Second, the generation of ground vibration is classified by this numerical simulation. The ground vibration is caused by inhomogeneous media, inhomogeneous force from the train or discrete structure, like an elevated bridge. This work does not investigate the boundary problem between the train body and the ground, thus is a qualitative basic study.

Multiple removal by wavepath migration is used for discriminating primary reflections from multiple reflections during the migration process. Both the take-off and incidence angles are computed from the data so that rays can be traced from both the source and the receiver positions. An event is a primary reflections as long as the rays intersect and the intersection point has a reflection traveltime equal to the observed traveltime. In contrast，an event is a multiple reflection if the intersection point of the rays has a reflection traveltime different from he observed traveltime. Numerical results for a three-layer model show that multiple removal by wavepath migration can clearly resolve the true interfaces and effectively remove the false structures generated by multiple energy. Numerical results show that the method can effectively remove the migration artifacts generated by surface-related multiples. When near-offset data are not used, the migrated images is nearly free of multiple-related artifacts. However, the method becomes less effective with decreasing source-receiver offset of the traces.

Reverse time migration method with forward modelling zero-offset seismic data is applied to the prediction ahead of tunnel face. The numerical model with soft rock mass zone and fault zone are used to test the reverse time migration for locating the positions of fault zone ahead of tunnel face. The numerical simulation results have proven that reverse time migration with zero-offset data is exactly able to image the fault zone in un-excavation ahead of tunnel.