text: The 9th International Symposium on Recent Advances in Exploration Geophysics (RAEG 2005) | Earthdoc
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  The 9th International Symposium on Recent Advances in Exploration Geophysics (RAEG 2005) — Recommend this title to your library
  The 9th International Symposium on Recent Advances in Exploration Geophysics (RAEG 2005)
  Conference date: 28 Mar 2005 - 29 Mar 2005
  Authors Q. Chen, L. Li and Y. Chen
  https://doi.org/10.3997/2352-8265.20140062
  Based on schematically formulation of the vibrations induced by moving trains, this paper analyses the waveforms along the Datong-Qinhuangdao railroad in Northern China recorded in the suburban Huairou district of Beijing on March 8, 2003. It is illustrated that vibrations induced by train, except traditional recognized noises and interference effects, could be used as a seismic source to detect crustal structures with its advantage of abundant frequency spectrum,repeatability and no additional harm to the environment. It will bring lights to the traditional exploration seismology with the further studies of signal processing and interpretation methods, and related models and new observing systems.
  A Meshless Method and its Applications in Seismic Exploration
  Authors X. Jia and T. Hu
  https://doi.org/10.3997/2352-8265.20140063
  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.
  True Tri-Axial Test Using Distinct Element Method
  Authors J. Takekawa, Y. Sanada, Y. Yamada, Y. Ashida and H. Kusumi
  https://doi.org/10.3997/2352-8265.20140064
  Many kinds of rock test for engineering properties have been carried out to obtain rock kinetic properties. However it is difficult to visualize inside rock specimen during laboratory tests. This study's objective is to reconstruct true tri-axial tests using Distinct Element Method (DEM). At first, tri-axial compression test is carried out to know correlations between analytical properties and rock properties obtained through compression test like cohesion and internal friction angle. Next, true tri-axial test is simulated using properties above. The result of the simulation, it was clarified that the simulation result can reconstruct a laboratory test qualitatively.
  Development of the Earthquake Information System for Local Regions Using Minimum Seismic Network and Neural Network Technology
  Authors Y. Oda, K. Ishii, T. Ishizaki, T. Iwatate and M. Yu
  https://doi.org/10.3997/2352-8265.20140065
  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.
  Comparison of Absorbing Boundary Conditions and Numerical Modeling of Electromagnetic Wave for Multi Component GPR
  By Y. Sanada
  https://doi.org/10.3997/2352-8265.20140066
  In this paper, numerical modeling of electromagnetic (EM) wave for multi component GPR is developed. For numerical modeling, the FDTD (Finite Difference Time Domain) method is used. In the accurate wave propagation modeling, efficient absorbing boundary condition is very important. The perfect matched layer (PML) and Cerjan's absorbing condition is compared. The result shows PML is more effective than Cerjan's one. The EM reflected waves are simulated with two different antenna arrays with varying structure. The results indicate that multi component GPR obtains more information than conventional one component GPR.
  Analysis of Ground Vibration near a Railroad
  Authors M. Katou, H. Kanda, Y. Sanada and T. Matsuoka
  https://doi.org/10.3997/2352-8265.20140067
  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.
  A New Method to Seek the Process of Tsunami Generation - Newly Discovered Waves at the 2003 Tokachi-Oki Earthquake
  Authors H. Mikada and H. Matsumoto
  https://doi.org/10.3997/2352-8265.20140068
  At the 2003 Tokachi-oki earthquake of M8, seafloor phenomena such as a generation process of tsunami, seafloor uplifts, etc., were observed using a cabled observatory installed on the seafloor. The seafloor uplifts were observed not before the main shock but continuously after the main shock. The uplifts were 0.35,0.37, and 0.12 m for epicentral distances of 25.5, 31.4, and 81.8 km, respectively. Pressure fluctuations that took place co-seismically show about 100 times in amplitude to those observed as the uplifts. The uplift of the seafloor generated not only tsunami but high amplitude acoustic waves. Both the tsunami and acoustic waves were generated by the uplift and superposed to each other. After the main shock, a continuous uplift of the seafloor is observed at the all three pressure gauge locations and the rate of uplift was about 0.004 m/day. These phenomena may imply that there was a change in the state of friction on the plate boundary interface by the main shock. In this paper, we demonstrate what was observed using ocean bottom pressure gauges installed right above the focal area of the earthquake, and then discuss these phenomena in tsunami generation and in post-seismic slip processes.
  Multiple Removal by Wavepath Migration
  Authors Y. Liu, X. Chang and H. Sun
  https://doi.org/10.3997/2352-8265.20140069
  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.
  Objective Function Migration with Wavepath
  https://doi.org/10.3997/2352-8265.20140070
  We present a new least -squares migration method called least-squares wavepath migration (LSWM) or Objective Function Migration with Wavepath. The proposed method combines an iterative conjugate gradient solver with a stationary-phase wave path migration operator. Numerical tests demonstrate that (a) LSWM is computationally more efficient than and almost as accurate as Kirchhoff least-squares migration, and (b) many of the artifacts seen in wavepath migration images are suppressed after several conjugate gradient iterations. Previous results showed that 3-D wavepath migration is up to 100 times as fast as a standard 3-D Kirchhoff migration, but sometimes at the cost of reduced quality. With the proposed LSWM method, the image quality in wavepath migration can be improved at an acceptable increase in computational cost.
  Reverse Time Migration of Tunnel Seismic Reflected Data
  By X. Chang
  https://doi.org/10.3997/2352-8265.20140071
  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.
  Study on Simulation Algorithm of Borehole Radar for Well Logging and its Application to Measuring Volumetric Water Content of Formation
  Authors S. Lin, L. Zhao, Y. Sanada and Y. Ashida
  https://doi.org/10.3997/2352-8265.20140072
  Borehole radar becomes useful tool for high resolution probing of both deep oil reservoir and shallow subsurface. In order to improve understanding of the electromagnetic wave propagation in single hole, a pseudo-spectral time domain (PSTD) algorithm that allows us to model the full EM wavefield for rotational symmetric structure was developed. This technique is based on Finite Difference Time Domain (FDTD) method and uses fast Fourier transform (FFT) algorithm for spatial derivatives. The PSTD algorithm can reduce grid number and save computer memory for large-scale forward calculation and inversion. The simulation provides a method for processing of field data. The measured volumetric water content from borehole radar data gave good agreement with other logging curves, such as resistivity logging curve.