We have studied a reflection imaging technique in which passive seismic signals from other construction activity such as drilling noise are used as a source signal in order to develop a low-cost measurement method inside underground gallery. In this paper, we show an application to data sets from the Mizunami Underground Research Laboratory of Japan Atomic Energy Agency. The drilling was conducted with a main purpose of extraction rock core samples for rock mechanics research. The three-dimensional reflection imaging results and comparison with borehole observation are shown in this paper. Firstly, we show a concept and advantages of utilization of drilling noise inside narrow underground gallery, and explain analytical technique based on correlation analysis of three-component waveforms by which we can theoretically obtain three-dimensional imaging results even with only one single point observation. Next, we describe an outline of the data acquisition at Mizunami, and characteristics of observed drilling noise waveforms. Then, we show results of reflection imaging by using drilling noise signals, and compare those with fractures based on borehole observations. We discuss relationship between the estimated three-dimensional reflection images and possible spatial distribution of fractures. These results infer that the proposed technique can be useful tool when we need additional geological information in order to achieve maximum safety at reasonable low-cost in case at a narrow space such as for a high level radioactive waste repository development.

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 Numerical simulation of seismic waves in a model realistically accommodating arbitrary shaped topography is important in exploration geophysics, earthquake disaster mitigation, environmental/civil engineering applications, etc. Finite difference method (FDM) can be applied to fairly complex model with simple manner using rotated staggered grid (RSG) technique. However, RSG scheme requires many grid points per a minimum wavelength to assure the accuracy sufficient for the simulation of surface waves propagating on an arbitrary-shaped ground surface. On the other hand, a Hamiltonian particle method (HPM) has been applied to simulate surface wave propagation taking advantage of the simplicity in the accommodation of free-surfaces. Although the particle method successfully simulated surface wave propagation, the accuracy of the method for modeling Rayleigh waves has not been verified quantitatively.
 In the present study, we investigate the accuracy of a Hamiltonian particle method, which is one of the particle methods, for the simulation of Rayleigh wave propagation. We calculate seismograms of the Lamb's problem using HPM and FDM with RSG (FDM-RSG), and compare them with those from the analytical approach. FDM-RSG solutions have the highest accuracy for amodel with a simple planar free surface aligned with the grid structure, however, suffer from the effect of inclined slope that may not be aligned with grids while those of HPM keep the accuracy enough to be applied in case of the inclined slope. We also investigate the accuracy of HPM for a range of the source frequencies and the offset distances. The results show that HPM can simulate surface wave propagation with smaller number of particles than FDM-RSG. Our numerical results indicate that HPM has some advantages over FDM-RSG in terms of the accuracy for modeling surface waves on arbitrary topography.

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 Heuristic algorithms, such as GA, have been frequently used in inversion of surface-wave phase velocity to S-wave profile in microtremor explorations. One of difficulty in the methods is to understand sensitivity of parameters. In this study, applicability of Markov Chain Monte Carlo method in surface-wave phase velocity inversion is examined. Numerical examinations of the method were conducted using synthetic phase velocity at frequencies from 0.1 to 2 Hz for fundamental mode of Rayleigh wave in a layered model of a deep sedimentary basin. The Metropolis-Hasting procedure was used to sample the model space, and final inverted model is derived from averaging S-wave velocity and thickness for the sampled models. The inverted model was similar to the true model with regardless of observed standard deviations of the phase velocity. We also estimated resolutions for the model parameters using their standard deviations of the sampled models, and showed that the resolution depends on the standard deviations of the synthetic phase velocity. Furthermore numerical tests were conducted using the phase velocity in different frequency range. The distributions of the parameters in deep part of the S-wave velocity profile show low resolutions in the absence of the phase velocity at low frequency. Shallow part of S-wave velocity profile was also in low resolution when the high frequency phase velocity data were not included in the inversion. We also evaluated variations of one-dimensional amplification factors for S-wave using the sampled models for the phase velocity with different standard deviations. The dominant periods and their amplitudes do not depend on the observed errors of the phase velocity, while amplification factors at high frequency are significantly different from each other. We furthermore applied the method to actual phase velocity data obtained in a microtremor exploration in the Kanto basin, Japan. The inverted S-wave velocity profile is almost the same as that from an existing inversion method. The numerical experiments and the actual application clearly demonstrate a high applicability of the method in the phase velocity inversion.

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 A Multi-channel seismic reflection (MCS) method is known to be applied for physical oceanographic researches in recent years. This has opened up the new field of ‘Seismic Oceanography’. However, there are several specifications of MCS survey because most of survey data of this research is secondary use for underground survey. Therefore, it is difficult to interpret of common oceanic structure in different surveys. We analyzed MCS data on an identical survey line in Izu-Ogasawara region for comparison of MCS specification effect. In this paper, we focus on the characteristics for seismic image of oceanic structure without the effect of time lapse. Results show the remarkable difference in oceanic structure, although the underground image is similar profile. We also compared the depth effect of airgun and streamer on the identical survey line in Izu-Ogasawara region. The results indicate the similar profile in general structure. We identified the similar distribution of amplitude spectrum in oceanic structure for survey direction, although the response beneath the seafloor is different. Our results indicate the first MCS case study for oceanic fine study.

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 We measured the complex resistivities of sericite ores and host rocks which were sampled from the Awashiro sericite mine, central Japan, where sericite veins can be detected as low resistivity and high chargeability anomalies by the IP method. The result indicates that sericite hardly show the IP effect although it has low resistivity. On the other hand, the IP effect is observed in the host rocks. We also measured the complex resistivities of three kinds of andesite samples which differ in the degree of alteration, the artificial samples which mixed pyrite particles with pure sericite, and six kinds of sericite ores which differ in water content and mineral content. The result of these measurements is as follows; 1) The IP effect is hardly observed in sericite itself, 2) the strength of the IP anomaly is influenced by the content of metallic mineral like pyrite produced by hydrothermal alteration, 3) the frequency dependency of complex resistivity changes with kinds of sericite ore. It is certain that the IP method is effective in investigation of sericite ores in this mine because they are usually accompanied by a lot of pyrite. It is expected that discrimination of sericite ores can be performed on site by the SIP method if the relationship between the frequency dependency of complex resistivity and mineral content in a sericite ore becomes clear.

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[in Japanese]

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