Appropriate restriction of a migration aperture decreases migration noise of a seismic migration method. The Fresnel volume migration chooses the migration aperture considering the Fresnel volume of the received elastic wave. The Fresnel volume is regarded as the energy propagation space and determined by the propagating wave frequency and the incident angle to the receiver. Robust estimation of the Fresnel volume position leads to robust migration aperture restriction. Thus, estimation of the incident angle with maximum accuracy enhances signal to noise ratio of the Fresnel volume migration result.
 In recent researches, some Fresnel volume migration methods estimate incident angles using three components slowness vectors in TSP, VSP, etc. Other methods estimate incident angles by analyzing the correlation of one component sensors for surface seismic data. Though, these method may miss-choose incident angles either when signal to noise ratio does not reach to estimate and when analyzing sensor receives several waves at the same time.
 In this study, we used τ-p transforms to estimate incident angles stably even in such cases. We applied the Fresnel volume migration using τ-p transforms to 2D simulation data of point and slope model. As the result, we find that two methods are robust methods to use for the Fresnel volume migration. Also, comparing with slowness vector method, τ-p transform estimation can restrict migration aperture more stably.

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 For prevention of disastrous slope failures, it is important to be able to monitor changes in water content within a slope in both time and space, because permeating water reduces the strength of the foundation of the slope and increases the likelihood of landslides and base rock collapses. We tested the effectiveness of geoelectrical measurements for monitoring movement of moisture within the slope of an experimental embankment, performing monthly resistivity surveys during the year starting February 1, 2011. The results clearly show seasonal changes in the resistivity structure of the embankment, with resistivity becoming low in the summer wet season and high in the winter dry season. This seasonal resistivity pattern is a response mainly to changing temperature, while shorter-term changes in resistivity correspond to changes in water content caused by rain. The change in temperature-compensated resistivity is inversely proportional to the change in volumetric water content, indicating that changes in water content in the embankment can be estimated by resistivity monitoring in conjunction with temperature correction. We also conducted continuous geoelectrical measurements for a two-week period that included an episode of heavy rain. The result shows that a sudden change in water content accompanying the rain was detectable by the resistivity change. It is difficult to derive water content quantitatively from geoelectrical data because the resistivity change does not have a linear relation to the water content change, but resistivity monitoring using the geoelectrical method is effective for monitoring changes in water content in zones of low saturation within the slope of an embankment.

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 Group velocity measurement by cross correlating long-term microtremor data has exploded. However, since it has been shown that the Green's function can be retrieved from cross correlation function, we examine the behavior of amplitude of cross correlation function in this study. A theoretical framework by Tsai(2011) shows that the signal part of cross correlation function depends on parameters of subsurface structure and amplitude of noise part is proportional to the root of the data amount for the analysis. Comparing this theoretical result with the observed cross correlation functions obtained from half a year microtremor data in the southern Kanto region, the maximum amplitudes of the observed cross correlation functions also show constant values for the data amount for the analysis, while the noise parts show a dependency for the root of the data amount. We also found that amplitude values fit a power law in which the noise level at long period is relatively low to the noise at short period. However, the maximum amplitude values also show a similar tendency to the noise amplitude especially in the short periods probably due to the lack of the data amount for the analysis. These results allow us to estimate a required data amount for the cross correlation analysis and we can estimate the minimum data in observations for seismic interferometry quantitatively. We also show the effect of data processing focusing on amplitudes of signal and noise of cross correlation functions. Although time domain normalization such as 1-bit normalization is very effective, it requires an appropriate filter for raw microtremor data in advance. We also show that the shorter window for the data division of long-term microtremor data into segments gives better convergence of cross correlation functions. Finally we show attenuation of maximum amplitudes of cross correlation functions because a theoretical study suggests a possibility of the identification of the parameters of subsurface structure.

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 Cavities are caused by the deterioration of the social infrastructure inside the ground. As a result cave-in accidents occur. It may become the tragic incident such as occurrence of casualties. It is demanded that we prevent cavities in the ground before the cave-in occurs. Ground penetrating radar, seismic reflection survey and electric survey etc. are applied to explore the cavities inside the ground. However the geophysical exploration is not all-round about the exploration depth and resolution. Only gravity method or density logging is applied for the density of the ground. The exploration technology using cosmic ray muons can demand density distribution of the ground to support the needs for the cavity exploration. The muon telescope and the three-dimensional tomography technique are developed with the aim of geophysical exploration using cosmic ray muons to practical use as a method to make up for conventional geophysical exploration. In this paper, it is shown the principle of the exploration using muons, some result of the basic experiments and the feasibility for the engineering works.

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[in Japanese]

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