The low-precision computation at long-offset is problem in marine CSEM method. A high-precision linear filter program based on continuous Euler transformation was developed in order to solve Hankel transform for electric and electromagnetic method numerically. The continuous Euler transformation can accelerate the convergence of an alternating series. As a result of numerical evaluation, the new linear filter program based on continuous Euler transformation has better computation precision than conventional linear filter method based on convolution in spite of less number of filter coefficient. This new method will become a main method to calculate Hankel transform instead of conventional linear filter method.

View full abstract

  The monitoring of subsurface CO₂ migration and distribution is of great importance for the safety of assessment in the CO₂ geological storage project. Among the various monitoring techniques, seismic method has been selected as the most reliable method in many worldwide CO₂ storage projects. However, the velocity of the seismic attributes has ever been mainly utilized. On the other hand, an amplitude attenuation which is another significant seismic attribute has not ever been applied. Aiming more extensive utilization of the seismic attenuation, this paper provides the relationship between CO₂ saturation and the seismic attenuation. In cases where estimation of amount of CO₂ will be required in underground for the future CO₂ emission trading, utilization of the seismic attenuation in addition to the velocity can increase the accuracy for the estimating. At first, we present a relationship between CO₂ saturation and seismic velocity in the case of which CO₂ are distributed partially in the pore space of rock. Next, we provide a theoretical relationship between the seismic attenuation and CO₂ saturation by the standard linear solid model. We apply this theory to geophysical logging results of the Nagaoka site and predict the seismic attenuation. Also, we have found that the modified patchy theory is applicable to the Nagaoka site by our previous research. In order to validate the theoretical model and the methodology for the calculation, we applied to a laboratory dataset. This dataset is acquired using air, not using CO₂. However the model and calculation method can be applied regardless of the kind of fluid in pore space of rock. This data set provides the observed relationship between seismic velocity and CO₂ saturation and the observed between seismic attenuation and CO₂ saturation by using a same rock-core specimen. The result shows the good agreement between the computed attenuation by the model and the observed attenuation. Hence, we found that our model and the methodology are validated consequently. And we found that large seismic attenuation will be observed at the small CO₂ saturation. It is very useful to monitor the CO₂ leakage from the storage layer. Because it enables us to detect CO₂ leakage in the earlier stage of leakage due to being able to observe a small amount of CO₂ by using the attenuation.

View full abstract

  The effects of hydrostatic pressure on strain measurements by using fiber optic distributed sensor system were elucidated through laboratory experiments. Strains of metallic plates and a cylindrical core of Berea sandstone were measured under hydrostatic pressure by using a system that measures strains by the frequency shifts of Brillouin and Rayleigh scattering in an optical fiber, together with simultaneous measurements by strain gauges. The measurements revealed that the radial strain of the optical fiber due to the hydrostatic pressure considerably affect the frequency shifts as well as the strain along the fiber line. The effects of two strains on the frequency shifts, Δf, are expressed as Δf=AΔε_z+ BΔε_r. where Δf, ε_z, ε_r are the Brillouin or Rayleigh frequency shifts, the fiber strain along the line and radial directions, respectively. A and B are the coefficients corresponding to the effects of the two directional strains for the Brillouin or Rayleigh frequency shifts. A and B are determined as 0.0497±0.0043, 0.0346±0.0014 (MHz/με) for the Brillouin and 0.130±0.0096, 0.0707±0.0032 (GHz/με) for the Rayleigh. Our experimental technique was also successfully applied for monitoring the deformation of sandstone under static pressure. The results assure accurate strain measurements by means of the fiber optic distributed strain measurements system.

View full abstract

  Recently, Independent Component Analysis (ICA) has been studied and developed as a new method for multidimensional signal processing in many fields. The ability to detect a certain sound among plural sounds is called the Cocktail-party effect, which is well known as the issue for blind signal separation in acoustics. ICA has been applied to the separation for sounds/biomedical signals and an image processing in the engineering. However, there are not many applications of ICA in fields of exploration geophysics and seismology. To identify noise excited by cars and trains included in seismic data and to separate earthquake ground motions by aftershocks occurred simultaneously during large earthquakes are the issue for blind signal separation; therefore, the development of the Cocktail-party effect is expected in geophysical and seismological fields. As to the separation problem of railway-induced ground vibration mixing in seismic data, in this study, we investigated the waveform separation of earthquake ground motions and railway-induced ground vibrations by ICA, through numerical experiments used the observed data.
  At first, we applied ICA to synthetic waveforms mixed earthquake ground motions and railway-induced ground motions spatially without time lag under the condition of BSS, to separate the original signals from those synthetic waveforms. As a result, the earthquake ground motions and the railway-induced ground motions were well separated. And, in the case of distance ratio of 0.99 for two stations from a railway, the earthquake ground motions and the railway-induced ground motions were also well separated. Secondary, we applied ICA to waveforms mixed the observed earthquake ground motions and railway-induced ground motions. As a result, the earthquake ground motions and railway-induced ground motions separated were in good agreement with those observed, especially in the frequency range of 0.5 to 30Hz for the earthquake ground motions. Kurtosis which is the fourth-order cumulant of high order statistics for earthquake ground motions and railway-induced ground motions is large and those frequency distributions are in a shape of super-Gaussian type; therefore, we suggested that ICA is qualified for the separation problem of earthquake ground motions and railway-induced ground motions.

View full abstract

[in Japanese]

View full abstract