We derived an equation giving a relationship between the average depth of a stratum boundary and the power spectrum of theg gzz component of a gravity gradient tensor. It was found that the relationship was nonlinear to the wave number on a semi-logarithmic scale and that the average depth was estimated to be shallower than the actual one if a conventional linear approximation would be employed. We applied the equation obtained in this study to the gzz component of the gravity gradient tensor observed in the Kuju geothermal area of central Kyushu, Japan, and obtained results that are consistent with the average depth estimated by spectrum analysis of gravity anomaly. On the other hand, we found that the equation could not estimate the average depth of the stratum boundary at the lowest wave number range. In addition, it was shown that the wave number range giving the same average depth as that given by spectrum analysis of gravity anomaly shifts to the higher wave number range, and its range becomes broader.
The Taiwan Power Research Institute plans to perform a field experiment on sedimentary rocks at 2500 m depth at a CO2 geological storage site near a coastal area in Taichung-city. Application of electromagnetic methods is being planned to monitor changes in resistivity caused by injected CO2. It is necessary to measure the initial resistivity of the layers, before large-scale CO2 injection tests. For the CSMT (Controlled Source Magneto-Telluric) method, the exploration depth has been previously limited to less than 1000 m at most sites in Japan, because the distance between an electric current source and a survey site cannot be separated enough because of the influence of artificial noise. In this study, we conducted simulations using numerical models in order to evaluate the effectiveness to monitor the CO2 reservoir at a depth of 2500 m. As a result, the total amount of injected CO2 reached 100 Mton, apparent resistivity increased 5%. We conducted a field test at the site using the great depth CSMT method. We set the current source at a distance of 15 km away from the site and we measured apparent resistivity and phase data at the frequencies of 8192 - 0.015625 Hz using a new electromagnetic exploration instrument with high spectrum resolution controlled by GPS synchronization (Johmori et al., 2010). We could obtain good data by processing with a digital filter and average E/H vector for adjacent survey stations, regardless of the noises at the site. We could also measure the data at the low frequency of 0.0625 Hz by performing a near-field correction considering the signal source. Furthermore, the resistivity profiles obtained by the 1D and 2D inversion techniques matched well with the resistivity of boring core samples and electrical logging data from 2000 to 3000 m depth at the site. In conclusion, we confirmed the effectiveness of this technique.
The 2011 off the Pacific coast of Tohoku Earthquake occurred on March 11, 2011, and caused enormous damage on Japan. The Tokyo Bay and the Tone River downstream regions were widely damaged by liquefaction. In this research, we carried out a surface wave survey at the Tone River northern coast area in Katori, Chiba, Japan which belongs to the Tone River downstream region, and obtained the S-wave velocity (Vs) structure down to about 30 m below the ground surface. As a result, the subsurface of the survey area is classified into three layers; the sandy surface layer which has relatively high Vs, the low Vs silty layer, and the high Vs sandy basement layer. Using the obtained Vs distribution, we investigated whether 1.5 m below the ground surface of the survey area would liquefy or not when it will be shaken by the same scale earthquake as the 2011 off the Pacific coast of Tohoku Earthquake. As a result, it was presumed that liquefaction would not occur on most of the survey line. Moreover, the estimated domains where liquefaction occurred correlated well with those estimated by geological surveys and also well with marshy area distributions in Meiji Era. These results showed that liquefaction resistance estimation using Vs distribution was useful.
In this report, ground shaking characteristics in the residential area around Mashiki Town, Kumamoto Prefecture, Japan, are evaluated by dense microtremor array measurements. First, we conducted microtremor measurements at 365 sites in the residential land. H/V spectra were then calculated using the measurement records. Finally, fine distributions based on the peak (ridge) and trough frequencies of the H/V spectra were illustrated in the residential land. The relationships between the evaluated H/V spectra and the seismic damage will be useful in the future study on the strong motion estimation in the residential land of during the 2016 Kumamoto earthquake sequence.
Measurements of spontaneous electrical potential (or self-potential, SP) for mineral exploration have been conducted on land. More recently, for exploration of hydrothermal ore deposits (by observing marine electric fields), underwater SP surveys are also conducted. Observed electric field data have smaller amplitudes than those obtained on land. Therefore, the noise must be removed to emphasize self-potential signals in the observed data. However, conventional "stacking" techniques cannot eliminate coherent noise which has a coherent component among the parallel observed data. This paper describes the use of a proposed noise removal technique using independent component analysis (ICA), which can decompose multi-component mixed signals into independent signals.
We tested the application of ICA to electric field data measured using deep-towed self-potential exploration. The data were obtained far from hydrothermal active areas and ore deposits. Results show that we extracted and removed noise that is difficult to extract by conventional stacking. Then we found no large marine spontaneous electric field, and inferred the causes of noise. Subsequently, to the observed electric field data we added a hypothetical marine electric signal from ore deposits to simulate the SP field. After applying ICA to the artificial electric field data we identified and eliminated the noise The hypothetical signal was extracted more clearly than when using conventional stacking.
Based on successful application to the test data, we applied ICA to electric field data obtained by an autonomous underwater vehicle cruising in a hydrothermally active area. Results show that we extracted the marine electric field deriving from sub-seafloor sources. Such extraction was not possible using conventional stacking This report also presents discussion of the causes of noise.
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