Radioactive disequilibrium of thorium series nuclides in surface waters of the Seto Inland Sea     

Takashi Okubo 《Journal of Oceanography》1982,38(1):1-7

Surface samples of sea water collected in the Seto Inland Sea were analyzed for²³²Th and²²⁸Th. The concentrations of²³²Th were generally less than 2 dpm/1,0001 and these values are probably an upper limit for the²³²Th concentration in surface waters of the Seto Inland Sea. The²²⁸Th concentrations ranged between 4.2 to 42.3 dpm/1,0001. Remarkable seasonal and temporal variations in²²⁸Th concentrations were found, in comparison with the minimal variations in²²⁸Ra concentrations reported previously. The activity ratios of²²⁸Th/²²⁸Ra were about 0.18 in the southern part of the Kii and the Bungo Channels, and decreased markedly from the open ocean toward the central region of the Seto Inland Sea. The average value of the²²⁸Th/²²⁸Ra ratio in the central region of the Seto Inland Sea was 0.032±0.020. This suggests that removal residence time of²²⁸Th can be estimated to be about 34±22 days in surface waters of the Seto Inland Sea.  相似文献   

Expert System for Applicability of Tunnel Boring Machines in Japan   总被引：6，自引：0，他引：6  

S. Okubo  K. Fukui  W. Chen 《Rock Mechanics and Rock Engineering》2003,36(4):305-322

Summary ¶This paper outlines the development of a new expert system for assessing the applicability of tunnel boring machine (TBM) tunneling in Japan. Although a great deal of research on TBMs has been published, and the applicability of TBMs has been discussed, considerable differences in opinion still exist between authors. In this paper, we review previous studies and outline the present situation with particular focus on disc-cutter TBMs. Based on the knowledge acquired, we present an expert system for the applicability of TBMs, for use in pre-feasibility studies. Originally, we planned to construct the expert system on the basis of unified knowledge or rules without contradictions. However, it was found after several attempts that it is very difficult to unify knowledge because opinions vary considerably and TBMs are under continuous development. As a result, the expert system was divided into three stages. In stage A, the fulfillment of fundamental requirements is checked. Stage B is a simple expert system consisting of a minimal set of suitable rules as judged by the authors. Stage C incorporates the opinions of various other experts and the over-simplified and omitted points in stage B. The system is applied to 18 tunnels in Japan, and while the results provided by the expert system can certainly be improved, the method for accumulating knowledge and rules makes the system simple and easy to use, with very large scope for improvement and expansion.Received July 1, 2001; accepted December 9, 2002 Published online April 29, 2003  相似文献   

North polar region of Mars: Advances in stratigraphy, structure, and erosional modification   总被引：1，自引：0，他引：1  

Kenneth L. Tanaka  J. Alexis P. Rodriguez  Mary C. Bourke  Kenneth E. Herkenhoff  Chris H. Okubo 《Icarus》2008,196(2):318-358

We have remapped the geology of the north polar plateau on Mars, Planum Boreum, and the surrounding plains of Vastitas Borealis using altimetry and image data along with thematic maps resulting from observations made by the Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter spacecraft. New and revised geographic and geologic terminologies assist with effectively discussing the various features of this region. We identify 7 geologic units making up Planum Boreum and at least 3 for the circumpolar plains, which collectively span the entire Amazonian Period. The Planum Boreum units resolve at least 6 distinct depositional and 5 erosional episodes. The first major stage of activity includes the Early Amazonian (∼3 to 1 Ga) deposition (and subsequent erosion) of the thick (locally exceeding 1000 m) and evenly-layered Rupes Tenuis unit (Abrt), which ultimately formed approximately half of the base of Planum Boreum. As previously suggested, this unit may be sourced by materials derived from the nearby Scandia region, and we interpret that it may correlate with the deposits that regionally underlie pedestal craters in the surrounding lowland plains. The second major episode of activity during the Middle to Late Amazonian (1 Ga) began with a section of dark, sand-rich and light-toned ice-rich irregularly-bedded sequences (Planum Boreum cavi unit, Abb_c) along with deposition of evenly-bedded light-toned ice- and moderate-toned dust-rich layers (Planum Boreum 1 unit, Abb₁). These units have transgressive and gradational stratigraphic relationships. Materials in Olympia Planum underlying the dunes of Olympia Undae are interpreted to consist mostly of the Planum Boreum cavi unit (Abb_c). Planum Boreum materials were then deeply eroded to form spiral troughs, Chasma Boreale, and marginal scarps that define the major aspects of the polar plateau's current regional topography. Locally- to regionally-extensive (though vertically minor) episodes of deposition of evenly-bedded, light- and dark-toned layered materials and subsequent erosion of these materials persisted throughout the Late Amazonian. Sand saltation, including dune migration, is likely to account for much of the erosion of Planum Boreum, particularly at its margin, alluding to the lengthy sedimentological history of the circum-polar dune fields. Such erosion has been controlled largely by topographic effects on wind patterns and the variable resistance to erosion of materials (fresh and altered) and physiographic features. Some present-day dune fields may be hundreds of kilometers removed from possible sources along the margins of Planum Boreum, and dark materials, comprised of sand sheets, extend even farther downwind. These deposits also attest to the lengthy period of erosion following emplacement of the Planum Boreum 1 unit. We find no evidence for extensive glacial flow, topographic relaxation, or basal melting of Planum Boreum materials. However, minor development of normal faults and wrinkle ridges may suggest differential compaction of materials across buried scarps. Timing relations are poorly-defined mostly because resurfacing and other uncertainties prohibit precise determinations of surface impact crater densities. The majority of the stratigraphic record may predate the recent (<20 Ma) part of the orbitally-driven climate record that can be reliably calculated. Given the strong stratigraphic but loose temporal constraints of the north polar geologic record, a comparison of north and south polar stratigraphy permits a speculative scenario in which major Amazonian depositional and erosional episodes driven by global climate activity is plausible.  相似文献   

Is the Chandler period variable?   总被引：2，自引：0，他引：2  

Shuhei Okubo 《Geophysical Journal International》1982,71(3):629-646

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Time series analysis of natural remanent magnetization in deep-sea sediments     

Shuhei Okubo  Hitoshi Takeuchi 《Geophysical Journal International》1979,56(2):309-318

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Evaluations of subsurface flow for reconstructions of climate change using borehole temperature and isotope data in Kamchatka     

Akinobu Miyakoshi  Makoto Taniguchi  Yasukuni Okubo  Takeshi Uemura 《Physics of the Earth and Planetary Interiors》2005,152(4):335-342

Subsurface temperature is affected by heat advection due to groundwater flow and surface temperature changes. To evaluate their effects, it was implemented the measurements of temperature-depth profile (T-D profile) and the continuous monitoring of soil temperature in the southern part of Kamchatka which has not affected by human activity. Additionally, stable isotopic compositions of surface water and groundwater were analyzed. T-D profile and stable isotopic compositions show groundwater flow system is differ from the shallow aquifer to the deep aquifer. In the shallow aquifer, T-D profile suggests the existence of upward groundwater flux. On the other hand, the annual variation of soil temperature is divided into the large variation period (VP) and the stable period (SP) by the magnitude of daily and seasonal variation. VP and SP correspond to the summer and the winter season, respectively, and it considers that the difference between VP and SP is caused by the effect of snow cover. Therefore, the T-D profile is affected by not only upward groundwater flux but also the surface warming particularly in the summer season (VP).  相似文献   

An Analytical Investigation of a Variable-Compliance-type Constitutive Equation     

S. Okubo  K. Fukui 《Rock Mechanics and Rock Engineering》2006,39(3):233-253

Summary. A constitutive equation is proposed in which the compliance is assumed to monotonically increase as a load is applied. The primary feature of the constitutive equation is that the equation can be applied to various loading conditions such as constant stress rate, constant strain rate, creep, or relaxation. The second feature is that the equation has exact solutions under many loading conditions. The present paper shows the exact solutions for the constitutive equation and investigates the mutual relationships between the exact solutions for the different loading conditions. The third feature is that it is comparatively easy to find the constants in the constitutive equation. The present paper shows how to solve the constitutive equation for the constants, and the constants for some native Japanese rocks. The constitutive equation used in the present paper is extremely simple. Therefore, the equation can be easily implemented in almost any FEM code. It is likely that additional terms of the constitutive equation will prove necessary for practical usage. However, additional terms can be found very easily by finding higher-order approximations of experimental data.  相似文献