Acoustic density measurements of consolidating cohesive sediment beds by means of a non-intrusive “Micro-Chirp” acoustic system     

Ho Kyung Ha  Jerome P.-Y. Maa  Charles W. Holland 《Geo-Marine Letters》2010,30(6):585-593

A non-intrusive “Micro-Chirp” acoustic system and a signal-processing protocol have been developed to estimate the bulk density of consolidating cohesive sediment beds. Using high-frequency (300–700 kHz) Chirp acoustic waves, laboratory measurements were conducted with clay–water mixtures. Because acoustic echo strength is proportional to variations in acoustic impedance, and the speed of sound in the clay bed hardly changed during consolidation, the bulk density could be successfully estimated without disturbing the sediment bed. Based on acoustic signal analysis, this study demonstrates that the reflection coefficient and bulk density at the water–sediment interface increase with consolidation time, and that a single speed of sound value can be used for practical bulk density estimation in muddy environments.  相似文献   

A generic approach to modelling flexible confined boundary conditions in SPH and its application     

Shaohan Zhao  Ha H. Bui  Vincent Lemiale  Giang D. Nguyen  Felix Darve 《国际地质力学数值与分析法杂志》2019,43(5):1005-1031

In this paper, a new approach to applying confining stress to flexible boundaries in the smoothed particle hydrodynamics (SPH) method is developed to facilitate its applications in geomechanics. Unlike the conventional SPH methods that impose confining boundary conditions by creating extra boundary particles, the proposed approach makes use of kernel truncation properties of SPH approximations that occur naturally at free-surface boundaries. Therefore, it does not require extra boundary particles and, as a consequence, can be utilised to apply confining stresses onto any boundary with arbitrary geometry without the need for tracking the curvature change during the computation. This enables more complicated problems that involve moving confining boundaries, such as confining triaxial tests, to be simulated in SPH without difficulties. To further enhance SPH applications in elasto-plastic computations of geomaterials, a robust numerical procedure to implement Mohr-Coulomb plasticity model in SPH is presented for the first time to avoid difficulties associated with corner singularities in Mohr-Coulomb model. The proposed approach was first validated against two-dimensional finite element (FE) solutions for confining biaxial compression tests to demonstrate its predictive capability at small deformation range when FE solutions are still valid. It is then further extended to three-dimensional conditions and utilised to simulate triaxial compression experiments. Simulation results predicted by SPH show good agreement with experiments, FE solutions, and other numerical results available in the literature. This suggests that the proposed approach of imposing confining stress boundaries is promising and can handle complex problems that involve moving confining boundary conditions.  相似文献   

Spathian to Aegean (upper Lower Triassic to lower Middle Triassic) carbon isotope stratigraphy constrained by the conodont biostratigraphy of carbonates on top of a mid-oceanic seamount formed in the Panthalassic Ocean     

Thuy Thi Nhu Ha  Takumi Maekawa  Hideko Takayanagi  Yasufumi Iryu 《Island Arc》2021,30(1):e12391

The Taho Formation in western Shikoku Island, Japan, consists of Triassic carbonates that formed on a seamount in the Panthalassic Ocean. In order to investigate the stratigraphy and paleoceanography of this carbonate succession, we analyzed the biostratigraphy and chemostratigraphy of a 17.6 m-thick section of the upper Taho Formation at the stratotype area in Tahokamigumi, Seiyo City. This section comprises bioclastic limestone containing Triassic bivalves, ammonoids, and conodonts. We recognized six conodont zones (in ascending order): the Novispathodus pingdingshanensis, Novispathodus brevissimus, Triassospathodus symmetricus, Triassospathodus homeri, Chiosella timorensis, and Magnigondolella cf. alexanderi zones. Thus, the studied carbonate succession is latest Smithian to Aegean in age. A δ¹³C profile of this section shows elevated values during the lowest Spathian followed by a gradual negative excursion, a subsequent positive excursion near the Spathian–Aegean boundary, and relatively constant values during the Aegean. The characteristic series of negative and positive excursions correlates with other δ¹³C records for this period, including the peak of the upper Smithian–lowest Spathian positive excursion (P3), lower to middle Spathian negative excursion (N4), and middle Spathian–lowest Aegean positive excursion (P4). This represents a new high-resolution Spathian–Aegean δ¹³C record of the Panthalassic Ocean, for which ages are constrained by conodont biostratigraphy. The Taho δ¹³C profile exhibits a consistent positive offset of ~2 ‰ as compared with those from other regions (i.e., mostly in the Tethyan Ocean). This can be explained by preferential removal of ¹²C from seawater during photosynthesis and calcification by marine organisms over the platform, and/or the relatively high δ¹³C values of dissolved inorganic carbon in the Panthalassic Ocean due to less influence of ¹²C-enriched terrestrial waters and high marine organic production/burial as compared with the more restricted Tethyan Ocean.  相似文献   

Evaluation of multiple regression models using spatial variables to predict nitrate concentrations in volcanic aquifers          下载免费PDF全文

Youn‐Young Jung  Dong‐Chan Koh  Won‐Bae Park  Kyoochul Ha 《水文研究》2016,30(5):663-675

Multiple linear regression of spatial variables including land use, soil type, and topography was applied to predict nitrate concentration and evaluate major factors affecting nitrate occurrence in springs and wells in the southern and northern areas of Jeju volcanic island, Korea. Three types of contributing area surrogates (CAS), namely circle, semicircle, and wedge, were employed to calculate the spatial variables. The regression results showed R² of 0.81–0.84 for springs and 0.74–0.77 for wells; R² values for wedge and semicircular CAS were more than 10% higher than those for circular CAS. The R² of spring models was significantly affected by both the shape and size of CAS, with optimal radii of 150–250 m and 300–400 m in the southern and northern areas, respectively, corresponding to thinner upper basaltic aquifers, and implying shorter flow paths in the southern area. The most influential variables in springs were orchards and soil types related to agriculture including silty loam and silty clay loam, indicating that nitrate levels are strongly affected by N fertilization in cultivated areas. In contrast, wells showed much less sensitivity to both shapes and sizes of CAS, with less contribution of land use and soil type to the regression, which could be attributed to a mix of multiple aquifer zones and widely different factors in the installation and operation of wells. Field parameters of electrical conductivity (EC) and pH increased the R² up to 10%, suggesting that these can be useful when regression with spatial variables yields a lower R². The optimal spatial scales for prediction of nitrate concentration and spatial variables that significantly contribute to nitrate contamination can provide relevant criteria for establishing groundwater management policies, considering the increasing anthropogenic land‐use trends on the island, where groundwater is highly sensitive to changes in spatial variables. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

A comparison of climatological subseasonal variations in the wintertime storm track activity between the North Pacific and Atlantic: local energetics and moisture effect   总被引：1，自引：0，他引：1  

Sun-Seon Lee  June-Yi Lee  Bin Wang  Fei-Fei Jin  Woo-Jin Lee  Kyung-Ja Ha 《Climate Dynamics》2011,37(11-12):2455-2469

Distinct differences of the storm track?Cjet relationship over the North Pacific and North Atlantic are investigated in terms of barotropic and baroclinic energetics using NCEP-2 reanalysis data for the period of 1979?C2008. From fall to midwinter the Pacific storm track (PST) activity weakens following the southward shift of the Pacific jet, whereas the Atlantic storm track (AST) activity remains steady in position and intensifies regardless of the slight southward shift of the Atlantic jet. This study is devoted to seeking for the factors that can contribute to this conspicuous difference between the two storm tracks on climatological subseasonal variation by analyzing eddy properties and local energetics. Different eddy properties over the two oceans lead to different contribution of barotropic energy conversion to the initiation of storm tracks. In the North Atlantic, meridionally elongated eddies gain kinetic energy efficiently from stretching deformation of the mean flow in the jet entrance. On the other hand, the term associated with shearing deformation is important for the initiation of PST. Analysis of baroclinic energetics reveals that the intensification of the AST activity in midwinter is mainly attributed to coincidence between location of maximum poleward and upward eddy heat fluxes and that of the largest meridional temperature gradient over slight upstream of the AST. The relatively large amount of precipitable water and meridional eddy moisture flux along baroclinic energy conversion axis likely provides a more favorable environment for baroclinic eddy growth over the North Atlantic than over the North Pacific. In the meantime, the midwinter minimum of the PST activity is attributable to the southward shift of the Pacific jet stream that leads to discrepancy between core region of poleward and upward heat fluxes and that of meridional thermal gradient. Weakening of eddy-mean flow interaction due to eddy shape and reduction of moist effect are also responsible for the weakening of storm track activities in midwinter when the strongest baroclinicity exists over the North Pacific.  相似文献   

Future change of extreme temperature climate indices over East Asia with uncertainties estimation in the CMIP5     

Ye-Won Seo  Hojin Kim  Kyung-Sook Yun  June-Yi Lee  Kyung-Ja Ha  Ja-Yeon Moon 《Asia-Pacific Journal of Atmospheric Sciences》2014,50(1):609-624

How well the climate models simulate extreme temperature over East Asia and how the extreme indices would change under anthropogenic global warming are investigated. The indices studied include hot days (HD), tropical nights (TN), growing degree days (GDD), and cooling degree days (CDD) in summer and heating degree days (HDD) and frost days (FD) in winter. The representative concentration pathway 4.5 (RCP 4.5) experiments for the period of 2075–2099 are compared with historical simulations for the period of 1979–2005 from 15 coupled models that are participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5). To optimally estimate future change and its uncertainty, groups of best models are selected based on Taylor diagrams, relative entropy, and probability density function (PDF) methods previously suggested. Overall, the best models’ multi-model ensemble based on Taylor diagrams has the lowest errors in reproducing temperature extremes in the present climate among three methods. Selected best models in three methods tend to project considerably different changes in the extreme indices from each other, indicating that the selection of reliable models are of critical importance to reduce uncertainties. Three groups of best models show significant increase of summerbased indices but decrease of the winter-based indices. Over East Asia, the most significant increase is seen in the HD (336 ± 23.4% of current climate) and the most significant decrease is appeared in the HDD (82 ± 4.2%). It is suggested that the larger future change in the HD is found over in the Southeastern China region, probably due to a higher local maximum temperature in the present climate. All of the indices show the largest uncertainty over Southeastern China, particularly in the TN (~3.9 times as large as uncertainty over East Asia) and in the HD (~2.4). It is further noted that the TN reveals the largest uncertainty over three East Asian countries (~1.7 and 1.4 over Korea and Japan, respectively). These future changes in extreme temperature events have an important implication for energy-saving applications and human molarity in the future.  相似文献   

Future change of the Indian Ocean basin-wide and dipole modes in the CMIP5     

Jung-Eun Chu  Kyung-Ja Ha  June-Yi Lee  Bin Wang  Byeong-Hee Kim  Chul Eddy Chung 《Climate Dynamics》2014,43(1-2):535-551

The Indian Ocean sea surface temperature (SST) variability has been represented with the two dominant variability modes: the Indian Ocean basin-wide (IOBW) and dipole (IOD) modes. Here we investigate future changes of the two modes together with mean state and El Niño and Southern Oscillation (ENSO) relationship under the anthropogenic global warming using 20 coupled models that participated in the phase five of Coupled Model Intercomparison Project by comparing the historical run from 1950 to 2005 and the RCP 4.5 run from 2050 to 2099. The five best models are selected based on the evaluation of the 20 models’ performances in simulating the two modes and Indian Ocean basic state for the latest 56 years. They are capable of capturing the IOBW and IOD modes in their spatial distribution, seasonal cycle, major periodicity, and relationship with ENSO to some extent. The five best models project the significant changes in the Indian Ocean mean state and variability including the two dominant modes in the latter part of twenty-first century under the anthropogenic warming scenario. First, the annual mean climatological SST displays an IOD-like pattern change over the Indian Ocean with enhanced warming in the northwestern Indian Ocean and relatively weaker warming off the Sumatra–Java coast. It is also noted that the monthly SST variance is increased over the eastern and southwestern Indian Ocean. Second, the IOBW variability on a quasi-biennial time scale will be enhanced due to the strengthening of the ENSO–IOBW mode relationship although the total variance of the IOBW mode will be significantly reduced particularly during late summer and fall. The enhanced air-sea coupling over the Indian-western Pacific climate in response to El Nino activity in the future projection makes favorable condition for a positive IOD while it tends to derive relatively cold temperature over the eastern Indian Ocean. This positive IOD-like ENSO response weakens the relationship between the eastern Indian Ocean and El Nino while strengthens the relationship with western Indian Ocean. Third, the IOD mode, intrinsic coupled mode of the Indian Ocean may not be changed appreciably under the anthropogenic global warming.  相似文献   

Assessing sustainability of groundwater resources on Jeju Island,South Korea,under climate change,drought, and increased usage   总被引：2，自引：0，他引：2  

Aly I. El-Kadi  Suzanne Tillery  Robert B. Whittier  Benjamin Hagedorn  Alan Mair  Kyoochul Ha  Gi-Won Koh 《Hydrogeology Journal》2014,22(3):625-642

Numerical groundwater models were used to assess groundwater sustainability on Jeju Island, South Korea, for various climate and groundwater withdrawal scenarios. Sustainability criteria included groundwater-level elevation, spring flows, and salinity. The latter was studied for the eastern sector of the island where saltwater intrusion is significant. Model results suggest that there is a need to revise the current estimate of sustainable yield of 1.77?×?10⁶ m³/day. At the maximum extraction of 84  % of the sustainable yield, a 10-year drought scenario would decrease spring flows by 28 %, dry up 27 % of springs, and decrease hydraulic head by an island-wide average of 7 m. Head values are particularly sensitive to changes in recharge in the western parts of the island, due to the relatively low hydraulic conductivity of fractured volcanic aquifers and increased groundwater extraction for irrigation. Increases in salinity are highest under drought conditions around the current 2-m head contour line, with an estimated increase of up to 9 g/L under 100 % sustainable-yield use. The study lists recommendations towards improving the island’s management of potable groundwater resources. However, results should be treated with caution given the available data limitations and the simplifying assumptions of the numerical modeling approaches.  相似文献   

1951-2013年格尔木及周边地区日照时数气候变化特征分析_*     

郭晓宁  杨延华  李海凤  哈金龙 《沙漠与绿洲气象(新疆气象)》2015,9(6):62-67

利用柴达木盆地格尔木站及周边9个气象站近60a逐月、年日照时数、气温、云量、降水、相对湿度资料以及1951~2013年西太平洋副高面积指数资料,通过统计学方法、突变检验、小波分析等方法,对格尔木日照时数的气候变化特征进行了分析。结果表明：近60a来格尔木日照时数变化总体呈下降趋势,春季变化较平稳,夏、秋、冬三季呈明显的减少趋势。月日照时数5月最多、2月最少。年代际变化上世纪50年代和60年代较为平稳,70年代突然下降,80年代又有所增加,90年代比80年代略有减少。到了本世纪头十年,平均日照时数急剧减少。日照时数具有明显的5年和7年短周期,日照时数在2006年出现突变。从研究结果来看,自然因子大气环流、气温、总云量、低云量、相对湿度等因素是格尔木及周边地区日照时数变化的主要影响因子,人工增雨为主的人类活动和三江源地区生态保护工程的实施对格尔木及周边地区日照时数也可能是影响因子,但这还有待进一步研究证实。  相似文献   

Recurrence relations for computing complete P and SV seismograms     

Joseph Ha 《Geophysical Journal International》1984,79(3):863-873

Summary. A set of recurrence relations which are computationally more efficient than those of the reflection matrix method of Kennett & Kerry is presented for P - and SV -wave generation in a ( n + 1) layered medium. The recurrence relations contain no growing terms and thus provide a stable algorithm for computing complete P and SV synthetic seismograms. Our algorithm requires a fewer algebraic operations for computing the reflectivity and transmissivity coefficients, ranging from 15 per cent less for a source in the half-space to 30 per cent less for a source in the top layer, than the reflection matrix method.  相似文献