Distribution and Variability of the Meiobenthic Assemblages near the Korean Polymetallic Nodule Claim Area of the Clarion-Clipperton Fracture Zone (Subequatorial NE Pacific)

This study was carried out as part of a baseline long-term environmental project in the proposed mining areas for an environmental impact assessment of future mining in the Clarion-Clipperton Fracture Zone (CCFZ). The community structure and distributional pattern of meiobenthos were investigated in the deep-sea bottom of the Clarion-Clipperton Fracture Zone of the northeastern Pacific in July 1998, 1999, 2001, 2003 and August 2004, 2005. Twenty one meiobenthic groups were found at the stations in the study area. The most abundant meiobenthos comprised nematodes followed by benthic foraminiferans and harpacticoid copepods. The maximum density of meiobenthos was 306 ind/10 cm² at the station located at 11°N (water depth, 4833 m), and the minimum density was 6 ind/10 cm² at the station located at 14°N (water depth, 5037 m). Oligotrophic conditions in the CCFZ seem to directly reflect the lower standing stocks of meiobenthos in the CCFZ compared to other deep-sea plains of similar depth. The latitudinal distribution pattern of meiobenthos in the study area seemed to be related with surface water primary productivity, which was connected to the water circulation pattern of the Pacific Ocean near the Equator, diverging at 8ºN latitude and converging at 5°N. The horizontal distribution of meiobenthic organisms in the study area showed high densities at the stations within 135–136°W. The densities of meiobenthic organismas within the CCFZ were high at stations with few manganese nodules on the sediment surface at low-latitude sites. In 1998, the observed relative high values of meiobenthic abundance were at stations from 5° to 6°N. Other stations from 7° to 10°N showed no significant fluctuations during the interannual sampling periods. It is considered that the inter-annual fluctuation of meiobenthos abundance is intimately related with a regime shift that may have occurred in the north Pacific between 1998 and 1999, the El Niño period. Vertical distribution of meiobenthic organisms showed the highest individual numbers in the surface sediment layers of 0～2 cm depth and a steep decreasing trend as sediment becomes deeper at the stations of high latitude located in 16～17°N. Size distribution analyses showed that organisms that fit into the sieve mesh size of 0.063 mm were abundant.     

Flood modelling using the distributed event-based SCS-LR model in the Mediterranean Real Collobrier catchment

ABSTRACT

Event-based models are often used for flood prediction because they require fewer data than more complex models and account for a small number of parameters. We present the performance of such a model in simulating Mediterranean floods, with a focus on the initialization and on the impact of the rainfall uncertainties on the calibration of the model. The distributed event-based parsimonious Soil Conservation Service Lag-and-Route (SCS-LR) model was applied in the Real Collobrier catchment, France, which has a very high density of raingauges. The initial condition of the model was highly correlated with predictors, such as baseflow or soil water content. A reduction in the raingauge density can markedly change the calibration of the model. As the density of raingauges is generally low in most catchments, the uncertainties associated with rainfall measurement are thus expected either to mask the actual accuracy of the model, or to alter the model parameters.     

3D acoustic waveform inversion in the Laplace domain using an iterative solver

In this paper we propose a 3D acoustic full waveform inversion algorithm in the Laplace domain. The partial differential equation for the 3D acoustic wave equation in the Laplace domain is reformulated as a linear system of algebraic equations using the finite element method and the resulting linear system is solved by a preconditioned conjugate gradient method. The numerical solutions obtained by our modelling algorithm are verified through a comparison with the corresponding analytical solutions and the appropriate dispersion analysis. In the Laplace‐domain waveform inversion, the logarithm of the Laplace transformed wavefields mainly contains long‐wavelength information about the underlying velocity model. As a result, the algorithm smoothes a small‐scale structure but roughly identifies large‐scale features within a certain depth determined by the range of offsets and Laplace damping constants employed. Our algorithm thus provides a useful complementary process to time‐ or frequency‐domain waveform inversion, which cannot recover a large‐scale structure when low‐frequency signals are weak or absent. The algorithm is demonstrated on a synthetic example: the SEG/EAGE 3D salt‐dome model. The numerical test is limited to a Laplace‐domain synthetic data set for the inversion. In order to verify the usefulness of the inverted velocity model, we perform the 3D reverse time migration. The migration results show that our inversion results can be used as an initial model for the subsequent high‐resolution waveform inversion. Further studies are needed to perform the inversion using time‐domain synthetic data with noise or real data, thereby investigating robustness to noise.     

Modeling of damage,permeability changes and pressure responses during excavation of the TSX tunnel in granitic rock at URL,Canada

This paper presents numerical modeling of excavation-induced damage, permeability changes, and fluid-pressure responses during excavation of a test tunnel associated with the tunnel sealing experiment (TSX) at the Underground Research Laboratory (URL) in Canada. Four different numerical models were applied using a wide range of approaches to model damage and permeability changes in the excavation disturbed zone (EDZ) around the tunnel. Using in situ calibration of model parameters, the modeling could reproduce observed spatial distribution of damage and permeability changes around the tunnel as a combination of disturbance induced by stress redistribution around the tunnel and by the drill-and-blast operation. The modeling showed that stress-induced permeability increase above the tunnel is a result of micro and macrofracturing under high deviatoric (shear) stress, whereas permeability increase alongside the tunnel is a result of opening of existing microfractures under decreased mean stress. The remaining observed fracturing and permeability changes around the periphery of the tunnel were attributed to damage from the drill-and-blast operation. Moreover, a reasonably good agreement was achieved between simulated and observed excavation-induced pressure responses around the TSX tunnel for 1 year following its excavation. The simulations showed that these pressure responses are caused by poroelastic effects as a result of increasing or decreasing mean stress, with corresponding contraction or expansion of the pore volume. The simulation results for pressure evolution were consistent with previous studies, indicating that the observed pressure responses could be captured in a Biot model using a relatively low Biot-Willis’ coefficient, α ≈ 0.2, a porosity of n ≈ 0.007, and a relatively low permeability of k ≈ 2 × 10⁻²² m², which is consistent with the very tight, unfractured granite at the site.     

A case study on the influence of THM coupling on the near field safety of a spent fuel repository in sparsely fractured granite

In order to demonstrate the feasibility of geological disposal of spent CANDU fuel in Canada, a safety assessment was performed for a hypothetical repository in the Canadian Shield. The assessment shows that the maximum long term radionuclide release from such repository would meet international criteria for dose rate; however, uncertainties in the assumed evolution of the repository were identified. Such uncertainties could be resolved by the consideration of coupled Thermal-Hydro-Mechanical-Chemical (THMC) processes. In Task A of the DECOVALEX-THMC project, THM models were developed within the framework of the theory of poroelasticity. Such model development was performed in an iterative manner, using experimental data from laboratory and field tests. The models were used to perform near-field simulations of the evolution of the repository in order to address the above-mentioned uncertainties. This paper presents the definition and rationale of task A and the results of the simulations. From a repository safety point of view, the simulations predict that the maximum temperature would be well below the design target of 100°C; however, the stress on the container can marginally exceed the design value of 15 MPa. However, the most important finding from the simulations is that a rock damage zone could form around the emplacement borehole. Such damage zone can extend a few metres from the walls of the emplacement holes, with permeability values that are orders of magnitude higher than the initial values. The damage zone has the potential to increase the radionuclide transport flux from the geosphere; the effect of such an increase should be taken into account in the safety assessment and mitigated if necessary by the provision of sealing systems. Prepared for publication in Environmental Geology. DECOVALEX-THMC Special Issue.     

Correlations Between Shear Wave Velocity and In-Situ Penetration Test Results for Korean Soil Deposits

Shear wave velocity (V _S) can be obtained using seismic tests, and is viewed as a fundamental geotechnical characteristic for seismic design and seismic performance evaluation in the field of earthquake engineering. To apply conventional geotechnical site investigation techniques to geotechnical earthquake engineering, standard penetration tests (SPT) and piezocone penetration tests (CPTu) were undertaken together with a variety of borehole seismic tests for a range of sites in Korea. Statistical modeling of the in-situ testing data identified correlations between V _S and geotechnical in-situ penetration data, such as blow counts (N value) from SPT and CPTu data including tip resistance (q _t), sleeve friction (f _s), and pore pressure ratio (B _q). Despite the difference in strain levels between conventional geotechnical penetration tests and borehole seismic tests, it is shown that the suggested correlations in this study is applicable to the preliminary determination of V _S for soil deposits.     

Paleomagnetic results from deep-sea sediment of the Korea Deep Ocean Study (KODOS) area (northern equatorial Pacific) and their paleodepositional implications

Paleomagnetic properties of sediment cores were examined to reconstruct paleodepositional conditions in the Korea Deep Ocean Study (KODOS) area, located in the northeastern equatorial Pacific. The studied KODOS sediments have a stable remanent magnetization with both normal and reversed polarities, which are well correlated with the geomagnetic polarity timescale for the late Pliocene and Pleistocene. Average sedimentation rates are 1.56 and 0.88 mm/kiloyear for the Pleistocene and late Pliocene, respectively. Clay mineralogy and scanning electron microscope analyses of the sediments indicate that terrestrial material was transported to the deep-sea floor during these times. The variations of sedimentation rates with age may be explained by the onset of the northern hemisphere glaciation and subsequent climatic deterioration during the Pliocene and Pleistocene. For the Pleistocene, an increasing sedimentation rate implies that input of terrestrial materials was high, and also a high input of biogenic materials was detected as a result of increased primary production in the surface water. The down-core variations in paleomagnetic and rock-magnetic properties of the KODOS sediments were affected by dissolution processes in an oxic depositional regime. As shown by magnetic intensity and hysteresis parameters, the high natural remanent magnetization (NRM) stability in the upper, yellowish brown layers indicates that the magnetic carrier was in pseudo-single domain states. In the lower, dark brown sediments, only coarse magnetic grains survived dissolution and the NRM was carried by more abundant, multi-domain grains of low magnetic stability. The down-core variation of magnetic properties suggests that the KODOS sediments were subjected to dissolution processes resulting in a loss of the more stable components of the magnetic fraction with increasing core depth.     

野外采样数字装图法原理

数字装图法的基本思想,是在开阔地区用电子速测仪直接测定地形点的坐标,而在半开阔地区和隐蔽地区则用电子速测仪测定少量明显地物点作骨干点,再用勘丈记录计算法和勘丈绘图数字化法,将一块一块的数字图形装进图幅中。