Experimental investigation into methane production from hydrate-bearing clayey sediment by CO2/N2 replacement

The replacement of gas hydrate in clayey sediment by a CO2/N2 (20:80) gas mixture injection was experimentally studied to explore the influence of clay on the gas exchange behaviours in the gas hydrate. Clay (montmorillonite) and silica sand were mixed in three different proportions (clay mass ratios of 10%, 30% and 50%) to simulate the host sediments of natural gas hydrate while pure silica sand sediment was selected for comparison. Experimental results showed that clay hindered gas diffusion during the initial replacement stage and thus reduced the methane recovery rate. In the later stage, the gas exchange between CO2/N2 and methane in the hydrate structure might be subject to thermodynamic inhibition and geometric constraints of the clay interlayer. Moreover, the CO2 sequestration ratio was lowered significantly in the sediment with a 50% clay mass ratio. Therefore, it was determined that clay has an inhibitory effect on gas hydrate replacement by CO2/N2.

Preliminary Economic Evaluation of Deep-Sea REE Mud Mining

Deep-sea rare-earth element-rich mud (REE mud) distributes in pelagic clayey sediment column on ocean seafloor at 4,000–6,000 m deep. The thickness ranges 5–80 m and the burial depth 0–100 m. The REE contents range 600–2,250 ppm in the Pacific and one of the richest, maximum 6,500ppm, has been found near Minami-Tori-shima (Marcus Is.) in Japan’s exclusive economic zones. Assuming a conventional hydraulic excavation and lifting methods, the economy of REE mud mining near Minami-Tori-shima (Marcus Is.) is preliminary examined. Because the contents of valuables in REE mud are low and the ones of wastes are high, quite little chance for the economically feasible mining has been found from the result.

Looking through drumlins: testing the application of ground-penetrating rada

Ground-penetrating radar (GPR) is becoming a commonly applied technique in geomorphology. However, its use in the study of subglacial bedforms has yet to be fully explored and exploited. This paper presents the results of a GPR feasibility study conducted on a drumlinized terrain in Cumbria, UK, where five drumlins were investigated using multiple radar antenna frequencies. The site was selected for the presence of nearby bedrock outcrops, suggesting a shallow drumlinized diamict–bedrock contact and a permeable lithology. Despite the clayey sediment and unfavourable weather conditions, a considerable penetration depth of ~12 m was achieved when using a 50 MHz antenna, with a separation of 1 m, trace spacing of 1 m and 128-fold vertical stack. Results indicate that the drumlinized diamict is in direct erosional contact with the bedrock. While the internal drumlin geometry is generally chaotic on the stoss side, evidence of layering dipping downflow at an angle greater than the drumlin surface profile was found on the lee side. The inter-drumlin areas comprise ~4 m of infill sediment that masks part of the original drumlin profile. Overall, this study indicates that GPR can be deployed successfully in the study of glacial bedform sedimentary architecture.

Attenuation of TNT in seawater microcosms

The ability of two differing marine sediments (one clayey, the other sandy) to attenuate the explosive 2,4,6-trinitrotoluene (TNT), dissolved in intertidal seawater from the eastern English coast of the North Sea, was examined using aerobic microcosms. Analysis of the seawater from the microcosms revealed an initial sharp decline in TNT concentration with clayey sediment in both sterilized (to prevent microbial activity) and unsterilized microcosms. This effect did not occur to such a marked extent in similar sterile and non-sterile sandy sediment microcosms and was attributed mainly to sorption of TNT to the fine clay particles of the clayey sediment. As time progressed, the attenuation of TNT in microcosms containing either type of sediment was found to be less in those that had been sterilized compared with those where microbial action proceeded unhindered. Feeding the microcosms, (i.e. supplying extra carbon sources for the microbial communities), appeared to have a small, but perceptible, enhancing effect upon TNT dissipation. The attenuation of TNT was also measured in large microcosms containing 2.5 L of seawater and no sediment. Analysis of the seawater revealed a gradual decline in TNT concentration in non-sterile and fed microcosms compared to their sterile counterpart. Overall, this laboratory study showed that the attenuation of TNT is slow (half-life in seawater ca.1900 days; half-life sand sediment <700 days; half life in clay sediment 130 days) under conditions commonly encountered in coastal waters of the North Sea.

Mechanical and hydraulic behaviour of a soft inorganic clay treated with lime

A comprehensive series of laboratory tests was conducted on an inorganic clayey sediment in order to predict the mechanical behaviour of dredged sediments used in reclamation projects. The soil used was a Louiseville clay, which was mixed with various quantities of lime (0–10%) and had a varied water content (122–650%). Tests were carried out with special large cells and standard oedometers to look at both the compressibility and the hydraulic conductivity. Once pozzolanic reactions are begun, for a given curing time, a linear relationship between preconsolidation pressure and lime concentration is observed. Compressibility results indicate that it is possible to define a separate compression curve for each lime concentration and curing time. Adding lime will influence the hydraulic conductivity both by flocculation and by the formation of secondary minerals. If only flocculation occurs, an increase in hydraulic conductivity follows. If enough lime is added, the resulting secondary minerals will create major changes in the micropore network and this will decrease the hydraulic conductivity by up to one order of magnitude. The development of a secondary micropore network along with the cementation of large flocs induce significant increases in both the liquid and plastic limits. This results in a significant increase in the water sorption potential and higher Atterberg limits. Key words: dredged sediments, compressibility, hydraulic conductivity, lime, stabilization, microstructure.

Mise en évidence d'une erreur de fossilisation du champ magnétique terrestre dans un dépôt actuel

We present a study of the remanent magnetization of the so-called "ocres d'Apt" clayey sediment, resulting from an artificial sifting carried out industrially for obtaining pigments. Our investigation concerned 172 specimens of several hundred of cubic centimetres, corresponding to a sampling distributed over the whole area of four basins (approximately 600 m2) and 10 m along the axis of a channel. We show that the main carrier of the magnetization is hematite and that the primary magnetization is a detrital one. An extremely moderate thermal treatment (<88 °C) erases the intense viscous magnetization acquired since the sampling (10 years) but preserves a weak component of a primary magnetization, a differential analysis of which (comparison between sites), as well as laboratory drying experiments, shows that it was acquired before the complete drying out of the clay and that its direction is not modified, either by currents that existed during some phases of the sedimentation, or by the final drying out. The intrasite mean direction of the primary magnetization is approximately identical for three of the basins and for the channel, with a zero difference in declination with, but with an inclination that is 16 °shallower than, that of the Earth's magnetic field. As for the fourth basin, the declination coincides with, but the inclination is 13 °higher than, that of the field.