A new estimate of global methane flux from onshore and shallow submarine mud volcanoes to the atmosphere

A new estimate of global methane emission into the atmosphere from mud volcanoes (MVs) on land and shallow seafloor is presented. The estimate, considered a lower limit, is based on 1) new direct measurements of flux, including both venting of methane and diffuse microseepage around craters and vents, and 2) a classification of MV sizes in terms of area (km²) based on a compilation of data from 120 MVs. The methane flux to the atmosphere is conservatively estimated between 6 and 9 Mt y^–1. This emission from MVs is 3–6% of the natural methane sources and is comparable with ocean and hydrate sources, officially considered in the atmospheric methane budget. The total geologic source, including MVs, seepage from seafloor, microseepage in hydrocarbon-prone areas and geothermal sources, would amount to 35–45 Mt y^–1. The authors believe it is time to add this parameter in the Intergovernmental Panel on Climate Change official tables of atmospheric methane sources.GEM     

The mud volcanoes of Pakistan

Marine-geologic investigations on the Arabian Sea by Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) in 1995 and 1998, and land expeditions in 1998 and 1999 to the coastal regions of the Makran Desert/Pakistan have extended the knowledge of the aerial distribution of mud volcanoes. These structures rise from under-compacted formations within the regional accretionary prism, which is built by the subduction of the oceanic crust of the Arabian Sea and its km-thick sedimentary load. The occurrence of mud volcanoes is limited to the abyssal plain near the accretionary front, to the coastal region of the Makran Desert and to a region in the interior of the Desert to the south to southeast of the so-called Hinglay Synform. The location of mud volcanoes in Pakistan is clearly tied to fault systems. Mud volcanoes are conspicuously absent on the lower slope of the accretionary prism, where thick gas hydrate layers have developed. The presence of large gas plumes emerging from the seafloor landward of the gas hydrate stability zone at water depths of less than 800 m points to a redirection of fluids from depth, which might explain the absence of mud volcanoes along the lower slope.     

Multi-electrode resistivity survey for groundwater exploration in the Harare greenstone belt, Zimbabwe

A multi-electrode resistivity survey, carried out over metasedimentary strata and metavolcanics in the Harare greenstone belt in northeastern Zimbabwe as part of a groundwater resources investigation, illustrates the ability of this technique to produce high-resolution images of the subsurface, which are useful for groundwater resources assessment. The resistivity results provide a clear view of the thickness of the weathered regolith and of the distribution of the various lithological units. Using a combination of apparent formation resistivity and inferred depth of weathering, it is possible to characterize the various lithologies on the geophysical profiles. These assigned lithologies show excellent correlation with the mapped geology, and the main lithologies, metabasalt, meta-arenite, granodiorite and banded iron formation can be clearly identified. The banded iron formation is characterized by low resistivity values, while a combination of the depth of weathering and resistivity values are used to distinguish between the meta-arenite on one hand and the metabasalt and granodiorite on the other. The multi-electrode method is successful in identifying potentially favourable zones for obtaining groundwater, such as areas with a maximum depth of weathered regolith, zones of fracturing and faulting, and high porosity and permeability zones associated with lithological contacts.

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Resumen Un estudio de resistividad multi-electrodo que se llevó a cabo en estratos meta sedimentarios y metavolcánicos en el cinturón de rocas verdes de Harare, en la parte noreste de Zimbabwe, como parte de una investigación de recursos de agua subterránea, ilustra la capacidad de esta técnica para producir imágenes de alta resolución de la subsuperficie, las cuales son útiles para la evaluación de los recursos da agua subterránea. Los resultados de resistividad suministran una visión clara el espesor del regolito meteorizado y de la distribución de varias unidades litológicas. Mediante el uso de una combinación de resistividad aparente de formación y de una profundidad inferida de meteorización, es posible caracterizar las diferentes litologías en los perfiles geofísicos. Estas litologías asignadas muestran una excelente correlación con la cartografía geológica y se pueden identificar las litologías predominantes como metabasalto, meta-arenita, granodiorita y formación de hierro bandeado. Esta última se caracteriza por valores bajos de resistividad, mientras que se utiliza una combinación de profundidad de meteorización y valores de resistividad, para distinguir entre meta-arenita por un lado y metabasalto y granodiorita por el otro. El método multi-electrodo es exitoso para identificar zonas potencialmente favorables para captar agua subterránea, las cuales pueden ser áreas con una profundidad máxima de regolito meteorizado, o bien zonas de fracturamiento y fallamiento, o también zonas de porosidad y permeabilidad altas asociadas con contactos litológicos.

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Résumé La prospection électrique par la méthode de multiélectrodes, effectuée pour linvestigation des eaux souterraines cantonnés dans des roches métamorphiques dorigine sédimentaire et volcanique de la ceinture de Harare-Zimbabwe a mis en évidence les possibilités de cette technique de réaliser des images à grande résolution du sous-sol qui sont très utiles pour lévaluation de la ressource en eaux souterraines. Daprès la distribution de la résistance il résulte une image très claire de lépaisseur des regolithes altérés ainsi que de la distribution des différentes unités lithologiques. Daprès les profiles géophysiques on peut caractériser les différents lithologies en utilisant les résistivités apparentes de la formation respective et les informations sur la profondeur de la zone altérée. Les lithologies ainsi déterminées montrent une excellente corrélation avec les cartes géologiques et on peut très bien identifier les lithologies principales comme les metabasaltes et les meta-arenites, les granodiorites et les formations de fer rubanées. Cettes dernières sont caractérisées par des faibles valeurs de la résistivité, tandis que en utilisant les résistivité et la profondeur de la zone altérée on peut distinguer les meta-arenites de granodiorites et des metabasaltes. La méthode des multiélectrodes est donc capable didentifier des formations favorables à laccumulation en eau comme les zones de faille et des fractures, ainsi que les zones à grande porosité et perméabilité, associées aux contactes lithologiques.

     

Paleomagnetic and rock magnetic results from lower crustal rocks of IODP Site U1309: Implication for thermal and accretion history of the Atlantis Massif

Integrated Ocean Drilling Program (IODP) Expeditions 304/305 recovered a total of 1.4 km sequence of lower crustal gabbroic and minor ultramafic rocks from the Atlantis Massif oceanic core complex on the western flank of the Mid Atlantic Ridge (MAR) at 30°N. We conducted an integrated paleomagnetic and rock magnetic study on this sequence to help address the interplay between magmatism and detachment faulting. Detailed thermal and alternating-field demagnetization results demonstrate that stable components of magnetization of mainly reversed polarity with unblocking temperatures below the Curie temperature of magnetite are retained in gabbroic rocks at IODP Site U1309. Several samples also contain multicomponent remanences of both normal and reversed polarities that were acquired over sharply defined blocking temperature intervals, providing evidence for localized reheating of some intervals during both normal and reversed polarity periods. Results from a series of rock magnetic measurements corroborate the demagnetization behavior and show that titanomagnetites are the main magnetic carrier rocks recovered at Site U1309D. The overall magnetic inclination of Hole U1309D is -35°, implying significant (up to ~ 50° counterclockwise, viewed to the north) rotation of the footwall around a horizontal axis parallel to the rift axis (010°) may have occurred. The tectonic rotations inferred by the paleomagnetic data suggest that the original fault orientation dipped relative steeply toward the spreading axis and subsequently rotated to a shallower angle. Coupled with the newly published U–Pb zircon ages for Hole U1309D rocks [Grimes, C.B., John, B.E., Wooden, J.L., 2008. Protracted construction of gabbroic crust at a slow-spreading ridge: Constraints from ²⁰⁶Pb/²³⁸U zircon ages from Atlantis Massif and IODP Hole 1309D, (30°N, MAR). Geochem. Geophys. Geosyst. 9, Q08012. doi:1029/2008GC002063], the new paleomagnetic data provide temporal and thermal constraints on the accretion history of the Atlantis Massif.     

Application of Greek lignite as an additive for controlling rheological and filtration properties of water–bentonite suspensions at high temperatures: A review

This review paper presents the results of an extensive study investigating whether addition of 3% w/w Greek lignite to 6.42% w/w water–bentonite suspensions, after being exposed to high temperatures, can prevent gelation and control filtration characteristics. Two different bentonites and eight lignites from different Greek basins have been used while a commercial lignite product has been used as standard. The lignite-free bentonite suspensions heated to 177 °C for 16 h (thermal aging) thicken considerably, increasing the yield stress and the yield point. Fluid consistency and flow behavior indices also change while no significant change is observed for plastic viscosity. Thermal aging of the suspensions results in unacceptably high fluid loss values.Addition of Greek lignite to water–bentonite suspensions, followed by thermal aging, provided the rheological stability of the suspensions by maintaining the low yield stress/point regardless of the type of bentonite. Some of the lignites performed as well as their commercial counterpart. No specific trends for rheological improvement have been identified with respect to various characteristics of lignites such as contents of humic, fulvic acids, humins and other parameters such as specific surface area and cation exchange capacity.Furthermore, addition of lignite in most cases provided very good filtration control of the water–bentonite suspensions after exposure to 177 °C, with some Greek lignites being superior to the commercial product. The same lignite parameters examined for rheological control, were also examined to determine their effect on fluid loss of these suspensions for both bentonites. The content of humic and fulvic acids of two groups of lignites showed weak inverse correlations with the fluid loss volumes for both bentonites, while all other parameters did not seem to directly correlate with the effectiveness of the lignites.     

修正剑桥模型在DPC桩—土结构层中的应用分析

针对DPC桩-土结构层开展大型直剪试验,基于试验分析,在考虑DPC桩-土结构层注浆影响上引入临界应力比,构建了可描述应变软化的修正剑桥模型应力应变方程。直剪试验表明DPC桩-土结构层剪切特性表现为剪切软化,具有明显的峰值强度和残余强度,呈现出明显的结构性,其特征与超固结的黏土的剪切应力应变曲线特性类似。推导出的修正剑桥模型能较好的解释实验结果,所得出的结论对DPC桩的设计有一定的指导意义。      

Growth patterns and dynamics of mud cracks at different diagenetic stages and its geological significance

This paper discusses the growth stages,spatial structures,quantitative fitting relationships among various parameters,growth patterns and influencing factors of mud cracks by field survey,core observation and SEM analysis.The study shows that:(1) Mud crack growth can go through three stages,i.e.the syndiagenetic stage,the burial diagenetic stage(including early diagenetic stage,middle-late diagenetic stage) and the epidiagenetic stage.(2) Quantitative fitting relationships among various parameters allow a great significance to describe the spatial structure,the regional distribution and the growth environment of mud cracks.(3) Mud crack growth has three models,such as the unilateral growth model including the linear growth pattern,the curvilinear growth pattern and the bifurcation growth pattern,the multilateral growth model including the inteisectional growth pattern,the join growth pattern and the dispersed growth pattern,and the mixed growth model including the combination of any patterns listed above.(4) Modern mud crack growth usually undergoes four stages.Sand beds in sand-mud rhythmic strata can play a lubricative role on crack growth and provide enough sandy deposits for filling cracks.(5) Mud crack growth usually produces bifurcation and bifurcation angles which are mostly 120° or 90° that are related to sediment heterogeneity and released energy.(6)Factors affecting mud crack growth cover many aspects:clay content and salinity can control the number of mud cracks in different areas;terrain can control mud crack morphology;and different sedimentary cycles can control the growth patterns and filling models of mud cracks.     

IODP计划的新进展

综合大洋钻探计划(IODP)经过5年(2003-2008年)的过渡运行,在2009年真正实现了"多平台"钻探格局,美国"决心号"、日本"地球号"和欧洲联合体"特定任务平台"同时执行特定海区和地质条件的钻探航次.随着澳大利亚-新西兰联合体和印度的加入,IODP"大家庭"已有7个成员单位共24个国家,犬洋钻探的国际合作渐入"佳境".IODP钻探航次在2009年的重大进展包括:"地球号"在IODP第319航次首次实施立管钻探并成功取芯;经全面改造升级后的"决心号"首航赤道太平洋成功获取53 Ma以来古赤道太平洋连续沉积物断面;"特定任务平台"成功实施推迟2年多的新泽西陆架IODP第313航次等.我国科学家在近年来参加IODP航次、提交航次建议书、积极参加国际学术会议等方面明显活跃,大大提高我国科学家的国际学术竞争力,并在海洋科学的相关领域进入了国际前沿.     

差应变法地应力测量——以汶川地震断裂带科学钻探WFSD-1钻孔为例

岩心差应变分析法是汶川地震断层科学钻探(WFSD)项目地应力测量的主要手段之一, 该技术通过高压容器对取之于钻孔不同深度的实验样品施加围压, 观察先前由于失去地层应力而膨胀产生的裂纹重新闭合, 并通过裂纹闭合过程差应变曲线分析, 确定主应力大小和方向。WFSD-1孔12个岩心样品的DSA测试结果表明:断层上下盘的应力状态有明显差异, 区域地应力不仅随深度增加, 还与地质结构和岩石的物理性质等因素有关。