Coupling mechanisms in double sandbar systems. Part 2: impact on alongshore variability of inner‐bar rip channels

Double sandbar systems are common morphological features along sandy, wave‐dominated, micro‐ to meso‐tidal coastlines. In the companion paper, we demonstrated how various alongshore inner‐bar rip‐channel patterns can develop through morphological coupling to an alongshore‐variable outer bar. The simulated coupling patterns are, however, scarcely observed in the field. Instead, inner‐bar rip channels more often possess remarkably smaller and more variable alongshore length scales, suggesting that coupling mechanisms do not play a substantial role in the overall double‐sandbar dynamics. Here we use a numerical model to show that the relative importance of self‐organization and morphological coupling changes in favour of the latter with an increase in waterdepth variability along the outer‐bar crest. Furthermore, we find that the typical alongshore variability in inner‐bar rip‐channel scale is indicative of a mixture of self‐organization and morphological coupling rather than self‐organization alone. Morphological coupling may thus be more important to understanding and predicting the evolution of inner‐bar rip channels than previously envisaged. Copyright © 2010 John Wiley and Sons, Ltd.     

Fluid-pressure variations and fault creep in central California

Observations of the change of water level in wells near the San Andreas fault in centra California indicate that there are pore-pressure changes associated with the process of fault creep. The sense of water-level and pore-pressure changes may indicate the direction of creep propagation.     

Free convection effects on the oscillatory flow in the Stokes's problem past an infinite porous vertical limiting surface with constant suction — I

An analysis of Rayleigh's problem (also Stokes's problem) for the flow of a viscous fluid (e.g. of a stellar atmosphere) past an impulsively started infinite, vertical porous limiting surface (e.g. of a star) with constant suction, when the free stream velocity oscillates in time about a constant mean, has been carried out. On solving the coupled non-linear equations in approximate way, expressions for the mean velocity, the mean temperature, the mean skin-friction and the mean rate of heat transfer, expressed in terms of Nusselt number, are obtained. The effects of Grashof numberG, Eckert numberE and Prandtl numberP, on these quantities, is discussed for the cases of an externally heating and cooling of the limiting surface, by the free convection currents, and the variations of them are shown graphically.     

TWO YEARS' SURVEY WITH CESIUM VAPOUR MAGNETOMETER*

The present paper shows that the techniques developed and exploited by qualified geophysicists have brought high sensitivity magnetic surveys to an industrial stage. It will appear that for most mining and oil problems, either on land or sea, the necessary modern equipment already exists without developing it further in the future. In particular, the digitalization of data is practically finished for the magnetometer itself as well as for some navigation techniques.     

Evaluation of Interactions Between Oilfield Chemicals and Reservoir Rocks

Natural Resources Research - Sand failure and production occurs when the formation stress exceeds the strength of the formation, which is derived majorly from the natural material that cements the...     

Mantle(?) xenoliths in the carbonatites of northern Transbaikalia

The composition and nature of high-Cr minerals in lithic clasts from the carbonatites of the Veseloe occurrence, northern Transbaikalia, were considered. In order to determine their source, the Cr-bearing phases were compared with chromite, magnetite, and rutile from ultrabasic rocks, mantle xenoliths, and eclogites. It was suggested that the xenoclasts studied were formed at great depths, whereas the carbonatites were directly derived from the mantle rather than formed by the crustal differentiation of a silicate-carbonate melt.     

Evidence for focused hot fluid flow within the Britannia Field, offshore Scotland, UK

Fluid inclusion and scanning electron microscope‐cathodoluminescence evidence indicates focused hot, saline, diagenetic fluid flow within the Eastern Flank of the Britannia Field, offshore Scotland, UK. The fluid was sourced from the Andrew Salt Dome, 10 km to the east. The fluids, which promoted quartz cementation of the upper zones within the field, were up to ～30°C hotter and had salinities up to ～10 wt% NaCl equivalent higher than fluids from lower in the reservoir section. During diagenesis hot saline fluids migrated westwards as part of a radiating ‘diagenetic front’ from the Andrew Salt Dome. Structural dip associated with the Eastern Flank of the Fladen Ground Spur impeded the westward movement of the diagenetic fluid. The quartz cements from the upper and lower reservoir zones can be distinguished by morphology. In the upper zones the quartz cements have well‐developed macro‐crystalline zoning and heterogeneous luminescence across the grain. In the lower zones, the cements are much less developed, unzoned and very weakly luminescent. The diagenetic fluids were primarily focused into Zone 45 within the upper reservoir. Furthermore, within the Main Platform Area the most prolific producing zone is Zone 45, indicating the importance of this interval as a permeable flow unit during both diagenetic and production timescales. Within the Eastern Flank, the quartz overgrowths have a major impact on reservoir permeability and thus well productivity. The overgrowths are most extensive in the originally clean sandstones with low clay content. Clay in optimum volumes (5–10%) can inhibit nucleation of the damaging quartz overgrowths without having a detrimental effect on pore connectivity. These observations provide a predictive concept for use in the search for relative reservoir sweetspots within the degraded Eastern Flank.     

Trace-element composition and zoning in clinopyroxene- and amphibole-group minerals: Implications for element partitioning and evolution of carbonatites

The present work is a first comprehensive study of the trace-element composition and zoning in clinopyroxene- and amphibole-group minerals from carbonatites, incorporating samples from 14 localities worldwide (Afrikanda, Aley, Alnö, Blue River, Eden Lake, Huayangchuan, Murun, Oka, Ozernaya Varaka, Ozernyi, Paint Lake, Pinghe, Prairie Lake, Turiy Mys). The new electron-microprobe data presented here significantly extend the known compositional range of clinopyroxenes and amphiboles from carbonatites. These data confirm that calcic and sodic clinopyroxenes from carbonatites are not separated by a compositional gap, instead forming an arcuate trend from nearly pure diopside through intermediate aegirine–augite compositions confined to a limited range of CaFeSi₂O₆ contents (15–45 mol%) to aegirine with < 25 mol% of CaMgSi₂O₆ and a negligible proportion of CaFeSi₂O₆. A large set of LA-ICPMS data shows that the clinopyroxenes of different composition are characterized by relatively low levels of Cr, Co and Ni (≤ 40 ppm) and manifold variations in the concentration of trivalent lithophile and some incompatible elements (1–150 ppm Sc, 26–6870 ppm V, 5–550 ppm Sr, 90–2360 ppm Zr, and nil to 150 ppm REE), recorded in some cases within a single crystal. The relative contribution of clinopyroxenes to the whole-rock Rb, Nb, Ta, Th and U budget is negligible. The major-element compositional range of amphiboles spans from alkali- and Al-poor members (tremolite) to Na–Al-rich Mg- or, less commonly, Fe-dominant members (magnesiohastingsite, hastingsite and pargasite), to calcic–sodic, sodic and potassic–sodic compositions intermediate between magnesio-ferrikatophorite, richterite, magnesioriebeckite, ferri-nyböite and (potassic-)magnesio-arfvedsonite. In comparison with the clinopyroxenes, the amphiboles contain similar levels of tetravalent high-field-strength elements (Ti, Zr and Hf) and compatible transition elements (Cr, Co and Ni), but are capable of incorporating much higher concentrations of Sc and incompatible elements (up to 500 ppm Sc, 43 ppm Rb, 1470 ppm Sr, 1230 ppm Ba, 80 ppm Pb, 1070 ppm REE, 140 ppm Y, and 180 ppm Nb). In some carbonatites, amphiboles contribute as much as 25% of the Zr + Hf, 15% of the Sr and 35% of the Rb + Ba whole-rock budget. Both clinopyroxenes and amphiboles may also host a significant share (~ 10%) of the bulk heavy-REE content. Our trace-element data show that the partitioning of REE between clinopyroxene (and, in some samples, amphibole) and the melt is clearly bimodal and requires a revision of the existing models assuming single-site REE partitioning. Clinopyroxenes and amphiboles from carbonatites exhibit a diversity of zoning patterns that cannot be explained exclusively on the basis of crystal chemistry and relative compatibility of different trace-element in these minerals. Paragenetic analysis indicates that in most cases, the observed zoning patterns develop in response to removal of selected trace elements by phases co-precipitating with clinopyroxene and amphibole (especially magnetite, fluorapatite, phlogopite and pyrochlore). With the exception of magnesiohastingsite–richterite sample from Afrikanda, the invariability of trace-element ratios in the majority of zoned clinopyroxene and amphibole crystals implies that fluids are not involved in the development of zoning in these minerals. The implications of the new trace-element data for mineral exploration targeting REE, Nb and other types of carbonatite-hosted rare-metal mineralization are discussed.     

On the existence of a second scale of convection in the upper mantle

Summary. This paper is concerned with an examination of the possibility that there might exist a small scale of convective circulation beneath the oceanic lithosphere. Recent suggestions that this might be the case have been made in an effort to understand why the bathymetry of the sea-floor deviates from the prediction of boundary layer theory for ages in excess of about 100 Ma. The energy source which sustains the secondary motion is supposed to be found in the steep temperature gradient near the planetary surface which is itself presumably maintained by the large-scale convective circulation associated with plate creation and destruction. Here we investigate the extent to which the temperature dependence of viscosity may act so as to stabilize the upper boundary layer against disruption by such secondary instability. If the viscosity profile is monotonie and the asymptotic upper mantle viscosity is about 10²²poise, as suggested by post-glacial rebound data, then the existence of the second scale is extremely unlikely. On the other hand, if a sufficiently pronounced low viscosity zone does exist under old sea-floor then the development of such a second scale cannot be ruled out completely. Some recently obtained geophysical evidence is reviewed which suggests that this is unlikely to be the case.     

Structural characteristics of zooplankton of the Shardarinskoe Reservoir and their use in water quality assessment

Data on the Shardarinskoe Reservoir are used to assess the indicator role of zooplankton community under unstable hydrological regime. The structural characteristics of zooplankton featured significant seasonal variations. The averaged indices characterized reservoir water quality at the level of mesotrophic and eutrophic water bodies. The possible use of cyclopidas to characterize toxic pollution of water is discussed.