Impacts of Methane on Carbon Dioxide Storage in Brine Formations

In the context of geological carbon sequestration (GCS), carbon dioxide (CO₂) is often injected into deep formations saturated with a brine that may contain dissolved light hydrocarbons, such as methane (CH₄). In this multicomponent multiphase displacement process, CO₂ competes with CH₄ in terms of dissolution, and CH₄ tends to exsolve from the aqueous into a gaseous phase. Because CH₄ has a lower viscosity than injected CO₂, CH₄ is swept up into a ‘bank’ of CH₄‐rich gas ahead of the CO₂ displacement front. On the one hand, this may provide a useful tracer signal of an approaching CO₂ front. On the other hand, the emergence of gaseous CH₄ is undesirable because it poses a leakage risk of a far more potent greenhouse gas than CO₂ if the cap rock is compromised. Open fractures or faults and wells could result in CH₄ contamination of overlying groundwater aquifers as well as surface emissions. We investigate this process through detailed numerical simulations for a large‐scale GCS pilot project (near Cranfield, Mississippi) for which a rich set of field data is available. An accurate cubic‐plus‐association equation‐of‐state is used to describe the non‐linear phase behavior of multiphase brine‐CH₄‐CO₂ mixtures, and breakthrough curves in two observation wells are used to constrain transport processes. Both field data and simulations indeed show the development of an extensive plume of CH₄‐rich (up to 90 mol%) gas as a consequence of CO₂ injection, with important implications for the risk assessment of future GCS projects.     

Structure-from-motion photogrammetry analysis of historical aerial photography: Determining beach volumetric change over decadal scales

Historical aerial photographs are an invaluable tool in shoreline mapping and change detection in coastal landscapes. We evaluate the extent to which structure-from-motion (SfM) photogrammetric methods can be applied to quantify volumetric changes along sandy beaches, using archival imagery. We demonstrate the application of SfM-derived digital surface models (DSMs) at East Beach and Lady Bay in southwest Victoria, Australia, using photographic datasets taken in 1969, 1977 and 1986, and compare them to LiDAR-derived DSMs acquired at both sites in 2007. The SfM approaches resulted in two entire and two partial suitable DSMs out of six datasets. Good-quality DSMs were spatially continuous with a good spread of ground control points (GCPs) near the beach at Lady Bay, whereas unsuitable DSMs were mostly restricted by poor distribution and number of GCPs in spatially segmented areas of East Beach, due to limited overlapping of images, possible poor quality of GCPs and also the propagation of errors in the derived point clouds. A volume of approximately 223 000 ± 72 000 m³ was deposited at Lady Bay between 1969 and 2007, despite minimal erosion observed near the breakwater. The partially suitable dataset of East Beach indicated that beach erosion of at least 39 m³ m⁻¹ occurred immediately to the east of the seawall after 1977. We also discuss the drawbacks and strengths of SfM approaches as a benchmark of historical erosion assessments along sandy beaches. © 2020 John Wiley & Sons, Ltd.     

Climatic and structural controls on Late-glacial and Holocene rockfall occurrence in high-elevated rock walls of the Mont Blanc massif (Western Alps)

In the Mont Blanc massif (European Western Alps), rockfalls are one of the main natural hazards for alpinists and infrastructure. Rockfall activity after the Little Ice Age is well documented. An increase in frequency during the last three decades is related to permafrost degradation caused by rising air temperatures. In order to understand whether climate exerts a long-term control on rockfall occurrence, a selection of paleo-rockfall scars was dated in the Glacier du Géant basin [>3200 m above sea level (a.s.l.)] using terrestrial cosmogenic nuclides. Rockfall occurrence was compared to different climatic and glacial proxies. This study presents 55 new samples (including replicates) and 25 previously-published ages from nine sampling sites. In total, 62 dated rockfall events display ages ranging from 0.03 ± 0.02 ka to 88.40 ± 7.60 ka. Holocene ages and their uncertainties were used to perform a Kernel density function into a continuous dataset displaying rockfall probability per 100 years. Results highlight four Holocene periods of enhanced rockfall occurrence: (i) c. 7–5.7 ka, related to the Holocene Warm Periods; (ii) c. 4.5–4 ka, related to the Sub-boreal Warm Period; (iii) c. 2.3–1.6 ka, related to the Roman Warm Period; and (iv) c. 0.9–0.3 ka, related to the Medieval Warm Period and beginning of the Little Ice Age. Laser and photogrammetric three-dimensional (3D) models of the rock walls were produced to reconstruct the detached volumes from the best-preserved rockfall scars (≤0.91 ± 0.12 ka). A structural study was carried out at the scale of the Glacier du Géant basin using aerial photographs, and at the scale of four selected rock walls using the 3D models. Two main vertical and one horizontal fracture sets were identified. They correspond respectively to alpine shear zones and veins opened-up during long-term exhumation of the Mont Blanc massif. Our study confirms that climate primarily controls rockfall occurrence, and that structural settings, coincident at both the massif and the rock wall scales, control the rock-wall shapes as well as the geometry and volume of the rockfall events. © 2020 John Wiley & Sons, Ltd.     

著名科学家莱昂·诺波夫(Leon Knopoff)逝世

莱昂·诺波夫(1925~2011),是一位对物理学、地震学和音乐领域做出贡献的国际著名科学家,于2011年1月20日在妻子和3个孩子的陪伴下于家中辞世。莱昂·诺波夫1949年在加州理工学院获得物理学和数学博士学位,翌年开始在洛杉矶加州大学任教。1960年洛杉矶加州     

Sediment routing hypothesis for pool‐riffle maintenance

This paper provides comprehensive evidence that sediment routing around pools is a key mechanism for pool‐riffle maintenance in sinuous upland gravel‐bed streams. The findings suggest that pools do not require a reversal in energy for them to scour out any accumulated sediments, if little or no sediments are fed into them. A combination of clast tracing using passive integrated transponder (PIT) tagging and bedload traps (positioned along the thalweg on the upstream riffle, pool entrance, pool exit and downstream riffle) are used to provide information on clast pathways and sediment sorting through a single pool‐riffle unit. Computational fluid dynamics (CFD) is also used to explore hydraulic variability and flow pathways. Clast tracing results provide a strong indication that clasts are not fed through pools, rather they are transported across point bar surfaces, or around bar edges (depending upon previous clast position, clast size, and event magnitude). Spatial variations in bedload transport were found throughout the pool‐riffle unit. The pool entrance bedload trap was often found to be empty, when the others had filled, further supporting the notion that little or no sediment was fed into the pool. The pool exit slope trap would occasionally fill with sediment, thought to be sourced from the eroding outer bank. CFD results demonstrate higher pool shear stresses (τ ≈ 140 N m^–2) in a localized zone adjacent to an eroding outer bank, compared to the upstream and downstream riffles (τ ≈ 60 N m^–2) at flows of 6 · 2 m³ s^–1 (≈ 60% of the bankfull discharge) and above. There was marginal evidence for near‐bed velocity reversal. Near‐bed streamlines, produced from velocity vectors indicate that flow paths are diverted over the bar top rather than being fed through the thalweg. Some streamlines appear to brush the outer edge of the pool for the 4 · 9 m³ s^–1 to 7 · 8 m³ s^–1 (between 50 and 80% of the bankfull discharge) simulations, however complete avoidance was found for discharges greater than this. Copyright © 2013 John Wiley & Sons, Ltd.     

A Stream‐Based Methane Monitoring Approach for Evaluating Groundwater Impacts Associated with Unconventional Gas Development

Gaining streams can provide an integrated signal of relatively large groundwater capture areas. In contrast to the point‐specific nature of monitoring wells, gaining streams coalesce multiple flow paths. Impacts on groundwater quality from unconventional gas development may be evaluated at the watershed scale by the sampling of dissolved methane (CH₄) along such streams. This paper describes a method for using stream CH₄ concentrations, along with measurements of groundwater inflow and gas transfer velocity interpreted by 1‐D stream transport modeling, to determine groundwater methane fluxes. While dissolved ionic tracers remain in the stream for long distances, the persistence of methane is not well documented. To test this method and evaluate CH₄ persistence in a stream, a combined bromide (Br) and CH₄ tracer injection was conducted on Nine‐Mile Creek, a gaining stream in a gas development area in central Utah. A 35% gain in streamflow was determined from dilution of the Br tracer. The injected CH₄ resulted in a fivefold increase in stream CH₄ immediately below the injection site. CH₄ and δ¹³C_CH4 sampling showed it was not immediately lost to the atmosphere, but remained in the stream for more than 2000 m. A 1‐D stream transport model simulating the decline in CH₄ yielded an apparent gas transfer velocity of 4.5 m/d, describing the rate of loss to the atmosphere (possibly including some microbial consumption). The transport model was then calibrated to background stream CH₄ in Nine‐Mile Creek (prior to CH₄ injection) in order to evaluate groundwater CH₄ contributions. The total estimated CH₄ load discharging to the stream along the study reach was 190 g/d, although using geochemical fingerprinting to determine its source was beyond the scope of the current study. This demonstrates the utility of stream‐gas sampling as a reconnaissance tool for evaluating both natural and anthropogenic CH₄ leakage from gas reservoirs into groundwater and surface water.     

Kinematic dynamo action in a sphere: Effects of periodic time-dependent flows on solutions with axial dipole symmetry

Choosing a simple class of flows, with characteristics that may be present in the Earth's core, we study the ability to generate a magnetic field when the flow is permitted to oscillate periodically in time. The flow characteristics are parameterised by D, representing a differential rotation, M, a meridional circulation, and C, a component characterising convective rolls. The dynamo action of all solutions with fixed parameters (steady flows) is known from earlier studies. Dynamo action is sensitive to these flow parameters and fails spectacularly for much of the parameter space where magnetic flux is concentrated into small regions, leading to high diffusion. In addition, steady flows generate only steady or regularly reversing oscillatory fields and cannot therefore reproduce irregular geomagnetic-type reversal behaviour. Oscillations of the flow are introduced by varying the flow parameters in time, defining a closed orbit in the space ( D,?M ). When the frequency of the oscillation is small, the net growth rate of the magnetic field over one period approaches the average of the growth rates for steady flows along the orbit. At increased frequency time-dependence appears to smooth out flux concentrations, often enhancing dynamo action. Dynamo action can be impaired, however, when flux concentrations of opposite signs occur close together as smoothing destroys the flux by cancellation. It is possible to produce geomagnetic-type reversals by making the orbit stray into a region where the steady flows generate oscillatory fields. In this case, however, dynamo action was not found to be enhanced by the time-dependence. A novel approach is being taken to solve the time-dependent eigenvalue problem where, by combining Floquet theory with a matrix-free Krylov-subspace method, we can avoid large memory requirements for storing the matrix required by the standard approach.     

Determining short-term changes in the hydraulic properties of a sandy-loam soil by a three-run infiltration experiment

ABSTRACT

Soil structure-dependent parameters can vary rapidly as a consequence of perturbing events such as intense rainfall. Investigating their short-term changes is therefore essential to understand the general behaviour of a porous medium. The aim of this study is to gain insight into the effects of wetting, perturbation and recovery processes through different sequences of Beerkan infiltration experiments performed on a sandy-loam soil. Two different three-run infiltration experiments (LHL and LLL) were carried out by pouring water at low (L, non-perturbing) and high (H, perturbing) heights above the soil surface and at short time intervals (hours, days). The results demonstrate that the proposed method allows one to capture short-term variations in soil structure-dependent parameters. The developed methodology is expected to simplify the parameterization of hydrological models with temporally variable soil hydraulic properties.