Comments on the origin of ocellar quartz-gabbros

The contention that ocellar quartz-gabbros and other similar rocks are the result of isochemical recrystallization of hydrothermally altered volcanic rocks, rather than the hybridization of basic rocks in contact with acid magma, is critically discussed.     

Characterisation and multifaceted anisotropy assessment of Corvio sandstone for geological CO2 storage studies

We present a comprehensive characterisation of the physical, mineralogical, geomechanical, geophysical, and hydrodynamic properties of Corvio sandstone. This information, together with a detailed assessment of anisotropy, is needed to establish Corvio sandstone as a useful laboratory rock‐testing standard for well‐constrained studies of thermo–hydro–mechanical–chemical coupled phenomena associated with CO₂ storage practices and for geological reservoir studies in general. More than 200 core plugs of Corvio sandstone (38.1 and 50 mm diameters, 2:1 length‐to‐diameter ratio) were used in this characterisation study, with a rock porosity of 21.7 ± 1.2%, dry density 2036 ± 32 kg m^?3, and unconfined compressive and tensile strengths of 41 ± 3.28 and 2.3 ± 0.14 MPa, respectively. Geomechanical tests show that the rock behaves elastically between ～10 and ～18 MPa under unconfined conditions with associated Young's modulus and Poisson's ratio of 11.8 ± 2.8 GPa and 0.34 ± 0.01 GPa, respectively. Permeability abruptly decreases with confining pressure up to ～10 MPa and then stabilises at ～1 mD. Ultrasonic P‐ and S‐wave velocities vary from about 2.8–3.8 km s^?1 and 1.5–2.4 km s^?1, respectively, over confining and differential pressures between 0.1 and 35 MPa, allowing derivation of associated dynamic elastic moduli. Anisotropy was investigated using oriented core plugs for electrical resistivity, elastic wave velocity and attenuation, permeability, and tracer injection tests. Corvio sandstone shows weak transverse isotropy (symmetry axis normal to bedding) of <10% for velocity and <20% for attenuation.     

Analysis of fracture induced scattering of microseismic shear-waves

Fractures are pervasive features within the Earth’s crust and have a significant influence on the multi-physical response of the subsurface. The presence of coherent fracture sets often leads to observable seismic scattering enabling seismic techniques to remotely locate and characterise fracture systems. In this study, we confirm the general scale-dependence of seismic scattering and provide new results specific to shear-wave propagation. We do this by generating full waveform synthetics using finite-difference wave simulation within an isotropic background model containing explicit fractures. By considering a suite of fracture models having variable fracture density and fracture size, we examine the widening effect of wavelets due to scattering within a fractured medium by using several different approaches, such as root-mean-square envelope analysis, shear-wave polarisation distortion, differential attenuation analysis and peak frequency shifting. The analysis allows us to assess the scattering behavior of parametrised models in which the propagation direction is either normal or parallel to the fracture surfaces. The quantitative measures show strong observable deviations for fractures size on the order of or greater than the dominant seismic wavelength within the Mie and geometric scattering regime for both propagation normal and parallel to fracture strike. The results suggest that strong scattering is symptomatic of fractures having size on the same order of the probing seismic wave.     

<Emphasis Type="Italic">Spartina alterniflora</Emphasis> Biomass Allocation and Temperature: Implications for Salt Marsh Persistence with Sea-Level Rise

To predict the impacts of climate change, a better understanding is needed of the foundation species that build and maintain biogenic ecosystems. Spartina alterniflora Loisel (smooth cordgrass) is the dominant salt marsh-building plant along the US Atlantic coast. It maintains salt marsh elevation relative to sea level by the accumulation of aboveground biomass, which promotes sediment deposition and belowground biomass, which accretes as peat. Peat accumulation is particularly important in elevation maintenance at high latitudes where sediment supply tends to be limited. Latitudinal variation in S. alterniflora growth was quantified in eight salt marshes from Massachusetts to South Carolina. The hypothesis that allocation to aboveground and belowground biomass is phenotypically plastic was tested with transplant experiments among a subset of salt marshes along this gradient. Reciprocal transplants revealed that northern S. alterniflora decreased allocation to belowground biomass when grown in the south. Some northern plants also died when moved south, suggesting that northern S. alterniflora may be stressed by future warming. Southern plants that were moved north showed phenotypic plasticity in biomass allocation, but no mortality. Belowground biomass also decomposed more quickly in southern marshes. Our results suggest that warming will lead northern S. alterniflora to decrease belowground allocation and that belowground biomass will decompose more quickly, thus decreasing peat accumulation. Gradual temperature increases may allow for adaptation and acclimation, but our results suggest that warming will lower the ability of salt marshes to withstand sea-level rise.     

The One-Way Wave Equation: A Full-Waveform Tool for Modeling Seismic Body Wave Phenomena

The study of seismic body waves is an integral aspect in global, exploration and engineering scale seismology, where the forward modeling of waves is an essential component in seismic interpretation. Forward modeling represents the kernel of both migration and inversion algorithms as the Green’s function for wavefield propagation and is also an important diagnostic tool that provides insight into the physics of wave propagation and a means of testing hypotheses inferred from observational data. This paper introduces the one-way wave equation method for modeling seismic wave phenomena and specifically focuses on the so-called operator-root one-way wave equations. To provide some motivation for this approach, this review first summarizes the various approaches in deriving one-way approximations and subsequently discusses several alternative matrix narrow-angle and wide-angle formulations. To demonstrate the key strengths of the one-way approach, results from waveform simulation for global scale shear-wave splitting modeling, reservoir-scale frequency-dependent shear-wave splitting modeling and acoustic waveform modeling in random heterogeneous media are shown. These results highlight the main feature of the one-way wave equation approach in terms of its ability to model gradual vector (for the elastic case) and scalar (for the acoustic case) waveform evolution along the underlying wavefront. Although not strictly an exact solution, the one-way wave equation shows significant advantages (e.g., computational efficiency) for a range of transmitted wave three-dimensional global, exploration and engineering scale applications.     

Detecting the near-surface redox front in crystalline bedrock using fracture mineral distribution,geochemistry and U-series disequilibrium

Oxidizing conditions normally prevail in surface waters and near-surface groundwaters, but there is usually a change to reducing conditions in groundwater at greater depth. Dissolved O₂ originally present is consumed through biogenic and inorganic reactions along the flow paths. Fracture minerals participate in these reactions and the fracture mineralogy and geochemistry can be used to trace the redox front. An important task in the safety assessment of a potential repository for the disposal of nuclear waste in crystalline bedrock, at an approximate depth of 500 m in Sweden, is to demonstrate that reducing conditions can be maintained for a long period of time. Oxygen may damage the Cu canisters that host nuclear waste; additionally, in the event of a canister failure, oxidizing conditions may increase the mobility of some radionuclides. The present study of the near-surface redox front is based on mineralogical (redox-sensitive minerals), geochemical (redox-sensitive elements) and U-series disequilibrium investigations of mineral coatings along open fractures. The fractures have been sampled along drill cores from closely spaced, 100 m deep boreholes, which were drilled during the site investigation work in the Laxemar area, south-eastern Sweden, carried out by the Swedish Nuclear Fuel and Waste Management Co. (SKB). The distribution of the redox-sensitive minerals pyrite and goethite in open fractures shows that the redox front (switch from mainly goethite to mainly pyrite in the fractures) generally occurs at about 15–20 m depth. Calcite leaching by recharging water is indicated in the upper 20–30 m and positive Ce-anomalies suggest oxidation of Ce down to 20 m depth. The U-series radionuclides show disequilibrium in most of the samples, indicating mobility of U during the last 1 Ma. In the upper 20 m, U is mainly removed (due to oxidation) or has experienced complex removal and/or deposition. At depths of 35–55 m, both deposition and removal of U are indicated. Below 55 m, recent deposition of U is generally indicated which suggests removal of U near surface (oxidation) and deposition of U below the redox front. Scattered goethite occurrences below the general redox front (down to ca 80 m) and signs of U removal at 35–55 m mostly correlate with sections of high transmissivity (and/or high fracture frequencies). This shows that highly transmissive fractures are generally required to allow oxygenated groundwaters at depth greater than ca 30 m. Removal of U (oxidation) below 55 m within the last 300 ka is not observed. Although penetration of glacial waters to great depths has been confirmed in the study area, their potential O₂ load seems to have been reduced near the surface.     

Modelling converted seismic waveforms in isotropic and anisotropic 1-D gradients: discontinuous versus continuous gradient representations

Over the past decade, there have been numerous receiver function studies directed at imaging the lithosphere-asthenosphere boundary (LAB). Although it is generally accepted that receiver function phases observed in these studies are derived from physical mode conversions at depth within the lithosphere-asthenosphere transition, it is still debatable as to whether these phases are directly indicative of the LAB. This is because interpretation of receiver function LAB signals relies on understanding the elastic characteristics of the Earth??s outer thermal boundary layer. The main issues for receiver function imaging are the sharpness of the elastic material property transition and, more importantly, what specifically are the material gradients. To test the various transition models, a forward modelling approach is required that allows accurate waveform synthetics for a range of discontinuous and continuous gradients in anisotropic, elastic media. We present a derivation of the reflection and transmission response for continuous one-dimensional (1-D) gradients in generally anisotropic elastic media. We evaluate the influence of 1-D isotropic and anisotropic elastic gradients on the seismic waveform by comparing numerical results of models for discontinuous and continuous transitions. The results indicate that discontinuous representations using layers each with uniform parameters and with thicknesses on the order of approximately 1/3 to 1/8 of the dominant seismic wavelength can be used to accurately model P-to-S and S-to-P mode conversions due to continuous transitions of both isotropic and anisotropic elastic properties. From a practical point of view, when comparing synthetic modelling with observation, this constraint can be relaxed further. The presence of signal noise and/or the result of receiver function stacking techniques will likely obscure these subtle waveform e ff ects. Hence this study suggests that accurate synthetic waveforms for LAB transitions can be modelled with discontinuous gradient representations using a reasonable number of discrete transition layers with layer thicknesses no greater than 1/2 to 1/3 the dominant seismic wavelength.