Geophysical characterization of gas hydrate systems of the South Shetland margin (Antarctica)

During the last few decades, interest in gas hydrates has been increasing significantly because of their economic potential as future energy source and their potential role in geohazards and global climate change. The global climate change is a particularly sensitive issue for the Polar Regions, such as Antarctica. In the South Shetland margin (Antarctic Peninsula), the occurrence of a potential gas hydrate reservoir has been demonstrated from the analysis of geophysical data acquired during three Italian Antarctic cruises. In order to enhance the knowledge of gas hydrate systems, I analyzed Ocean Bottom Seismometer (OBS) and coincident multi-channel seismic (MCS) data acquired in 2004. The main objectives of this thesis are to estimate the distributions and concentrations of gas hydrate and free gas within the sediments, and to investigate the system’s petrophysical properties. Travel time inversion and forward modeling of OBS data were performed to estimate detailed P- and S-wave velocity fields. The P-wave velocity field was determined by the inversion of refractions and reflections in OBS data, while the S-wave velocity field was obtained by ray-tracing forward modeling of the converted S-waves from the horizontal components of OBS data. Several velocity models were tested in order to reduce the errors caused by the spatial drift of the OBS from the MCS line during sinking, and the errors from inversion. The final velocity model shows that P-wave velocity increases gradually with depth down to the bottom simulating reflection (BSR) at approximately 510-650 m below the seafloor. The layer with high P-wave velocity of 2.0-2.1 km/s just above the BSR can be associated with the presence of gas hydrates. Below the BSR, a low velocity layer of 1.4-1.6 km/s is observed, which indicates the presence of free gas. From the analysis of critical refractions in OBS data, the base of free gas layer (BGR) occurs at a depth varying between 80-160 m below the BSR. Forward modeling of converted S-waves in OBS data allows us to obtain Poisson’s ratio estimates. We observe that Poisson’s ratios are fairly uniform within each layer and they show good agreement with previous study performed in this area. The comparison of Poisson’s ratio indicates that the gas hydrate reservoir shows no significant regional variations. The resulting velocity fields were translated in terms of gas hydrate and free gas concentrations, using a modified Biot-Geerstma-Smit theory. The results show that hydrate concentration in the layer just above the BSR ranges from 10% to 15% of total volume, and free gas concentration is approximately 0.3% to 0.8% of total volume assuming a uniform gas distribution. Part of this research related to the OBS analysis and gas-phase estimation, has been published in the international journal Energies (Song et al., 2018) and included in the Appendix 1

5,5′-Di-4-pyridyl-2,2′-(p-phenyl­ene)di-1,3,4-oxadiazole

In the crystal structure of the title compound, C20H12N6O2, the mol­ecules are located on centres of inversion. The complete mol­ecule is almost planar, with a maximum deviation from the mean plane of 0.0657 (1) Å for the O atom. In the crystal, mol­ecules are stacked into columns elongated in the a axis direction. The centroid–centroid distances between the aromatic rings of the mol­ecules within the columns are 3.6406 (1) and 3.6287 (2) Å. Mol­ecules are additionally connected via weak inter­molecular C—H⋯N hydrogen bonding

(μ-5-Carb­oxy-1H-imidazole-4-carboxyl­ato-κ4 N 1,O 5:N 3,O 4)bis­[amminesilver(I)]

In the title compound, [Ag2(C5H2N2O4)(NH3)2], each of the two AgI atoms is coordinated by two N atoms from an ammonia mol­ecule and a 5-carb­oxy-1H-imidazole-4-carboxyl­ate ligand in an almost linear geometry, and by one carboxyl­ate O atom with a weak inter­action. The Ag atoms are assembled into a linear tetra­mer through Ag⋯Ag inter­actions. Each Ag tetra­mer is linked by four 5-carb­oxy-1H-imidazole-4-carboxyl­ate ligands, forming a puckered chain. The complex involves a strong intra­molecular O—H⋯O hydrogen bond

4-Hydr­oxy-6-[(4-hydr­oxy-1-oxo-1,2-dihydro­phthalazin-6-yl)carbon­yl]phthalazin-1(2H)-one

In the crystal structure of the title compound, C17H10N4O5, the mol­ecules lie on twofold axes (through the ketone bridge C and O atoms). The dihedral angle between the two phthalazine rings is 52.25 (1)°. In the crystal, inter­molecular N—H⋯O and O—H⋯O inter­actions link the mol­ecules

Feasibility and principle analyses of morphing airfoil used to control flight attitude

Morphing airfoil technology can enable an aircraft to adapt its shape to enhance mission performance and replace the traditional flap, ailerons, elevator and rudders to optimize flight attitude control efficiency. A set of optimal airfoil shapes are obtained aimed to minimize the aerodynamic drag character by optimizing morphing configurations at different under the two-dimensional steady-flow simulation. The traditional airfoil and morphing airfoil at different are compared. It is proved that morphing wing can be used instead of a traditional wing. Couples of traditional control surface and morphing airfoil are chosen to simulate and analyze the aerodynamic difference. The flow mechanism is described on the basis of aerodynamic simulations performed by CFX. It is demonstrated why the morphing wing can provide the same with a small

Characterization and Correction of the Scattering Background Produced by Dust on the Objective Lens of the Lijiang 10-cm Coronagraph

Scattered light from the objective lens, directly exposed to the intense sunlight, is a dominant source of stray light in internally occulted coronagraphs. The variable stray light, such as the scatter from dust on the objective lens, can produce varying scattering backgrounds in coronal images, significantly impacting image quality and data analysis. Using data acquired by the Lijiang 10-cm Coronagraph, the quantitative relationship between the distribution of dust on the objective lens and the resulting scattering backgrounds background is analyzed. Two empirical models for the scattering background are derived, and used to correct the raw coronal data. The second model, which depends on three parameters and performs better, shows that the scattering-background distribution varies with angle, weakens with increasing height, and enhances with increasing dust level on the objective lens. Moreover, we find that the dust on the center of the objective lens can contribute more significantly to the scattering background than on the edge. This study not only quantitatively confirms the significant impact of the stray light produced by dust on the objective lens of the coronagraph, but also corrects the coronal data with this stray light for the first time. Correcting for dust-scattered light is crucial for the high-precision calibration of ground-based coronagraph data, enabling a more accurate analysis of coronal structures. Furthermore, our model is envisioned to support the provision of reliable observational data for future routine coronal magnetic-field measurements using ground-based coronagraphs.Comment: 18 pages, 14 figrue