The wind-induced shaping and migration of an isolated dune: A numerical experiment

Numerical experiments are carried out to simulate the development and migration of a barchan dune starting with a conical pile of sand. Such an experiment is done in three steps: (1) computation of the steady-state wind field over and around a barchan using the numerical meso-scale simulation model FITNAH, whereby the horizontal variation of the friction velocity is also calculated; (2) computation of the sand transport using the friction velocity in the transport formula by Lettau and Lettau (1978); (3) computation of the erosion and deposition rates as the divergence of the sand transport, where a special treatment is used for the slip-face of the barchan dune. Adding these rates to the height field leads to a different shape of the dune after a time step t _h . Then this procedure has to be repeated for the next time step t _h .The results are in good agreement with observations: the initial pile of sand develops wings (horns) and a slip-face between them. In addition, flow separation over the lee-side can be simulated. Finally, the tendency to form a barchan in equilibrium is considered.     

Achievements of the Earth orientation parameters prediction comparison campaign

Precise transformations between the international celestial and terrestrial reference frames are needed for many advanced geodetic and astronomical tasks including positioning and navigation on Earth and in space. To perform this transformation at the time of observation, that is for real-time applications, accurate predictions of the Earth orientation parameters (EOP) are needed. The Earth orientation parameters prediction comparison campaign (EOP PCC) that started in October 2005 was organized for the purpose of assessing the accuracy of EOP predictions. This paper summarizes the results of the EOP PCC after nearly two and a half years of operational activity. The ultra short-term (predictions to 10 days into the future), short-term (30 days), and medium-term (500 days) EOP predictions submitted by the participants were evaluated by the same statistical technique based on the mean absolute prediction error using the IERS EOP 05 C04 series as a reference. A combined series of EOP predictions computed as a weighted mean of all submissions available at a given prediction epoch was also evaluated. The combined series is shown to perform very well, as do some of the individual series, especially those using atmospheric angular momentum forecasts. A main conclusion of the EOP PCC is that no single prediction technique performs the best for all EOP components and all prediction intervals.     

Rotational evaluation of a long-period spherical harmonic ocean tide model

By exchanging angular momentum with the solid earth, tidal variations in ocean currents and sea level cause the rotation of the solid earth to change. Observations of earth rotation variations can therefore be used to evaluate ocean tide models. The rotational predictions of a spherical harmonic ocean tide model that is not constrained by any type of data are compared here to the predictions of numerical ocean tide models and to earth rotation observations from which atmospheric and non-tidal oceanic effects have been removed. The spherical harmonic ocean tide model is shown to account for the observed variations at the fortnightly tidal period in polar motion excitation but not in length-of-day. Overall, its long-period polar motion excitation predictions fit the observed tidal signals better than do the predictions of the numerical ocean tide models studied here. It may be possible to improve its agreement with length-of-day observations by tuning certain model parameters, as was done to obtain the close agreement reported here between the modeled and observed polar motion excitation; alternatively, the discrepancy in length-of-day may point to the need to revise current models of mantle anelasticity and/or models of the oceanic response to atmospheric pressure variations.     

Monitoring CO2 saturation using time‐lapse amplitude versus offset analysis of 3D seismic data from the Ketzin CO2 storage pilot site,Germany

The injection of CO₂ at the Ketzin pilot site commenced in June 2008 and was terminated in August 2013 after 67 kT had been injected into a saline formation at a depth of 630–650 m. As part of the site monitoring program, four 3D surface seismic surveys have been acquired to date, one baseline and three repeats, of which two were conducted during the injection period, and one during the post‐injection phase. The surveys have provided the most comprehensive images of the spreading CO₂ plume within the reservoir layer. Both petrophysical experiments on core samples from the Ketzin reservoir and spectral decomposition of the 3D time‐lapse seismic data show that the reservoir pore pressure change due to CO₂ injection has a rather minor impact on the seismic amplitudes. Therefore, the observed amplitude anomaly is interpreted to be mainly due to CO₂ saturation. In this study, amplitude versus offset analysis has been applied to investigate the amplitude versus offset response from the top of the sandstone reservoir during the injection and post‐injection phases, and utilize it to obtain a more quantitative assessment of the CO₂ gaseous saturation changes. Based on the amplitude versus offset modelling, a prominent decrease in the intercept values imaged at the top of the reservoir around the injection well is indeed associated solely with the CO₂ saturation increase. Any change in the gradient values, which would, in case it was positive, be the only signature induced by the reservoir pressure variations, has not been observed. The amplitude versus offset intercept change is, therefore, entirely ascribed to CO₂ saturation and used for its quantitative assessment. The estimated CO₂ saturation values around the injection area in the range of 40%–60% are similar to those obtained earlier from pulsed neutron‐gamma logging. The highest values of 80% are found in the second seismic repeat in close vicinity to the injection and observation wells.     

Detection of fluid mud layers using tuning fork,dual-frequency echo sounder,and chirp sub-bottom measurements

Ocean Dynamics - Many ports and navigation channels around the world suffer from silting and the presence of fluid mud layers, which can cause reductions in nautical depth. In this context, the...     

Time-frequency analysis based on curvelet transforms with time skewing

Oil and gas exploration gradually changes to the deep and complex areas. The quality of seismic data restricts the effective application of conventional time-frequency analysis technology, especially in the case of low signal-to-noise ratio. To address this problem, we propose a curvelet-based time-frequency analysis method, which is suitable for seismic data, and takes into account the lateral variation of seismic data. We first construct a kind of curvelet adapted to seismic data. By adjusting the rotation mode of the curvelet in the form of time skewing, the scale parameter can be directly related to the frequency of the seismic data. Therefore, the curvelet coefficients at different scales can reflect the time-frequency information of the seismic data. Then, the curvelet coefficients, which represent the dominant azimuthal pattern, are converted to the time-frequency domain. Since the curvelet transform is a kind of sparse representation for the signal, the screening process of the dominant coefficient masks most of the random noise, which enables the method to adapt for the low signal-to-noise ratio data. Results of synthetic and field data experiments using the proposed method demonstrate that it is a good approach to identify weak signals from strong noise in the time-frequency domain.     

Insights into the formation of silica‐rich achondrites from impact melts in Rumuruti‐type chondrites

Ancient, SiO₂‐rich achondrites have previously been proposed to have formed by disequilibrium partial melting of chondrites. Here, we test the alternative hypothesis that these achondrites formed by fractional crystallization of impact melts of Rumuruti (R) chondrites. We identified two new melt clasts in R chondrites, one in Pecora Escarpment (PCA) 91241 and one in LaPaz Icefield (LAP) 031275. We analyzed major, minor, and trace element concentrations, as well as oxygen isotopes, of these two clasts and a third one that had been previously recognized (Bischoff et al. 2011) as an impact melt in Dar al Gani (DaG) 013. The melt clast in PCA 91241 is an R chondrite impact melt closely resembling the one previously recognized in DaG 013. The melt clast in LAP 031275 has an L chondrite provenance. We show that SiO₂‐rich melts could form from the mesostases of R chondrite impact melts. However, their CI‐normalized rare earth element patterns are flat, whereas those of ancient SiO₂‐rich achondrites (Day et al. 2012; Srinivasan et al. 2018) and those of disequilibrium partial melts of chondrites (Feldstein et al. 2001) have positive Eu anomalies from preferential melting of plagioclase. Thus, we conclude that ancient SiO₂‐rich achondrites were probably formed by disequilibrium partial melting (due to an internal heat source on their parent bodies), rather than from impact melts.     

A Multi-Trajectory Chemical-Transport Vectorized Gear Model: 3-D Simulations and Model Validation

A new multi-trajectory highly vectorizable Gear chemical-transport model is discussed and tested against measurements from 25 sites in Europe for a two-week summer period. The simulations are compared with measurement data and with results of a Quasi-Steady-State Approximation (QSSA) based Lagrangian model using the EMEP mechanism.The model is developed from the Lagrangian EMEP model. The Regional Atmospheric Chemistry Mechanism (RACM) is used as the models chemical mechanism. To solve the stiff chemical rate equations, a sparse-matrix highly vectorizable Gear algorithm is used. The meteorological data used to run the model are obtained from the HIgh Resolution Limited Area Model (HIRLAM).The vectorized Gear based model improves the accuracy of the numerical solution for the chemistry compared with the QSSA based model. The differences between the two models are explained by alternative approaches of the chemical modules employed in the model: a Gear algorithm versus a QSSA solver, photolysis rate parameters in RACM are calculated from a radiation transfer code versus parameterized photolysis rate parameters used in the Lagrangian EMEP model, the RACM versus the EMEP atmospheric chemical mechanisms and two methods of aggregating the emissions of Volatile Organic Compounds (VOC). Photolysis rate parameters calculated from radiation codes (which are more accurate than simple parameterizations) and the Gear algorithm (which is a benchmark solver compared with the QSSA solver) are recommended for atmospheric chemistry modeling because of the high sensitivity of ozone concentrations to the chemical reaction scheme and to the photolysis rates (Stockwell and Goliff, 2004).     

Primitive olivine‐phyric shergottite NWA 5789: Petrography,mineral chemistry,and cooling history imply a magma similar to Yamato‐980459

Knowledge of Martian igneous basaltic compositions is crucial for constraining mantle evolution, including early differentiation and mantle convection. Primitive magmas provide direct information about their mantle source regions, but most Martian meteorites either contain cumulate olivine or crystallized from fractionated melts. The recently discovered Martian meteorite Northwest Africa (NWA) 5789 is an olivine‐phyric shergottite. NWA 5789 has special significance among the Martian meteorites because it appears to represent one of the most magnesian Martian magmas known, other than Yamato (Y) 980459. Its most magnesian olivine cores (Fo₈₅) are in Mg‐Fe equilibrium with a magma of the bulk rock composition, suggesting that the bulk represents a magma composition. Based on the Al/Ti ratio of its pyroxenes, we infer that the rock began to crystallize at a high pressure consistent with conditions in Mars’ lower crust/upper mantle. It continued and completed its crystallization closer to the surface, where cooling was rapid and produced a mesostasis of radiating sprays of plagioclase and pyroxene. The mineralogy, petrology, mineral chemistry, and bulk rock composition of NWA 5789 are very similar to those of Y‐980459. The similarities between the two meteorites suggest that NWA 5789 (like Y‐980459) represents a primitive, mantle‐derived magma composition. They also suggest the possibility that NWA 5789 and Y‐980459 formed in the same lava flow. However, based on the mineralogy and texture of its mesostasis, NWA 5789 must have cooled more slowly than Y‐980459. NWA 5789 will help elucidate the igneous geology and geochemistry of Mars.