Study of the transfer between libration point orbits and lunar orbits in Earth–Moon system     

Yu Cheng  Gerard Gómez  Josep J. Masdemont  Jianping Yuan 《Celestial Mechanics and Dynamical Astronomy》2017,128(4):409-433

This paper is devoted to the study of the transfer problem from a libration point orbit of the Earth–Moon system to an orbit around the Moon. The transfer procedure analysed has two legs: the first one is an orbit of the unstable manifold of the libration orbit and the second one is a transfer orbit between a certain point on the manifold and the final lunar orbit. There are only two manoeuvres involved in the method and they are applied at the beginning and at the end of the second leg. Although the numerical results given in this paper correspond to transfers between halo orbits around the \(L_1\) point (of several amplitudes) and lunar polar orbits with altitudes varying between 100 and 500 km, the procedure we develop can be applied to any kind of lunar orbits, libration orbits around the \(L_1\) or \(L_2\) points of the Earth–Moon system, or to other similar cases with different values of the mass ratio.  相似文献   

Transmission + reflection anisotropic wave-equation traveltime and waveform inversion     

Shihang Feng  Gerard T. Schuster 《Geophysical Prospecting》2019,67(2):423-442

A transmission + reflection wave-equation traveltime and waveform inversion method is presented that inverts the seismic data for the anisotropic parameters in a vertical transverse isotropic medium. The simultaneous inversion of anisotropic parameters and ε is initially performed using transmission wave-equation traveltime inversion method. Transmission wave-equation traveltime only provides the low-intermediate wavenumbers for the shallow part of the anisotropic model; in contrast, reflection wave-equation traveltime estimates the anisotropic parameters in the deeper section of the model. By incorporating a layer-stripping method with reflection wave-equation traveltime, the ambiguity between the background-velocity model and the depths of reflectors can be greatly mitigated. In the final step, multi-scale full-waveform inversion is performed to recover the high-wavenumber component of the model.  We use a synthetic model to illustrate the local minima problem of full-waveform inversion and how transmission and reflection wave-equation traveltime can mitigate this problem. We demonstrate the efficacy of our new method using field data from the Gulf of Mexico.  相似文献   

Species selection and assessment of eco-engineering effects of seedlings for biogeomorphological landscape experiments     

Ivar R. Lokhorst  Sjoukje I. de Lange  Gerard van Buiten  Sanja Selaković  Maarten G. Kleinhans 《地球表面变化过程与地形》2019,44(14):2922-2935

Landscape experiments of fluvial environments such as rivers and deltas are often conducted with live seedlings to investigate effects of biogeomorphological interactions on morphology and stratigraphy. However, such experiments have been limited to a single species, usually alfalfa (Medicago sativa), whereas important environments in nature have many different vegetation types and eco-engineering effects. Landscape experimentation would therefore benefit from a larger choice of tested plant species. For the purpose of experimental design our objective was to identify fast-germinating and fast-growing species and determine their sensitivity to flow conditions during and after settling, their maximum growth, hydraulic resistance and added bank strength. We tested germination time and seedling growth rate of 18 candidate species with readily available seeds that are fast growing and occur at waterlines, plus Medicago sativa as a control. We selected five species that germinate and develop within days and measured properties and eco-engineering effects depending on plant age and density, targeting typical experimental conditions of 0–0.3 m/s flow velocity and 0–30 mm water depth. Tested eco-engineering effects include bank strength and flow resistance. We found that Rumex hydrolapathum can represent riparian trees. The much smaller Veronica beccabunga and Lotus pedunculatus can represent grass and saltmarsh species as they grow in dense patches with high flow resistance but are readily erodible. Sorghum bicolor grows into tall, straight shoots, which add significantly to bank strength, but adds little flow resistance and may represent sparse hardwood trees. Medicago sativa also grows densely under water, suggesting a use for mangroves and perhaps peat. In stronger and deeper flows the application of all species changes accordingly. These species can now be used in a range of landscape experiments to investigate combined effects on living landscape patterns and possible facilitation between species. The testing and treatment methodology can be applied to new species and other laboratory conditions. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd. © 2019 The Authors Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd © 2019 The Authors Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.  相似文献   

Automated Analyte Separation by Ion Chromatography Using a Cobot Applied to Geological Reference Materials for Li Isotope Composition     

Marie L. Kuessner  Alkiviadis Gourgiotis  Gerard Manhs  Julien Bouchez  Xu Zhang  Jrme Gaillardet 《Geostandards and Geoanalytical Research》2020,44(1):57-67

The demand for large and reliable data sets on isotopic composition has increased in geochemistry and environmental sciences over recent years. We present an automated ion chromatographic separation method using a robotic pipetting arm, termed ‘ChemCobOne’, to reduce sample separation time. Its performance was tested for lithium isotope separation in geological reference materials using a single‐step separation with HCl (0.2 mol l^?1) and a 2 ml resin volume. This refined lithium purification method does not forfeit precision, accuracy or purity compared with manual sample processing. In addition, a δ⁷Li value for NASS‐6 of 30.99 ± 0.50‰ (2s) (95% CI = 0.14‰, n = 44) was determined and the first δ⁷Li values for the granite rock reference material GS‐N (?0.57 ± 0.25‰ (2s), 95% CI = 0.15‰, n = 15), and for the soil reference material NIST SRM 2709a (?0.37 ± 0.67‰ (2s), 95% CI = 0.15‰, n = 63) are proposed.  相似文献   

Pilot assessment of depth related distribution of macrofauna in surf zone along Dutch coast and its implications for coastal management     

Gerard Janssen  Hans Kleef  Saskia Mulder  & Peter Tydeman 《Marine Ecology》2008,29(S1):186-194

Surf zones are highly dynamic, physically stressful parts of sandy beach ecosystems. The high wave energy of surf zones has in the past severely hampered ecological surveys of these systems. Here we used a novel technique to collect fauna from this environment along the Dutch coast. A large vehicle in the form a tripod that drives along the sandy seafloor and supports a sampling platform 11 m above the water line can collect both infaunal (grabs) samples and pull beam trawls for epibenthos. The distribution and diversity of macrofauna were studied at different depths in the surf zone along the Dutch coast. Species diversity and abundance increased with increasing depth of the water column. This increase was especially noticeable on the seaward side of the outer breaker bar. Within the surf zone, in the trough between the two breaker bars, there were spots of high diversity and abundance of macrobenthic infauna. Moreover, the area is also important for epibenthic and fish species, like the commercially important flatfish sole. Spatial patterns of species richness and abundance across an onshore-offshore gradient from the beach to seawards of the breakers suggest the presence of faunal zonation in this environment. The high abundance recorded in troughs was primarily caused by patches of juvenile Sand mason Lanice conchilega. The management implications of these results are that we suggest to protect the surf zone, including the trough between the two breaker bars, as a potential area of high diversity and abundance and to reconsider the objectives of the EU-Habitat Directive and the Water Framework Directive for the coastal area.  相似文献   

In Situ measurement of transverse isotropy in shallow-water marine sediments     

Patricia A. Berge  Subhashis Mallick  Gerard J. Fryer  Noël Barstow  Jerry A. Carter  George H. Sutton  John I. Ewing 《Geophysical Journal International》1991,104(2):241-254

  相似文献   

Seismic waves in stratified anisotropic media   总被引：4，自引：0，他引：4  

Gerard J. Fryer  L. Neil Frzzer 《Geophysical Journal International》1984,78(3):691-710

Summary. The response of a structure composed of anisotropic strata can be built up from the reflection and transmission properties of individual interfaces using a slightly modified version of the recursion scheme of Kennett. This scheme is conveniently described in terms of scatterer operators and scatterer products. The effects of a free surface and the introduction of a simple point source at any depth can be accommodated in a manner directly analogous to the treatment for isotropic structures. As in the isotropic case the results so obtained are stable to arbitrary wavenumbers. For isotropic media, synthetic seismograms can be constructed by computing the structure response as a function of frequency and radial wavenumber, then performing the appropriate Fourier and Hankel transforms to obtain the wavefield in time-distance space. Such a scheme is convenient for any system with cylindrical symmétry (including transverse isotropy). Azimuthally anisotropic structures, however, do not display cylindrical symmétry; for these the transverse component of the wavenumber vector will, in general, be non-zero, with the result that phase, group, and energy velocities may all diverge. The problem is then much more conveniently addressed in Cartesian coordinates, with the frequency-wavenumber to time-distance transformation accomplished by 3-D Fourier transform.  相似文献   

The orientation of pre-transient coronal magnetic fields     

Gerard Trottet  R. M. MacQueen 《Solar physics》1980,68(1):177-186

Loop-like white light coronal transients are generally believed to be nearly planar sheets which are thin compared to the loop extent; however, this picture may be questioned since virtually no observations (of the more than 100 transient events observed during 1973–74 Skylab period) show such loops edge-on. From the group of transient events studied by Munro etal. (1979) for which definite surface associations exist, we find loop transients are strongly correlated with filament regions where the filament axis was oriented north-south. From direct soft X-ray observations of an expanding arch, the possible identification of the soft X-ray signature of footpoints of transient loops, and monochramatic observations of low coronal loops, we infer that loop-like coronal transients have their origin in low-lying coronal loops nearly co-planar with the north-south aligned filament axis. The situation with respect to non-loop events is less clear; such events apparently often arise from more complex filament geometries. Possible reasons for the preference of transients to arise from north-south filament-oriented regions are discussed.The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

The structure of coronal magnetic loops     

Riccardo Giachetti  Gerard Van Hoven  Claudio Chiuderi 《Solar physics》1977,55(2):371-386

We present the second part of a complete theory for the plasma and field structure of a cool coronal arch, corresponding to those observed in the EUV from Skylab. The global magneto-hydrodynamic (MHD) stability of a previously described equilibrium-loop model is evaluated, and compared with that of an unmodified ambient force-free field. The influence of the photospheric boundary condition is also evaluated, producing a specification of stability limits which depend on the relative field and plasma pressures and scale widths. The resulting restrictions on the allowable field configuration of a coronal loop are then compared with observed values. The implications of this general method for deducing small-scale coronal magnetic-field structure from the measured plasma profile of an emissive feature are also described.  相似文献   

Apollo 15 and 16 results of the integrated geochemical experiment     

I. Adler  J. I. Trombka  P. Lowman  R. Schmadebeck  H. Blodget  E. Eller  L. Yin  R. Lamothe  G. Osswald  J. Gerard  P. Gorenstein  P. Bjorkholm  H. Gursky  B. Harris  J. Arnold  A. Metzger  R. Reedy 《Earth, Moon, and Planets》1973,7(3-4):487-504

A number of experiments carried in orbit on the Apollo 15 and 16 spacecraft were used in the compositional mapping of the lunar surface. The observations involved measurements of secondary (fluorescent) X-rays, gamma rays and alpha particle emissions. A large scale compositional map of over 20% of the lunar surface was obtained for the first time. It was possible to demonstrate significant chemical differences between the mare and the highlands, to find specific areas of high radioactivity and to learn something about the composition of the Moon's hidden side.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April 1973.  相似文献