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61.
W.A. Mulder 《Geophysical Prospecting》2014,62(1):17-33
Migration velocity analysis with the constant‐density acoustic wave equation can be accomplished by the focusing of extended migration images, obtained by introducing a subsurface shift in the imaging condition. A reflector in a wrong velocity model will show up as a curve in the extended image. In the correct model, it should collapse to a point. The usual approach to obtain a focused image involves a cost functional that penalizes energy in the extended image at non‐zero shift. Its minimization by a gradient‐based method should then produce the correct velocity model. Here, asymptotic analysis and numerical examples show that this method may be too sensitive to amplitude peaks at large shifts at the wrong depth and to artefacts. A more robust alternative is proposed that can be interpreted as a generalization of stack power and maximizes the energy at zero‐subsurface shift. A real‐data example is included. 相似文献
62.
63.
A numerical model has been developed for the simulation of turbidity currents driven by nonuniform, non cohesive sediment and flowing over a complex three dimensional submarine topography. The model is based on an alternative approach known as Cellular Automata paradigm. The model is validated by comparing a simulation with a reported field-scale event. The chosen case is a turbidity current which occurred in Capbreton Canyon and was initiated by a storm in December 1999. Using data from recent oceanographic cruises, the deposit of the event has been precisely described, which constrain values of model parameters. The model simulates the 1999 turbidity current over the actual canyon topography and related turbidite using three different types of particle. The model successfully simulates areas of erosion and deposition in the canyon. It predicts the vertical and longitudinal grain size evolution, and shows that the fining-up sequence can be deposited by several phases of deposition and erosion related to the current energetic variation during its evolution. This result could explain the presence of intrabed contacts or the frequent lack of facies in Bouma sequences. 相似文献
64.
Migration for velocity and attenuation perturbations 总被引:1,自引:0,他引:1
Migration maps seismic data to reflectors in the Earth. Reflections are not only caused by small‐scale variations of the velocity and density but also of the quality factor that describes attenuation. We investigated scattering due to velocity and attenuation perturbations by computing the resolution function or point‐spread function in a homogeneous background model. The resolution function is the migration image of seismic reflection data generated by a point scatterer. We found that the resolution function mixes velocity and attenuation parameter perturbations to the extent that they cannot be reconstructed independently. This is true for a typical seismic setting with sources and receivers at the surface and a buried scatterer. As a result, it will be impossible to simultaneously invert for velocity and attenuation perturbations in the scattering approach, also known as the Born approximation. We proceeded to investigate other acquisition geometries that may resolve the ambiguity between velocity and attenuation perturbations. With sources and receivers on a circle around the scatterer, in 2D, the ambiguity disappears. It still shows up in a cross‐well setting, although the mixing of velocity and attenuation parameters is less severe than in the surface‐to‐surface case. We also consider illumination of the target by diving waves in a background model that has velocity increasing linearly with depth. The improvement in illumination is, however, still insufficient to remove the ambiguity. 相似文献
65.
Laurent Millet Charline Giguet-Covex Valérie Verneaux Jean-Claude Druart Thierry Adatte Fabien Arnaud 《Journal of Paleolimnology》2010,44(4):963-978
This paper presents the recent history of a large prealpine lake (Lake Bourget) using chironomids, diatoms and organic matter
analysis, and deals with the ability of paleolimnological approach to define an ecological reference state for the lake in
the sense of the European Framework Directive. The study at low resolution of subfossil chironomids in a 4-m-long core shows
the remarkable stability over the last 2.5 kyrs of the profundal community dominated by a Micropsectra-association until the beginning of the twentieth century, when oxyphilous taxa disappeared. Focusing on this key recent period,
a high resolution and multiproxy study of two short cores reveals a progressive evolution of the lake’s ecological state.
Until AD 1880, Lake Bourget showed low organic matter content in the deep sediments (TOC less than 1%) and a well-oxygenated
hypolimnion that allowed the development of a profundal oxyphilous chironomid fauna (Micropsectra-association). Diatom communities were characteristic of oligotrophic conditions. Around AD 1880, a slight increase in the
TOC was the first sign of changes in lake conditions. This was followed by a first limited decline in oligotrophic diatom
taxa and the disappearance of two oxyphilous chironomid taxa at the beginning of the twentieth century. The 1940s were a major
turning point in recent lake history. Diatom assemblages and accumulation of well preserved planktonic organic matter in the
sediment provide evidence of strong eutrophication. The absence of profundal chironomid communities reveals permanent hypolimnetic
anoxia. From AD 1995 to 2006, the diatom assemblages suggest a reduction in nutrients, and a return to mesotrophic conditions,
a result of improved wastewater management. However, no change in hypolimnion benthic conditions has been shown by either
the organic matter or the subfossil chironomid profundal community. Our results emphasize the relevance of the paleolimnological
approach for the assessment of reference conditions for modern lakes. Before AD 1900, the profundal Micropsectra-association and the Cyclotella dominated diatom community can be considered as the Lake Bourget reference community, which reflects the reference ecological
state of the lake. 相似文献
66.
Origin and mechanism of Subantarctic Mode Water formation and transformation in the Southern Indian Ocean 总被引:1,自引:0,他引:1
The sources and pathways of mode waters and lower thermocline waters entering the subtropical gyre of the Indian Ocean are
examined. A Lagrangian analysis is performed on an eddy-admitting simulation of the Global Ocean performed by the DRAKKAR
Group (NEMO/OPA), which captures the main observed features. We trace the subducted mode water’s pathways, identify their
formation regions and trace whether their source waters come from the Atlantic, Pacific or Indian sectors of the Southern
Ocean. Three main sites for mode waters ventilation in the Indian sector are identified with different circulation pathways
and source water masses: (a) just north of Kerguelen, where 4.2 Sv of lighter Subantarctic Mode Waters (SAMW); σ
0 ∼ 26.5) are exported—originating in the Atlantic and Agulhas Retroflection regions; (b) SW of Australia, where 6.5 Sv of
medium SAMW (σ
0 ∼ 26.6) are ventilated—originating in the southern and denser Agulhas Retroflection region; (c) SW of Tasmania and along
the South Australian coast, where 3 Sv of denser SAMW (σ
0 ∼ 26.75) are ventilated—originating from three sources: Leeuwin Current waters, Tasman Sea (Pacific) waters and Antarctic
Surface Waters. In all cases, modelled mode waters were last ventilated in the Indian Ocean just north of the deepest winter-mixed
layers. For the waters subducted SW of Australia, the last ventilation site extends even further north. Waters ventilated
in the deepest mixed layers north of the Subantarctic Front are then re-ventilated 5 years later southwest of Australia. The
model results raise new hypotheses that revisit the classical picture of the SAMW formation and transformation, where a large
homogeneous mixed layer is subducted and ‘slides’ equatorward, essentially maintaining the T/S characteristics acquired at
the surface. Firstly, the last ventilation of the modelled mode waters is not in the region of the deepest mixed layers, as
previously thought, but further north in regions of moderate meso-scale eddy activity. Secondly, the model shows for the first
time a significant source region for Indian Ocean mode waters coming from deep winter-mixed layers along the south Australian
coast. Finally, this analysis shows how the mode water characteristics are modified after subduction, due to internal eddy
mixing. The simulation shows that resolved eddies have a strong impact on the mixed layer properties and that isopycnal eddy
mixing also contributes to the generation of more homogeneous mode water characteristics in the interior. 相似文献
67.
68.
We present a theoretical weakly nonlinear analysis of the dynamics of an inviscid flow submitted to both rotation and precession of an unbounded cylindrical container, by considering the coupling of two Kelvin (inertial) waves. The parametric centrifugal instability known for this system is shown to saturate when one expands the Navier–Stokes equation to higher order in the assumed small precession parameter (ratio of precession to rotation frequencies) with the derivation of two coupled Landau equations suitable to describe the dynamics of the modes. It is shown that an azimuthal mean flow with differential rotation is generated by this modes coupling. The time evolution of the associated dynamical system is studied. These theoretical results can be compared with water experiments and also to some numerical simulations where viscosity and finite length effects cannot be neglected. 相似文献
69.
Rémy Thibaud Géraldine Del Mondo Thierry Garlan Ariane Mascret Christophe Carpentier 《Transactions in GIS》2013,17(5):742-762
Defining a model for the representation and the analysis of spatio‐temporal dynamics remains an open domain in geographical information sciences. In this article we investigate a spatio‐temporal graph‐based model dedicated to managing and extracting sets of geographical entities related in space and time. The approach is based on spatial and temporal local relations between neighboring entities during consecutive times. The model allows us to extract sets of connected entities distant in time and space over long periods and large spaces. From GIS concepts and qualitative reasoning on space and time, we combine the graph model with a dedicated spatial database. It includes information on geometry and geomorphometric parameters, and on spatial and temporal relations. This allows us to extend classical measurements of spatial parameters, with comparisons of entities linked by complex relations in space and time. As a case study, we show how the model suggests an efficient representation of dunes dynamics on a nautical chart for safe navigation. 相似文献
70.
Use of LIDAR in landslide investigations: a review 总被引:13,自引:0,他引:13
Michel Jaboyedoff Thierry Oppikofer Antonio Abellán Marc-Henri Derron Alex Loye Richard Metzger Andrea Pedrazzini 《Natural Hazards》2012,61(1):5-28
This paper presents a short history of the appraisal of laser scanner technologies in geosciences used for imaging relief
by high-resolution digital elevation models (HRDEMs) or 3D models. A general overview of light detection and ranging (LIDAR)
techniques applied to landslides is given, followed by a review of different applications of LIDAR for landslide, rockfall
and debris-flow. These applications are classified as: (1) Detection and characterization of mass movements; (2) Hazard assessment
and susceptibility mapping; (3) Modelling; (4) Monitoring. This review emphasizes how LIDAR-derived HRDEMs can be used to
investigate any type of landslides. It is clear that such HRDEMs are not yet a common tool for landslides investigations,
but this technique has opened new domains of applications that still have to be developed. 相似文献