Perched lobe formation in the Gulf of Cadiz: Interactions between gravity processes and contour currents (Algarve Margin, Southern Portugal)     

E. Marchès  T. Mulder  M. Cremer  T. Garlan 《Sedimentary Geology》2010,229(3):81-94

The Gulf of Cadiz is swept by the strong saline Mediterranean Outflow Water (MOW). On the Algarve Margin (South Portugal), this current has constructed fine-grained contourite drifts. This margin is dissected by the Portimao Canyon and three short channels that only incise the upper slope, and are absent on a terrace located at mid-slope depths along the Algarve Margin. High-resolution seismic profiles and sediment cores highlight the original architecture of the sedimentary deposits on this terrace. Coarse-grained lenticular chaotic bodies formed during major relative sea-level lowstands are intercalated within the drift. The lobate shape and sandy nature of the lenticular chaotic bodies and their location at the mouths of the three short channels suggest they are gravity-generated deposits that are perched on the middle continental slope.In the Gulf of Cadiz, the interaction between contour current and gravity processes is strongly controlled by climatic variations and relative sea-level changes during the late Quaternary. During cold periods when sea-level was low, erosion intensified on the continental shelf and the deepest part of MOW was active. Sediment was transported downslope through the channels and deposited on sedimentary lobes perched on the mid-slope terrace. During warm periods when relative sea-level was high, the supply of sediment from the shelf was shut off and the shallowest part of MOW was more active. Contourite drifts fill the channels and bury the sandy lobes.  相似文献   

岩浆成矿体系的热演化和剥露史的数字模拟     

傅清平  BrentI.A.Mclnnes  PeterJ.Davies 《地球科学》2004,29(5):555-562

通过对岩浆冷却过程的数字模拟研究,揭示出岩浆在冷却成矿过程中的温度分布和变化规律及影响因素．在此基础上,进一步应用高精度的温龄计组合来限定岩浆成矿体系的热演化和剥露历史,精确地计算出岩浆的初始侵位时间和深度、矿物结晶时间、冷却速率、冷却和暴露地表时间,以及剥露和剥蚀速率等重要参数,并将模拟结果应用于斑岩铜矿床的成矿研究中．研究表明,将精确的年龄测试手段与计算机模拟技术相结合,可为定量研究岩浆矿床的热演化和剥露史、深入了解矿床的成因机制提供一种有效方法．  相似文献   

The oldest tylosaurine mosasaur (Reptilia; Lacertilia) from the Late Cretaceous of Belgium: Hermann von Meyer (1860) revisited     

Mulder  Eric W.A.  Mai  Hartmut 《Geologie en Mijnbouw》1999,78(2):207-213

A mosasaurid parietal from the Late Cretaceous of Belgium, originally described by Von Meyer (1860), turns out to be assignable to the tylosaurine Hainosaurus cf. bernardi Dollo 1885. A small sediment sample, extracted from the bone, was analysed for coccoliths and revealed its early Late Campanian age. This is thus the earliest record of a tylosaurine mosasaur from Belgium.  相似文献   

The effect of geometry and tidal forcing on hydrodynamics and net sediment transport in semi-enclosed tidal basins     

Thomas Boelens  Henk Schuttelaars  George Schramkowski  Tom De Mulder 《Ocean Dynamics》2018,68(10):1285-1309

A new depth-averaged exploratory model has been developed to investigate the hydrodynamics and the tidally averaged sediment transport in a semi-enclosed tidal basin. This model comprises the two-dimensional (2DH) dynamics in a tidal basin that consists of a channel of arbitrary length, flanked by tidal flats, in which the water motion is being driven by an asymmetric tidal forcing at the seaward side. The equations are discretized in space by means of the finite element method and solved in the frequency domain. In this study, the lateral variations of the tidal asymmetry and the tidally averaged sediment transport are analyzed, as well as their sensitivity to changes in basin geometry and external overtides. The Coriolis force is taken into account. It is found that the length of the tidal basin and, to a lesser extent, the tidal flat area and the convergence length determine the behaviour of the tidally averaged velocity and the overtides and consequently control the strength and the direction of the tidally averaged sediment transport. Furthermore, the externally prescribed overtides can have a major influence on tidal asymmetry in the basin, depending on their amplitude and phase. Finally, for sufficiently wide tidal basins, the Coriolis force generates significant lateral dynamics.  相似文献   

Sediment failure types,preconditions and triggering factors in the Gulf of Cadiz     

D. Leynaud  T. Mulder  V. Hanquiez  E. Gonthier  A. Régert 《Landslides》2017,14(1):233-248

A series of morphological structures, such as scars and escarpments related to seafloor instabilities, were observed in the Gulf of Cadiz using multibeam bathymetry and acoustic imagery. According to the geometry of the slide scars, the slope angle, the surrounding seafloor morphology and the mechanical parameters of the sediment, we suggest the likely mechanisms initiating the failures for the different types of observed structures. Most of the small-scale sediment failures (≤2 km²) seem directly related to dome-like structures (where slopes are steep) or are located in the vicinity of such structures (fluid flows). It appears that progressive deformation or fluid flow related to the growing of dome-like structures may have weakened the sediments sufficiently to bring 7°-steep slopes to metastable conditions (with a factor of safety close to 1.0). The other instability types are likely related to high-magnitude (Ms?>?6) earthquakes, which are prone to occur in this area (located in the neighbourhood of the 1755 Lisbon earthquake area). Some particular large-scale structures were observed among these seafloor features, for example on the Guadalquivir Bank. On this bank, a series of successive large scars (at least 4 km long), composed of multiple and very regular arcuate segments (1 km in diameter), were observed at different bathymetric levels (every 40 m). These structures might be related to a deep-rooted detachment zone (e.g. successive listric faults) and triggered by high-magnitude earthquakes or by accumulated displacement along a tectonic discontinuity. This would explain such a large-scale deformation, providing a regular escarpment of 40 m high without any sediment flow downslope, thereby suggesting an ongoing (or unfinished) deformation.  相似文献   

Subsurface offset behaviour in velocity analysis with extended reflectivity images     

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   

Simulating the 1999 Capbreton canyon turbidity current with a Cellular Automata model     

T. Salles  T. Mulder  M. Gaudin  M.C. Cacas  S. Lopez  P. Cirac 《Geomorphology》2008,97(3-4):516-537

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   

Migration for velocity and attenuation perturbations   总被引：1，自引：0，他引：1  

Bobby Hak  Wim A. Mulder 《Geophysical Prospecting》2010,58(6):939-952

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   

Fast and dynamic generation of linear octrees for geological bodies under hardware acceleration     

DENG Hao    ZHANG LiQiang  MAO XianCheng  MA JingTao  KANG ZhiZhong  PENG JunHuan  RUI XiaoPing & LIU Liu State Key Laboratory of Remote Sensing Science  Beijing Normal University  Beijing  China  School of Geoscience  Environmental Engineering  Central South University  Changsha  PTV America  Inc. Broadway Plaza Ste  Tacoma  WA  USA 《中国科学:地球科学(英文版)》2010,(1)

In the application of 3D Geoscience Modeling,we often need to generate the volumetric representations of geological bodies from their surface representations.Linear octree,as an efficient and easily operated volumetric model,is widely used in 3D Geoscience Modeling.This paper proposes an algorithm for fast and dynamic generation of linear octrees of geological bodies from their surface models under hardware acceleration.The Z-buffers are used to determine the attributes of octants and voxels in a fast way,a...  相似文献   

Applying essentially non‐oscillatory interpolation to controlled‐source electromagnetic modelling*     

M. Wirianto  W.A. Mulder  E.C. Slob 《Geophysical Prospecting》2011,59(1):161-175

Modelling and inversion of controlled‐source electromagnetic (CSEM) fields requires accurate interpolation of modelled results near strong resistivity contrasts. There, simple linear interpolation may produce large errors, whereas higher‐order interpolation may lead to oscillatory behaviour in the interpolated result. We propose to use the essentially non‐oscillatory, piecewise polynomial interpolation scheme designed for piecewise smooth functions that contains discontinuities in the function itself or in its first or higher derivatives. The scheme uses a non‐linear adaptive algorithm to select a set of interpolation points that represent the smoothest part of the function among the sets of neighbouring points. We present numerical examples to demonstrate the usefulness of the scheme. The first example shows that the essentially non‐oscillatory interpolation (ENO) scheme better captures an isolated discontinuity. In the second example, we consider the case of sampling the electric field computed by a finite‐volume CSEM code at a receiver location. In this example, the ENO interpolation performs quite well. However, the overall error is dominated by the discretization error. The other examples consider the comparison between sampling with essentially non‐oscillatory interpolation and existing interpolation schemes. In these examples, essentially non‐oscillatory interpolation provides more accurate results than standard interpolation, especially near discontinuities.  相似文献