Amplitude Preserving Kirchhoff Migration: A Traveltime Based Strategy     

Gajewski  Dirk  Coman  Radu  Vanelle  Claudia 《Studia Geophysica et Geodaetica》2002,46(2):193-211

Amplitude versus offset information is a key feature to seismic reservoir characterization. Therefore amplitude preserving migration was developed to obtain this information from seismic reflection data. For complex 3-D media, however, this process is computationally expensive. In this paper we present an efficient traveltime based strategy for amplitude preserving migration of the Kirchhoff type. Its foundations are the generation of traveltime tables using a wavefront-oriented ray-tracing technique, and a generalized moveout relation for 3-D heterogeneous media. All required quantities for the amplitude preserving migration are computed from coarsely gridded traveltime tables. The migration includes the interpolation from the coarsely gridded input traveltimes onto the fine migration grid, the computation of amplitude preserving weight functions, and, optionally, the evaluation of an optimized migration aperture. Since ray tracing is employed for the traveltime computation the input velocity model needs to be smooth, i.e. velocity variations of spatial dimensions below the wavelength of the considered reflection signals are removed. Numerical examples on simple generic models validate the technique and an application to the Marmousi model demonstrates its potential to complex media. The major advantage of the traveltime based strategy consists of its computational efficiency by maintaining sufficient accuracy. Considerable savings in storage space (10⁵ and more for 3-D data with respect to no interpolation at all) can be achieved. The computational time for the stack can be substantially reduced (up to 90% in 3-D) with the optimized migration aperture since only those traces are stacked which really contribute to the image point under consideration.  相似文献   

Eustatic and climatic controls on the development of the Wahiba Sand Sea, Sultanate of Oman   总被引：3，自引：0，他引：3  

Dirk Radies  Frank Preusser  Albert Matter  Maria Mange† 《Sedimentology》2004,51(6):1359-1385

Aeolian sand sea accumulations can serve as valuable archives of climate change in continental environments. The Wahiba Sand Sea is situated at the northern margin of the area presently affected by Indian Summer Monsoon Circulation and it records environmental changes associated with this major climatic boundary over the last 160 000 years. The internal stratigraphy and evolution of the sand sea is investigated using a combination of outcrop, borehole, seismic and luminescence data. Proximity to the Indian Ocean means that the sand sea succession shows the influence of sea level changes on the sedimentary architecture and composition of the dune deposits. During the last two glacial periods, low global sea level was associated with a high input of bioclastic grains, reflecting the significance of subaerially exposed shelf areas as one of the main sources of aeolian sediment. The onset of aeolian sediment transport and deposition was related to the breakdown of stabilizing vegetation during arid periods that equate with sea level lowstands. The preservation of aeolian sediments by the formation of supersurfaces and associated palaeosoils took place during times of increased wetness and elevated groundwater tables. This interplay of constructive and destructive periods greatly influenced the sedimentary architecture. Oscillations of wet and dry periods between 160 000 and 130 000 years and 120 000–105 000 years ago are attributed to the evolution of a wet aeolian system. Younger periods of aeolian deposition around and after the last glacial maximum were characterized by dry aeolian conditions. No soil horizons developed during these times.  相似文献   

Rising methane gas bubbles form massive hydrate layers at the seafloor   总被引：3，自引：0，他引：3  

Matthias Haeckel  Erwin Suess  Dirk Rickert 《Geochimica et cosmochimica acta》2004,68(21):4335-4345

Extensive methane hydrate layers are formed in the near-surface sediments of the Cascadia margin. An undissociated section of such a layer was recovered at the base of a gravity core (i.e. at a sediment depth of 120 cm) at the southern summit of Hydrate Ridge. As a result of salt exclusion during methane hydrate formation, the associated pore waters show a highly elevated chloride concentration of 809 mM. In comparison, the average background value is 543 mM.A simple transport-reaction model was developed to reproduce the Cl⁻ observations and quantify processes such as hydrate formation, methane demand, and fluid flow. From this first field observation of a positive Cl⁻ anomaly, high hydrate formation rates (0.15-1.08 mol cm⁻² a⁻¹) were calculated. Our model results also suggest that the fluid flow rate at the Cascadia accretionary margin is constrained to 45-300 cm a⁻¹. The amount of methane needed to build up enough methane hydrate to produce the observed chloride enrichment exceeds the methane solubility in pore water. Thus, most of the gas hydrate was most likely formed from ascending methane gas bubbles rather than solely from CH₄ dissolved in the pore water.  相似文献   

Investigation of the pyrolytic liberation of molecular nitrogen from Palaeozoic sedimentary rocks     

Bernhard M. Krooss  Lothar Friberg  Yves Gensterblum  Jan Hollenstein  Dirk Prinz  Ralf Littke 《International Journal of Earth Sciences》2005,94(5-6):1023-1038

Open-system non-isothermal pyrolysis up to 1,200°C in combination with elemental analysis was used to study the thermal liberation of molecular nitrogen (N₂) from sedimentary rocks and kerogen concentrates of Palaeozoic age from the Central European Basin system and an Eocene shale (Liaohe Basin, China) with a high content (36%) of ammonium feldspar (buddingtonite). The N/C_org (atomic) ratios of the kerogen concentrates ranged from 0.005 to 0.014, which represents the range commonly observed for coals. Bulk N/C_org ratios of the Palaeozoic shales extended from 0.035 to 0.108, indicating the presence of significant amounts of inorganic nitrogen. Namurian A and A-B (CnA; CnA-B) samples typically exhibited the earliest onset of N₂ generation with intense, characteristic peaks around 600°C. N₂ liberation from the buddingtonite-rich sample occurred at higher temperatures, with a broad peak around 700°C. Pyrograms of the kerogen concentrates showed no or strongly reduced N₂ generation in the 500–700°C range. On-line isotope-specific analysis of the pyrolytically liberated N₂ on one sample revealed a variability of ∼10‰ in the δ¹⁵N values and a steady increase in δ¹⁵N with temperature during the main phase of N₂ generation.  相似文献   

Effect of rock fragment embedding on the aeolian deposition of dust on stone‐covered surfaces     

Dirk Goossens 《地球表面变化过程与地形》2005,30(4):443-460

Many stone‐covered surfaces on Earth are subject to aeolian deposition of atmospheric dust. This study investigates how the deposition of dust is affected when rock fragments become gradually more embedded in the ground or, inversely, become more concentrated on the surface. Experiments were executed in an aeolian dust wind tunnel with eight different types of pebbles. The following parameters were measured: dust deposition on the pebbles, dust deposition between and underneath pebbles, total dust deposition (pebbles + inter‐pebble space), and the fraction, of total deposition, of dust caught by the pebbles alone. The absolute amount of dust deposition and the dust deposition density (dust deposition per unit surface) were studied for each parameter. The effects exerted by pebble size, pebble flattening, pebble elongation and wind speed were also investigated. Dust patterns on and around pebbles were also studied via flow visualization. The absolute amount of dust settling on pebbles decreases the more that the pebbles become embedded. Dust deposition density on pebbles, on the other hand, increases with embedding. The more pebbles become embedded in the soil, the more efficient the process of dust deposition on pebbles becomes. Dust deposition between and underneath pebbles increases with pebble embedding. Dust deposition density between and underneath pebbles is maximum at 50 per cent embedding, showing that in this area dust deposition is most efficient when pebbles are halfway embedded. Total deposition slightly decreases the more pebbles become embedded, but total dust deposition density increases with embedding. Aerodynamic flow separation and diverging and converging airflow play an important role in the process of dust deposition on stone‐covered surfaces. The more pebbles protrude above the soil, the more they act as an obstacle and the more they disturb the air and dust flow creating scouring zones, flow separation bubbles and shelter areas for the dust. All these effects diminish as pebbles become more embedded in the soil. However, perturbations in dust patterns remain visible until pebbles have disappeared entirely. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

An automated ray method for diffraction modelling in complex media     

Dirk Klaeschen  Wolfgang Rabbel  Ernst R. Flueh 《Geophysical Journal International》1994,116(1):23-38

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Microseismic activity in the Hellenic Volcanic Arc, Greece, with emphasis on the seismotectonic setting of the Santorini–Amorgos zone     

Marco Bohnhoff  Martina Rische  Thomas Meier  Dirk Becker  George Stavrakakis  Hans-Peter Harjes 《Tectonophysics》2006,423(1-4):17

The volcanic arc of the Hellenic subduction zone with its four volcanic centers is of major relevance when evaluating the seismovolcanic hazard for the Aegean region. We present results from a 22-station temporary seismic network (CYCNET) in the central Hellenic Volcanic Arc (HVA). CYCNET recordings allow to analyze the level and spatio-temporal evolution of microseismic activity in this region for the first time. A total of 2175 events recorded between September 2002 and July 2004 are analyzed using statistical methods, cluster analysis and relative relocation techniques. We identify distinct regions with significantly varying spatio-temporal behavior of microseismicity. A large portion of the seismic activity within the upper crust is associated with the presence of islands representing horst structures that were generated during the major Oligocene extensional phase. In contrast, the central part of the Cyclades metamorphic core complex remains aseismic considering our magnitude threshold of 1.8 except one spot where events occur swarm-like and with highly similar waveforms.The highest activity in the study area was identified along the SW–NE striking Santorini–Amorgos zone. Within this zone the submarine Columbo volcano exhibits strong temporal variations of seismic activity on a high background level. This activity is interpreted to be directly linked to the magma reservoir and therein the migration of magma and fluids towards the surface. NE of Columbo where no volcanic activity has yet been reported we observe a similar seismicity pattern with small-scaled activity spots that might represent local pathways of upward migrating fluids or even developing volcanic activity within this zone of crustal weakness. In contrast, the Santorini and Milos volcanic complexes do not show significant temporal variations and low to moderate background activity, respectively. Relating our results to the distribution of historical earthquakes and the GPS-derived horizontal velocity field we conclude that the Santorini–Amorgos zone is presently in the state of right-lateral transtension reflecting a major structural boundary of the volcanic arc subdividing it into a seismically and volcanically quiet western and an active eastern part.  相似文献   

Engineering-Seismological Analysis of Site Effects in the Area of Cologne     

Sergey Tyagunov  Dirk Hollnack  Friedemann Wenzel 《Natural Hazards》2006,38(1-2):199-214

The study was implemented in the framework of the DFNK (Deutsches Forschungsnetz Naturkatastrophen – German Research Network Natural Disasters) project. The area around the city of Cologne was chosen as an object for assessment of seismic hazard and risk. A comprehensive geo-database was compiled for the area, which allows using a computational approach for analysis of site amplification functions and probable surface effects. Taking into account peculiarities of the local soil conditions, the analysis of site effects included both amplification of ground motions and liquefaction potential. These phenomena, having different nature, are interrelated and can considerably contribute to seismic hazard. This paper presents some results obtained on the base of computational analysis using geo-modeling.  相似文献   

Establishing a chronology for lacustrine sediments using a multiple dating approach—A case study from the Frickenhauser See, central Germany     

Dirk Enters  Gerald Kirchner  Bernd Zolitschka 《Quaternary Geochronology》2006,1(4):249-260

In order to establish a reliable chronology for lacustrine sediments of the Frickenhauser See (central Germany) different dating methods have been applied. A total of 17 AMS ¹⁴C dates, all within the last 2000 years, were supplemented with ¹³⁷Cs/²¹⁰Pb dating and varve counting of the uppermost sediments (131 years). The age–depth model for the Frickenhauser See has to cope with highly variable sedimentation rates and overlapping probability distributions of calibrated ¹⁴C dates. The uncertainty of calibrated ¹⁴C dates could be considerably reduced by including the stratigraphic relationship of the dated samples, the age information derived from short-lived isotopes and varve counting as well as an upper and lower limit of realistic sedimentation rates as ‘a priori’ information in the calibration procedure. Sets of possible age combinations obtained by repeated sampling from the modified probability distributions were used to calculate continuous age–depth relationships based on monotonic smoothing splines. The obtained age–depth model for the sediment record of the Frickenhauser See represents the average of over 16,000 such model runs and suggests a drastic increase in sedimentation rates from around 1–2 mm a⁻¹ (200–1000 AD) to over 25 mm a⁻¹ for the period between 1100 and 1300 AD. From then on, sedimentation rates exhibit relatively stable values around 3–9 mm a⁻¹. ‘Conventional’ age–depth models such as general polynomial regression or cubic splines either do not include the obtained age-information in a satisfying manner (the model being too “stiff”) or exhibit “swings” causing age-reversals in the model. Although the age–depth relationships obtained for monotonic smoothing splines and mixed-effect regression are generally very similar, they differ in their respective sedimentation rates as well as in their uncertainties. Mixed-effect regression resulted in much higher sedimentation rates of more than 37 mm a⁻¹. These results suggest that monotonic smoothing splines give better control of the age–depth model characteristics and are well suited in situations, where the integrity of ¹⁴C dates is high, i.e. the dated material represents the age of the respective layer.  相似文献   

Non-linear primary-multiple separation with directional curvelet frames   总被引：2，自引：0，他引：2  

Felix J. Herrmann  Urs Böniger  Dirk Jacob Verschuur 《Geophysical Journal International》2007,170(2):781-799

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