An implementation ofN-body chain regularization     

Seppo Mikkola  Sverre J. Aarseth 《Celestial Mechanics and Dynamical Astronomy》1993,57(3):439-459

The chain regularization method (Mikkola and Aarseth 1990) for high accuracy computation of particle motions in smallN-body systems has been reformulated. We discuss the transformation formulae, equations of motion and selection of a chain of interparticle vectors such that the critical interactions requiring regularization are included in the chain. The Kustaanheimo-Stiefel (KS) coordinate transformation and a time transformation is used to regularize the dominant terms of the equations of motion. The method has been implemented for an arbitrary number of bodies, with the option of external perturbations. This formulation has been succesfully tested in a generalN-body program for strongly interacting subsystems. An easy to use computer program, written inFortran, is available on request.  相似文献   

Metallicity effects on open cluster dynamics     

Jarrod R. Hurley  Christopher A. Tout  Sverre J. Aarseth  Onno R. Pols 《Monthly notices of the Royal Astronomical Society》2004,355(4):1207-1216

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A Time-Transformed Leapfrog Scheme     

Seppo Mikkola  Sverre Aarseth 《Celestial Mechanics and Dynamical Astronomy》2002,84(4):343-354

We present a time-transformed leapfrog scheme combined with the extrapolation method to construct an integrator for orbits in N-body systems with large mass ratios. The basic idea can be used to transform any second-order differential equation into a form which may allow more efficient numerical integration. When applied to gravitating few-body systems this formulation permits extremely close two-body encounters to be considered without significant loss of accuracy. The new scheme has been implemented in a direct N-body code for simulations of super-massive binaries in galactic nuclei. In this context relativistic effects may also be included.  相似文献   

Deep crustal structure and rheology of the Gascoyne volcanic margin,western Australia     

Frey  Øivind  Planke  Sverre  Symonds  Phillip A.  Heeremans  Michel 《Marine Geophysical Researches》1998,20(4):293-311

Interpretation of deep seismic reflection data across the Gascoyne Margin reveals six distinct seismic facies units related to the tectono-magmatic breakup history. On the outer Exmouth Plateau four large scale units are identified: (1) an extensively block-faulted upper crust; (2) a middle-crustal unit of discontinuous, undulatory reflectors; (3) a reflection-free deep crustal unit; and (4) a lower-crustal band of low-frequency, high-amplitude reflectors. Two additional units are found near the continent-ocean boundary (COB); (5) seaward-dipping reflectors (SDR); and (6) landward-dipping reflectors in the lower crust below the SDR. The lower-crustal high-reflectivity band, located near the top of a high-velocity unit (V_p > 7 kms^–1), is interpreted as magmatic underplating. There is a spatial correlation between the underplated area and the presence of extensive upper-crustal block-faulting and intrusive rocks in the shallow crust. The undulatory middle-crustal reflector unit is also only identified in the outer plateau area, and is interpreted as a zone in which the upper-crustal faults terminate. The inner parts of the margin consist of a deep basin showing little upper-crustal faulting and no evidence of middle crustal deformation or underplating. Theoretical modeling of the effect of rifting and magmatic underplating on crustal strength profiles suggests that the brittle-ductile transition may migrate at least 5 km upwards during several million years after the underplating event. Based on the seismic interpretation and crustal strength modeling we propose that the seismic structure of the outer Exmouth Plateau is severely modified by a transient change in the crustal rheological structure associated with magmatic underplating.  相似文献   

Bioassessment Techniques for Monitoring of Eutrophication and Nutrient Limitation in Coastal Ecosystems   总被引：3，自引：0，他引：3  

Jens Erik Lyngby  Sverre Mortensen  Nick Ahrensberg 《Marine pollution bulletin》1999,39(1-12):212-223

Bioassesment by the use of the macroalga, Ulva lactuca L., was carried out in the Limfjord, Denmark, to assess the significance of nitrogen and phosphorus as limiting factors for primary production during 1985, 1993, 1994 and 1995 and for the detection of changes in eutrophication levels.

Minimum and critical tissue concentrations for nitrogen and phosphorus in macroalgae were identified. The concentrations of nitrogen were generally below the critical concentration level in June–October in 1985, 1993, and 1995 but in 1994 nitrogen was only limiting for primary production in short periods. Only in early spring in 1985 and 1993 were the tissue concentrations of phosphorus below the critical concentration level, whereas in 1994 up to 3–4 months showed phosphorus limited growth, indicating that significant changes in limitation patterns can occur between different years.

It was concluded that the use of biomonitoring techniques is well suited as a bioassessment method for direct detection and for providing a time-integrated measure of nutrient availability in coastal waters, and thus for assessing ecosystem health with regard to eutrophication. It is recommended that biomonitors and the concept of critical tissue concentrations should be used in environmental management and incorporated in future monitoring programmes.  相似文献   

Comparison of wave and current measurements to NORA10 and NoNoCur hindcast data in the northern North Sea   总被引：1，自引：1，他引：0  

Kjersti Bruserud  Sverre Haver 《Ocean Dynamics》2016,66(6-7):823-838

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Salt diapirs without well defined boundaries – a feasibility study of semi‐automatic detection     

Anglique Berthelot  Anne H. S. Solberg  Erlend Morisbak  Leiv‐J Gelius 《Geophysical Prospecting》2011,59(4):682-696

Prestack depth imaging of seismic data in complex areas such as salt structures requires extensive velocity model updating. In many cases, salt boundaries can be difficult to identify due to lack of seismic reflectivity. Traditional amplitude based segmentation methods do not properly tackle this problem, resulting in extensive manual editing. This paper presents a selection of seismic attributes that can reveal texture differences between the salt diapirs and the surrounding geology as opposed to amplitude‐sensitive attributes that are used in case of well defined boundaries. The approach consists of first extracting selected texture attributes, then using these attributes to train a classifier to estimate the probability that each pixel in the data set belongs to one of the following classes: near‐horizontal layering, highly‐dipping areas and the inside of the salt that appears more like a low amplitude area with small variations in texture. To find the border between the inside of the salt and the highly‐dipping surroundings, the posterior probability of the class salt is input to a graph‐cut algorithm that produces a smooth, continuous border. An in‐line seismic section and a timeslice from a 3D North Sea data set were employed to test the proposed approach. Comparisons between the automatically segmented salt contours and the corresponding contours as provided by an experienced interpreter showed a high degree of similarity.  相似文献   

Bayesian estimation of reservoir properties—effects of uncertainty quantification of 4D seismic data     

Kjersti Solberg Eikrem  Geir Nævdal  Morten Jakobsen  Yan Chen 《Computational Geosciences》2016,20(6):1211-1229

This paper shows a history matching workflow with both production and 4D seismic data where the uncertainty of seismic data for history matching comes from Bayesian seismic waveform inversion. We use a synthetic model and perform two seismic surveys, one before start of production and the second after 1 year of production. From the first seismic survey, we estimate the contrast in slowness squared (with uncertainty) and use this estimate to generate an initial estimate of porosity and permeability fields. This ensemble is then updated using the second seismic survey (after inversion to contrasts) and production data with an iterative ensemble smoother. The impact on history matching results from using different uncertainty estimates for the seismic data is investigated. From the Bayesian seismic inversion, we get a covariance matrix for the uncertainty and we compare using the full covariance matrix with using only the diagonal. We also compare with using a simplified uncertainty estimate that does not come from the seismic inversion. The results indicate that it is important not to underestimate the noise in seismic data and that having information about the correlation in the error in seismic data can in some cases improve the results.  相似文献   

On some uncertainties related to the short term stochastic modelling of ocean waves     

Sverre Haver  Torgeir Moan 《Applied Ocean Research》1983,5(2):93-108

A short-term model for the representation of ocean waves is considered. The model is described and the associated uncertainties are briefly reviewed. Emphasis is placed on a discussion of the validity of the basic assumptions regarding stationarity and Gaussianity, together with an elaboration on uncertainties related to the shape of the spectral density function. The basis for the present investigation is 4586 time series for the sea surface elevation, representing a rather wide range of different sea states. Conclusions presented subsequently rest to some extent on the assumption that the actual time series reproduce the true time fluctuations of the surface with a sufficient accuracy. A verification of this assumption requires a very extensive investigation, and could not be included within the scope of this work.  相似文献   

Gas hydrate and free gas indications within the Cenozoic succession of the Bjørnøya Basin, western Barents Sea     

Jan Sverre Laberg  Karin Andreassen 《Marine and Petroleum Geology》1996,13(8):921-940

Multichannel seismic data, containing high-amplitude reflections from Cenozoic sediments of the Bjørnøya Basin, southwestern Barents Sea, have been studied, inferring the existence of gas hydrate and free gas. The Cenozoic succession comprises Late Palaeocene and Early Eocene claystones and siltstones and locally also some sandstones overlain by Late Pleistocene glaciogenic sediments. The inferred gas hydrate and free gas accumulations are mainly located in the vicinity of larger faults which can be followed up to base Tertiary level, and which seem to have controlled the geographical distribution of the accumulations. Free gas accumulations are inferred to occur most frequently within the Late Palaeocene strata that occur below the gas hydrate stability zone, and indicate that relatively small gas leakages from deeper accumulations have dominated. Larger gas leakages have probably led to gas migration up into the gas hydrate stability zone and, together with the increasing thickness of the hydrate stability zone towards the north, control the distribution of the suspected gas hydrates. The inferred gas leakages are closely related to the Cenozoic evolution of the Barents Sea, and are probably caused by gas expansion due to the removal of up to 1 km of sediments from the Barents Sea shelf and/or reservoir tilting during the Late Cenozoic glaciations which affected this area.  相似文献