Electromagnetic fields of a vertical magnetic dipole above a laterally inhomogeneous thin layer of conductive overburden     

Richard G. Geyer  James R. Wait 《Pure and Applied Geophysics》1978,116(1):181-197

An analytical formulation is developed for the resultant electromagnetic field of an oscillating vertical magnetic dipole located over a thin conductive sheet of infinite extent. The sheet is characterized by a conductivity-thickness product or conductance d that may be a function of the horizontal coordinates. The system of integral equations arising in the general formulation is simplified greatly when azimuthal symmetry prevails. Numerical results for a Gaussian variation of d in the radial direction are presented for the case of a symmetrically located source. These results are for the fields at the level of the source dipole over the conductive sheet. It is shown that the quadrature response of the sheet is enhanced when there is rapid variation of the conductance. The null in the resultant wave tilt is also found to be shifted toward the direction of increasing conductance.  相似文献   

Organic biomarkers for tracing carbon cycling in the Gulf of Papua (Papua New Guinea)   总被引：1，自引：0，他引：1  

K.A. Burns  P. Greenwood  R. Benner  D. Brinkman  G. Brunskill  S. Codi  I. Zagorskis 《Continental Shelf Research》2004,24(19):2373-2394

Sediment traps were deployed in the Gulf of Papua in June–July 1997, to determine fluxes of organic matter and inorganic elements from the photic zone to deeper waters at the base of the continental slope and in the northern Coral Sea. Three stations, ranging from 900 to 1500 m depth, had “shallow” traps at 300 m below the water surface and “deep” traps set 100 m above the bottom. Infiltrex II water samplers collected particulate and dissolved organic matter from the Fly, Purari and Kikori rivers, and near-surface water from the shelf of the Gulf of Papua. Samples were analysed for molecular organic biomarkers to estimate the sources of organic carbon and its cycling processes.Dry weight fluxes from the shallow traps ranged from 115 to 181 mg m⁻² day⁻¹ and particulate organic carbon (POC) fluxes ranged from 1.2 to 1.9 mM OC m⁻² d⁻¹ with molar organic carbon to particulate nitrogen ratios (C/N) ranging from 6.0 to 6.5. Fluxes in deep traps were likely influenced by both early diagenesis and entrapment of resuspended shelf sediments. Dry weight fluxes in deep traps ranged from 106 to 574 mg m⁻² day⁻¹ and POC fluxes ranged from 0.6 to 1.5 mM OC m⁻² d⁻¹, with C/N ratios ranging from 8.5 to 10.8. ¹³C/¹²C ratios were −20.2‰ to −21.7‰ in all trap samples, indicating that most of the settling POC was “marine-derived”. Shallow traps had δ¹⁵N values of 6.3‰ to 7.2‰ while the values in deep traps were 4.9–5.0‰, indicating the N-rich near-surface OC was less degraded than that in the deep traps. The biogenic lipids consisted of hydrocarbon, sterol and fatty acid biomarkers indicative of marine zooplankton, phytoplankton and bacteria. Sterol markers for diatoms and dinoflagellates were abundant in the water samples. Highly branched isoprenoid alkenes, usually attributable to diatoms, were also detected in both water and shallow traps. Traces of C₂₆–C₃₄ n-alcohols indicative of land–plant biomarkers, were found in river water samples and in the shallow sediment traps. A large unresolved complex mixture (UCM) of hydrocarbons, and a uniform distribution of n-alkanes, indicative of petroleum hydrocarbons, were also detected in the traps. Hopane and sterane biomarkers detected in the trap oil were characteristic of a marine carbonate source, and the aromatic hydrocarbon composition distinguished at least two different oil signatures.We concluded that mass and POC fluxes were similar to those reported for other continental shelves and marginal oceans in tropical and subtropical regions. There was a dramatic decrease in POC as particles sank, due to zooplankton repackaging and photochemical and bacterial decomposition. Carbon isotopic and biomarker patterns showed most of the POC in the sediment traps was marine-sourced with only traces of terrestrial input. There was a significant flux of petroleum, which may signal the existence of natural petroleum seeps in this region.  相似文献   

Torn between extremes: the ups and downs of phytoplankton     

A.?BeckmannEmail author  I.?Hense 《Ocean Dynamics》2004,54(6):581-592

We present a model to investigate the success and resulting patterns of phytoplankton migration, based on motility which depends exclusively on the internal energy and nutrient state of the cells. The model consists of migrating and non-migrating sub compartments for phytoplankton, and migration is a function of the prescribed threshold values for internal quotas. The different modes of phytoplankton behavior are evaluated in the framework of a coupled physical–biological model that includes wind-driven up- and downwelling. The results show that (1) migration is almost always advantageous with respect to biomass, (2) a wide variety of migration patterns (e.g., subsurface maxima, surface-avoidance behavior) can be reproduced by a relatively simple treatment of motility, (3) multiple deep chlorophyll maxima can be explained as the result of certain threshold values in combination with negligible vertical movement of the water, and (4) descending tongues of high phytoplankton concentration may be caused by migratory behavior and not necessarily by subduction due to frontal convergence. Thus, our model offers explanations for a large variety of observed phytoplankton distributions and migration patterns.The comments of two anonymous reviewers are gratefully acknowledged.  相似文献   

Complex Site Effects in Thessaloniki (Greece): II. 2D SH Modelling and Engineering Insights     

Dimitrios?RaptakisEmail author  Konstantia?Makra  Anastasios?Anastasiadis  Kyriazis?Pitilakis 《Bulletin of Earthquake Engineering》2004,2(3):301-327

This paper presents results of numerical modelling of site response for Thessaloniki, obtained with two different 2D methods; a finite difference and a finite element method. Ground motion across a 2D model of the subsoil of the city has been simulated for vertically incident SH waves. The predominance of locally generated surface waves is very clear in the synthetic seismograms of a weak event and of stronger ones. These results are then compared with the observations in time domain and frequency domain. The role of the soil formations with high attenuation in the lateral propagation and the effect of the differential motion close to the lateral variations are also pinpointed. The stronger events were finally used to compute strong ground motion in order to reveal and to discuss practical engineering aspects such as peak ground acceleration value, the most familiar indicator in seismic norms, the soil to rock spectral coefficients for the period bandwidth of interest, and the aggravation factor in terms of 2D to 1D response spectra as a useful ruler to account for complex site effects.  相似文献   

One to One Resonance at High Inclination     

R. Brasser  D. C. Heggie  S. Mikkola 《Celestial Mechanics and Dynamical Astronomy》2004,88(2):123-152

We report results from long term numerical integrations and analytical studies of particular orbits in the circular restricted three-body problem. These are mostly high-inclination trajectories in 1 : 1 resonance starting at or near the triangular Lagrangian L₅ point. In some intervals of inclination these orbits have short Lyapunov times, from 100 to a few hundred periods, yet the osculating semi-major axis shows only relatively small fluctuations and there are no escapes from the 1 : 1 resonance. The eccentricity of these chaotic orbits varies in an erratic manner, some of the orbits becoming temporarily almost rectilinear. Similarly the inclination experiences large variations due to the conservation of the Jacobi constant. We studied such orbits for up to 10⁹ periods in two cases and for 10⁶ periods in all others, for inclinations varying from 0° to 180°. Thus our integrations extend from thousands to 10 million Lyapunov times without escapes of the massless particle. Since there are no zero-velocity curves restricting the motion this opens questions concerning the reason for the persistence of the 1 : 1 resonant motion. In the theory sections we consider the mechanism responsible for the observed behavior. We derive the averaged 1 : 1 resonance disturbing function, to second order in eccentricity, to calculate a critical inclination found in the numerical experiment, and examine motion close to this inclination. The cause of the chaos observed in the numerical experiments appears to be the emergence of saddle points in the averaged disturbing potential. We determine the location of several such saddle points in the (, ) plane, with being the mean longitude difference and the argument of pericentre. Some of the saddle points are illustrated with the aid of contour plots of the disturbing function. Motion close to these saddles is sensitive to initial conditions, thus causing chaos.This revised version was published online in October 2005 with corrections to the Cover Date.  相似文献   

不同抗震标准下的海洋石油平台设计地震动参数研究   总被引：3，自引：0，他引：3  

彭艳菊  唐荣余  吕悦军  沙海军 《世界地震工程》2004,20(4):119-126

在海洋石油平台抗震标准方面,中国与美国规范之间存在一定的差异。本文首先介绍了两国抗震规范中的相关内容。以渤海某油田为例,根据渤海及邻区的地震构造、地震活动环境及地震动衰减关系,利用概率方法进行地震危险性分析;采用一维等效线性化方法,根据场地土动力性能试验资料,确定不同抗震标准下的设计地震动参数。最后分析不同抗震标准下的地震动参数关系,以及对工程设计的影响。结合我国渤海与美国南加利福尼亚地区的地震活动性对比分析,对我国海洋石油平台的设防地震水准提出建议。  相似文献   

S-wave velocity structures of sediments estimated from array microtremor records and site responses in the near-fault region of the 1999 Chi-Chi,Taiwan earthquake     

Satoh?ToshimiEmail author  Hiroshi?Kawase  Tomotaka?Iwata  Sadanori?Higashi  Sato?Toshiaki  Huey-Chu?Huang 《Journal of Seismology》2004,8(4):545-558

The 1999 Chi-Chi, Taiwan earthquake, M_W = 7.6, caused severe damage in the near-fault region of the earthquake. In order to evaluate site effects in the near-field strong motions we estimate S-wave velocity structures of sediments at four sites using array records of microtremors. We also recalculated S-wave velocity structures at other four sites previously reported. To show the validity of the estimated S-wave velocity structures we separate empirical site responses from aftershock records using the generalized inversion method and show the agreement between empirical and theoretical site responses. We also show an observed fact that suggests soil nonlinearity during the Chi-Chi earthquake by comparing horizontal-to-vertical spectral ratios (HVRs) for main shock records with HVRs for aftershock records. Then we calculate one-dimensional equivalent-linear site responses using the estimated S-wave velocity structures and the main shock records observed on the surface. It is found that site amplification due to thick (about 6 km) sediments is one of the important factors for explaining the long-period velocity pulses of about 5 to 10 sec observed at sites in the footwall during the Chi-Chi earthquake. It is also found that the theoretical site responses of shallow soft sediments at sites that sustained severe damage in the hanging wall shows significant amplification around 1 sec. As the amplitude of velocity pulses with period around 1 sec is most critical in causing damage to ordinary buildings of moderate heights, our results suggest that the 1-sec period velocity pulses, amplified by the site response of shallow sediments should contribute to the severe damage during the Chi-Chi earthquake.  相似文献   

The Apsidal Antialignment of the HD 82943 System     

Jianghui Ji  Hiroshi Kinoshita  Lin Liu  Guangyu Li  Hiroshi Nakai 《Celestial Mechanics and Dynamical Astronomy》2003,87(1-2):113-120

We perform numerical simulations to explore the dynamical evolution of the HD 82943 planetary system. By simulating diverse planetary configurations, we find two mechanisms of stabilizing the system: the 2:1 mean motion resonance (MMR) between the two planets can act as the first mechanism for all stable orbits. The second mechanism is a dynamical antialignment of the apsidal lines of the orbiting planets, which implies that the difference of the periastron longitudes ₃ librates about 180° in the simulations. We also use a semi-analytical model to explain the numerical results for the system under study.  相似文献   

Period variations of the Chandler wobble   总被引：1，自引：0，他引：1  

H. Jochmann 《Journal of Geodesy》2003,77(7-8):454-458

Variations in the period of the Chandler wobble have been discussed since its discovery by Chandler in 1892. Various authors engaged in the investigation of polar motion time series suggest both a variable and an invariable period. It cannot be resolved by the analysis of time series whether the Chandler period is variable. By studying the influence of mass redistributions on the Chandler period it has been found that it is in fact variable, but the magnitude of such variation is much smaller than that found by polar motion time series analysis. For the currently available time series of polar motion, it is sufficient to assume an invariable Chandler period. AcknowledgmentsUseful discussions with Dr. F. Barthelmes and Dr. K. Fleming are gratefully appreciated.  相似文献   

工程场地地震动相干函数的数值模拟   总被引：5，自引：1，他引：4  

李杰  廖松涛 《地震工程与工程振动》2003,23(2):12-17

在确定性波动有限元分析基础上,结合随机振动分析的虚拟激励原理,可以形成工程场地随机波动分析方法。该方法将随机输入下的波动分析问题转换为多个虚拟激励下的确定性波动分析组合问题,从而可以方便地获得场地波动观测量之间的谱密度矩阵,进而计算给出工程场地的地震动相干函数。本文阐述了随机波动分析的基本原理,提出了该方法的正确性验证标准。将建议方法分别应用于具有一致随机激励与非一致随机激励的复杂工程场地的地震动相干函数分析之中,讨论了受局部场地条件影响的地震动相干函数的若干特征。  相似文献