Conductivity structure of crust in the Tangshan seismic area and the possibility of exploring potential seismic sources by magnetotelluric method   总被引：1，自引：0，他引：1  

Xinling Qin  L. B. Pedersen  Yulin Zhao  Ping Zhang  Fuye Qian  Wei Qian  Zhengnan Li  Jingxian Du 《地震学报(英文版)》1992,5(2):355-365

It is shown by the result of digital magnetotelluric soundings in the Tangshan seismic area and its surrounding regions that the crust under the surface conductive sediments is divided into two layers,i. e., the resistive upper crust and the conductive lower crust. The upper crust wherein the Tangshan main shock and most of the aftershocks occurred is a convex lens-like body which is cut by faults at the east, south and west sides. The focus of the mainshock was located at the position of maximum thickness of the resistive upper crust while the spatial variation of Curie point isothermal surface and the deepest limit of the depths of aftershocks coincide with the downward depression of the bottom of the resistive upper crust. Thus, the Tangshan main shock and most of its aftershocks were related closely to the resistive upper crust from the view points of either vertical layering or lateral variations. And there were only a very few aftershocks in the conductive lower crust. The mechanical property of the rocks transforms from being brittle in the upper crust into ductile in the lower crust mainly due to the combination of different factors, e.g., increase of confining pressure, change in minerals, rise in temperature as well as stabilization of slips by high pore—pressure. A small amount of water and a rise in temperature may lead to a decrease of the electric resistivity within the rock while a change in the static pressure and mineral content within the rock causes very little change in the electric resistivity. Thus it is deduced that a resistive upper crust and a more conductive lower crust from the view points of either vertical or lateral variations are related to the brittle and ductile properties respectively. Hence it is possible that there is a relationship between the electric structure of the crust and zones of potential seismic hazard. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 354–363, 1991. The project is supported by the Chinese Joint Seismological Science Foundation, and the Swedish International Development Authority.  相似文献   

Cenozoic uplift of Nuussuaq and Disko, West Greenland—elevated erosion surfaces as uplift markers of a passive margin   总被引：1，自引：0，他引：1  

Johan M. Bonow  Peter Japsen  Karna Lidmar-Bergstrm  James A. Chalmers  Asger Ken Pedersen 《Geomorphology》2006,80(3-4):325-337

Remnants of a high plateau have been identified on Nuussuaq and Disko, central West Greenland. We interpret the plateau as an erosion surface (the summit erosion surface) formed mainly by a fluvial system and graded close to its former base level and subsequently uplifted to its present elevation. It extends over 150 km east–west, being of low relative relief, broken along faults, tilted westwards in the west and eastwards in the east, and having a maximum elevation of ca. 2 km in central Nuussuaq and Disko. The summit erosion surface cuts across Precambrian basement rocks and Paleocene–Eocene lavas, constraining its age to being substantially younger than the last rift event in the Nuussuaq Basin, which took place during the late Maastrichtian and Danian. The geological record shows that the Nuussuaq Basin was subjected to subsidence of several kilometres during Paleocene–Eocene volcanism and was transgressed by the sea later during the Eocene. By comparing with results from apatite fission track analysis and vitrinite reflectance maturity data, it is suggested that formation of the erosion surface was probably triggered by an uplift and erosion event starting between 40 and 30 Ma. Surface formation was completed prior to an uplift event that started between 11 and 10 Ma and caused valley incision. This generation of valleys graded to the new base level and formed a lower erosion surface, at most 1 km below the summit erosion surface, thus indicating the magnitude of its uplift. Formation of this generation of valleys was interrupted by a third uplift event also with a magnitude of 1 km that lifted the landscape to near its present position. Correlation with the fission-track record suggests that this uplift event started between 7 and 2 Ma. Uplift must have been caused initially by tectonism. Isostatic compensation due to erosion and loading and unloading of ice sheets has added to the magnitude of uplift but have not significantly altered the configuration of the surface. It is concluded that the elevations of palaeosurfaces (surfaces not in accordance with present climate or tectonic conditions) on West Greenland's passive margin can be used to define the magnitude and lateral variations of Neogene uplift events. The striking similarity between the landforms in West Greenland and those on many other passive margins is also noted.  相似文献   

海南岛万泉河口沉积物有机碳、 氮同位素的特征及其环境意义^*   总被引：4，自引：0，他引：4  

葛晨东  王颖  T.F.Pedersen  O.Slaymaker 《第四纪研究》2007,27(5):845-852

对海南岛万泉河口沙美内海柱样K3和K4沉积物进行生物地球化学指标总有机碳(TOC)、总氮(TN)、有机碳同位素(δ¹³ C)、氮同位素(δ¹⁵ N)的分析,以及²¹⁰ Pb定年了解其近代变化特征,追踪沉积物有机质来源的变化,提取沉积环境演变及人类活动的信息。K3中TOC的变化介于0.51 ％ ~1.12 ％ ,K4变化介于0.43 ％ ~1.49 ％ ,而其随时间的变化反映了流域内人类活动的历史。17世纪TOC含量明显增加记录了明末清初开始流域内广泛种植农作物的活动,19世纪以来TOC含量逐渐增加,至现代达最高值,对应了工业革命以来人类活动的增强。K3和K4的δ¹³ C值变化分别在-24.38 ‰ ~-21.02 ‰ 之间和-24.74 ‰ ~-20.17 ‰ 之间,δ¹⁵ N值变化分别在2.20 ‰ ~4.62 ‰ 之间和2.39 ‰ ~5.02 ‰ 之间,映证了本区沉积物有机质是陆源物质和海洋物质混合的特点。明末清初开始流域内广泛种植玉米等农作物造成δ¹³ C值增加和δ¹⁵ N值减少; 自19世纪以来至现在,该区的经济活动增加,人口的持续增长,尤其是近50年来化肥农药的大量使用,造成陆源输入的有机碳含量大幅增加,使δ¹³ C值减少和δ¹⁵ N比值增加。 δ¹³ C值自19世纪以来就逐渐变轻,说明陆源和海洋有机碳的比例发生了变化,较轻的陆源有机碳比例相对增加,较重的海洋有机碳比例相对减少,反映了玉带滩沙坝的增长、沙美内海封闭、海水交换作用减少的状况。C/N比值分别从底部的16.19和17.36到顶部减小为11左右,自19世纪以来呈明显减小的趋势也反映了沙美内海泻湖自生的藻类有机质供应越来越多,也说明了其逐渐封闭、水动力条件越来越弱,与外海的交换作用逐渐减少的状况。  相似文献   

Transport,sloughing and settling rates of estuarine macrophytes: Mechanisms and ecological implications     

M.R. Flindt  C.B. Pedersen  C.L. Amos  A. Levy  A. Bergamasco  P.L. Friend 《Continental Shelf Research》2007

The study of plant-bound nutrient transport has been largely neglected in estuaries. Lately however, it has been shown that nutrients bound to macroalgae and seagrasses can constitute a major part of the nutrient transport in shallow tidal estuaries. Organic detritus in estuaries comes from various sources. This paper looks into the source of detritus from sloughing, and the transport behaviour of plant detritus under unidirectional flows. In order to determine the extent of the sloughing of macrophytes, the threshold current velocities for the traction and resuspension of the most common submersed macrophytes in Venice Lagoon (Ulva lactuca, Enteromorpha sp., Ceramium rubrum, Cladophora sp., and Chaetomorpha linum) were studied in a laboratory flume. It was found that all macrophytes subjected to flows of 1.5–3.0 cm s⁻¹ move initially as bed load. The threshold for suspension of the macrophyte tissue was at current speeds >3 cm s⁻¹. The exception was the filamentous macroalgae, C. linum, which moved as bed load at all current speeds. This implies that the advection of plant-bound nutrients in Venice Lagoon is widespread and takes place over virtually all stages of the tide.  相似文献   

Determination of the regularization level of truncated singular-value decomposition inversion: The case of 1D inversion of MT data     

Laust B. Pedersen 《Geophysical Prospecting》2004,52(4):261-270

The total mean‐square error (MSE) of the estimated model, defined as the sum of the standard model variance and the bias variance, is used to define the truncation level of the singular‐value decomposition to give a reasonable balance between model resolution and model variance. This balance is determined largely by the data and no further assumptions are necessary except that the bias terms are estimated sufficiently well. This principle has been tested on the 1D magnetotelluric inverse problem with special emphasis on high‐frequency radio magnetotelluric (RMT) data. Simulations clearly demonstrate that the method provides a good balance between resolution and variance. Starting from a homogeneous half‐space, the best solution is sought for a fixed set of singular values. The model variance is estimated from the sum of the inverse eigenvalues squared, up to a certain threshold, and the bias variance is estimated from the model projections on the remaining eigenvectors. By varying the threshold, the minimum of the MSE is found for an increasing number of fixed singular values until the number of active singular values becomes greater than or equal to the estimated number. As a side‐effect, the depth of penetration of a given set of measurements can be estimated very efficiently by simply noting at which depth the final model deviates little from the starting homogeneous half‐space model. A suite of synthetic data is inverted and an example of inversion of one site is shown to illustrate how the truncation is carried out as the non‐linear inversion process proceeds. A field example with a profile across a plume of contaminated groundwater in the Netherlands shows good agreement with the electrical resistivity obtained in a nearby borehole.  相似文献   

Fluid flow in sedimentary basins: model of pore water flow in a vertical fracture   总被引：2，自引：0，他引：2  

TOM Pedersen  KNUT Bjørlykke 《Basin Research》1994,6(1):1-16

Open fractures provide high-permeability pathways for fluid flow in sedimentary basins. The potential for flow along permeable or open fractures and faults depends on the continuity of flow all the way to the surface except in the case of convective flow. Upward flowing fluid cools and may cause cementation due to the prograde solubility of quartz, but in the case of carbonates such flow may cause dissolution. The rate and duration of these processes depend on the mechanisms for sustaining fluid flow into the fracture, the geometries of fracture and sedimentary beds intersected, permeability, pressure and temperature gradients. Heat loss to the adjacent sediments causes sloping isotherms which can induce non-Rayleigh convection. To analyse these problems we have used a simple model in which a single fracture acts as a pathway for vertically moving fluid and there is no fluid transport across the walls of the fracture except near its inlet and outlet. Four mechanisms for fluid flow into the lower part of the fracture are considered: decompression of pore water; compaction of intersected overpressared sediments; focusing of compaction water derived from sediments beneath the fracture; and finally focusing of pore water moving through an aquifer. Water derived from the basement is not considered here. We find that sustained flow is unlikely to have velocities much higher than 1–100 m/yr, and the flow is laminar. The temperature of the fluid expelled at the top of the fracture increases by less than 1% and the vertical temperature gradient in the fracture remains close to the geothermal gradient. Where hot water is introduced from basement fractures (hydrothermal water) during tectonic deformation, much higher velocities may be sustained in the overlying sediments, but here also this depends on the permeability near the surface. Most of the cooling of water with (ore) mineral precipitation will then occur near the surface. In most cases, pore water decompression and sediment compaction will yield only very limited pore water flux with no significant potential for cementation or heating of the sediments adjacent to the fracture. Focusing of compaction water from sediments beneath the fracture or from an intersected aquifer can yield fluxes high enough to cement an open fracture significantly but the flow must be sustained for a very long time. For velocities of 1–100 m/yr, it takes typically 0.3–30 Myr to cement a fracture by 50%. The highest velocities may be obtained when a fracture extends all the way to the surface or sea floor. When a fracture does not reach the sediment surface, the flow velocity is reduced by the displacement of water in the sediments near the top of the fracture. The flow into the fracture from the sediments may often be rate limiting rather than the flow on the fracture. Sedimentary rocks only a few metres from the fracture will receive a much lower flux than the fracture. The fracture will therefore close due to cementation before significant amounts of silica can be introduced into adjacent sandstones. The isotherm slope in the adjacent sediments will in most cases be less than 10–20°. Non-Rayleigh convection velocities in the sediments adjacent to the fracture are too small to cause any significant diagenetic reactions such as quartz cementation. These quantifications of fluid flow in fractures in sedimentary basins are important in terms of constraining models for diagenesis, heat transport and formation of ore minerals in a compaction-driven system.  相似文献   

Collisional tectonics of the Baltic Shield in the northern Gulf of Bothnia from seismic data of the BABEL project     

Karsten Gohl  Laust B. Pedersen 《Geophysical Journal International》1995,120(1):209-226

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An airborne tensor VLF system. From concept to realization1     

Laust B. Pedersen  Wei Qian  Lars Dynesius  Ping Zhang 《Geophysical Prospecting》1994,42(8):863-883

Radio signals from very low frequency (VLF) transmitters distributed world-wide have been used for several decades to study the lateral variations of the electrical conductivity in the upper few hundred metres of the earth's crust. Traditionally, in airborne applications, the total magnetic fields from one or two transmitters are measured to form the basis for construction of maps that primarily show those conductive structures that are parallel or subparallel to the direction to the transmitters. The tensor VLF technique described in this paper makes use of all signals available in a predefined frequency band to construct transfer functions relating the vertical magnetic field and the two horizontal magnetic field components. These transfer functions are uniquely determined for a particular measuring site and contain information about the lateral conductivity variations in all directions. First experiences with real field data, acquired during a test survey in Sweden, show that maps of the so-called peaker, the spatial divergence of the transfer functions, give an image of the conducting structures. Most of the structures can be correlated to small valleys filled with conducting sediments or valleys underlain by conductive fracture zones in the crystalline rocks.  相似文献   

A palynological and sedimentological study of Cretaceous floodplain deposits of the Atane Formation at Skansen and Igdlunguaq, Disko, West Greenland     

Eva Bundgaard Koppelhus  Gunver Krarup Pedersen 《Cretaceous Research》1993,14(6)

A palynological and sedimentological study has been carried out on the Cretaceous fluvial and deltaic Atane Formation of West Greenland. Two localities, Skansen and Igdlunguaq on the southern coast of Disko island, have been studied. The sediments are divided into two genetic facies associations interpreted as representing deposition in fluvial channels and on a floodplain. The facies indicate that most of the sediments on the floodplain accumulated in swamps or shallow lakes, whereas abundant spores and pollen indicate the presence of vegetated land nearby. The palynomorph assemblages recovered consist of 72 species of spores and pollen grains of bryophytes, pteridophytes, gymnosperms and angiosperms. The palynomorphs from Skansen and most of those from Igdlunguaq indicate a maximum age-range from late Albian to Cenomanian for the successions sampled, although a mid Cenomanian age seems most likely. The highest horizon examined at Igdlunguaq may, however, be late Cenomanian or Turonian in age. The assemblages compare well with palynofloras from North America and Northwest Europe.  相似文献   

Channelling contribution to tipper vectors: a magnetic equivalent to electrical distortion   总被引：2，自引：0，他引：2  

Ping Zhang  Laust B. Pedersen  Marianne Mareschal  Michel Chouteau 《Geophysical Journal International》1993,113(3):693-700

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