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1.
Oceanic electromagnetic studies: A review 总被引：3，自引：0，他引：3
Nick A. Palshin 《Surveys in Geophysics》1996,17(4):455-491
The review covers the main results of the oceanic electromagnetic studies presented during last 4–5 years. Seafloor electromagnetic observations are dedicated to studies in solid Earth geophysics and oceanography. Thus, a large range of objectives and targets are under investigation. Technological and theoretical advances provide an increase of quantity and quality of the collected seafloor data. The host conductivity models for oceanic crust and mantle are being developed; the possibility of obtaining useful structural constraints in heterogeneous environment are discussed. Significant oceanographic results were derived from electromagnetic field observations; these appeared to be a new powerful method for study of long-term large-scale ocean variability. 相似文献
2.
根据电磁场理论,推导了磁偶源和接收点均位于海水中时层状海底模型的频域电磁场响应一般表达式,并通过此式,得到了海水为均匀半空间和有限海水深度两种情况下,垂直磁偶极装置、中心回线和重叠回线分别置于均匀半空间海底表面时的瞬变电磁响应（磁场和感应电压）表达式. 这些表达式将瞬变响应和海底的电导率等参数有机联系在一起,为海底瞬变电磁法的正演计算和反演解释提供了理论基础. 仿真计算表明,海水的存在不仅使得瞬变响应曲线形态发生变化,而且影响其对海底电导率的分辨能力. 相似文献
3.
We report on strong coast effect distortions observed for broadband marine magnetotelluric (MT) data collected on the forearc offshore northeastern Japan. Eight days of horizontal electric and magnetic fields recorded at eight seafloor stations and the horizontal magnetic fields from a land remote station were processed with a robust multiple-station algorithm, yielding good MT responses and inter-station transfer functions at periods of 7–10,000 s. Transverse electric (TE) mode responses have cusps in apparent resistivity and negative phases at periods around 1000 s, while the transverse magnetic (TM) mode responses are galvanically depressed below the TE responses. An analysis of inter-station transfer functions confirms that the apparent resistivity cusps are a magnetic field, rather than electric field, phenomenon, consisting of an amplitude minimum and rapid phase change around a characteristic frequency. Poynting vectors for a TE coast effect model study illustrate that the anomalous phases are associated with energy diffusing back up to the seafloor from below, after being turned around from its usual downward propagating trajectory by inductive coupling between the conductive ocean and the resistive seafloor along the continental margin. We show that the characteristic frequency and position of the TE mode apparent resistivity cusps are determined by a relatively simple combination of the electrical resistivity of the seafloor, the depth of the ocean, and the distance from the coastline. By including coastlines and bathymetry in 2D inversion, we recover the seafloor conductivity structure along the forearc, demonstrating that broadband data can constrain the thickness of conductive forearc sediments and the underlying high resistivity associated with the mantle wedge and subducting oceanic lithosphere. 相似文献
4.
Marine electromagnetic induction studies 总被引：2，自引：0，他引：2
S. C. Constable 《Surveys in Geophysics》1990,11(2-3):303-327
In reviewing seafloor induction studies conducted over the last seven years, we observe a decline in single-station magnetotelluric (MT) experiments in favour of large, multinational, array experiments with a strong oceanographic component. However, better instrumentation, processing techniques and interpretational tools are improving the quality of MT experiments in spite of the physical limitations of the band limited seafloor environment, and oceanographic array deployments are allowing geomagnetic depth sounding studies to be conducted. Oceanographic objectives are met by the sensitivity of the horizontal electric field to vertically averaged motional currents, providing the same information, at much greater reliability and much lower cost, as an array of continuously operating current meter moorings.The seafloor controlled source method has now become, if not routine, at least viable. Prior to 1982, only one seafloor controlled source experiment has been conducted; now at least three groups are involved in the experimental aspects of this field. The horizontal dipole-dipole configuration is favoured, although a variant of the magnetometric resistivity method utilising a vertical electric transmitter has been developed and deployed. By exploiting the characteristics of the seafloor environment, source receiver spacings unimaginable on land can be achieved; on a recent deployment dipole spacings of 90 km were used with a clear 24 Hz signal transmitted through the seafloor. This, and prior experiments, show that the oceanic upper mantle is characteristically very resistive, 105 m at least. This resistive zone is becoming apparent from other experiments as well, such as studies of the MT response in coastal areas on land.Mid-ocean ridge environments are likely to be the target of many future electromagnetic studies. By taking available laboratory data on mineral, melt and water conductivity we predict to first order the kinds of structures the EM method will help us explore. 相似文献
5.
We discuss the interpretation method and results of magnetotelluric soundings in combination with other geological and geophysical data. The interpretation method was developed by studying possible distortions in MTS curves using 3D-numerical modeling of the magnetotelluric field. Deep conductivity was studied by using longitudinal MTS curves below a period of 400 s, which are nearly unaffected by the induction effect due to marine electrical currents. Transverse curves were used to obtain more detail for the geoelectric model. Inversion of average longitudinal MTS curves resulted in a geoelectric section of the lithosphere down to a depth of 60 km. Anomalies of high conductivity in the lithosphere were detected and were found to produce certain effects in gravity and seismic velocities. MTS and seismic tomography data were used to determine the possible origin of the high conductivity anomaly and to estimate rock porosity and the concentration of magma melts. 相似文献
6.
海洋地球物理以物理学的思维与方法研究占地球三分之二面积的海洋系统.20世纪地球科学迅猛发展，它的重大进展是海底扩张说与板块构造说的出现和海底大洋的发现，以及前者所引发的地球科学思想革命，从固定论向活动论的思维转变.海底研究对于20世纪地球科学发展的贡献极为巨大，而海洋地球物理是推动海底科学研究的重要原动力.海洋地球物理在20世纪地球科学的发展中有过辉煌的成就，占有十分重要的地位；在新的21世纪里，海洋地球物理研究仍然保持着前沿科学的地位，继续推动着地球科学的进展.目前的海底探测主要还是依赖于声学探测技术.水下声学定位技术是实现水下探测系统精确定位和海底高精度探测的基础.传统性的海洋地震探测技术是研究海底构造与海洋岩石圈深部结构和寻找海底矿产的主力技术，它近年来无论在海上采集技术还是数据处理技术方面都发展得很快.多波束测深、侧扫声呐测图和海底地层剖面测量等则是近数十年快速发展起来探测海底浅部结构信息的技术.这些技术已经在当代海底科学研究、海底资源勘查、海洋工程和海洋开发，以及海洋军事活动等方面发挥出极其重要的作用. 相似文献
7.
C.R. Brewitt-Taylor 《Physics of the Earth and Planetary Interiors》1975,10(2):151-158
A model for the coast-effect of geomagnetism is presented, in which the horizontal magnetic field induces currents in a circuit including a thin finite ocean. The currents flow horizontally across the ocean, vertically down into the earth, back through the deep interior of the earth, and vertically up to the ocean to complete the current loop. The upper layers of the earth are given non-zero conductivity, allowing the possibility of such current loops.A two-dimensional model involving such currents has been worked out analytically, and it is found that a significant induced magnetic field at the seafloor can be obtained with a reasonable conductivity in the earth's upper layers. A three-dimensional model has also been worked out numerically. It is found that the induced vertical component of magnetic field is of comparable magnitude to the horizontal component induced normal to the coast, whereas the horizontal component parallel to the coast is small. These relations are required to explain the observation of Parkinson arrows. 相似文献
8.
Uplift of old ocean seafloor relative to the familiar t1/2 curve is generally presumed to be a consequence of an additional heat source at the base of the oceanic lithosphere. Several geodynamical mechanisms have been invoked to explain this behavior. We have taken a petrological rather than geodynamical approach and considered whether the causes of the flattening can be found within the conductive portion of the lithosphere, where viscous creep is insignificant. Accordingly, self-consistent calculations of the phase assemblages of several candidate mantle compositions have been performed, subject to the constraint of experimentally determined calorimetric measurements. We have found that, in general, for temperatures below around 800°C and pressures greater than 10 kbar, the phase change from spinel peridotite to garnet peridotite has a significantly negative Clapeyron slope, which could cause some amount of uplift of old seafloor. The transition, for representative thermal profiles, should be depressed from about 35 km (at 40 Ma) to 45 km (at 160 Ma). Since spinel peridotite is of lower density the net effect is to raise the seafloor topography. The extent of the uplift, which depends primarily on the Al2O3 content of the peridotite and on the effective thermal diffusivity of the lithosphere, should be on the order of 140–220 m. 相似文献
9.
Kiyoshi Baba 《Surveys in Geophysics》2005,26(6):701-731
This review paper presents recent research on electrical conductivity structure in various marine tectonic settings. In at
least three areas, marine electromagnetic studies for structural exploration have increasingly progressed: (1) data accumulations,
(2) technical advances both for hardware and software, and (3) interpretations based on multidisciplinary approaches. The
mid-ocean ridge system is the best-studied tectonic setting. Recent works have revealed evidence of conductive zones of hydrothermal
circulation and axial magma chambers in the crust and partial melt zones of the mid-ocean ridge basalt source in the mantle.
The role of water or dissolved hydrogen and its redistribution at mid-ocean ridges is emphasized for the conductivity pattern
of the oceanic lithosphere and asthenosphere. Regions of mantle upwelling (hotspot or plume) and downwelling (subducting slab)
are attracting attention. Evidence of heterogeneity exists not only in the crust and the upper mantle, but also in the mantle
transition zone. Electrical conductive zones frequently overlap seismic low-velocity zones, but discrepancies are also apparent.
Some studies have compared conductivity models with the results of seismic and other studies to investigate the physical properties
or processes. A new laboratory-based conductivity model for matured oceanic lithosphere and asthenosphere is proposed. It
takes account of both the water distribution in the mantle as well as the thermal structure. It explains observed conductivity
patterns in the depth range of 60–200 km. 相似文献
10.
几年以前 ,我国尚未独立开展过海洋电磁探测 ,没有获取海洋岩石层电性资料的技术手段。然而 ,地下岩层的电性参数较之其他物性参数能更好地反映岩石的性质以及岩石所处的物理状态 ,因此迫切需要发展海底大地电磁探测技术。海底大地电磁探测是把仪器布置在海底 ,采集海底大地电磁场数据 ,从而研究海底以下不同深度上介质导电性的分布规律 ,达到了解地下不同深度地质情况的目的。经过两年的努力 ,我们已基本实现了海底大地电磁探测技术的研究目标 相似文献
11.
On the electrical conductivity of the oceanic lithosphere 总被引：1，自引：0，他引：1
Charles S. Cox 《Physics of the Earth and Planetary Interiors》1981,25(3):196-201
12.
On the Causes of Electrical Conductivity Anomalies in Tectonically Stable Lithosphere 总被引：2，自引：0，他引：2
Kate Selway 《Surveys in Geophysics》2014,35(1):219-257
Magnetotelluric (MT) data can image the electrical resistivity of the entire lithospheric column and are therefore one of the most important data sources for understanding the structure, composition and evolution of the lithosphere. However, interpretations of MT data from stable lithosphere are often ambiguous. Recent results from mineral physics studies show that, from the mid-crust to the base of the lithosphere, temperature and the hydrogen content of nominally anhydrous minerals are the two most important controls on electrical conductivity. Graphite films on mineral grain boundaries also enhance conductivity but are stable only to the uppermost mantle. The thermal profile of most stable lithosphere can be well constrained, so the two important unknowns that can affect the conductivity of a lithospheric section are hydrogen content and graphite films. The presence of both of these factors is controlled by the geological history of the lithosphere. Hydrogen in nominally anhydrous minerals behaves as an incompatible element and is preferentially removed during melting or high-temperature tectonothermal events. Grain-boundary graphite films are only stable to ~900 °C so they are also destroyed by high-temperature events. Conversely, tectonic events that enrich the lithosphere in incompatible elements, such as interaction with fluids from a subducting slab or a plume, can introduce both hydrogen and carbon into the lithosphere and therefore increase its electrical conductivity. Case studies of MT results from central Australia and the Slave Craton in Canada suggest that electrical conductivity can act as a proxy for the level of enrichment in incompatible elements of the lithosphere. 相似文献
13.
14.
A. V. Kuvshinov 《Surveys in Geophysics》2008,29(2):139-186
Electromagnetic induction in the Earth’s interior is an important contributor to the near-Earth magnetic and electric fields.
The oceans play a special role in this induction due to their relatively high conductivity which leads to large lateral variability
in surface conductance. Electric currents that generate secondary fields are induced in the oceans by two different processes:
(a) by time varying external magnetic fields, and (b) by the motion of the conducting ocean water through the Earth’s main
magnetic field. Significant progress in accurate and detailed predictions of the electric and magnetic fields induced by these
sources has been achieved during the last few years, via realistic three-dimensional (3-D) conductivity models of the oceans,
crust and mantle along with realistic source models. In this review a summary is given of the results of recent 3-D modeling
studies in which estimates are obtained for the magnetic and electric signals at both the ground and satellite altitudes induced
by a variety of natural current sources. 3-D induction effects due to magnetospheric currents (magnetic storms), ionospheric
currents (Sq, polar and equatorial electrojets), ocean tides, global ocean circulation and tsunami are considered. These modeling
studies demonstrate that the 3-D induction (ocean) effect and motionally-induced signals from the oceans contribute significantly
(in the range from a few to tens nanotesla) to the near-Earth magnetic field. A 3-D numerical solution based on an integral
equation approach is shown to predict these induction effects with the accuracy and spatial detail required to explain observations
both on the ground and at satellite altitudes.
On leave from Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation, Russian Academy of Sciences, 142190
Troitsk, Moscow region, Russia. 相似文献
15.
C.R. Brewitt-Taylor 《Physics of the Earth and Planetary Interiors》1976,13(1):P9-P14
Improved theoretical results have been obtained for an induction model involving vertical electric currents through the earth's upper layers, which was previously analysed by a method involving severe approximations. A numerical method has been used for this H-polarisation problem, which is described in detail. This type of induction is important when the half-width of the ocean is greater than the skin-depth in the sub-oceanic crust, which should be obtained from the average resistivity of the crust, rather than the average conductivity. 相似文献
16.
We consider geomagnetically induced currents (GICs) in power systems from the viewpoint of a geophysicist. Special attention is paid to the Finnish high-voltage power system, in which exact theoretical model calculations together with recordings have been performed for several years. We present several examples of theoretically computed GICs using different geophysical models for estimating the geoelectric field driving GICs. Statistical prediction of GICs is outlined referring to studies made in Finland. We show that a combination of GIC recordings at few sites with theoretical modelling of ionospheric currents and the earth's conductivity, and data of geomagnetic activity makes it possible to derive GIC statistics of the entire power system. Finally, we discuss requirements for a long-range prediction of GICs, which will obviously be a widely-studied topic in future. 相似文献
17.
本文在前人研究大陆岩石圈板块有效弹性厚度的基础上,建立研究海洋岩石圈板块有效弹性厚度的理论模型,推导出与大陆岩石圈不同的海洋岩石圈板块响应函数 Z(k,Te) 理论计算公式.并分析海洋岩石圈板块响应函数 Z(k,Te) 的特点.文中对实际的海洋测量数据的响应函数 Z(k,Te) 进行计算和分析,估算我国南海南沙海域和南海中央海盆岩石圈板块有效弹性厚度分别约为10 km和6~7 km. 相似文献
18.
There are some factors, such as the topographic relief, sedimentary thickness and thermal conductivity, magmatic activity and thermal cooling, influencing the seafloor heat flow and the evolution of lithosphere structure in southwest sub-basin (SWSB), South China Sea. On the base of the geological structure characteristic of SWSB this paper will discuss some other factors including thermal anomaly area, dike produced by magma intrusion and lithosphere relief, by modeling and calculating. Calculating results indicate partial areas where temperature is higher than vicinity in the lithosphere, which we call thermal anomaly here containing thermal anomaly area and dike in this paper, could decrease heat flow below, increase above, and gradually increase to two sides; heat flow in upwelling parts of lithosphere is usually higher than sinking parts, and in the middle is of a gradual transition. 相似文献
19.
A first application of a marine inductive source electromagnetic configuration with remote electric dipole receivers: Palinuro Seamount,Tyrrhenian Sea 
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下载免费PDF全文 Roxana Safipour Sebastian Hölz Marion Jegen Andrei Swidinsky 《Geophysical Prospecting》2018,66(7):1415-1432
We study a new marine electromagnetic configuration that consists of a ship‐towed inductive source transmitter and a series of remote electric dipole receivers placed on the seafloor. The approach was tested at the Palinuro Seamount in the southern Tyrrhenian Sea, at a site where massive sulphide mineralization has been previously identified by shallow drilling. A 3D model of the Palinuro study area was created using bathymetry data, and forward modelling of the electric field diffusion was carried out using a finite volume method. These numerical results suggest that the remote receivers can theoretically detect a block of shallowly buried conductive material at up to ～100 m away when the transmitter is located directly above the target. We also compared the sensitivity of the method using either a horizontal loop transmitter or a vertical loop transmitter and found that when either transmitter is located directly above the mineralized zone, the vertical loop transmitter has sensitivity to the target at a farther distance than the horizontal loop transmitter in the broadside direction by a few tens of metres. Furthermore, the vertical loop transmitter is more effective at distinguishing the seafloor conductivity structure when the vertical separation between transmitter and receiver is large due to the bathymetry. As a horizontal transmitter is logistically easier to deploy, we conducted a first test of the method with a horizontal transmitter. Apparent conductivities are calculated from the electric field transients recorded at the remote receivers. The analysis indicates higher apparent seafloor conductivities when the transmitter is located near the mineralized zone. Forward modelling suggests that the best match to the apparent conductivity data is obtained when the mineralized zone is extended southward by 40 m beyond the zone of previous drilling. Our results demonstrate that the method adds value to the exploration and characterization of seafloor massive sulphide deposits. 相似文献
20.
R.G. Roberts 《Physics of the Earth and Planetary Interiors》1983,33(3):198-212
The composition of the upper mantle is of great significance to our understanding of plate tectonics and global evolution. Information about the physical properties of the Earth at upper mantle depths, including lateral variations in electrical conductivity, can be deduced from measurements of the electric and magnetic fields at the Earth's surface. Electromagnetic methods appear to give poorer resolution than do some other methods, for example seismics, but as they are sensitive to quite different properties of a medium they provide a different and complementary class of information.The basic theory of electromagnetic sounding methods is briefly reviewed below, and evidence regarding lateral conductivity inhomogeneities in the Earth's upper mantle is examined. While lateral electrical conductivity inhomogeneities appear to be the rule rather than the exception, the interpretation of electromagnetic data still presents difficulties and the results from many regions are not as yet unambiguous. Where the data are of sufficient resolution, a rapid increase in electrical conductivity can usually be identified within the upper mantle. The depth to this highly conductive zone is different in different tectonic environments, but is broadly consistent between analogous but widely separated tectonic environments. A comparatively shallow conducting region is found beneath the ocean lithosphere. The depth of this region is dependent on lithospheric age. Many of the more shallow conducting regions in both continental and oceanic environments are associated with high heat flow values and seismic low velocity zones. These highly conducting regions may be zones of partial melt. 相似文献