电磁感应研究的进展与展望

最近几年国内外电磁感应研究在以下几方面取得迅速发展:国际和区域合作大尺度电磁感应研究;海洋电磁测深;能源、资源勘探和环境监测等应用研究;理论分析和实验研究;数据采集系统.     

Tectonic interpretation of regional conductivity anomalies

This review paper selects key results from electromagnetic induction studies of a variety of distinctive tectonic phenomena in the top 200 km of the Earth. Its main theme is that electromagnetic data are essential for an understanding of tectonism involving partial melting, recycling of large volumes of fluids (CO₂ and H₂O) and underthrusting of metasedimentary rocks. The wide variety of tectonic regimes in which these processes are known to be important is reflected in the choice of case studies. A discussion of conductivity models for young oceanic lithosphere and asthenosphere is followed by results from induction studies across the S.E. Australian passive margin, the North American active margin, the Ryukyu Island-Arc and the Oregon Cascades continental arc. The importance of partial melting and free fluid movement i apparent in these regions. Terrain accretion and/or continent-continent collisions recorded at palaeosuture zones in Ireland, Germany and Scandinavia have left distinctive conductivity structures. These are often associated with grain-boundary graphite either in weakly-metamorphosed black shales in underthrust sedimentary basins or precipitated from CO₂-rich fluids. They are discussed in the context of the evolution of mature continental crust. All of the case studies are based on experiments published since 1989 in which the electromagnetic results have been central to an integrated geophysical and geological interpretation.     

An introduction to electromagnetic induction in the ocean

The governing equations for the induction of electromagnetic fields in the ocean by ionospheric and oceanic sources are presented. A uniformly conducting layered model and a nonuniformly conducting thin-sheet model are discussed with reference to the interpretation of fields observed in the ocean. A procedure for the separation of the electric field continuum into parts of ionospheric and oceanic origin is presented.     

Electrical Structure in Marine Tectonic Settings

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.     

Lead isotope study of orogenic lherzolite massifs

Orogenic lherzolites allow for almost “in-situ” observation of mantle isotopic heterogeneities on a restricted geographical scale, in contrast to basalts for which melting processes have averaged original mantle compositions over uncertain scales. Pb isotopes from whole rocks and clinopyroxenes from the massifs of Lherz (Pyrenees), Lanzo (Alps), Beni Bousera (Morocco) and Zabargad (Red Sea) show internal heterogeneities that encompass the entire range of variation observed in oceanic basalts. Some depleted lherzolites have a very unradiogenic composition similar to that of the most depleted ridge tholeiites. Pyroxenites from mafic layers generally have more radiogenic compositions, some of them comparable to the most radiogenic oceanic island results. The isotopic differences between lherzolites and pyroxenites vanish where layers are very closely spaced ( < 2 cm). In this case, the lherzolites may have equilibrated with the more Pb-rich pyroxenites through solid-state diffusion under mantle conditions. These results directly illustrate the smallest scales at which Pb isotopic heterogeneity may survive within the mantle.The genesis of these heterogeneities are discussed within the framework of the “marble cake” mantle model [1], where lherzolites are residues left over after oceanic crust extraction, whereas pyroxenites represent either basaltic or cumulate portions of the oceanic crust, reinjected by subduction and stretched by solid-state mixing during mantle convection. The Pb isotope data suggest that each massif was involved in several cycles of convective overturn, segregation and reinjection of the oceanic crust, during periods well over 1 Ga.If the upper mantle is made of interlayered radiogenic and unradiogenic layers, basalt heterogeneities may result from preferential melt-extraction from different layers depending on the degree of melting, as well as from large-scale, plume-related mantle heterogeneities. Orogenic lherzolites therefore allow direct observation of disseminated small-scale heterogeneities previously inferred from observations of oceanic basalts from seamounts and ridges.     

Temporal variations in free core nutation period

Based on the nearly diurnal resonance in the tidal gravity observations,the temporal variations in period of the Earth's free core nutation (FCN) are investigated by using the tidal gravity observations of 18-year duration recorded continu-ously with a superconducting gravimeter (SG) at Brussels. The effects of the global oceanic tide loading and local barometric pressure on the SG observations have been removed by using eleven high-precision global digital models of oceanic tides and barometric pressure me...     

全球海洋碳循环模式MOM4_L40对碳和营养物自然分布的模拟

本文介绍了国家气候中心发展的一个全球海洋碳循环环流模式,并分析评估了该模式的基本性能.该模式是在美国地球物理流体动力学实验室(GFDL,Geophysical Fluid Dynamics Laboratory)的全球海洋环流模式MOM4(Modular Ocean Model Version 4)基础上发展的一个垂直方向40层、包含生物地球化学过程的全球三维海洋碳循环环流模式,简称为MOM4_L40(Modular Ocean Model Version 4 With 40Levels).该模式在气候场强迫下长期积分1000年,结果分析表明,与观测相比,模式较好地模拟了海洋温度、盐度、总二氧化碳、总碱、总磷酸盐的表面和垂直分布特征.模拟的海洋总二氧化碳分布与观测基本相符,表层为低值区,其下为高值区,高值区域位于10°S—60°N之间,但2000m以上模拟值较观测偏小,2000m以下模拟值较观测偏大.总体来说,MOM4_L40模式是一个可信赖的海洋碳循环过程模拟研究工具.     

Seasonal effects of non-tidal oceanic mass shifts in observations with superconducting gravimeters

In order to achieve a consistent combination of terrestrial and satellite-derived (GRACE) gravity field variations reductions of systematic perturbations must be applied to both data sets. At the same time evidence needs to be provided that these reductions are both necessary and sufficient. Based on the OMCT and the ECCO model the gravity effect of non-tidal oceanic mass shifts is computed for various sites equipped with a superconducting gravimeter (SG) and esp. the long-periodic contributions are studied. With these oceanic models the dynamic ocean response to atmospheric pressure loading is automatically computed, and thus goes beyond the more simplistic concepts of an inverted barometer, or alternately a rigid ocean, which is a clear advantage.The findings so far are ambiguous: for instance the systematic seasonal change of about 10 nm/s² in gravity for mid-European stations is presently not found in the observed gravity variations. Generally, the order of magnitude of the total effect of 22–27 nm/s² is surprisingly large for inland stations. In some data sections the reduction leads to the removal of some of the larger residuals. The results obtained for the South-African station Sutherland differ. Here the modelled seasonal variation caused by the non-tidal oceanic mass redistribution and gravity residuals generally correlate, and thus by the reduction an improvement of the signal-to-noise ratio in the gravity observations is achieved.An explanation for the different results might be found in the global hydrological models. Such a model is needed in order to remove the effect of large-scale variations in continental water storage in the gravity observations. This reduction plays a greater role for European stations than for the South African site. A critical impact of the land-sea-mask used in the oceanic models and the subsequent insufficient resolution of the North and Baltic Sea on the computations at the mid-European sites could not be confirmed.From a comparison between the OMCT and the ECCO model substantial discrepancies in some regions of the earth emerge, while both predict variations at inland stations in Europe, South Africa, and Asia of similar magnitude. We currently hesitate to recommend including this reduction in the routine processing of SG data because the seasonal order of magnitude for inland stations is unexpectedly large and partly significant deviations between the modelled oceanic effects exist. If the order of magnitude proves to be correct universally, this reduction has to be applied.     

The subcontinental versus suboceanic debate,I Lead-neodymium-strontium isotopes in primary alkali basalts from a shield area the Ahaggar volcanic suite

Pb, Nd and Sr isotopic compositions have been determined in lherzolite-xenolith-bearing alkali-basalts from the center of the African shield. The present data are very similar to those reported for ocean-island basalts and do not support the hypothesis of different mantle sources for alkali-basalts from continental and oceanic areas. From these observations and on the basis of data obtained for xenolith in kimberlite and for tholeiitic continental basalts one may infer the following terrestrial mantle structure: whereas oceanic tholeiites would originate in upper oceanic mantle, oceanic and continental alkali basalts would come from the lower mantle and tholeiitic continental basalts from the continental lithosphere.     

Regional ocean tide loading modelling around the Iberian Peninsula

We developed a new 1/12° resolution oceanic tide model in the complex region that surrounds the Iberian Peninsula. The model, named IBER01, allows us to obtain more accurate tidal loading computations for precise geodetic and gravimetric observations in this area. The modelling follows the scheme of data assimilation (coastal tide gauge, bottom pressure sensors and TOPEX/Poseidon altimetry) into a hydrodynamical model, which is based on two-dimensional barotropic depth averaged shallow-water equations. Detailed bathymetry data and quadratic bottom friction with a specific drag coefficient for the region have been considered. Improved ocean load maps for the Iberian Peninsula are obtained for eight harmonic constituents (Q1, P1, O1, K1, N2, M2, S2 and K2), after computing the load effect (Newtonian attraction and elastic contribution) using IBER01 and six present-day global oceanic tide models for comparison. The results achieved verify the quality of the new model. Our ocean loading computations reduce considerably the discrepancies between the theoretical Earth tide parameters and those from observations at the level of 0.3%.     

Impact of micro-organisms activity on the natural remanent magnetization of the young oceanic crust

Microbes have been widely reported in the deep subseafloor environment. Still it is difficult to detect a global chemical signature of bacterial activity in the oceanic crust. We carried out experiments up to 355 days exposing very young oceanic basalts to anaerobe sulfate reducing organisms in an in-vitro marine environment. The Natural Remanent Magnetization of samples was monitored during the whole duration of experiments and within this time frame the most magnetized sub-samples lost up to 30% of their original signal. Scanning electron microscope observations show cycling of iron from the titanomagnetites to iron sulfide phases. Our results suggest that microbes can have a major and fast impact on the magnetization of young oceanic basalts and could contribute to a global signal as the central anomaly magnetic high seen along ridges axis.     

Using the Regional Ocean Modeling System (ROMS) to improve the ocean circulation from a GCM 20<Superscript>th</Superscript> century simulation

Global coupled climate models are generally capable of reproducing the observed trends in the globally averaged atmospheric temperature. However, the global models do not perform as well on regional scales. Here, we present results from a 20-year, high-resolution ocean model experiment for the Atlantic and Arctic Oceans. The atmospheric forcing is taken from the final 20 years of a twentieth-century control run with a coupled atmosphere–ocean general circulation model. The ocean model results from the regional ocean model are validated using observations of hydrography from repeat cruises in the Barents Sea. Validation is performed for average quantities and for probability distributions in space and time. The validation results reveal that, though the regional model is forced by a coupled global model that has a noticeable sea ice bias in the Barents Sea, the hydrography and its variability are reproduced with an encouraging quality. We attribute this improvement to the realistic transport of warm, salty waters into the Barents Sea in the regional model. These lateral fluxes in the ocean are severely underestimated by the global model. The added value with the regional model that we have documented here lends hope to advance the quality of oceanic climate change impact studies.     

海底表面磁源瞬变响应建模及海水影响分析

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.     

泉州重力潮汐观测的分析

为了研究海潮与固体潮的相互作用,对近海的泉州台用三台重力仪（二台LaGoste-G型、一台GS型）为期四个半月的观测资料进行了初步分析,探讨了仪器的性能及误差来源。由分析得出: 1.由LaCoste-G型584和589重力仪观测得到的结果是一致的,三台仪器对于主要潮波O₁和M₂波的观测结果也相符较好,其互差不超过8‰。利用Schwiderski图和我国等潮图作海潮改正后的潮汐因子接近理论模型值。但由于泉州为近海台,大的余差可能反映了局部地区海潮的影响。 2.LaCoste-G型重力仪的误差主要来自灵敏度的变化,应在基本台上作定期标定。 3.GS-207重力仪的观测结果受气压的影响较大,特别是K₁和S₂波,经初步分析得出约7微伽/毫巴。     

三维板块几何形态对大陆深俯冲动力学的制约

大陆深俯冲及超高压变质作用是大陆动力学的重要研究内容,前人进行了系统的地质、地球物理观测以及数值模拟研究.然而,自然界中大陆板块的俯冲、碰撞及造山过程大部分具有明显的沿走向的差异性,这种典型的三维特征可能很大程度上依赖于会聚大陆板块的初始几何学和运动学特征.本文采用三维高分辨率的动力学数值模拟方法,建立了方形大陆板块和楔形大陆板块两种不同的俯冲-碰撞模型,并且俯冲大陆板块侧面与大洋俯冲带相邻.数值模拟结果揭示大洋板块可以持续地俯冲到地幔之中,而大陆板块俯冲到一定深度处,其前端的俯冲板块将发生断离,并进而造成残余的大陆板块俯冲角度的减小.方形大陆俯冲板块的断离深度约为150km,而楔形大陆俯冲板块的断离深度较大,约250~300km,这很大程度上取决于俯冲带中大洋板块的牵引力和大陆板块的负浮力之间的竞争关系.同时,无论方形还是楔形大陆板块俯冲模型中,板块断离后,侧向的大洋俯冲板块仍可以拖曳约60~70km宽的大陆边缘岩石圈持续向下俯冲,揭示了新西兰东部的洋-陆空间转换俯冲带的动力学机制.并且,数值模型与喜马拉雅造山带和秦岭—大别—苏鲁造山带进行了对比,进而对其高压-超高压岩石空间展布沿走向的差异性特征和机制提供了一定的启示.     

电离层-均匀地球模型中地表水平电偶极子激发的电磁场

本文在电离层-均匀地球模型中使用Debye势方法,在电磁场一般性边界条件下导出了地表水平电偶极子源在地球介质中激发的电磁场的表达式.对球Bessel函数使用指数函数近似,并采用加速算法,计算了地表及地下的电磁场.其正确性得到理论计算和实际观测数据的多方面支持.结果表明,在收发距离大于1000 km,电磁场强度很稳定,几乎没有衰减的迹象;电磁场受电离层的影响很大,由于电离层高度升高或电导率减小引起的电磁场幅度的减少可以达到50%,但是由电场和磁场得到的视电阻率变化却很小.     

Implications of island arc rotations to the studies of marginal terranes

Paleomagnetic studies of rocks from the Bonin and Mariana Island arcs indicate that these island arcs have undergone substantial rotation and northward translation since their formation 40 to 45 Ma. These rotations are inconsistent with existing models of marginal basin and island arc formation. The data presently available suggest to us that the Mariana and Bonin island arcs rotated as one tectonic element at the margin of the Philippine Sea and Pacific plate. These observations demonstrate that large vertical axis rotations are present in the island arc environment, prior to any interaction with a continental landmass. Since many circum-Pacific marginal terranes have been assigned island arc origins, these pre-accretion rotations should be considered when interpreting paleomagnetic results for possible microplate reconstructions. Knowledge of the structural and rotational styles of oceanic pre-accretionary deformations may prove useful for separating these pre-accretion effects from those imposed by the accretionary process in future studies aimed at deciphering the geologic history of island arc marginal terranes.     

海洋潮汐对重力潮汐观测的影响

本文论述了海潮对重力固体潮观测的影响,把整个褶积计算分为近洋区及远洋区两部分,其中近洋区采用格林函数进行数值积分的方法,远洋区则利用全球海潮潮高的球函数展开式。着重讨论了远洋区的实施方案,导出了必要的工作公式,并就加速远洋区级数收敛问题作了探讨。利用导得的公式,计算了M₂海洋潮波对中国境内重力观测的影响。     

Modern trends in the solution of forward and inverse 3D electromagnetic induction problems

The main results obtained during the last 5–8 yr in the solution of forward and inverse problems of 3D induction studies are summarized. The up-to-date status of 3D modelling is presented and prospective improvements in the formulation and numerical solution of forward problems are discussed. Approximate techniques and practical aspects of 3D modelling are specially considered. The general scheme of 3D interpretation of electromagnetic geophysical observations is outlined and realistic formalized approaches to solving 3D inverse problems, namely direct inversion and formalized model fitting, are studied.     

Time domain electromagnetic‐induced polarisation: extracting more induced polarisation information from grounded source time domain electromagnetic data

Electrical induced polarisation surveys are used to detect chargeable materials in the earth. For interpretation of time domain electrical‐induced polarisation data a common procedure is to first invert the direct current data (electric current on time) to recover conductivity and then invert the induced polarisation data (current off‐time) to recover chargeability. This direct current‐induced polarisation inversion procedure assumes that the off time data are free of secondary electromagnetic induction effects. To comply with this, early time data are often discarded or not recorded. For mid‐time data, an electromagnetic decoupling technique, which removes electromagnetic induction in the observations, needs to be implemented. Usually, responses from a half‐space or a layered earth are subtracted. Recent capability in three‐dimensional time domain electromagnetic forward modelling and inversion allows to revisit these procedures. In a Time domain electromagnetic‐induced polarisation survey, a high sampling rate allows early time channels of the electromagnetic data to be recorded. The recovery of chargeability then follows a three‐step workflow: (i) invert early time channel time domain electromagnetic data to recover the three‐dimensional conductivity; (ii) use that conductivity to compute the time domain electromagnetic response at later time channels and subtract this fundamental response from the observations to extract the induced polarisation responses, and (iii) invert the induced polarisation responses to recover a three‐dimensional chargeability. This workflow effectively removes electromagnetic induction effects in the observations and produces better chargeability and conductivity models compared with conventional approaches. In a synthetic example involving a gradient array, we show that the conductivity structure obtained from the early time channel data, which are usually discarded, is superior to that obtained from the steady state direct current voltages. This adds a further reason to collect these electromagnetic data.