BSR热流的三维地貌校正和流体汇聚探测(英文)

从天然气水合物稳定区底界的地震似海底反射BSR(Bottom Simulating Reflector)深度计算得到的BSR热流包含了海底地貌(热流在凹地型会聚,在凸地形发散)和增生楔内部流体活动的影响。从BSR热流中移除地貌效应的贡献就能揭示出流体是否发生了汇聚。在难以使用解析方法计算地貌效应的复杂海底区域,三维有限元方法可以高精度的模拟地貌对背景热流的影响,从而可以对BSR热流进行地貌效应校正,得到平坦地形条件下的BSR热流,并进一步通过与背景热流值的对比,识别目前仪器所不能探测的流体汇聚区。在北卡斯卡底(Cascadia)俯冲边缘陆坡中部的研究区应用该方法,显示黄瓜岭(Cucumber Ridge)高地及其周围的海底热流正异常显著(高出背景热流值10-20%),同时这些区域在地震成像上与海底的裂隙系统相对应,指示了流体沿着这些高渗透率通道进行汇聚,并且很可能导致较高的水合物富集度。     

Heat flow anomalies and their interpretation

More than 10,000 heat flow determinations exist for the earth and the data set is growing steadily at about 450 observations per year. If heat flow is considered as a surface expression of geothermal processes at depth, the analysis of the data set should reveal properties of those thermal processes. They do, but on a variety of scales. For this review heat flow maps are classified by 4 different horizontal scales of 10ⁿ km (n = 1, 2, 3 and 4) and attention is focussed on the interpretation of anomalies which appear with characteristic dimensions of 10^{(n − 1)} km in the respective representations.The largest scale of 10⁴ km encompasses heat flow on a global scale. Global heat loss is 4 × 10¹³ W and the process of sea floor spreading is the principal agent in delivering much of this heat to the surface. Correspondingly, active ocean ridge systems produce the most prominent heat flow anomalies at this scale with characteristic widths of 10³ km. Shields, with similar dimensions, exhibit negative anomalies.The scale of 10³ km includes continent wide displays. Heat flow patterns at this scale mimic tectonic units which have dimensions of a few times 10² km, although the thermal boundaries between these units are sometimes sharp. Heat flow anomalies at this scale also result from plate tectonic processes, and are associated with arc volcanism, back arc basins, hot spot traces, and continental rifting. There are major controversies about the extent to which these surface thermal provinces reflect upper mantle thermal conditions, and also about the origin and evolution of the thermal state of continental lithosphere.Beginning with map dimensions of 10² km thermal anomalies of scale 10¹ km, which have a definite crustal origin, become apparent. The origin may be tectonic, geologic, or hydrologic. Ten kilometers is a common wavelength of topographic relief which drives many groundwater flow systems producing thermal anomalies. The largest recognized continental geothermal systems have thermal anomalies 10¹ km wide and are capable of producing hundreds of megawatts of thermal energy.The smallest scale addressed in this paper is 10¹ km. Worldwide interest in exploiting geothermal systems has been responsible for a recent accumulation of heat flow data on the smallest of scales considered here. The exploration nature of the surveys involve 10's of drillholes and reveal thermal anomalies having widths of 10⁰ km. These are almost certainly connected to surface and subsurface fluid discharge systems which, in spite of their restricted size, are typically delivering 10 MW of heat to the near surface environment.     

地幔对流的数值模拟及其与表面观测的关系

本文从基本的热对流方程出发,并结合地幔对流特点,特别考虑到自重及非线性影响,探讨地幔对流及其与表面观测的关系,发展了相应的数值方法.结果表明,计算得到的长波大地水准面、地表地形、板块速度场水平散度与观测值符合程度较好.上、下地幔的非绝热温度异常与由地震层析得到的地震波速异常显示一定的相关性.地幔内部的流动呈现复杂形态,反映了高瑞利数对流的特征.     

张裂型盆地地热参数的垂向变化与琼东南盆地热流分布特征

海洋热流数据是开展海洋地球动力学研究和油气资源评价的基础数据.为深入认识琼东南盆地的地热特征,本文首先利用耦合沉积作用与岩石圈张裂过程的数值模型分析了张裂型盆地主要地热参数的垂向变化特征;并通过钻孔资料的详细分析,获得了琼东南盆地44口钻孔的热流数据;结合海底地热探针获取的热流数据,对琼东南盆地地热特征及其主要影响因素进行了简要分析.结果表明:沉积作用的热披覆效应对表层热流有较明显的抑制作用,由于沉积物生热效应与披覆效应的共同作用,同一钻孔处海底表层热流与钻孔深度3000~4000m处热流或与海底间的平均热流差异很小,可以一起用于分析琼东南盆地的热流分布特征;莺歌海组、乐东组热导率随深度变化小于黄流组及其下地层热导率的变化,钻孔沉积层平均热导率约为1.7 W·(m·K)-1,钻孔地层生热率一般低于2.5μW·m-3,平均生热率为1.34μW·m-3,平均地温梯度主要介于30~45℃/km,热流介于50~99mW·m-2,陆架区热流主要集中于60~70mW·m-2,深水区钻孔具有较高的地温梯度和热流值;从北部陆架与上陆坡区往中央坳陷带,热流值从50~70mW·m-2,增高为65~85mW·m-2,并且往东有升高趋势,在盆地东部宝岛凹陷、长昌凹陷与西沙海槽北部斜坡带构成一条热流值高于85mW·m-2的高热流带.进一步分析认为,琼东南盆地现今热流分布特征是深部热异常、强烈减薄岩石圈的裂后冷却作用、晚期岩浆热事件、地壳与沉积层的生热贡献以及沉积作用的热披覆效应等多种主要因素综合作用的结果.     

Electrical conductivity anomalies in the earth

Anomalies of short-period geomagnetic variations have been found in various regions over the world. It is known that such anomalies arise from electromagnetic induction within an electrical conductivity anomaly or from local perturbation of induced electric currents by a conductivity anomaly. In order to investigate a regional electric state in the Earth, conductivity anomaly (CA) studies based on anomalous behaviors of geomagnetic variations have been extensively undertaken, as well as studies based on magnetotelluries in which induced currents are directly used.Some of the geomagnetic variation anomalies, however, turned out to be caused by surface conductors, such as sea water and sediments. Anomalies of this sort have been intensively studied and classified into coast, island, peninsula, and strait effects in the case of sea effects. Three-dimensional conduction or channelling of induced electric currents is sometimes observed in the cases of sediments and some crustal conductivity anomalies. However, anomalies of such surface origins often provide some information of the underground conductivity structure.Electrical conductivity anomalies can be classified into two types: anomalies originating in the crust and in the upper mantle. Many of crustal anomalies are well correlated with metamorphic belts, fracture zones, and hydrated layers, and magnetic and gravity anomalies are also often found over the conductivity anomalies. Most of mantle anomalies have been interpreted mainly in terms of high temperature and partial melting, since conductivity anomalies coincide well with anomalies in heat flow and seismic wave velocities.     

从卫星红外亮温与大地热流的关系看地震前的热红外异常

一些强地震前地表可产生热异常是人们比较一致的认识,但能否用卫星遥感技术观测到地下热异常则存在很大的分歧.本文试图通过分析热红外亮温与大地热流值的关系,探讨利用卫星遥感技术观测地震热异常的可能性及其存在的一些问题.研究结果表明:（1）在绝大多数情况下,亮温随大地热流值的升高而升高,升高的速率平均为0.057℃/mW·m^-2.如果地震引起的大地热流异常为100 mW·m^-2,则有可能产生卫星红外平均亮温约5.7℃的异常.（2）不同地区亮温随大地热流值变化速率不同,即使地震前出现了卫星热红外亮温异常,但不同地区表现也是不同的.（3）在一些地区一些季节,亮温与大地热流值关系不明显,可能是亮温数据受气象因素干扰所致.这说明地震前的热红外亮温异常是复杂的,甚至有时是难于观测到的.     

Magnetovariational measurements in the Cook Strait region of New Zealand

Magnetovariational measurements have been made at 10 sites on the northern side of the Cook Strait, New Zealand. Single-station transfer functions have been calculated for the sites and indicate that the effect of induction in the shallow water of the Cook Strait is most important at around 1000 s period. At longer periods the effect of induced currents in the Pacific Ocean predominates.A two-dimensional electrical conductivity model including local conductivity structure has been shown to satisfy the measured responses at sites about 60–80 km distance from the strait. Closer to the strait the inductive process is strongly three-dimensional. A simple d.c. line current model of current flow has been shown to reproduce some of the features of the observed responses.Induction arrows indicate the existence of conductivity anomalies associated with a known lateral seismic boundary and with one of the two principal faults in the region.     

Thermal structure about southwest sub-basin of South China Sea

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.     

Geophysical observations pertaining to solid-state convection in the terrestrial planets

We present a broad-based review of the observational evidence that pertains to or otherwise implies solid-state convection to be occurring (or have occurred) in the interiors of the terrestrial planets.For the Earth, the motion of the plates is prima facie evidence of large-scale mantle convection. Provided we understand upper-mantle thermal conductivity correctly, heat flow beneath the old ocean basins may be too high to be transported conductively from the upper mantle through the base of the lithosphere and therefore convection on a second smaller scale might be operative. The horizontal scale of plate dimensions implies, due to typical cell aspect ratios observed in convection, that the motion extends to the core-mantle boundary. Improved global data coverage and viscoelastic modeling of isostatic rebound due to Pleistocene deglaciation imply a uniform mantle viscosity, and thus indicate that whole-mantle convection could exist. Additionally, there is some seismic evidence of lithospheric penetration to depths deeper than 700 km. We discuss some salient features and assumption boundedness of arguments for convection confined to the upper mantle and for convection which acts throughout the mantle since the vertical length scale has a profound effect upon the relevance of geophysical observations. The horizontal form of mantle convection may be fully three-dimensional with complex planform and, therefore, searching for correlative gravity patterns in the ocean basins may not be useful without additional geophysical constraints. Many long-wavelength gravity anomalies may arise from beneath the lithosphere and must be supported dynamically, although thermal convection is not a unique explanation. Topography is an additional geophysical constraint, but for wavelengths greater than a few hundred kilometers, a general lack of correlation exists between oceanic residual gravity and topography, except at specific locations such as Hawaii. Theoretical calculations predict a complex relationship between these two observational types. Oceanic gravity data alone shows no regular planform and there is no correlation with any small-scale convective pattern predicted by laboratory experiments.All of the observational evidence argues against Martian plate tectonics occurring now or over much of the history of this planet, but lack of plate tectonics is not an argument against interior convection. The Tharsis uplift on Mars may have resulted from convective processes in the mantle, and the present-day gravity anomaly associated with Tharsis must be supported by the finite strength of the lithosphere or by mantle convection. Stresses imparted by the present topographic load would be greater than a kilobar, in excess of long-term finite strength. Observed fracture patterns are probably a direct result of this load, and the key question concerns the level of resultant strain relief. The global topographic and geomorphic dichotomy between the northern and southern hemisphere required a solid-state flow process to create the accompanying center-of-figure to center-of-mass offset.Lunar heat flow values, in analogy with oceanic heat flow on the Earth, strongly imply a convective mechanism of heat transport in the interior which, based on seismic Q values, is limited to the lower mantle. The presence of moonquakes in this region does not preclude solid-state convective processes. Lunar conductivity profiles provide no information on convection because of the difficulty in conductivity modeling, uniqueness of models, and the uncertainty in the conductivity-temperature relationship. The excess oblateness of the lunar figure over the hydrostatic value does not require convective support; in fact, such a mechanism is unlikely.The presence of a dipole magnetic field on Mercury does not provide a constraint on mantle convection unless its existence can be inextricably linked to a molten core. The non-hydrostatic shape of the equatorial figure, required for the observed $\text{3}\text{2}$ resonance between Mercury's rotational and orbital periods, is most likely related to surface processes, as opposed to convection. The $\text{3n}\text{2}$ resonance implies escape from a 2n resonance and, therefore, is related to the question of a molten core. Further dynamical data is needed to constrain interior models.Interpretation of limited radar imagery for the surface of Venus is enigmatic in terms of plate tectonics and therefore interior convection. Linear tensional and possibly compressional features are observed, but there are also crustal regions which appear to show large impact structures and are thus geologically old and may not have been recycled.     

Comments on thermal isostasy

The deformation of the lithosphere due to temperature anomalies caused by a heat source located below or within the layer is usually modelled as one of Pratt local isostasy. A more appropriate model is one of rheologically layered lithosphere comprising of a stiff viscoelastic or elastic layer overlying a weaker viscoelastic layer. The surface deformations are a result of not only the perturbations in body forces due to density changes, but thermal bending moments. In geophysically realistic situations the former contribution dominates. Pratt isostasy is attained if the stresses in the entire lithosphere are allowed to relax and this end state is not contingent upon the lithosphere being confined against horizontal deformation. In a rheologically layered lithosphere, even though the non-isostatic thermal stresses persist in the upper layer, the surface deformations are indistinguishable from that of local isostasy if the horizontal dimension of the heat source exceeds about three times the effective elastic thickness of the lithosphere.     

What compensates the topography of southern Norway? Insights from thermo-isostatic modeling

The origin of the high topography of the Norwegian Mountains is currently much debated. Several geophysical studies show that the uppermost mantle below southern Norway has anomalously low velocities as compared to other parts of the Baltic Shield. This study aims to shed lights on the structure of the lithospheric mantle below southern Norway by adapting and further refining a method based on isostatic and thermal equilibrium to compute temperature, temperature-related density and synthetic S-wave velocity in stable continental domains. The one-dimensional steady-state heat equation is used with topographic, Moho depth, crustal density and surface heat flow data. A condition of local isostasy is assumed and geoid undulations are used to constrain the range of possible lithosphere models.Results derived from this method suggest a thickening of the thermal lithosphere below southern Norway from west to east. The western part is found to have higher temperatures, lower densities and lower synthetic S-wave velocities than the eastern part, compatible with results from a recent P-wave travel time residual study. Comparison of the synthetic shear-velocity profiles beneath southwestern Norway with velocity profiles inverted from Rayleigh wave dispersion data suggests that the higher temperatures associated with a thinner lithosphere can explain parts of the seismic low-velocity anomaly.The inferred lithospheric structure is sensitive to uncertainties in the crustal input model, but the main features remain undisturbed by changes in the input data. The results show that the lithosphere of southwestern Norway can be in local isostatic equilibrium, if it is thinner and warmer than the lithosphere of eastern Norway. The present-day high topography may therefore be partially sustained by lower densities in the mantle lithosphere.     

Groundwater temperature evolution in the subsurface urban heat island of Cologne,Germany

Long‐term heating of shallow urban aquifers is observed worldwide. Our measurements in the city of Cologne, Germany revealed that the groundwater temperatures found in the city centre are more than 5 K higher than the undisturbed background. To explore the role of groundwater flow for the development of subsurface urban heat islands, a numerical flow and heat transport model is set up, which describes the hydraulic conditions of Cologne and simulates the transient evolution of thermal anomalies in the urban ground. A main focus is on the influence of horizontal groundwater flow, groundwater recharge and trends in local ground warming. To examine heat transport in groundwater, a scenario consisting of a local hot spot with a length of 1 km of long‐term ground heating was set up in the centre of the city. Groundwater temperature‐depth profiles at upstream, central and downstream locations of this hot spot are inspected. The simulation results indicate that the main thermal transport mechanisms are long‐term vertical conductive heat input, horizontal advection and transverse dispersion. Groundwater recharge rates in the city are low (<100 mm a^?1) and thus do not significantly contribute to heat transport into the urban aquifer. With groundwater flow, local vertical temperature profiles become very complex and are hard to interpret, if local flow conditions and heat sources are not thoroughly known. Copyright © 2014 John Wiley & Sons, Ltd.     

The preliminary heat flow map of Europe and some of its tectonic and geophysical implications

The heat flow map of Europe was derived from 2605 existing observations, which for this purpose were supplemented by numerous results of deep borehole temperatures, gradients and local heat flow patterns. In areas without data the heat flow field was extrapolated on the basis of the regional tectonic structure and the observed correlation of heat flow and the age of the last tectono-thermal event. The heat flow pattern as obtained in the map may be described by two components: (i) regional part and (ii) local part of the measured surface geothermal activity. The regional part of the heat flow field in Europe is dominated on the whole by a general north-east to south-west increase of the geothermal activity, which is an obvious consequence of the tectonic evolution, the major heat flow provinces corresponding thus to the principal tectonic units. The geothermal fine structure (local part) superimposing the former is mainly controlled by local tectonics, especially by the distribution of the deep reaching fracture zones and by the hydrogeological parameters. The correlation between the heat flow pattern and the crustal structure allows some preliminary geophysical implications: (a) areas of the increased seismicity may be connected with the zones of high horizontal temperature gradient, (b) increased surface heat flow may be generally observed in the zones of weakened crustal thickness, (c) there are considerable regional variations in the calculated temperature on the Moho-discontinuity, as well as in the upper mantle heat flow contribution.     

FINITE-ELEMENT ANALYSIS OF SHALLOW TEMPERATURE ANOMALIES*

Finite-element analysis has been used in calculating shallow temperature anomalies due to both the presence of salt domes and near-surface faults. The results of the numerical analysis are in good agreement with field observations reported earlier by Poley and van Steveninck. The calculations show how surface emissivity must be responsible for the pronounced temperature anomalies above salt domes. Faulting is a mechanism that disorders the layering sequence of sedimentary deposits. If the disordered sedimentary layers show notable differences in heat conductivity, the layers are thick enough and the displacements along the fault plane large enough, temperature anomalies close to the free surface may be apparent because of the surface-emissivity mechanism. In order to explain certain characteristic temperature anomalies where a fault reaches or approaches the free surface, an additional mechanism has to be assumed; namely a distortion of texture, and consequently a change in the heat conductivity of the rock material surrounding the fault plane. The consequences of both a reduced and an increased heat conductivity of the surroundings of a near-surface fault have been calculated, and they show remarkable agreement with temperature anomalies observed in various parts of the world.     

基于约束三维重力反演的南极大陆地壳密度结构研究

地壳密度是表征其物质组成以及结构特征的一个基本参数,是深入理解构造演化、地球动力学等问题的重要基础.本文利用最新发布的布格重力异常数据,以现有的三维S波速度模型、地表热流以及岩石学数据作为约束条件,采用引入深度加权函数的三维重力反演算法,获取了南极大陆三维精细密度结构模型.研究结果显示,南极大陆地壳尺度范围内密度异常在-0.25～0.20g·cm^-3之间.大致以南极横断山脉为界,受中生代与新生代多期拉张活动的影响,西南极上地壳密度异常以低值为主,与东南极克拉通的高密度异常呈现明显对比关系.通过对比发现,反演的密度结构与岩石圈强度信息存在较好的相关性.进一步分析西南极裂谷系统的密度结构与岩石圈强度,认为受新生代拉张活动和岩浆活动影响的局部地区除外,热沉降作用很可能使得地壳趋于冷却.此外,西南极的低密度特征延伸到南极横断山脉,结合现有的研究结果认为可能需要多种机制来解释南极横断山脉的隆升过程.     

Long-term temperature monitoring in a borehole drilled into the Nojima Fault, southwest Japan

Abstract Long-term monitoring of temperature distribution in an active fault zone was carried out using the optical fiber temperature-sensing technique. An optical fiber cable was installed in a borehole drilled into the Nojima Fault in Awaji Island, south-west Japan, and the temperature profile to a depth of 1460 m had been measured for 2.5 years (July 1997–January 2000). Although the obtained temperature records showed small temporal variations due to drifts of the measurement system all along the cable, local temperature anomalies were detected at two depths. One at around 80 m seems to correspond to a fracture zone and may be attributed to groundwater flow in the fracture zone. This anomaly had been stable throughout the monitoring period, whereas the other anomaly at around 500 m was a transient one. The water level in the borehole could be estimated from the diurnal temperature variations in the uppermost part of the borehole and may provide information on the hydrological characteristics of the fault zone, which is connected to the borehole through perforations on the casing pipe. Except for these minor variations, the temperature profile had been very stable for 2.5 years. The conductive heat flow calculated from this profile and the thermal conductivity measured on core samples increases with depth, probably resulting from errors in thermal conductivity due to sampling problems and/or from advective heat transfer by regional groundwater flow. Assuming that the middle part of the borehole (less fractured granite layer) is least affected by these factors, heat flow at this site is estimated to be approximately 70 mW/m².     

Correlation analysis of geothermal data for the sedimentary basins of India

Correlation coefficients are calculated from the available geothermic data for the sedimentary basins of India to reveal some significant association between: (1) heat flow and thermal conductivity, (2) heat flow and geothermal gradient, (3) geothermal gradient and thermal conductivity, and (4) heat flow and related crustal thickness. The observed negative correlationship between heat flow and thermal conductivity for all Indian sedimentary basins suggests corresponding changes in crustal thickness.     

南海北部大陆边缘盆地地热特征与油气富集

本文报道了莺歌海盆地、北部湾盆地共148个新测热导率数据,根据收集的钻井温度数据新增计算65个大地热流数据;结合前人研究成果绘制了南海北部大陆边缘沉积盆地的地温梯度图、大地热流分布图;系统归纳了南海北部大陆边缘油气勘探成果.结果表明,南海北部大陆边缘珠江口盆地、琼东南盆地、北部湾盆地、莺歌海盆地的平均热流值分别为68.7±11 mW/m²、71.1±13 mW/m²、65.7±8.9 mW/m²、74.7±10 mW/m²,属于典型的“热盆”.热流区域分布特征总体上受大地构造背景控制,随地壳厚度从北向南,由陆架到陆坡区逐渐减薄而增高,水热活动与岩浆活动等是引起局部高热流异常的原因.盆地地温场的差异控制和约束了油气分布富集规律,从研究区油气勘探成果中可以发现,该区域的气田多发育于高热流盆地（凹陷）,而中-低热流盆地（凹陷）则多孕育油田,油气田具有“北油南气”的分布特征.     

Depth anomalies in the Pacific: Active,fossil and precursor

Previously unreported depth anomalies in the central and eastern Pacific are described. Some of these depth anomalies exist over hot spots and propagating spreading ridges; they are not limited to the area of active volcanism but extend beyond it, into the areas toward which the volcanism is propagating. These areas may be “precursor” topographic features, showing up areas of impending or potential mid-plate volcanism or spreading. A distinction can be made between active depth anomalies and fossil ones. Gravity anomalies and high heat flow values can be correlated with active depth anomalies in one area, supporting the arguments favoring a thinning of the lithosphere as their underlying cause.     

Effects of finite rifting times on the development of sedimentary basins

Most thermo-mechanical models for the development of sedimentary basins have assumed that the rifting responsible for the formation of the basin occurred instantaneously and have examined the post-rift development of the basin. This assumption greatly simplifies the mathematical treatment, but is not in accord with what is found in nature, where 10-to 50-m.y. rifting events commonly accompany the formation of sedimentary basins and continental margins. The effects of a finite rifting time on the development of sedimentary basins are examined using an analytic technique which allows an arbitrary rifting history in both time and space and which considers the effects of both vertical and horizontal heat transfer. This technique allows the thermal structure of the lithosphere to be calculated throughout the rifting event and thus permits the subsidence history and surface heat flow of the developing basin to be traced.The effect of a finite-duration extension event is that heat is lost during rifting increasing the syn-rift subsidence at the expense of the post-rift. Lateral heat flow, which was not included in previous studies of the effect of finite rifting times, has a significant effect on the subsidence history, distribution of sediments and thermal history. In particular, the post-rift subsidence is decreased by more than 25% for a 20-m.y. rifting event and by more than 10–15% for a rifting event as short as 10 m.y. This will significantly decrease the subsidence rates in the post-rift stage and implies that inferences concerning the structure, development and thermal history of the basin derived from using “β-curves” to interpret backstripped subsidence can be greatly in error.Variations in syn-rift sediment accumulation and lithospheric thermal structure at the end of rifting resulting from different rifting histories can interact with other factors, such as the flexural response of the lithosphere to sediment loading, to affect the final width of the basin, the total amount of sediments that accumulate and the basin stratigraphy.