Mid-mantle deformation inferred from seismic anisotropy

With time, convective processes in the Earth's mantle will tend to align crystals, grains and inclusions. This mantle fabric is detectable seismologically, as it produces an anisotropy in material properties--in particular, a directional dependence in seismic-wave velocity. This alignment is enhanced at the boundaries of the mantle where there are rapid changes in the direction and magnitude of mantle flow, and therefore most observations of anisotropy are confined to the uppermost mantle or lithosphere and the lowermost-mantle analogue of the lithosphere, the D" region. Here we present evidence from shear-wave splitting measurements for mid-mantle anisotropy in the vicinity of the 660-km discontinuity, the boundary between the upper and lower mantle. Deep-focus earthquakes in the Tonga-Kermadec and New Hebrides subduction zones recorded at Australian seismograph stations record some of the largest values of shear-wave splitting hitherto reported. The results suggest that, at least locally, there may exist a mid-mantle boundary layer, which could indicate the impediment of flow between the upper and lower mantle in this region.     

Boundary-layer mantle flow under the Dead Sea transform fault inferred from seismic anisotropy

Lithospheric-scale transform faults play an important role in the dynamics of global plate motion. Near-surface deformation fields for such faults are relatively well documented by satellite geodesy, strain measurements and earthquake source studies, and deeper crustal structure has been imaged by seismic profiling. Relatively little is known, however, about deformation taking place in the subcrustal lithosphere--that is, the width and depth of the region associated with the deformation, the transition between deformed and undeformed lithosphere and the interaction between lithospheric and asthenospheric mantle flow at the plate boundary. Here we present evidence for a narrow, approximately 20-km-wide, subcrustal anisotropic zone of fault-parallel mineral alignment beneath the Dead Sea transform, obtained from an inversion of shear-wave splitting observations along a dense receiver profile. The geometry of this zone and the contrast between distinct anisotropic domains suggest subhorizontal mantle flow within a vertical boundary layer that extends through the entire lithosphere and accommodates the transform motion between the African and Arabian plates within this relatively narrow zone.     

Small-scale structure of the geodynamo inferred from Oersted and Magsat satellite data

The 'geodynamo' in the Earth's liquid outer core produces a magnetic field that dominates the large and medium length scales of the magnetic field observed at the Earth's surface. Here we use data from the currently operating Danish Oersted satellite, and from the US Magsat satellite that operated in 1979/80, to identify and interpret variations in the magnetic field over the past 20 years, down to length scales previously inaccessible. Projected down to the surface of the Earth's core, we found these variations to be small below the Pacific Ocean, and large at polar latitudes and in a region centred below southern Africa. The flow pattern at the surface of the core that we calculate to account for these changes is characterized by a westward flow concentrated in retrograde polar vortices and an asymmetric ring where prograde vortices are correlated with highs (and retrograde vortices with lows) in the historical (400-year average) magnetic field. This pattern is analogous to those seen in a large class of numerical dynamo simulations, except for its longitudinal asymmetry. If this asymmetric state was reached often in the past, it might account for several persistent patterns observed in the palaeomagnetic field. We postulate that it might also be a state in which the geodynamo operates before reversing.     

结构增强扩散在地震资料构造解释中的应用

为了提高地震资料构造解释的效率和准确性,提出结构增强扩散方法提高地震资料质量.将地震资料转换为地震图像,通过提取地震图像结构张量,使用双边滤波器求取梯度值,结构增强扩散方法根据地震图像纹理不连续的方向和强度信息自动判断阈值,实现在断点处保持不连续特征的同时,增强地震同相轴的连续性,从而提高地震图像横向分辨率和突出地震资...     

Crustal rheology of the Himalaya and Southern Tibet inferred from magnetotelluric data

The Cenozoic collision between the Indian and Asian continents formed the Tibetan plateau, beginning about 70 million years ago. Since this time, at least 1,400 km of convergence has been accommodated by a combination of underthrusting of Indian and Asian lithosphere, crustal shortening, horizontal extrusion and lithospheric delamination. Rocks exposed in the Himalaya show evidence of crustal melting and are thought to have been exhumed by rapid erosion and climatically forced crustal flow. Magnetotelluric data can be used to image subsurface electrical resistivity, a parameter sensitive to the presence of interconnected fluids in the host rock matrix, even at low volume fractions. Here we present magnetotelluric data from the Tibetan-Himalayan orogen from 77 degrees E to 92 degrees E, which show that low resistivity, interpreted as a partially molten layer, is present along at least 1,000 km of the southern margin of the Tibetan plateau. The inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in Southern Tibet.     

应用地震数据体结构特征法预测油层分布规律

塔河油田八区的主产层为碳酸盐岩裂缝-缝洞型储层,具有很强的非均质性,给储层的油气预测带来了一定的困难.应用传统的储层预测方法很难准确预测出油区内油藏的油水界面,从而很难弄清储层的分布、发育规律.针对塔河油田八区特殊的地质条件,采用了地震数据体结构特征法对该区奥陶系储层进行了油气预测,并对油区内、外的区域进行了有利区块的划分.在有利区块中设计的18口钻探井位,经钻井证实均获得了较高的油气产量,取得了很好的经济效益.     

应用地震数据体结构特征法预测油层分布规律

塔河油田八区的主产层为碳酸盐岩裂缝-缝洞型储层,具有很强的非均质性,给储层的油气预测带来了一定的困难.应用传统的储层预测方法很难准确预测出油区内油藏的油水界面,从而很难弄清储层的分布、发育规律.针对塔河油田八区特殊的地质条件,采用了地震数据体结构特征法对该区奥陶系储层进行了油气预测,并对油区内、外的区域进行了有利区块的划分.在有利区块中设计的18口钻探井位,经钻井证实均获得了较高的油气产量,取得了很好的经济效益.     

Melting of the Earth's lithospheric mantle inferred from protactinium-thorium-uranium isotopic data

The processes responsible for the generation of partial melt in the Earth's lithospheric mantle and the movement of this melt to the Earth's surface remain enigmatic, owing to the perceived difficulties in generating large-degree partial melts at depth and in transporting small-degree melts through a static lithosphere. Here we present a method of placing constraints on melting in the lithospheric mantle using 231Pa-235U data obtained from continental basalts in the southwestern United States and Mexico. Combined with 230Th-238U data, the 231Pa-235U data allow us to constrain the source mineralogy and thus the depth of melting of these basalts. Our analysis indicates that it is possible to transport small melt fractions--of the order of 0.1%--through the lithosphere, as might result from the coalescence of melt by compaction owing to melting-induced deformation. The large observed 231Pa excesses require that the timescale of melt generation and transport within the lithosphere is small compared to the half-life of 231Pa (approximately 32.7 kyr). The 231Pa-230Th data also constrain the thorium and uranium distribution coefficients for clinopyroxene in the source regions of these basalts to be within 2% of one another, indicating that in this setting 230Th excesses are not expected during melting at depths shallower than 85 km.     

A perovskitic lower mantle inferred from high-pressure, high-temperature sound velocity data

The determination of the chemical composition of Earth's lower mantle is a long-standing challenge in earth science. Accurate knowledge of sound velocities in the lower-mantle minerals under relevant high-pressure, high-temperature conditions is essential in constraining the mineralogy and chemical composition using seismological observations, but previous acoustic measurements were limited to a range of low pressures and temperatures. Here we determine the shear-wave velocities for silicate perovskite and ferropericlase under the pressure and temperature conditions of the deep lower mantle using Brillouin scattering spectroscopy. The mineralogical model that provides the best fit to a global seismic velocity profile indicates that perovskite constitutes more than 93 per cent by volume of the lower mantle, which is a much higher proportion than that predicted by the conventional peridotitic mantle model. It suggests that the lower mantle is enriched in silicon relative to the upper mantle, which is consistent with the chondritic Earth model. Such chemical stratification implies layered-mantle convection with limited mass transport between the upper and the lower mantle.     

Magma flow inferred from magnetic fabrics in Wanning gabbro pluton and diabase dykes, Hainan

Measurements of anisotropy of magnetic susceptibility (AMS) were performed on a gabbro pluton and 7 diabase dykes in the Wanning area, southeast Hainan Island. Rock magnetism showed that pseudo-single domain (PSD) to multidomain (MD) Ti-poor magnetite carries the magnetic fabric in the gabbro pluton whereas MD Ti-poor magnetite carries the magnetic fabric in the diabase dykes. The corrected anisotropy degree (Pj ) in most specimens was found to be less than 1.2 which is indicative of a possible flow-related magnetic fabric. The AMS eigenvectors within each site are generally well grouped. The maximum susceptibility axes (K1 ) of the gabbro pluton are inclined towards the north at low angles (< 30°). K1 axes of the diabase dykes are inclined towards the NNW and SSE with dip angles of ≤ 30°. From this study, it appears that the emplacement mode of the gabbro pluton was characterized by intrusion from the north to the south at a low angle whereas the diabase dykes were emplaced from the NNW to the SSE at low to moderate angles. This was verified by comparison of the rock fabric to the magnetic fabric. All of this evidence leads to the conclusion that the Wanning gabbro pluton and diabase dykes were the products of pulsative intrusion from the same magma chamber in the area far to the north of Wanning, which suggests that basic rocks may exist beneath the Indosin- ian granite in the area to the north of Wanning.     

Significant dissipation of tidal energy in the deep ocean inferred from satellite altimeter data

How and where the ocean tides dissipate their energy are long-standing questions that have consequences ranging from the history of the Moon to the mixing of the oceans. Historically, the principal sink of tidal energy has been thought to be bottom friction in shallow seas. There has long been suggestive evidence, however, that tidal dissipation also occurs in the open ocean through the scattering by ocean-bottom topography of surface tides into internal waves, but estimates of the magnitude of this possible sink have varied widely. Here we use satellite altimeter data from Topex/Poseidon to map empirically the tidal energy dissipation. We show that approximately 10(12) watts--that is, 1 TW, representing 25-30% of the total dissipation--occurs in the deep ocean, generally near areas of rough topography. Of the estimated 2 TW of mixing energy required to maintain the large-scale thermohaline circulation of the ocean, one-half could therefore be provided by the tides, with the other half coming from action on the surface of the ocean.     

Resistance to mantle flow inferred from the electromagnetic strike of the Australian upper mantle

Seismic anisotropy is thought to result from the strain-induced lattice-preferred orientation of mantle minerals, especially olivine, owing to shear waves propagating faster along the a-axis of olivine crystals than along the other axes. This anisotropy results in birefringence, or 'shear-wave splitting', which has been investigated in numerous studies. Although olivine is also anisotropic with respect to electrical conductivity (with the a-axis being most conductive), few studies of the electrical anisotropy of the upper mantle have been undertaken, and these have been limited to relatively shallow depths in the lithospheric upper mantle. Theoretical models of mantle flow have been used to infer that, for progressive simple shear imparted by the motion of an overriding tectonic plate, the a-axes of olivine crystals should align themselves parallel to the direction of plate motion. Here, however, we show that a significant discrepancy exists between the electromagnetic strike of the mantle below Australia and the direction of present-day absolute plate motion. We infer from this discrepancy that the a-axes of olivine crystals are not aligned with the direction of the present-day plate motion of Australia, indicating resistance to deformation of the mantle by plate motion.     

炸药震源和可控震源资料相位差异校正方法

在山前带或城区等复杂地表的地震勘探中,为提高地震资料采集的质量,经常选用炸药震源和可控震源联合激发的工作方式.但两种震源采集的地震资料在相位方面存在明显差异.常规的地表一致性反褶积,基本上没有考虑子波的相位特性,其主要目标是校正子波的振幅谱,由此降低了地震资料处理的质量.提出了基于多道自相关函数统计平均的两步法统计子波反褶积方法,并用估算的最小相位子波得出的反褶积算子对地震数据进行了反褶积处理.研究表明,该方法估算出来的最小相位子波受反射系数影响小,在复杂地表地震资料处理中,可达到压缩地震子波、消除子波相位差异的目的,使资料真正实现同相叠加,有效提高了地震资料的信噪比和分辨率,并在一定程度上实现了相对振幅保持.     

炸药震源和可控震源资料相位差异校正方法

在山前带或城区等复杂地表的地震勘探中,为提高地震资料采集的质量,经常选用炸药震源和可控震源联合激发的工作方式。但两种震源采集的地震资料在相位方面存在明显差异。常规的地表一致性反褶积,基本上没有考虑子波的相位特性,其主要目标是校正子波的振幅谱,由此降低了地震资料处理的质量。提出了基于多道自相关函数统计平均的两步法统计子波反褶积方法,并用估算的最小相位子波得出的反褶积算子对地震数据进行了反褶积处理。研究表明,该方法估算出来的最小相位子波受反射系数影响小,在复杂地表地震资料处理中,可达到压缩地震子波、消除子波相位差异的目的,使资料真正实现同相叠加,有效提高了地震资料的信噪比和分辨率,并在一定程度上实现了相对振幅保持。     

应用地震数据体结构特征法预测普光气田储层含气性

针对普光气田碳酸盐岩储层埋藏深,储集空间复杂多样,含气类型变化大,传统的一些油气预测方法已经不能取得较好应用效果的特点,首先引入了地震数据体结构特征油气预测方法,以每个地震道的地震数据元素之间的相互关系（即数据体结构特征）为研究对象,通过提取每一地震道的振幅数值等参数,研究其数据组合、排列特征与含油气性的关系,达到预测油气层的目的。最后,将地震数据体结构特征法用于普光气田储层含气性预测,结果表明,该方法受储层性质影响较小,适用于非均质性很强的碳酸盐岩储层的含气性预测。     

抽象论域到数据结构的转换

给出了函数式语言到过程式语言转换的关键技术,即抽象论域到具体数据结构的转换技术.抽象论域的转换是抽象表达式到具体表达式转换、模式匹配处理的前提.函数式语言到过程式语言的转换,解决了函数式语言在具体实现和时间方面存在的大量动态函数复制的问题.这种转换技术可用于开发指称语义到解释器、指称语义到编译器、属性文法到过程式语言、属性文法到YACC(Yet Another Computer Compilier)输入文件的各种自动生成器.     

Internal structure of a cold dark molecular cloud inferred from the extinction of background starlight

Stars and planets form within dark molecular clouds, but little is understood about the internal structure of these clouds, and consequently about the initial conditions that give rise to star and planet formation. The clouds are primarily composed of molecular hydrogen, which is virtually inaccessible to direct observation. But the clouds also contain dust, which is well mixed with the gas and which has well understood effects on the transmission of light. Here we use sensitive near-infrared measurements of the light from background stars as it is absorbed and scattered by trace amounts of dust to probe the internal structure of the dark cloud Barnard 68 with unprecedented detail. We find the cloud's density structure to be very well described by the equations for a pressure-confined, self-gravitating isothermal sphere that is critically stable according to the Bonnor-Ebert criteria. As a result we can precisely specify the physical conditions inside a dark cloud on the verge of collapse to form a star.     

Spreading-rate dependence of melt extraction at mid-ocean ridges from mantle seismic refraction data

A variety of observations indicate that mid-ocean ridges produce less crust at spreading rates below 20 mm yr(-1) (refs 1-3), reflecting changes in fundamental ridge processes with decreasing spreading rate. The nature of these changes, however, remains uncertain, with end-member explanations being decreasing shallow melting or incomplete melt extraction, each due to the influence of a thicker thermal lid. Here we present results of a seismic refraction experiment designed to study mid-ocean ridge processes by imaging residual mantle structure. Our results reveal an abrupt lateral change in bulk mantle seismic properties associated with a change from slow to ultraslow palaeo-spreading rate. Changes in mantle velocity gradient, basement topography and crustal thickness all correlate with this spreading-rate change. These observations can be explained by variations in melt extraction at the ridge, with a gabbroic phase preferentially retained in the mantle at slower spreading rates. The estimated volume of retained melt balances the approximately 1.5-km difference in crustal thickness, suggesting that changes in spreading rate affect melt-extraction processes rather than total melting.     

下扬子地区盆地的四层楼结构及其动力学机制

下扬子地区震旦纪以来经历了不同的大地构造与地球动力学背景 ,盆地演化相应地经历了震旦纪—中三叠世海盆→晚三叠世—中侏罗世沿江前陆盆地→晚侏罗世—早白垩世火山岩盆地→晚白垩世—早第三纪陆相伸展盆地。下扬子地区震旦纪—中三叠世海盆发育于伸展性被动大陆边缘。沿江前陆盆地形成于扬子与华北板块碰撞活动中的前陆变形带上。火山岩盆地出现于区域性走滑剪切和环太平洋岩浆弧背景下。库拉—太平洋板块高角度、高速正面向东亚大陆下的俯冲 ,造成了岩石圈上拱拉张 ,从而产生了晚白垩世—早第三纪的陆相伸展盆地。随着西太平洋边缘弧后盆地的出现及印度与欧亚板块的碰撞 ,中国东部遭受近东西向挤压 ,从而结束了下扬子地区的盆地演化历史。     

由振动台试验结果反推原型结构地震反应注记

由结构模型的振动台试验结果来考察真实结构的动力特性,是研究工程结构地震反应的试验研究方法之一,如何正确处理试验模型与原型的相似关系以及如何由模型反应正确推导原型反应是很重要的问题。通过对15层框一桁架结构的振动台试验模型的地震反应,采用2种方法反推原型结构地震反应并加以对比,从而考察和探讨模型与原型相似关系的准确性,指出了由模型反推原型结构地震反应的注意事项和建议。