Uniqueness of mainly dipolar magnetic fields recovered from directional data

We show that a knowledge of either the signed or the unsigned direction of a potential field on a given smooth surface S , which separates the space into a volume containing the sources and a volume free of sources, sometimes gives enough information for the whole field to be recovered within the free volume, except for a constant multiplier (positive, for the signed case). We show that the best parameter to be considered on the surface S is the number n of loci where the field is known to be either zero (no direction) or normal to the surface. In the case of sources lying outside S ('external-sources' directional problem) we prove that the dimension of the space of solutions is no larger than n –1. This implies uniqueness for the external-sources directional problem when n = 2. In the case of sources lying inside S ('internal-sources' directional problem), we distinguish fields with monopole sources (such as the gravitational field) from those without monopole sources (such as the magnetic field). For gravitational fields, we show that the dimension of the space of solutions cannot exceed n . We note that the only situation of interest is the one for which n = 1, which implies in practice that the surface S is an isopotential and that the problem has a unique solution. For magnetic fields, we show that the dimension of the space of solutions cannot exceed n –1. It follows that the problem has a unique solution when n = 2. This shows in particular that a geomagnetic field with only two poles (south and north magnetic poles) can be recovered, except for a constant multiplier (positive, for the signed case) from directional data gathered at the Earth's surface. Finally, we note that our results are not restricted to the 3-D space and can readily be extended to two dimensions and higher dimensions.     

环南极区域古水深演化初探

重构南极区域的古水深和历史地质格局,首先需要对研究区域进行板块重构,即在特定的时间点将洋壳和陆壳恢复到他们原来的地理位置上。本文采用Greiner阐述的欧拉旋转法进行板块重构。欧拉极参数主要参考Muller等基于大西洋-印度洋热点参考系的有限欧拉极,并将部分参数进行了必要的转换和统一。对于指定的重构年代,地壳年龄比重建年代年轻的区域需被移除。本文借助于Matlab等编程工具,恢复了南极及周边区域自130 Ma以来15个特定时间段的位置。综合多种数据,将现代水深根据热驱动地壳沉降、沉积作用和均衡作用的影响进行调整,对30°S以南区域的古水深进行重构,绘制了研究区域不同年代的古水深图,分析了南大洋自130 Ma以来的形成演化过程。     

A simple method of representing azimuthal anisotropy on a sphere

We describe a method of expressing azimuthally anisotropic surface wave velocities on the Earth using a local and smooth spherical-spline parametrization. Anisotropy in the Earth leads to azimuthally varying Love and Rayleigh wave velocities that can be expressed as (cos 2ζ, sin 2ζ) and (cos 4ζ, sin 4ζ) perturbations to the isotropic velocities, where ζ is the direction of surface-wave propagation. The strength of the perturbations varies laterally, and a current goal of seismic tomography is the detailed global mapping of these variations. Several parametrizations have previously been used to describe azimuthally varying velocities. The representation proposed here uses spherical splines and is designed to describe smooth variations in both the strength and geometry of azimuthal anisotropy. The method builds on a simple geometrical approximation for the local azimuth of propagation expressed at the defining spline knot points. It avoids the singularities at the poles that result when azimuthal variations are parametrized using traditional scalar spherical harmonics. Compared with a generalized spherical-harmonic expansion of the tensor fields that represent 2ζ and 4ζ azimuthal variations smoothly on a sphere, the new method offers the advantages of local geographical support and simplicity of implementation.     

Plate kinematics of the Western Mediterranean region during the Oligocene and Early Miocene

The tectonic history of the Western Mediterranean region during the Oligocene and Early Miocene is illustrated through a series of plate reconstructions, from chron C13n to chron C6n. The reconstructions are based on a new interpretation of published magnetic anomaly data and two-ships seismic data, and their integration with known geological constraints, in order to determine style and timing of the backarc extension processes in the Liguro-Provençal, Valencia and Algerian basins. In particular, a reinterpretation of the regional magnetic anomaly field allowed the calculation of the instantaneous Euler poles associated with the motion of 11 microplates relative to Eurasia and Iberia. Furthermore, a quantitative analysis of the acoustic basement morphology and the balancing of deep crustal profiles were used to estimate the closure and pre-rift rotation angles associated with the Euler poles of opening of the Ligurian, Provençal, Valencia and Algerian basins. This rigorous reconstruction of the geometry of the pre-rift continental margins of Iberia and Eurasia could furnish important insights into the study of Mediterranean tectonics for older times.     

Palaeomagnetic results from two basic volcanic formations in the western Himalayas and a Phanerozoic polar wandering curve for India

Summary. We present palaeomagnetic results from the Durgapipal and Rudraprayag formations, which are basic volcanic formations in the Lesser Himalayas of Uttar Pradesh State. NRM measurements and AF demagnetization stability tests were made on specimens cored from oriented block samples collected at representative sites. Mean stable remanent magnetic directions were used for calculating the Virtual Geomagnetic Pole (VGP) positions; where necessary tectonic corrections were applied.
The virtual geomagnetic north poles were found to be located at:

• (a). 

  Durgapipal (Permian): λ _p = 10° S, L_p = 42° W;
• (b). 

  Rudraprayag (Silurian-Devonian): λ _p = 30° S, L_p = 12° W.

A new, continuous Phanerozoic apparent polar wandering curve for the Indian subcontinent has been plotted from the available palaeomagnetic data and the VGP positions reported in this paper. As a result, the gap in the Indian palaeomagnetic data from the Lower Carboniferous to the Cambrian has been partially filled. The locations of the pole positions for the two formations on the Phanerozoic polar wandering curve for the Indian subcontinent, have been found to coincide with the stratigraphic ages assigned to them on the basis of rather limited geological and palaeontological evidence.
The Cambrian and Permian poles for the Salt Range in the NW Himalayas and the Permian pole for the Kumaon Himalayas are grouped along with the pole positions of contemporaneous formations of the Peninsular Shield. The palaeomagnetic data thus suggests that the two formations are autochthonous in nature.     

The Nama Group revisited

The Nama Group of southern Namibia is a candidate for the Terminal Proterozoic Global Stratotype Section and Point (GSSP). Desirable characteristics of a GSSP include a well-preserved index-fossil assemblage, little deformation or metamorphism. well-constrained isotopic ages, stable-isotope records and magnetostratigraphic control. The age of the Nama Group sediments is now constrained to between 570 and 510 Ma. Assuming the Gondwana assembly was nearly complete at this same time, there is a discrepancy between the previously published Nama poles, a revised 550-510 Ma apparent polar wander path for Gondwana and the preceding supercontinental assemblages of Rodinia and Panottia. For these reasons, the Nama Group sediments were resampled in an effort to evaluate the potential of detailing the magnetostratigraphy of the Nama Group and resolving the discrepancy between the Nama poles and the APWP of Gondwana. Collectively, both the previous studies of the Nama Group and this one show a complex series of overprints and no easily discernible primary direction of magnetization. We therefore urge caution in using the Nama Group poles in any tectonic models of the Neoproterozoic-Early Palaeozoic. Specifically, the N1 component of magnetization, previously identified as a primary magnetization, was discovered in a younger suite of samples. Therefore, previous tectonic models that used the N1 magnetization direction as representative of the time of Nama deposition should be revised in light of these recent findings.     

Autoregressive estimation of complex eigenfrequencies in low frequency seismic spectra

b
The fact that a seismogram can be represented as an autoregressive (AR) time series and the use of Prony's method enable us to obtain the complex frequencies from the poles of the corresponding AR process. A frequency-domain formulation which employs a time-domain tapering technique is devised so that spectral peaks can be analysed individually, or in small groups. Statistical analysis of the estimators and several examples from two recent earthquakes illustrate the method.     

An objective method for smoothing palaeomagnetic data

Summary. This paper describes a new method of smoothing noisy data, such as palaeomagnetic directions, in which the optimum degree of smoothing is determined objectively from the internal evidence of the data alone. As well as providing a best-fitting smooth curve, the method indicates, by means of confidence limits, which oscillations or fluctuations in the fitted curve are real. The procedure, which is illustrated by an analysis of palaeomagnetic decimation directions from Lake Windermere, has potential applications throughout the Earth Sciences. It may be used in any investigation requiring the estimation of a smooth function from noisy data, provided certain basic assumptions are reasonably satisfied.     

湖南省经济增长极研究

本文借助增长极理论,研究了湖南空间开发的基本格局,阐明了地域发展必须以经济水准较高的城市为成长核心,运用多元分析方法确定了不同层次的增长极,最后指出本世纪内湖南城市经济建设重点应是长沙、株洲、湘潭城市群和岳阳市。     

Palaeomagnetism of the Lower Ordovician Orthoceras Limestone, St. Petersburg, and a revised drift history for Baltica in the early Palaeozoic

Palaeomagnetic investigation of Lower Ordovician limestone in the vicinity of St. Petersburg yields a pole position at latitude 34.7°N, longitude 59.1°E ( dp / dm =5.7°/6.4°). A probable primary remanence origin is supported by the presence of a field reversal. The limestone carries one other remanent magnetization component associated with a Mesozoic remagnetization event.
An apparent polar wander path is compiled for Baltica including the new result, ranging in age from Vendian to Cretaceous. Ages of the published Lower to mid-Palaeozoic palaeomagnetic pole positions are adjusted in accordance with the timescale of Tucker & McKerrow (1995). The new Arenig result is the oldest of a series of Ordovician and Silurian palaeomagnetic pole positions from limestones in the Baltic region. There are no data to constrain apparent polar wander for the Tremadoc, Cambrian and latest Vendian. If the Fen Complex results, previously taken to be Vendian in age ( c . 565 Ma), are reinterpreted as Permian remagnetizations, an Early Ordovician–Cambrian–Vendian cusp in the polar wander path for Baltica is eliminated. The apparent polar wander curve might then traverse directly from poles for Vendian dykes on the Kola peninsula ( c . 580 Ma) towards our new Arenig pole ( c . 480 Ma). The consequence of this change in terms of the motion of Baltica in Cambrian times is to reduce significantly a rotational component of movement.
The new Arenig pole extends knowledge of Ordovician apparent polar wander an increment back in time and confirms the palaeolatitude and orientation of Baltica in some published palaeogeographies. Exclusion of the Fen Complex result places Baltica in mid- to high southerly latitudes at the dawn of the Palaeozoic, consistent with faunal and sedimentological evidence but at variance with some earlier palaeomagnetic reconstructions.     

A weighted least-squares fit of the Australian apparent polar wander path for the last 100 Myr

Summary. Recent versions of the Australian apparent polar wander path (APWP) for the late Mesozoic and Tertiary show considerable variation. Re-examination of the Australian igneous data suggests that they are more reliable than assumed by some recent authors. The trajectory of the Australian APWP is defined by fitting the position of a set of poles including both igneous and laterite/overprint data. This allows the dated igneous poles to be used to determine age as a function of distance along the trajectory. Both the trajectory and the age are fitted by means of weighted least-squares regression, and are given approximate confidence limits.
Age is best fitted in the Australian case as a linear function of distance along the APWP. This result contrasts with that of Idnurm, who suggested a variable rate of polar wander during the Tertiary. The new APWP is in better agreement with hot-spot data. Dating of New Caledonian laterites by the new APWP gives a result consistent with geological evidence, while dating by reference to Idnurm's path does not. Large non-dipole components or significant true polar wander are not needed to explain the Australian Tertiary APWP.     

求算界限温度日期的一种新方法──样条函数插值法

本文根据某站点各月平均气温,通过计算机,用样条函数插值法模拟年温变化曲线,进而直接求算出主要界限温度初终日期、间隔日数及其相应积温,并与用滑动平均法得出的结果进行了比较检验。     

孤立冰块流拖曳系数的优化辨识

流拖曳系数是海冰动力学模型中的重要参数,实际海冰运动时受到水流的底面剪切作用和侧面正压力作用,相应的拖曳系数分别为摩拖曳系数和形拖曳系数。利用实验室波流水槽设备,对长方平底、长方粗糙底和圆饼型淡水冰试样进行拖曳运动试验,获得26组冰样运动数据。根据这些试验数据,利用动量法原理和数学优化方法构造了摩拖曳系数和形拖曳系数的辨识模型,将摩拖曳系数和形拖曳系数两项分开。在此基础上,分析了冰样粗糙度指标(支承长度率和冰厚均方根偏差)对这两个系数的影响;发现了摩拖曳系数同冰样水平尺寸和垂直尺寸之比的关系,该关系能将冰山和浮冰的摩拖曳系数归结到同一条曲线上。     

计算机绘制粒度分布曲线的插值方法

张力样条是在三次样条函数中加入张力系数而构成。根据相同的粒度数据用两种方法在计算机上所作的粒度分布曲线图形的对比,显示出张力样条方法比三次样条方法有明显的优越性,它消除了插值曲线的不必要的摆动,在保持曲线平滑的前提下,能更准确地表达沉积物的粒度特征。用张力样条结合将离散点连成光滑曲线的阿克玛方法进行插值,使复杂的粒度分布曲线的插值同时保持准确性和平滑性,在沉积物粒度的数据的计算机处理方面有明显的意义。     

陕北黄土高原景观动态的植被覆盖季节响应

半干旱地区生态环境状态与景观动态变化息息相关,为深入探讨景观动态的生态响应,本研究选择陕北黄土高原为研究区,分别就景观动态的3种类型 (景观类型间变化、类型内组分变化及未变化) 与植被覆盖状态进行分析。其中植被覆盖状态主要通过植被覆盖值高低及年内植被恢复期、维持期与转折点等曲线特征体现。结果发现研究区景观类型的动态变化并不显著,而各动态类型与植被覆盖的相关程度各不相同,植被覆盖不仅和景观类型有关,亦与景观动态类型有关,而在植被覆盖对景观动态的响应上,则体现出由于景观动态类型的差异,造成同一景观类型的植被覆盖季节响应模式和强度完全不同。     

沙粒跃移轨迹参数的统计研究

高速频闪摄影从实验技术上加强了对沙粒跃移运动及其参数的研究,但因拍摄沙粒的成像点不完整又存在局限性,用曲线拟合高速频闪摄影的跃移沙粒轨迹可以补足缺失的成像点。本文对高速摄影的沙粒轨迹进行曲线拟合,从拟合曲线反求出沙粒的起跳角θ₀、上升段跃高H₁、上升段跃距L₁、沉降角θ_t、下降段跃高H₂、下降段跃距L₂,并对各参数进行统计分析,得出各参数间的函数关系式,为进一步研究颗粒跃移运动过程提供基础。     

深圳快速城市化中地形对景观垂直格局的影响

基于RS 和GIS 的支持, 以深圳市为案例研究地形对景观格局及其垂直分布的影响, 进行景观格局时空动态分析。文章指出, 人类活动对土地的干扰程度表现为从低平地区向高 峻地区递减, 从而导致在以山地占主体的盐田等区在城市化前期低于地势平坦的福田等区; 随着城市的进一步加深, 福田等区多样性指数则快速降低, 盐田等区相对降低很少, 或还处 于上升状态。多样性指数在高程带和坡度带上多时段变化表明: 在垂直空间上, 土地利用结 构的垂直分异是景观格局垂直分异内在原因, 多样性指数的空间曲线和时间曲线具有反“U” 形库兹涅茨曲线特征, 但是这种曲线变化特征需要到一定的城市化阶段才能形成, 并且曲线 特征随城市化的加深而更加明显; 在时间上, 低海拔和低坡度地区的多样性指数的动态特征 也有类似库兹涅茨曲线的反“U”形, 而且随城市化的加深, 具有这种特征的高程带和坡度 带均在上移。     

Remanent magnetization of Precambrian and Cretaceous kimberlites in South Africa

Summary. Middle Precambrian and Cretaceous kimberlites were collected from three sites (Premier, Montrose and National) and two sites (Wesselton and Koffyfontein) in South Africa respectively. The natural remanent magnetization of these rocks remains stable to both alternating field and thermal demagnetization. The virtual geomagnetic pole-positions derived from the directions of stable remanence of the Precambrian rocks can be correlated with palaeomagnetic poles obtained from other Middle-Late Precambrian rocks in Africa. The Cretaceous poles for the Wesselton and the Koffyfontein rocks coincide with other Cretaceous poles.     

基于 NDVI 物候特征的华南地区冬种马铃薯遥感提取方法

马铃薯是华南地区的特色冬种农作物,其地块的“早稻―晚稻―冬种马铃薯”三季种植模式具有特有的植被指数时间序列曲线特征。利用这一特征,提出一种基于 NDVI 时间序列数据和 SAM 的冬种马铃薯种植面积提取方法。以广东省惠州市稔平半岛为研究区,冬种马铃薯面积为研究对象,采用 2011 年 HJ-1 A/B CCD 遥感数据为主要数据,计算每一景影像的 NDVI 后以时间为坐标轴排列成 NDVI 时间序列数据集,在此提取冬种马铃薯种植区的 NDVI 时间序列参考曲线,使用光谱角度匹配（SAM）方法,计算每个像元的 NDVI 时间序列曲线与 NDVI时间序列参考曲线的光谱夹角值,根据 Rule 图像的统计参数确定夹角阈值,达到快速有效地在遥感影像上提取冬种马铃薯对应像元的目的。结果表明：研究区总体提取精度为 82.70%,重点种植区域提取精度为 93.75%,可见基于 NDVI 物候特征的 SAM 方法能够有效提取研究区冬种马铃薯的种植面积。