从单层位出发建立曲面上的位场转换解释系统(一)

本文从单层位出发,导出了曲面上位场转换所需的基本公式:单层场强分量及单层场强分量的一阶与二阶导数表达式。再从联系磁位和引力位的泊松公式出发,导出了“磁化方向与磁场分量方向互换定理“,从而建立了一个多功能的曲面上的位场转换解释系统,给出了使用该系统处理与解释实际资料的例子。     

变磁化方向化磁极理论

发展了磁偶层位势理论,证明沿任意方向磁化的磁偶层位势函数(1)在全空间有意义;(2)在曲面S上连续并有确定的极限;(3)当U(P)|_s、M~0(Q)给定时,M(Q)有唯一解,在此基础上,结合磁化方向与磁场分量方向互换定理,建立变磁化方向化磁极理论。     

井中三分量磁测的模量反演

井中三分量磁测解释通常要考虑磁性体的磁化方向,但是在剩磁或退磁影响过大的情况下,磁化方向难以确定。由于磁异常模量具有不依赖或弱依赖磁性体磁化方向的特点,而且利用井中三分量数据可以直接计算模量值,因此,笔者尝试利用井中三分量磁测的模量进行磁性体反演。首先利用井中三分量磁测数据计算模量值,然后利用模量反演二度板状体和直立长方体模型的磁性参数和几何参数,并将该方法应用于新疆某铁矿区井中磁测资料的反演解释中。结果表明,该方法不受或弱受磁法方向影响,在无法准确确定磁化方向的情况下能较准确地判断异常体的位置;优于传统的单分量Za反演方法;现场应用效果良好。     

二维起伏地形条件下磁场分量互相转换和向上延拓的一种方法

本文从二维磁场矢量积分延拓公式出发,介绍了一个关于二维起伏地形条件下磁场各分量转换的迭代算法,从而找到了一条不同于以等效源法、单层位及双层位理论等为基础的磁场分量转换与向上延拓的途径。     

偶层场强分量表达式在求磁异常各阶导数上的应用

根据位场理论,将观测曲面上测得的△T(或Za)视为位函数,代入偶层位公式的极限形式,求解出对应于△T(或Za)的等效偶层的磁化强度J(α,β,γ)。将J(α,β,γ,)代入偶层场强分量表达式 (α)/(α)n(1/r)dS 只要给定了方向u,就可得到△T沿该方向的一阶导数。将J(α,β,γ)代入(α)T/(α)x,(α)T/(α)y,(α)T/(α)z的积分表达式就能得到△T的二阶导数。因为使用的是二重积分形式的导数的精确表达式,它克服了空间域差商求导的致命弱点。特别是该方法能将起伏地形曲面上的场值向上延拓(包括曲化平)和求一阶(或二阶)导数两个转换过程集中在一个积分表达式中,一次计算完毕。     

深度域地震层位在精细随机反演中的应用

地震解释层位是反射同相轴空间分布的直观体现,通常用于反演过程中的框架构建,在约束反演过程中控制着测井外推的方向和趋势。由于地震资料分辨率相对较低,地震层位作为反演框架约束时难以准确刻画井间储层横向对比关系,如果采用人工解释地质层位,工作量较为庞大。将地震解释层位信息与测井信息有机的结合起来,充分利用测井在纵向高密度采样的特点和地震资料在横向约束作用,形成以测井地质分层为支撑点来共同约束建立层位框架模型,再进行岩性随机反演,能够较好地解决随机反演中的"窜层"现象。通过实践,对砂泥岩薄互层的识别有了更好的预测效果。     

潞安矿区五阳煤矿8005工作面关键层分布特征及离层发育监测

以五阳煤矿8005工作面采煤沉陷治理工程为背景,对工作面地层中覆岩关键层的平、剖面分布特征进行分析研究,得出主注浆层位亚关键层4沿工作面推进方向先变薄再变厚的基本特征,并据此提出“单层—双层—单层”的过渡式注浆充填方式。在工作面推进过程中利用分布式光纤传感监测技术对覆岩全层位变形进行了连续观测,为工作面开采初期单层注浆工作的开展提供了重要参考依据。     

在点电(流)源场中21/2度地电模型场强和电位的解

本文给出在各向异性的21/2度地电模型中求解直流电场问题的一种方法.导出了关于场强的面积分方程式。平行于走向方向的场强分量用富氏级数来表示,而在垂直于走向的平面上场强级数的每一项均采用了一种分节法来求解.这里我们同时假定在分节上场强具有线性特征.本文最后给出了关于不同电流效应的数值计算实例.     

基于横波分裂的转换波各向异性分析技术及应用

当横波不平行也不垂直穿过裂缝时,会发生横波分裂现象。这里基于横波分裂理论,采用二维分量旋转方法进行转换波分裂分析。为了得到地层裂缝信息,提高宽方位转换波径向分量成像质量,利用宽方位转换波的径向和横向分量资料,完成了多层裂缝介质的各向异性分析。在此基础上,还完成了转换波方位各向异性校正研究。首先,利用分量旋转方法求出第一裂缝介质层的各向异性方向、快慢横波时延等参数;然后,将快、慢横波时延应用于慢波数据进行慢波时延补偿;最后,根据第一层的方位角将快波和补偿后的慢波旋转回原来的径向和横向分量方向。这样,就完成了第一裂缝层的各向异性分析,同时还得到第一层的裂缝方向、裂缝密度、补偿后的径向分量等数据。依此类推,进行以下裂缝层的各向异性分析。该转换波各向异性分析技术已应用于川西XC气田的宽方位三维三分量地震数据处理,不但得到了裂缝参数,而且还消除了宽方位转换波的方位各向异性影响,提高了转换波径向分量的成像质量。     

关于频率电磁测深几个问题的探讨(六)——频率电磁测深的电磁场分布与观测参量

分析了水平电偶源与垂直磁偶源的电磁场在半空间大地中的分布特点,指出观测各场分量的合适地面部位。为了定性解释,一般将场分量计算为单分量视电阻率或比值视电阻率;做定量解释时,可直接采用场强进行反演,以减小换算误差。基于当前电偶源与磁偶源的发射功率和电磁场衰减规律,在实际应用中,水平电偶源可探测更大深度(<3 000 m),垂直磁偶源探测浅部(<500 m)。      

地球坐标系内的磁场转换方法

为了满足卫星磁测资料解释的需要,作者提出了地球坐标系内的磁场转换方法,包括:（1）非同一球面上磁场换算同一球面上磁场的“曲化平”方法;（2）不同球面磁场互换的“延拓”方法;（3）由测量方向与磁化方向均与基本地磁场一致的场T,换算测量与磁化方向均指向地心的场Z,的“化向地磁极”方法。所有方法均建立在球谐分析的基础上,且均可以应用于地球表面的局部区域。     

Reliability of Relative Paleointensity Recorded in Chinese Loess–Paleosol Sediments

A detailed paleomagnetic and rock-magnetic investigation spanning loess L7 to paleosol S8 has been carried out at the Baoji and Xifeng sections. Results of anisotropy of magnetic susceptibility confirm that the studied loess–paleosol sediments retain primary sedimentary fabrics. Stepwise thermal demagnetization shows that two well-defined magnetization components can be isolated from both loess and paleosol specimens. A low-temperature component, isolated between 100°C and 200°C, is consistent with the present geomagnetic field direction, and a high-temperature component, isolated between 200–300°C and 620–680°C, includes clearly normal and reversed polarities. Isothermal remanent magnetization and thermomagnetic analyses indicate that characteristic remanent magnetization is mainly carried by magnetite and hematite. The Day plot, together with the stratigraphic variations of rock-magnetic parameters, shows that the uniformity of magnetic mineralogy and grain size fulfills the criteria for relative paleointensity (RPI) studies. RPI records have been constructed using natural remanent magnetization (NRM) intensity after thermal demagnetization at 300°C normalized by low-frequency magnetic susceptibility (NRM300/χ). The results show that the RPI record from the Baoji section, where pedogenesis is quite weak, is compatible with the stacked PISO-1500 paleointensity record, suggesting that it might reflect the paleointensity variation of the geomagnetic field. The RPI record from the Xifeng section, where pedogenesis is rather strong, indicates a clear dissimilarity with the stacked PISO-1500 paleointensity record, implying that it does not reflect the paleointensity variation of the geomagnetic field. Our new results show that the NRM300/χ from the strongly pedogenetic paleosols does not completely eliminate the pedogenetic (climatic) influence, so it might be unsuitable for a reliable paleointensity study.     

The paleomagnetic record from deep-sea sediment cores

The paleomagnetic record of deep-sea sediment cores is compared with that which would be expected from our knowledge of the Earth's magnetic field. It is found that some of the scatter in directions of magnetization obtained from deep-sea cores is removed in cores with very low sedimentation rate, the cause being that the secular variation of the Earth's magnetic field is more completely averaged out over the finite size of each sample, when these samples comprise a longer time span of sedimentation. Corrections have been applied to the results from a series of cores in order to obtain the inclination of the average direction of magnetization from the average inclination and the scatter of inclination. These corrected inclination values confirm the hypothesis that the average Earth's field over the past few million years has been similar to an axial dipole displaced towards the North Pole. The amount of displacement obtained was 168 km. The record of short-period polarity intervals within the Brunhes, the Matuyama and the Gauss epochs was studied. It was shown that these intervals are very scattered in position. It is thought that some hitherto undiscovered short-period polarity intervals may be responsible for part of the scatter, but it is also highly likely that many samples give spurious reversals (i.e., ones not caused by the Earth's magnetic field).The possible correlation between climatic changes and the Earth's magnetic field is examined. It is concluded that the cores which show correlations between direction and/or intensity of magnetization and climatic indicators, thus suggesting the possible correlation between climate and the Earth's magnetic field, are not accurately recording the relevant parameters of the Earth's magnetic field. The correlation must be caused by climatic effects which have a direct influence on the magnetization of the sediments.Very little is known about the mechanisms of magnetization in deep-sea sediment cores, and there are several unexplained phenomena, such as the fact that many cores have maximum susceptibilities which are vertical, and the fact that cores differ widely in their ability to record accurately the Earth's magnetic field.     

Multicomponent magnetization in the helmsdale granite, north Scotland; geotectonic implication

A palaeomagnetic study of the Helmsdale granite (U/Pb-420 m.y., K/Ar-400 m.y.), northeast Scotland, has revealed a multicomponent remanence dominated by two characteristic axes of magnetization. The suggested oldest of these magnetizations, the direction of which is nearly horizontal and directed N-S, is thought to have been acquired in Upper Silurian-Lower Devonian times. The existence of this shallow direction of magnetization discounts a recent hypothesis of a ca. 2000 km sinistral offset along the Great Glen Fault. The second component of magnetization appears to be partly carried by haematite that apparently formed through disintegration of biotite and/or plagioclase. This secondary magnetization has a direction that can be associated with a Permian-early Mesozoic age. Similar overprinted magnetizations are characteristic features also in the Devonian sedimentary sequences north of Helmsdale.     

Palaeomagnetic study of the Ronda peridotites (Betic Cordillera, southern Spain)

A palaeomagnetic study of the Ronda peridotites (southern Spain) has been carried out on 301 samples from 20 sites, spread along the three main outcrops of the ultrabasic complex: Ronda, Ojén and Carratraca massifs. Different lithologies and outcrops with different degrees of serpentinization have been sampled and analysed. Rock magnetic experiments have been carried out on a representative set of samples. These measurements include: Curie curves, hysteresis cycles, isothermal remanent magnetization (IRM) acquisition curves, thermal demagnetization of IRM imparted along three orthogonal axes and magnetic bulk susceptibility. Results indicate that magnetite is the main magnetic mineral present in the samples. Stepwise thermal and alternating field (AF) demagnetization of the natural remanent magnetization (NRM) reveals the presence of a characteristic remanent magnetization (ChRM) carried by magnetite, and in some sepentinized samples, a northward component with variable unblocking temperatures up to 250–575 °C. The appearance and the relative intensity of this northward component are strongly related to serpentinization degree. Taking into account the geological history of the peridotites, the ChRM has been considered as a thermo-chemical remanent magnetization acquired during the first serpentinization phase associated to the post-metamorphic cooling of this unit. On the basis of radiometric and fission track analysis, the ChRM is proposed to have been acquired between 20 and 17–18 Ma. The inclination of the mean direction of the ChRM statistically coincides with the expected inclination for stable Iberia during the Oligocene–Miocene. The declination of the ChRM differs from the expected declination, indicating clockwise block rotations of 41±12° about vertical axes since the cooling of the peridotites. When applying a compositional layering correction, the ChRM directions fail to pass this kind of fold test, thus, the compositional layering was not a palaeohorizontal during ChRM acquisition time. Normal and reversed polarities of the ChRM are reported, showing that at least one reversal of the Earth's magnetic field took place during ChRM acquisition process. A tentative polarity zonation within the peridotitic outcrops is also suggested. No evidence is found from these data for the previously proposed simultaneity between post-metamorphic cooling and rotation of the peridotites.     

Magnetic anomalies of two-dimensional bodies and algorithms for magnetic inversion of dykes and basement topographies

Generalized equations for the anomalies in any component of the earth’s magnetic field due to two-dimensional bodies of arbitrary magnetization are derived in terms of a new parameter, called the direction of measurement. Schemes for inverting the magnetic anomalies of arbitrarily magnetized dykes and basement topographies are then developed and the relevant computer software is presented. In both the schemes, the initial values of the parameters are calculated by the computer, so that the input merely consists of the anomalies and their distances. The differences in the observed and calculated anomalies are solved iteratively for the errors in initial values of the parameters.     

Numerical simulation of thermoremanent magnetization in intrusions

This paper is devoted to the possibility of inverse remanent magnetization that is acquired in intrusive rocks due to a magnetic field that is produced by a sum of the normal core field and an anomalous effect induced by solidified rocks. A computer program has been created to simulate the process of solidification of intrusive body rocks from its edges to the center. The computing results showed the possibility of remanent magnetization that is acquired, which can have a different direction as compared to the external magnetic field, up to the inverse one.     

NEW MAGNETOSTRATIGRAPHIC AND SEDIMENTOLOGIC RESULTS FROM TERTIARY SEDIMENTS OF THE HOH XIL BASIN, NORTHERN QINGHAI-TIBET PLATEAU: IMPLICATIONS FOR THE CENOZOIC TECTONIC HISTORY OF THE TIBET PLATEAU

NEW MAGNETOSTRATIGRAPHIC AND SEDIMENTOLOGIC RESULTS FROM TERTIARY SEDIMENTS OF THE HOH XIL BASIN, NORTHERN QINGHAI-TIBET PLATEAU: IMPLICATIONS FOR THE CENOZOIC TECTONIC HISTORY OF THE TIBET PLATEAU     

Natural electromagnetic radiation (EMR) and its application in structural geology and neotectonics

Natural electromagnetic radiation (EMR) impulses are emitted from rocks under stress. Electromagnetic emission may start during crystal deformation prior to and during the nucleation phase of nanocracks. The emission direction is either parallel with or normal to the crack surfaces. The EMR magnetic component is measured by the sensor or aerial of an instrument, the Cerescope, at frequencies from 5 to 50 kHz. Measurements at the surface show directions of recent stresses remarkably well. A calibration of EMR intensity in terms of stress magnitude is possible in tunnels, where the overburden pressure can be calculated. Two examples from the Upper Rhine Graben and NW India show EMR line measurements. In both cases, stress concentrations at fault or bedding surfaces can be detected. These surfaces can be regarded as tectonically active. Two further examples of EMR determinations in tunnels give more detailed information on the regional stress field. The example from the Swiss Jura fold-and-thrust belt shows directional results, with different directions beneath and above the regional detachment horizon at the base of the belt. The example from central Scandinavia shows a late Caledonian shear zone as a boundary between two recent stress domains, and gives absolute values of stress.     

Composite detrital and thermal remanent magnetization in tuffs from Aeolian Islands (southern Tyrrhenian Sea) revealed by magnetic anisotropy

This paper reports on the complex relation between rock emplacement and remanence acquisition in tuffs deposited by pyroclastic density currents, disclosed by systematic measurements of the anisotropy of magnetic susceptibility and natural remanent magnetization (NRM). Thermal demagnetization shows that the NRM consists of two components with different blocking-temperature spectra. The direction of the low-temperature component is consistent with the geocentric axial dipole value, whereas the high-temperature component has dispersed directions. The magnetic fabric is oblate, the magnetic foliation is close to the bedding and the lineations are generally dispersed along a girdle within the foliation plane. The directions of the magnetic lineation and the high-temperature remanence component of individual specimens are close to each other. This correspondence suggests that the high blocking-temperature grains acquired a remanence aligned to their long dimension before deposition, while cooling within the explosive cloud and the moving pyroclastic current. Thereafter, during deposition, the traction processes at the base of the current oriented the grains along the flow direction and affected both fabric and high-temperature remanence. This NRM component results from mechanical orientation of previously magnetized grains and is thus detrital in origin. A second, thermal component was then acquired during the cooling of the low blocking-temperature grains after deposition. These results show that NRM in fine-grained pyroclastic rocks is affected by the Earth’s magnetic field as well as the emplacement processes and that magnetic fabric data are essential to unravel its complex nature.