Effects of ionospheric horizontal gradients on differential GPS

This paper outlines the effect of horizontal ionospheric gradients on transionospheric path propagation such as for the case of GPS signals. The total electron content (TEC) is a function of time of day, and is much influenced by solar activity and also the receiving station location. To make the model applicable for long baselines, for which the ionosphere is not generally well correlated between receiving stations, the ionospheric gradients should be taken into account. In this work the signal path is determined using a modified ray-tracing technique together with a homing-in method. Results show that horizontal gradients can have a significant effect on GPS positioning for both single station positioning and differential GPS. For differential GPS, the ionospheric delay can, however, be either increased or decreased compared with the case of no gradient, depending on the gradient direction.     

中国地区地磁场水平梯度的计算与分析

根据2000年中国地区156个地磁台站和复测点的实测数据以及国境外的6个地磁台站实测数据,加上国境外36个IGRF点,分别利用Taylor多项式模型和曲面Spline模型,建立了中国地区地磁场总强度(F)、磁偏角(D)和磁倾角(I)的地磁模型,通过对两种模型中各分量的φ(纬度)和λ(经度)进行微分,计算得到F、D、I的水平梯度值,并绘制相应分量沿南北方向和东西方向的水平梯度图,将其与2000年IGRF10的水平梯度图作比较,最后对水平梯度的空间分布以及三种梯度图进行了分析比较.结果表明:F和I的水平梯度主要随纬度变化,F随纬度增加,变幅约为7.0～6.0 nT/min,在中部地区(25°～35°N,90°～120°E)强度达到最大;I随纬度的增加,变幅约为2'/min～1'/min;D的水平梯度随纬度和经度而变化,随着经度和纬度的增加,变幅分别约为0.4'/min～-0.3'/min和-0.1'/min～-0.4'/min.     

Optimization of a precise integration method for seismic modeling based on graphic processing unit

General purpose graphic processing unit(GPU) calculation technology is gradually widely used in various fields.Its mode of single instruction,multiple threads is capable of seismic numerical simulation which has a huge quantity of data and calculation steps.In this study,we introduce a GPU-based parallel calculation method of a precise integration method(PIM) for seismic forward modeling.Compared with CPU single-core calculation,GPU parallel calculating perfectly keeps the features of PIM,which has small bandwidth,high accuracy and capability of modeling complex substructures,and GPU calculation brings high computational efficiency,which means that high-performing GPU parallel calculation can make seismic forward modeling closer to real seismic records.     

Gravity and magnetic field interpretation based on transformations of horizontal gradients in the VECTOR system

A method of potential field processing based on the transformation of vectors of the total horizontal gradient in windows of various sizes is considered. The gradients are calculated at the centers of triangles, whose vertices are points of observations, as a rule, of gravity and magnetic fields. Averaging of horizontal gradients of the field rather than initial values of the field is the main distinction of this approach from the known methods. This procedure, referred to in this paper as vector scanning of the field, makes it possible to obtain layer distributions of field sources in a 3-D diagram that is a quasi-density model of the study medium within the framework of certain model concepts. The paper presents a model example demonstrating the possibility of separating the fields produced by two sources located on a vertical line and an example illustrating the application of this method to the interpretation of the gravity field in the zone of the geodynamic influence of the Urals.     

A spreadsheet method of estimating best-fit hydraulic gradients using head data from multiple wells

Hydraulic gradients from planar water tables, or piezometric surfaces, and horizontal flow regimes can be quickly and conveniently calculated from data sets involving numerous wells. The matrix-solving functions of a modem spreadsheet program (Excel) were used to determine the equation of a water-table plane, Ax + By + Cz - D = 0, and the equation coefficients were then used to determine the magnitude of the hydraulic gradient, according to gradient = square root of A2 + B2/C2, and its direction, according to alpha = arctan B/A, where alpha is the angle measured from the x-axis. A pre-prepared Excel file constructed to handle data from up to 20 wells at once is available for free downloading at www.geo.ku.edu/hydro/KUHydro.html.     

海洋拖曳式水平电偶源数值模拟与电场接收机研制

海洋天然气水合物资源调查中,电磁法已成为地震勘探最有效的补充手段之一,而我国海洋水合物电磁探测中的拖曳式电场接收机此前是空白.本文从方法原理、仪器设备和海洋试验等方面,对海洋拖曳式水平电偶极-偶极方法进行研讨.建立三维电阻率模型,进行数值模拟,讨论收发距、发射频率、拖曳装置离海底高度等参数与电场异常响应的关系,针对特定模型给出了最佳观测参数以指导仪器硬件设计与海洋测试.在此基础上,采用高级嵌入式计算机的智能化控制、前端低噪声斩波放大、高精度同步采集等技术,研制了由拖体、承压舱、采集电路、低噪声电场传感器等组成的海洋拖曳式水平轴向电场接收机,其系统噪声达到2 nV/√HZ@(1~100Hz).将自主研制的电场接收机用于国内首次拖曳式电偶极-偶极方法的海洋试验与评估,并对海试资料进行了处理与分析.研究结果表明,所研发的仪器达到了设计指标,能够用于海洋天然气水合物调查.     

A new 3-D ray tracing method based on LTI using successive partitioning of cell interfaces and traveltime gradients

We present a new method of three-dimensional (3-D) seismic ray tracing, based on an improvement to the linear traveltime interpolation (LTI) ray tracing algorithm. This new technique involves two separate steps. The first involves a forward calculation based on the LTI method and the dynamic successive partitioning scheme, which is applied to calculate traveltimes on cell boundaries and assumes a wavefront that expands from the source to all grid nodes in the computational domain. We locate several dynamic successive partition points on a cell's surface, the traveltimes of which can be calculated by linear interpolation between the vertices of the cell's boundary. The second is a backward step that uses Fermat's principle and the fact that the ray path is always perpendicular to the wavefront and follows the negative traveltime gradient. In this process, the first-arriving ray path can be traced from the receiver to the source along the negative traveltime gradient, which can be calculated by reconstructing the continuous traveltime field with cubic B-spline interpolation. This new 3-D ray tracing method is compared with the LTI method and the shortest path method (SPM) through a number of numerical experiments. These comparisons show obvious improvements to computed traveltimes and ray paths, both in precision and computational efficiency.     

基于印模法的地面磁电阻率数据三维非线性共轭梯度反演

本文提出了能提高异常体分辨能力,同时得到绝对电导率的地面磁电阻率数据三维反演方法.磁电阻率响应用准直流的低频磁场代替;数值模拟由频率域电场满足的Helmholtz方程出发,采用三维交错网格有限差分法;长直导线源作为发射源,其中源的计算包含在背景场中;结合地面磁电阻率数据各分量的特点,选择y分量进行反演研究;反演采用三维非线性共轭梯度反演技术,为了提高异常体的深度分辨能力,进行迭代重构反演;用印模法对初始模型进行重构,采用的是辅模型在浅部,元模型在深部的组合方式.从合成数据和实际数据的反演结果可以得到以下的认识：（1）由频率域麦克斯韦方程组出发,低频磁场数据反演可以直接得到电导率,而不是相对电导率之比;（2）采用印模法组合初始模型,进行迭代重构反演,可以提高地面磁电阻率数据反演对异常体的分辨能力,确定埋深位置,同时不会丧失对于浅部异常体的分辨能力;（3）在结合印模法的地面磁电阻率数据三维反演中,深部异常体的分辨能力受地表不均匀导电体影响较小;（4）确定印模深度可以采用上一次重构反演结束时的模型变化量,通过相邻两次重构反演结束时的模型变化量之差来确定迭代重构是否终止.因为静磁场与重力场在数学上的相似性,本文的反演方法可以被运用到重力场等位场的地面数据的反演中.

     

The iteration method for downward continuation of a potential field from a horizontal plane

This paper introduces an iteration method for the downward continuation of potential‐field data from a horizontal plane, and compares it with the conventional frequency‐domain method (Fourier transform) using 2D and 3D model tests. The paper evaluates the two methods in terms of the results, i.e. downward‐continuation distance and stability. The iteration method proves to be more stable and able to downward continue the potential for a greater distance than the Fourier transform method.     

An alternative approach in establishing relation between vertical and horizontal gradients of 2D potential field

The relation in which the vertical and horizontal gradients of potential field data measured along a profile across a two‐dimensional source are a Hilbert transform pair is re‐established using complex domain mathematics. In addition, a relation between the measured field and its vertical gradient in terms of a closed‐form formula is also established. The formula is based on hypersingular or Hadamard's finite‐part integral. To estimate the vertical gradient directly from the field data, Linz's algorithm of computing Hadamard's finite‐part integral is implemented. Numerical experiments are conducted on synthetically generated total magnetic intensity data with a mild level of noise contamination. A model of a magnetically polarised vertical thin sheet buried at a finite depth within a non‐magnetic half‐space was considered in generating the synthetic response. The results from numerical experiments on the mildly noise‐contaminated synthetic response are compared with those from using classical Fourier and robust regularised Hilbert transform‐based techniques.     

A numerical method to calculate the electromagnetic field of a horizontal current dipole

Summary A computer program is described to calculate the field of an oscillating current dipole over a horizontally stratified medium for the quasi-static case. Multi-layer master curves for frequency sounding can be calculated with this program. Examples of these curves are presented.Contribution No. 90 of the Geophysical Institute, Swiss Federal Institute of Technology, Zürich.     

水位变化对干涸湖底沉积物有机碳矿化的影响

人为干扰和气候变化会改变湖泊水位状态,明确不同水位条件下湖泊沉积物有机碳矿化特征及其影响因素,对了解内陆水生态系统碳循环具有重要意义.为探究干旱区典型盐湖沉积物有机碳矿化速率对水位变化的响应,以巴里坤湖干涸湖底原状沉积物为研究对象,初步探究了0（T1）、-9（T2）、-23（T3）、-34（T4）和-45 cm（T5）水位处理对沉积物有机碳矿化速率的影响.结果表明,T1、T2和T3处理有机碳矿化速率在试验初期较高（0~10 d）,10 d后缓慢下降,T4和T5处理有机碳矿化速率呈先增加后降低趋势;T1（1.718 μmol/（m²·s））与T3（1.784 μmol/（m²·s））处理有机碳矿化速率不存在显著差异,T1处理有机碳矿化速率是T2、T4和T5处理的1.09、3.31和3.57倍,不同处理有机碳累积矿化量表现为T3 > T1 > T2 > T4 > T5.有机碳累积矿化量（C_t）占沉积物有机碳（C₀）的比例（C_t/C₀）介于0.012~0.044之间,沉积物有机碳潜在排放量（C_i）占C₀的比例（C_i/C₀）介于0.018~0.045之间;水位降低,沉积物有机碳矿化常数（k值）减小,T1处理k值最大（0.137 d）,T4处理最小（0.032 d）.线性方程C_r=0.008x+0.488能较好地模拟有机碳矿化速率（C_r）与水位（x）的关系;不同水位处理有机碳矿化速率与模拟柱中沉积物5 cm温度呈显著的指数函数关系,T4、T5处理有机碳矿化温度敏感系数（Q₁₀）显著高于T1、T2和T3处理,即水位降低增加了巴里坤湖干涸湖底沉积物Q₁₀.因此,就巴里坤湖干涸湖底沉积物而言,水位从0 cm降至-45 cm时有机碳矿化速率降低,Q₁₀增加;反之水位上升则会促进有机碳矿化分解,Q₁₀降低.水位持续下降抑制有机碳矿化可能是维持干旱区盐湖沉积物碳库稳定的机制之一.     

GOCE卫星SGG数据系统误差的综合标定方法

介绍了GOCE卫星重力梯度数据系统误差的常用求定方法,提出了一种联合卫星轨迹交叉点不符值和现有重力场模型的系统误差综合标定方法.给出了分步解算和整体平差两种解算方法及相应的计算步骤.分步解算是先利用卫星轨迹交叉点不符值确定含尺度影响的偏差漂移项,然后对观测值进行偏差漂移改正,并利用现有重力场模型计算尺度和偏差,最后对偏...     

Effect of pressure gradients and line-tying on the magnetic stability of coronal loops

Abstract

In this paper we study the stability of an idealised magnetostatic coronal loop, incorporating both the effect of line-tying, due to the dense photosphere, and of pressure gradients. The stability equations may be solved analytically for our particular equilibrium. From the marginally stable case, the critical conditions separating instability from stability are derived. It is found that stretching or twisting a loop eventually makes it kink unstable.     

多层倾斜层状介质P-SV转换波转换点计算的最佳角度搜索法

转换波转换点的位置对转换波道集的抽取和叠加都非常重要.目前对于单一倾斜反射界面或水平层状介质模型,已经能较好地计算转换波转换点的位置.本文针对多层倾斜层状介质,提出最佳角度搜索法求取P-SV转换波转换点的位置,此方法是通过搜索最符合Snell定律的入射角和反射角来确定最佳的P-SV转换波转换点的位置.在搜索最佳转换点位置的同时,该方法也能给出最佳的P-SV波的传播路径.本文在单层和双层倾斜层状介质模型中测试了该方法,获得了精确的P-SV转换波转换点的位置和P-SV波的传播路径.     

温度对数字石英水平摆倾斜仪观测资料的影响

以数字石英水平摆倾斜仪为研究对象,通过温度实验和地震台站观测,分析温度对水平摆倾斜仪输出信号的影响,从定量角度给出其影响频段,分析表明,温度对长周期地震观测仪器的影响是普遍存在且不可忽略的,开展温度对地震观测资料影响的定量研究,有助于科学评价观测仪器温度效应。     

A direct method of interpreting gravity and magnetic anomalies: The case of a horizontal cylinder

Summary A new method of interpreting the gravity and magnetic anomalies is introduced with special reference to the magnetic anomalies of a horizontal cylinder. The method consists of calculating the functions of the anomaly and its distance from an arbitrary point. These form a simple linear equation with coefficients related to the parameters defining the body. Since each observation forms a separate linear equation, the required normal equations are formed by the method of least squares and solved for the coefficients and hence for the various parameters defining the target. The discussion here is confined to the vertical magnetic anomalies. The application of the method to horizontal and total field anomalies of two dimensional bodies is also outlined.     

Thermospheric mass density derived from CHAMP satellite precise orbit determination data based on energy balance method

In this article, the energy balance method is used to retrieve thermospheric mass density from CHAMP satellite precise orbit determination(POD) data during 2007–2009. The retrieved thermospheric mass densities are compared with those from accelerometer data and an empirical model. The main conclusions are as follows:(1) Thermospheric mass density can be retrieved from POD data by the energy balance and semi-major axis decay methods, whose results are consistent.(2) The accuracy of the retrieved densities depends on the integration time period, and the optimal period for CHAMP density retrieval from POD data is about 20 minutes.(3) The energy balance method can be used to calibrate accelerometer data.(4) The accuracy of retrieving thermospheric density from POD data varies with satellite altitude and local time.     

利用改进的SRTI法研究中尺度电离层行波扰动

针对现有单站电离层行波扰动指数（Single Receiver TID Index,SRTI）探测中尺度电离层行波扰动时探测周期范围有限的问题,本文提出了一种改进的SRTI方法,以提高探测扰动周期的适用范围.介绍了利用SRTI和改进的SRTI探测中尺度电离层行波扰动的原理,从理论上分析比较了二者探测扰动周期范围的能力,并采用美国加州南部的5个IGS站2012年年积日189的GPS观测数据,对电离层扰动进行了分析.结果显示在21：30—23：00 LT时,扰动观测序列存在周期为15 min和39 min的扰动波,但是现有SRTI方法只探测到了前者,而改进的SRTI法很好地探测到了两个周期的扰动波,表明其对短周期和较长周期的扰动均具有较好的灵敏性.利用改进的SRTI法计算的扰动观测序列,应用傅里叶变换相位差分法估计了电离层扰动的传播参数,其结果总体上与前人研究成果一致,这表明改进的SRTI法完全可以应用于电离层行波扰动的研究中,而且由于计算简单,在处理大量数据时具有一定优势.