影响面波勘探精度的因素探讨

分析了影响面波勘探精度的主要因素,针对计算面波相速度的精度问题,讨论了常用测量台间以相速度方法的优缺点及其适用范围,并提出通过测量台间格林函数进行相位校正,以得到两台之间的精解相位。以合成的高频理论地震图作为记录信号,并在理论地震图上叠加了２０％的随机噪声,利用这些方法分析理论信号,数字实验表明：在相干频率范围内,格林函数方法能够精确测量台间相速度,而互相关法和窄带通滤波互相关法所得结果较为离散。     

利用面波测定台间衰减系数

使用相匹配滤波技术提取基阶面波振型的相位,然后再利用时变滤波进行 振幅谱校正,以此分离出基阶振型面波信号。对合成面波理论地震图的处理结果表 明,该方法可精确地从含高阶（最高阶数为５）的综合地震图中分离出基阶面波,与利 用同一模型合成的基阶面波相比,分离出的基阶面波的振幅谱畸变很小,根据中国数 字化地震台网（ＣＤＳＮ）记录的长周期面波资料,对与震源位于同一大圆弧上的北京。 海拉尔台观测的瑞雷波进行处理,分离出基阶振型信号,然后利用两台的基阶振型信 号进行反褶积得到台间格林函数,由格林函数计算得到台间周期为１３～１４２ ｓ的面 波衰减系数和群速度,所得结果也较为平稳。     

Green's function interpolations for prestack imaging

A new interpolation method is presented to estimate the Green's function values, taking into account the migration/inversion accuracy requirements and the trade-off between resolution and computing costs. The fundamental tool used for this technique is the Dix hyperbolic equation (DHE). The procedure, when applied to evaluate the Green's function for a real source position, uses the DHE to derive the root-mean-square velocity, v _RMS, from the precomputed traveltimes for the nearest virtual sources, and by linear interpolation generates v _RMS for the real source. Then, by applying the DHE again, the required traveltimes and geometrical spreading can be estimated.
The inversion of synthetic data demonstrates that the new interpolation yields excellent results which give a better qualitative and quantitative resolution of the imaging sections, compared with those carried out by conventional linear interpolation. Furthermore, the application to synthetic and real data demonstrates the ability of the technique to interpolate Green's functions from widely spaced virtual sources. Thus the proposed interpolation, besides improving the imaging results, also reduces the overall CPU time and the hard disk space required, hence decreasing the computational effort of the imaging algorithms.     

东北地区背景噪声的Rayleigh和Love波相速度层析成像

本文利用中国数字地震台网位于东北地区的122个宽频地震台站的18个月记录的三分量连续地震噪声数据,采用互相关方法提取了Rayleigh和Love波经验格林函数,并利用时频自动分析技术获取了相应的相速度频散曲线.通过反演频散曲线,获得了Rayleigh和Love波周期为8~35s的二维相速度分布.结果表明,东北地区相速度的分布存在横向和垂向的不均匀性.短周期的相速度分布同地表地质构造密切相关,松辽盆地及山间沉积盆地呈现低速异常,而大兴安岭、小兴安岭及东部的一些山岭显示高速异常.随着周期的增加,位于中间的松辽盆地变为高低速相间,两侧的造山带呈现低速异常.这种异常的转变,可能是受构造活动或者莫霍面深度的影响.另外,在周期为20~35s频段内,Rayleigh和Love波同一周期的相速度在松辽盆地和位于吉林地区的郯庐断裂带表现不一致,表明可能存在径向各向异性.     

埋地目标体矢量电磁散射的一种快速正演算法

利用积分方程方法以及半空间并矢格林函数的快速算法对埋地目标体矢量电磁散射进行正演计算。首先,利用半空间电磁并矢格林函数建立起埋地目标体的体积分方程。然后通过将空间偏导转移至格林函数谱域积分的积分号之外,并采用离散复镜像方法来近似余下的零阶索末菲积分,进一步得到并矢格林函数各个分量的闭合形式。由于避免了对索末菲积分的繁琐数值计算,使得生成反应矩阵和计算散射场时由半空间并矢格林函数计算带来的瓶颈问题得到较好的克服,因而极大地提高了埋地目标体电磁响应正演计算效率,同时也能保证足够的精度。     

云南地区的背景弹性波场分析

利用云南省地震台网44个台站记录的2008年1-9月连续波形数据进行互相关计算,得到了台站间的格林函数,并获取对应的频散曲线,据此分析了该地区的区域背景弹性波场的来源及分布.研究发现,该地区的区域背景弹性波场有着明显的方向性,15s信号的总体传播方向是从东南向西北,也就是说主要来自云南省的东南方向,据此推测区域背景弹性...     

地球表面质量负荷的静态响应

本文给出了单位点质量负荷作用在球状成层地球模型上的解。对较新的G-D1066A地球模型求出了n直到10000阶的负荷勒夫数,并利用这组负荷勒夫数计算了格林函数的值。简述了如何利用格林函数计算地球对表面任何负荷的响应。讨论了负荷潮研究在地球物理和海洋学方面的一些应用。     

格林函数的奇异性处理

格林函数在解决电磁场和直流电场问题的积分方程法和边界单元法等方法中有着广泛的应用.在应用格林函数时会遇到格林函数奇异性的问题,即其在源区域的内部,当场点和源点重合时,会碰到对格林函数为无穷大,积分为奇异积分.对于不同问题中遇到的不同形式的格林函数奇异性问题的各种处理方法进行了分析和评述.对于标量格林函数奇异性问题处理方法有：挖去法、级数展开法、绕开法和解析法等.对于并矢格林函数奇异性问题处理方法有：分量处理法、源并矢法和拟源并矢法等处理方法.通过对实际工作中所遇到的新方法新问题中的不同形式的格林函数的研究,提供了奇异性问题的处理的方法和途径.     

Local and controlled prestack depth migration in complex areas

Prestack depth migrations based on wavefield extrapolation may be computationally expensive, especially in 3D. They are also very dependent on the acquisition geometry and are not flexible regarding the geometry of the imaging zone. Moreover, they do not deal with all types of wave, considering only primary reflection events through the model. Integral approaches using precalculated Green's functions, such as Kirchhoff migration and Born-based imaging, may overcome these problems. In the present paper, both finite-difference traveltimes and wavefront construction are used to obtain asymptotic Green's functions, and a generalized diffraction tomography is applied as an example of Born-based acoustic imaging. Target-orientated imaging is easy to perform, from any type of survey and subselection of shot/receiver pairs. Multifield imaging is possible using Green's functions that take into account, for instance, reflections at model boundaries. This may help to recover parts of complex structures which would be missing using a paraxial wave equation approach. Finally, a numerical evaluation of the resolution, or point-spread, function at any point of the depth-migrated section provides valuable information, either at the survey planning stage or for the interpretation.     

山西地震台网地震预警时间及盲区半径分析

影响地震预警能力的关键技术指标是预警时间及预警盲区。通过分析山西地震台网勘选完成的1 101个预警台站布局,计算地震台站平均台间距,根据预警盲区半径与台间距关系式得出盲区半径。根据计算结果及相关分布图,分析认为,在预警区内,平均台间距及盲区半径分布与地震活动的相关性较高,勘选完成的地震预警台站布局比较合理。     

Green's functions and response functions in geochemical modelling

The use of Green's functions and response functions in modelling atmospheric and oceanic tracers is discussed. It is shown that low-resolution Green's function representations can describe tracer distributions more accurately than low-resolution models. The Green's function formalism is also used to discuss the problem of determining surface CO₂ sources from observations of surface CO₂ concentrations.     

The Green￠s function of the harmonic horizontal force applied at the interior of the saturated half-space soil

By using integral transform methods, the Green's functions ofhorizontal harmonic force applied at the interior of the saturated half-space soil are obtained in the paper. The general solutions of the Biot dynamic equations in frequency domain are established through the use of Hankel integral transforms technique. Utilizing the above- mentioned general solutions, and the boundary conditions of the surface of the half-space and the continuous con-ditions at the plane of the horizontal force, the solutions of the boundary value problem can be determined. By the numerical inverse Hankel transforms method, the Green's functions of the harmonic horizontal force are obtainable. The degenerate case of the results deduced from this paper agrees well with the known results. Two numerical examples are given in the paper.     

植被电磁散射的半空间模型研究

本文给出了地面植被电磁散射的半空间模型研究方法.在以往的相关文献中,均采用自由空间格林函数求解单个叶片散射体的散射场,本文利用半空间并矢格林函数求出了单个散射体的散射场,然后运用Monte Carlo方法模拟生成地面植被层,得到了在半空间下植被层的电磁散射特性,计算出了单、双站雷达散射截面,并与自由空间下的散射场做了相互对比.结果表明,在半空间格林函数下得到的散射场更为适用于描述地面植被的电磁散射特性.     

大气重力格林函数的理论计算

重力测量中需要扣除大气的影响.大气负荷对重力测量的影响可以分为大气质量变化引起的直接效应和大气负荷引起的地球变形带来的间接效应,大气负荷对重力观测值的直接影响,相对于间接效应量级较大.本文从理论上研究了大气负荷对重力观测的直接影响,仿照Farrell定义的负荷格林函数,引入大气重力格林函数,用来表示大气压变化对于重力观测的直接引力影响.在前人的基础上,本文采用了更为精细的大气模型,考虑大气温度随高程的变化,用离散褶积的方法求得了大气重力格林函数的理论值.实际计算时还要考虑地表温度、台站高程、周围地形等因素的影响,本文讨论了这些因素对大气重力格林函数的影响.考虑地表温度、台站高程、地形改正等各种影响因素以及地球变形引起的间接效应后,对台站周围区域积分即可求得大气变化引起的理论重力信号.     

层土动力Green函数数值计算中若干问题的探讨

动力Ｇｒｅｅｎ子数数值计算技术是动力边界元法得以实施的关键技术之一。本文 无对两类层状介质动力Ｇｒｅｅｎ函数的频域解进行了比较,在此基础上,文中着重探讨了在土－结构相互作用问题中应用显式动力Ｇｒｅｅｎ函数时的若干数值计算问题,包括土层的子层离散和波数的计算以及半空间的模拟等。     

秦岭-大别及邻区背景噪声的瑞利波层析成像

秦岭—大别造山带西起青藏高原东北缘,东至郯庐断裂带,是华北板块和扬子板块之间的碰撞造山带.本文收集陕、豫、皖、赣、湘、鄂、渝等区域地震台网的160个宽频带地震台站连续两年地震背景噪声数据,用双台站互相关算法获得瑞利面波经验格林函数,提取相速度频散曲线,并根据面波层析成像反演得到秦岭—大别及邻区周期8~35 s范围内相速度分布图像.结果显示,大别地块在14 s相速度分布图中呈现低速异常,与8 s相速度分布图中的高速异常形成鲜明对比,反映大别HP/UHP(high pressure/ultrahigh pressure metamorphic rocks,高压/超高压变质岩)的影响仅存在于上地壳.25 s相速度图中,大致以太行一武陵重力梯度带为界,东部以高速异常为主,西部以低速异常为主,反映了地壳东薄西厚的结构特征.14~35 s相速度分布图显示郯庐断裂带南段东西两侧的显著差异,佐证了郯庐断裂带发生大规模左行平移运动时,其南段可能切入壳幔边界.同时,郯庐断裂带南段可能存在一个热物质上涌的通道,熔融的热物质通过该通道上升,混入大别地区的中下地壳,造成了红安一大别造山带的差异隆升.南秦岭与四川盆地东北部表现为低速异常,是否与青藏高原物质东流或者南秦岭的拆沉有关,还有待于进一步深入研究.     

饱和层状场地中任意凹陷地形对入射SV波的散射

利用饱和土层的精确动力刚度矩阵和动力格林(Green)函数,采用间接边界元法,在频域内求解了层状饱和场地中任意凹陷地形对入射SV波的散射问题.通过自由场反应分析,求得凹陷地形表面各点的位移和各单元的应力响应;同样计算了虚拟分布荷载的格林影响函数,求得相应的位移和应力响应;根据边界条件确定虚拟分布荷载,将自由场位移响应和...     

Decoupled elastic prestack depth migration

This paper presents a new decoupled form of the formula for common-shot or common-receiver amplitude-preserving elastic prestack depth migration (PreSDM), which can be used for estimating angle-dependent elastic reflection coefficients in laterally inhomogeneous anisotropic media. The multi-shot or multi-receiver extension of this formula is suitable for automated prestack amplitude-versus-angle (AVA) elastic inversion of ocean-bottom cable (OBC), walkaway VSP (WVSP) or standard towed-cable data at any subsurface location. The essence of the theory is a systematic application of the stationary-phase principle and high-frequency approximations to the basic elastic Green's theorem. This leads to nonheuristic explicit wave mode decoupling and scalarization of vector PreSDM. Used in combination, ray-trace and finite-difference (FD) eikonal solvers create a useful tool to calculate accurate Green's function travel time and amplitude maps. Examples of synthetic OBC data and applications to field WVSP data show that the new imaging technique can produce a clear multi-mode elastic image.     

Stochastic multi-site generation of daily weather data

Spatial autocorrelation is a correlation between the values of a single variable, considering their geographical locations. This concept has successfully been used for multi-site generation of daily precipitation data (Khalili et al. in J Hydrometeorol 8(3):396–412, 2007). This paper presents an extension of this approach. It aims firstly to obtain an accurate reproduction of the spatial intermittence property in synthetic precipitation amounts, and then to extend the multi-site approach to the generation of daily maximum temperature, minimum temperature and solar radiation data. Monthly spatial exponential functions have been developed for each weather station according to the spatial dependence of the occurrence processes over the watershed, in order to fulfill the spatial intermittence condition in the synthetic time series of precipitation amounts. As was the case for the precipitation processes, the multi-site generation of daily maximum temperature, minimum temperature and solar radiation data is realized using spatially autocorrelated random numbers. These random numbers are incorporated into the weakly stationary generating process, as with the Richardson weather generator, and with no modifications made. Suitable spatial autocorrelations of random numbers allow the reproduction of the observed daily spatial autocorrelations and monthly interstation correlations. The Peribonca River Basin watershed is used to test the performance of the proposed approaches. Results indicate that the spatial exponential functions succeeded in reproducing an accurate spatial intermittence in the synthetic precipitation amounts. The multi-site generation approach was successfully applied for the weather data, which were adequately generated, while maintaining efficient daily spatial autocorrelations and monthly interstation correlations.     

Piezomagnetic fields produced by dislocation sources

Tectonomagnetic modeling based on the linear piezomagnetic effect is reviewed with special attention to dislocation models. Stacey's scheme was the prototype for such modeling, as proposed in his first seismomagnetic calculations in 1964. The linear piezomagnetic law is presented, in which the stress-induced magnetization is expressed as a linear combination of stress components. The Gauss law for magnetic field and the Cauchy-Navier equation for static elastic equilibrium are combined through linear piezomagnetism and the Hooke law to yield the basic equation for piezomagnetic potential. A representation theorem for its solution is given by surface integrals of the displacement and its normal derivative over the strained body.A Green's function method is developed to compute the piezomagnetic field produced by a dislocation surface in an elastic half-space. Volterra's formula for piezomagnetic potential is derived by modifying Stacey's scheme for tectonomagnetic modeling. The Green's functions for the problem are called elementary piezomagnetic potentials, which are defined as potentials produced by elementary dislocations. Special consideration is required to construct the elementary piezomagnetic potentials, because the stress field around a point dislocation has a singularity of orderr ^–3. The integral representing elementary piezomagnetic potentials is not uniformly convergent. Owing to inappropriate convergency, the Green's functions obtained in an earlier study led to a puzzling outcome. Revised Green's functions give consistent results with those obtained so far by numerical integrations. Generally the piezomagnetic field produced by dislocation sources is weak in the case of a homogeneous earth model. Two enhancement effects for piezomagnetic signals are suggested: one due to inhomogeneous magnetization and the other via bore-hole observations.