任意平面分层介质中正演计算的快速方法

在非均匀介质电磁散射、逆散射等领域中，正演计算速度直接影响着反演计算效率。快速高精度的正演计算方法有利于反演效率的改善。对于任意平面分层介质情形，传统高效的数值模式匹配法的正演计算效率并不很高。本文通过首先推得每一从向分层中场量的汉克尔变换表达，再用快速傅里叶变换来完成汉克尔变换计算，从而得到更高效的计算方法。它避免了数值本征模式的求解，特别适用于处理平面分层介质中纵向分层较多的情形。对于其它非均     

Cylindrical FDTD Analysis of LWD Tools Through Anisotropic Dipping-Layered Earth Media

Electrical logging-while-drilling (LWD) tools are commonly used in oil and gas exploration to estimate the conductivity (resistivity) of adjacent Earth media. In general, Earth media exhibit anisotropic conductivities. This implies that when LWD tools are used for deviated and horizontal drilling, the resulting borehole problem may include dipping-layered media with dipping beds having full 3 times 3 conductivity tensors. To model this problem, we describe a 3-D cylindrical finite-difference time-domain (FDTD) algorithm extended to fully anisotropic conductive media and implemented with cylindrical perfectly matched layers to mimic open-domain problems. The 3-D FDTD algorithm is validated against analytical results in simple formations, showing good agreement, and used to simulate the response of LWD tools through anisotropic dipping beds for various values of anisotropic conductivities and dipping angles     

Numerical Mode-Matching Method for Tilted-Coil Antennas in Cylindrically Layered Anisotropic Media With Multiple Horizontal Beds

Tilted-coil antennas (TCAs) have been proposed to increase the directional sensitivity and anisotropy sensitivity of well-logging tools used in oilfield exploration. In this paper, we simulate TCAs in 3-D cylindrically layered and anisotropic earth formations with multiple horizontal beds using an extended numerical mode-matching (NMM) approach. The field components are expanded in terms of longitudinal (vertical) eigenmodes to facilitate the analysis of transverse electric and transverse magnetic fields, which are coupled in this case. The perfectly matched layer is incorporated into the NMM formulation to mimic the Sommerfeld radiation condition in the longitudinal (vertical) direction. NMM results are compared with 3-D simulation results using finite-difference time-domain method and a pseudoanalytical approach based on Sommerfeld integrals (for problems where the latter is applicable), showing very good agreement.     

An FDTD algorithm with perfectly matched layers for generaldispersive media

A three-dimensional (3-D) finite difference time domain (FDTD) algorithm with perfectly matched layer (PML) absorbing boundary condition (ABC) is presented for general inhomogeneous, dispersive, conductive media. The modified time-domain Maxwell's equations for dispersive media are expressed in terms of coordinate-stretching variables. We extend the recursive convolution (RC) and piecewise linear recursive convolution (PLRC) approaches to arbitrary dispersive media in a more general form. The algorithm is tested for homogeneous and inhomogeneous media with three typical kinds of dispersive media, i.e., Lorentz medium, unmagnetized plasma, and Debye medium. Excellent agreement between the FDTD results and analytical solutions is obtained for all testing cases with both RC and PLRC approaches. We demonstrate the applications of the algorithm with several examples in subsurface radar detection of mine-like objects, cylinders, and spheres buried in a dispersive half-space and the mapping of a curved interface. Because of their generality, the algorithm and computer program can be used to model biological materials, artificial dielectrics, optical materials, and other dispersive media     

Numerical solution of Green's function of potential field ofeccentric point source in axisymmetric inhomogeneous media

An analysis has been performed for a borehole direct-current resistivity boundary value problem in axisymmetric inhomogeneous media. In order to solve the problem of eccentric excitative sources in actual electrical logging, the authors apply an efficient numerical mode matching (NMM) theory to present Green's function of the potential field of an eccentric point source. The basic and higher order modes are described by applying numerical eigenmode expansion, and the local reflection and transmission matrices are used to match the boundary condition of potential field on every planar layer. Some typical examples are calculated and analyzed. The first example is concerned with a 16-in normal tool, which results are in excellent agreement with the data published previously in references. The second example about the micro laterolog-3 point electrodes is calculated to display a range of different eccentric effects. Some interesting and useful results are obtained     

The 3-D multidomain pseudospectral time-domain algorithm for inhomogeneous conductive media

A three-dimensional (3-D) multidomain pseudospectral time-domain (PSTD) method with a strongly well-posed perfectly matched layer (PML) is developed as an accurate and flexible tool for the simulation of electromagnetic wave propagation and scattering in inhomogeneous and conductive media. This approach allows for an accurate treatment of curved geometries by multidomain decomposition and curvilinear coordinate transformation. Numerical experiments show the results agree excellently with analytical solutions and results of other well-known algorithms, and demonstrate a remarkable improvement in accuracy and efficiency over the FDTD method. The 3-D multidomain PSTD algorithm is then applied to calculate radar cross sections (RCS).     

Three-dimensional source localization in a waveguide

This paper deals with three-dimensional (3-D) passive localization of a narrowband point source in a 2½-dimensional waveguide using an array of sensors. Two different maximum likelihood (ML) procedures for estimating the source range, depth, and direction-of-arrival (DOA) based on the normal mode representation of the received data are studied. In the first procedure, ML estimation of range and depth is applied on the data collected by a vertical array, and DOA is estimated using the ML algorithm on the data received by a separate, horizontal array. In the second procedure, the ML algorithm is applied on the data received by a two-dimensional (2-D), hybrid array for simultaneously estimating of all three source location parameters. Our study shows that although a horizontal array is sufficient for 3-D localization, to reduce sensitivity of the localization algorithm, a 2-D array should be used. The presented performance analysis of the two algorithms enables one to determine the performance losses in using the stage-wise, suboptimal algorithm relative to the optimal one in any given scenario. Numerical examples with channel parameters, which are typical to shallow water source localization, show performance losses of 0-3 dB. Simulation results of the two ML algorithms and their comparison with the Cramer-Rao bound (CRB) support the theory     

On the solution of a class of large body problems with partialcircular symmetry (multiple asymmetries) by using a hybrid-dimensionalfinite-difference time-domain (FDTD) method

This paper presents an efficient method to solve a large body scattering problem, viz. a paraboloid reflector antenna system, with only partial circular symmetry. The asymmetry in the system is introduced by two factors, viz. the microstrip feed and an inhomogeneous radome. The paper presents a novel approach, based on the equivalence and reciprocity principles and the “equivalent” aperture theory, to overcome the asymmetry problem. The technique thereby enables substantial computational efficiencies by analyzing the majority of the three-dimensional (3-D) computational domain in an effective two-dimensional (2-D) simulation, with the remainder being analyzed using a 3-D algorithm     

Multidomain pseudospectral time-domain simulations of scattering by objects buried in lossy media

A multidomain pseudospectral time-domain (PSTD) method with a newly developed well-posed PML is introduced as an accurate and flexible tool for the modeling of electromagnetic scattering by 2-D objects buried in an inhomogeneous lossy medium. Compared with the previous single-domain Fourier PSTD method, this approach allows for an accurate treatment of curved geometries with subdomains, curvilinear mapping, and high-order Chebyshev polynomials. The effectiveness of the algorithm is confirmed by an excellent agreement between the numerical results and analytical solutions for perfectly conducting as well as permeable dielectric cylinders. The algorithm has been applied to model various ground-penetrating radar (GPR) applications involving curved objects in a lossy half space with an undulating surface. This multidomain PSTD algorithm is potentially a very useful tool for simulating antennas near complex objects and inhomogeneous media.     

A 3-D polarized reversed Monte Carlo radiative transfer model for Millimeter and submillimeter passive remote sensing in cloudy atmospheres

This paper introduces a three-dimensional (3-D) polarized radiative transfer model that has been developed to assess the influence of cirrus clouds on radiances measured by the Earth Observing System Microwave Limb Sounder (EOS-MLS) instrument. EOS-MLS is on the Aura satellite, which launched in July 2004. The radiative transfer model uses a reversed Monte Carlo algorithm and has been incorporated in the Atmospheric Radiative Transfer Simulator 1.1.x software package. The model will be used to study aspects of the scattering problem that are not considered in the existing operational EOS-MLS cloudy-sky forward model, including the influence of nonspherical, oriented hydrometeors, and 3-D inhomogeneous cloud structure. This paper presents the radiative transfer algorithm and example model results, which demonstrate significant 3-D and polarization effects. Although the development of this model was motivated by the EOS-MLS mission, it is also directly applicable to ground-based and down-looking geometries.     

An approach of three-dimensional vector radiative transfer (3-D-VRT) equation for inhomogeneous scatter media

To solve a three-dimensional vector radiative transfer (3-D-VRT) equation for the model of spatially inhomogeneous scatter media, the finite enclosure of the scatter media is geometrically divided, in both the vertical z and horizontal (x,y) directions, to form very thin multiboxes. The zeroth-order emission, first-order Mueller matrix of each thin box, and an iterative approach of high-order radiative transfer are applied to deriving high-order scattering and emission of whole inhomogeneous scatter media. Numerical results of polarized brightness temperature at microwave frequency from an inhomogeneous scatter model such as vegetation canopy are calculated and discussed.     

Microwave Subsurface Imaging Using Direct Finite-Difference Frequency-Domain-Based Inversion

We have developed a new algorithm for electromagnetic inverse scattering problems in inhomogeneous media using finite-difference frequency-domain (FDFD) forward modeling, referred to as the FDFD-based inversion method. The key issue of this method is to build a linear expression for the inverse problem from an FDFD forward model by using Born approximation to neglect mutual coupling between scattered pixels and to then solve for the inverse coefficient matrix. An important advantage of this matrix-based method is that there is no need to specify a Green's function. As such, this inverse scattering algorithm is easily implemented and is robust to the heterogeneity in the background. We test the algorithm with a microwave subsurface object detection application using cross-well radar. The new method is compared with conventional inversion using Green's function-based Born approximation. Numerical experiments are presented for a 2-D borehole geometry for buried object detection in uniform soil and in multilayered soil backgrounds.      

Multiconductor Transmission Lines in Inhomogeneous Bi-Anisotropic Media

An analysis method for multiconductor transmission lines embedded in inhomogeneous bi-anisotropic media is presented. The Maxwell-Boffi equations, accompanied by the Maxwell-Minkowski ones, are expressed as Taylor series of the operating frequency where the zeroth- and first-order terms are retained. The reduced problem is solved using the potential formulation through the static 2-D free-space Green's function. The method of moments is employed to numerically solve the set of quasi-TEM integral equations, to produce the Telegrapher's equations, and to derive the circuit parameters in matrix form. The procedure is augmented by providing in closed form the field-coupling integrals.     

A 3-D hybrid finite element/boundary element method for the unifiedradiation and scattering analysis of general infinite periodic arrays

We present a rigorous frequency domain variational 3-D electromagnetic formulation for the general nonself-adjoint infinite periodic array problem. The hybrid method described combines the vector finite element and Floquet boundary element techniques. It is general in the sense that it is applicable to infinite periodic arrays of the open or aperture-types. It is thus effective for modeling both the scattering and radiation performance of diverse FSS, absorber, and phased-array structures. The technique accurately handles arbitrarily complicated 3-D geometries, lossy inhomogeneous media and internal as well as external excitations. These analyses can be applied to general skewed grids under arbitrary scan and polarization conditions     

Source localization in a waveguide using polynomial rooting

Source localization in acoustic waveguides involves a multidimensional search procedure. We propose a new algorithm in which the search in the depth direction is replaced by polynomial rooting. Using the proposed algorithm, range and depth estimation by a vertical array requires a 1-D search procedure. For a 3-D localization problem (i.e., range, depth, and direction-of-arrival (DOA) estimation), the algorithm involves a 2-D search procedure. Consequently, the proposed algorithm requires significantly less computation than other methods that are based on a brute-force search procedure over the source location parameters. In order to evaluate the performance of the algorithm, an error analysis is carried out, and Monte-Carlo simulations are performed. The results are compared with the Cramer-Rao bound (CRB) and to the maximum likelihood (ML) simulation performance. The algorithm is shown to be efficient, while being computationally simpler than the ML or the Bartlett processors. The disadvantage of the algorithm is that its SNR threshold occurs in lower SNR than in the ML algorithm     

A rigorous coupled-wave analysis and crossed-diffraction gratinganalysis of radiation and scattering from three-dimensionalinhomogeneous objects

A rigorous coupled-wave analysis (RCWA) algorithm useful for studying the radiation and scattering from three-dimensional (3-D) inhomogeneous objects is developed. A spherical electromagnetic (EM) state variable (SV) form of Maxwell's equations is presented. The RCWA method is used to numerically study EM dipole radiation within and exterior to an inhomogeneous spherical shell     

A 2-D recursive inverse adaptive algorithm

In this paper, a 2-D form of the recently proposed recursive inverse (RI) adaptive algorithm is introduced. The filter coefficients can be updated along both the horizontal and vertical directions on a 2-D plane. The proposed approach uses a variable step size and avoids the use of the inverse autocorrelation matrix in the coefficient update equation, which leads to an improved and more stable performance. Performance of the 2-D RI algorithm is compared to that of the 2-D RLS algorithm in an image deconvolution and an adaptive line enhancer problem settings. The simulation results show that the proposed 2-D RI algorithm leads to an improved performance compared to that of the 2-D RLS algorithm.     

DIRECT SOLUTION BY 2-D AND 3-D FINITE ELEMENT METHOD ON FORWARD PROBLEM OF LOW FREQUENCY CURRENT FIELDS IN INHOMOGENEOUS MEDIA

The paper adopts finite element method to analyze the forward problem of low-frequency current fields in inhomogeneous media. Firstly, the direct solution of 2-D and 3-D scalar potential is given. Secondly, the technique of covering finite elements for problems with movement has been presented; namely, when the place of testing point moved, the meshing data will be produced automatically to avoid re-meshing and distortion of the mesh. Thirdly the free and prescribed potential method is used to make the finite element coefficient matrices. Then this paper provides the result of a validity test obtained by simulating the laterolog-3 logging, compared with the numerical model-matching method. Finally, the MLL response is calculated.     

Three-Dimensional Through-Wall Imaging Under Ambiguous Wall Parameters

A through-wall imaging problem for a 3-D geometry is considered. Scatterers are located beyond a wall represented by a dielectric slab whose features are unknown or known with some degree of uncertainty. A two-step imaging procedure is presented. First, the thickness and the dielectric permittivity of the wall are estimated by a simple procedure which takes into account that actual measurements concern the total scattered field (i.e., the field reflected by the wall plus the one scattered by the obscured scatterers). Then, the problem is cast as a linear inverse scattering problem and solved by means of a truncated-singular value decomposition algorithm. In particular, a 2-D sliced approach is employed to obtain the 3-D scene. Numerical examples are shown to assess the effectiveness of the reconstruction procedure.     

粒子群法在三维航迹规划及优化中的应用

有效航迹规划是对敌纵深目标攻击成功的可靠保证,为了在复杂的地形和敌方火力威胁环境中生成最优的三维航迹,提出了一种利用粒子群法来优化三维航迹规划的方法,同时,利用动态窗对原航迹规划中新出现的威胁进行航迹动态规划。根据飞行器的飞行性能,通过引入最小威胁曲面概念生成三维航迹搜索空间,再利用一个有限项的多项式函数来逼近最小威胁曲面中的三维航迹在二维水平面内的投影,将原来的规划问题简化为在一个一元函数多项式系数空间中的搜索寻优。仿真结果显示,利用粒子群法优化的静态航迹规划能有效减小搜索空间,提高规划效率,同时,动态航迹规划能回避新威胁。