摘要:

A method is presented for modeling the wideband, frequency domain electromagnetic (EM) response of a three‐dimensional (3‐D) earth to dipole sources operating at frequencies where EM diffusion dominates the response (less than 100 kHz) up into the range where propagation dominates (greater than 10 MHz). The scheme employs the modified form of the vector Helmholtz equation for the scattered electric fields to model variations in electrical conductivity, dielectric permitivity and magnetic permeability. The use of the modified form of the Helmholtz equation allows for perfectly matched layer ( PML) absorbing boundary conditions to be employed through the use of complex grid stretching. Applying the finite difference operator to the modified Helmholtz equation produces a linear system of equations for which the matrix is sparse and complex symmetrical. The solution is obtained using either the biconjugate gradient (BICG) or quasi‐minimum residual (QMR) methods with preconditioning; in general we employ the QMR method with Jacobi scaling preconditioning due to stability. In order to simulate larger, more realistic models than has been previously possible, the scheme has been modified to run on massively parallel (MP) computer architectures. Execution on the 1840‐processor Intel Paragon has indicated a maximum model size of 280 × 260 × 200 cells with a maximum flop rate of 14.7 Gflops. Three different geologic models are simulated to demonstrate the use of the code for frequencies ranging from 100 Hz to 30 MHz and for different source types and polarizations. The simulations show that the scheme is correctly able to model the air‐earth interface and the jump in the electric and magnetic fields normal to discontinuities. For frequencies greater than 10 MHz, complex grid stretching must be employed to incorporate absorbing boundaries while below this normal (real) grid stretching can

ISSN:0048-6604    

DOI:10.1029/95RS02815

年代:1996

数据来源: WILEY

摘要:

Since all ocean wave components contribute to the second‐order scattering of a high‐frequency radio wave by the sea surface, it is theoretically possible to estimate the ocean wave spectrum from first‐ and second‐order scattering in the Doppler spectrum measured with an HF ocean radar. To extract the wave spectral information, however, it is necessary to solve a nonlinear integral equation. This paper describes in detail how to solve the nonlinear integral equation without linearization or approximation. We show that the problem of solving the nonlinear integral equation can be converted into a nonlinear optimization problem. An algorithm to find the optimal solution is described. Examples of the algorithm applied to simulated data and measured data are shown. The wave frequency spectrum can be estimated even if the Doppler spectrum is available in only a single direction. In this case, however, the solution of the two‐dimensional wavenumber spectrum tends to converge to a spectrum that is symmetrical to the beam direction. Even if the wave spectrum is dominant in a single direction, the solution may give two peaks in the wavenumber spectrum. One of them is the true peak and the other is the mirror image of it with respect to the beam direction. This ambiguity can be avoided by using Doppler spectra measured in at least two different directions. Although there is still some room for improvement in the practical application of this method, it can be applied to estimate the wave directional spectrum up to a rather high frequency, or Bragg

ISSN:0048-6604    

DOI:10.1029/95RS02439

年代:1996

数据来源: WILEY

摘要:

We develop an asymptotic ray theory for transient diffusive electromagnetic fields in isotropic media. The formulation is first derived in the time Laplace transform domain by introducing an ansatz procedure, whereby appropriate expansions for the electric and the magnetic field strengths are substituted in the original field equations. We arrive at consistent recurrence formulas for the sequences of field amplitude vectors that multiply appropriately chosen asymptotic sequences of algebraic powers of the Laplace transform parameter. These representations differ for the two types of fields (electric and magnetic fields) and for the two types of sources (electric current source and magnetic current source). The exponential part of the field expressions contains the diffusive equivalent of the eikonal function in the asymptotic ray theory of wave propagation. This function satisfies the diffusive equivalent of the eikonal equation. Next, we derive the transport equations for the vectorial electric and magnetic field amplitudes of the successive orders. Transient field representations within the asymptotic ray approximation are then obtained by carrying out the inverse Laplace transformation to the time domain by inspection. The ray approximation thus obtained is asymptotic for “early times”. We consider as an example the case of the electric and the magnetic dipole radiation in a homogeneous medium. Here an exact solution exists, which we show to exhibit the structure of the original ansatz but with a finite number of terms. The asymptotic ray theory for transient diffusive electromagnetic fields is expected to lend itself to important applications in surface, surface‐to‐borehole, and crosswell transient electromagnetic pros

ISSN:0048-6604    

DOI:10.1029/95RS02593

年代:1996

数据来源: WILEY

摘要:

In this paper, we present an approach to the nonlinear inverse scattering problem using the extended Born approximation (EBA) on the basis of methods from the fields of multiscale and statistical signal processing. By posing the problem directly in the wavelet transform domain, regularization is provided through the use of a multiscale prior statistical model. Using the maximum a posteriori (MAP) framework, we introduce the relative Cramér‐Rao bound (RCRB) as a tool for analyzing the level of detail in a reconstruction supported by a data set as a function of the physics, the source‐receiver geometry, and the nature of our prior information. The MAP estimate is determined using a novel implementation of the Levenberg‐Marquardt algorithm in which the RCRB is used to achieve a substantial reduction in the effective dimensionality of the inversion problem with minimal degradation in performance. Additional reduction in complexity is achieved by taking advantage of the sparse structure of the matrices defining the EBA in scale space. An inverse electrical conductivity problem arising in geophysical prospecting applications provides the vehicle for demonstrating the analysis and algorithmic techniques developed in this

ISSN:0048-6604    

DOI:10.1029/95RS03130

年代:1996

数据来源: WILEY

摘要:

This paper gives a review of the advantages offered by an integral representation using diffracting component waves in solving wave propagation problems, deterministic as well as stochastic, in multiscale media. The component waves are constructed in such a way that they account for the diffraction on local inhomogeneities. As a result, the method presented here describes propagation effects which pertain to nonzero values of the wave parameter of the local inhomogeneities embedded in the smoothly inhomogeneous background medium. The technique involves in a natural way the concepts of complex rays and complex caustics. In random problems the method describes the influence of diffraction by local random inhomogeneities on the field also in the near‐caustic areas. The method given is particularly suitable for solving HF wave propagation problems in the disturbed ionospher

ISSN:0048-6604    

DOI:10.1029/95RS02452

年代:1996

数据来源: WILEY

摘要:

The electromagnetic radiation due to a pulsed traveling wave current along a polygon wire structure located above a lossless half‐space is investigated. The Cagniard‐de Hoop technique with polar variables is used in order to derive closed form expressions for the electric field in the two‐media configuration. Numerical results for the electric field above a lossless half‐space are presented for a four wire polygon structure in which a known traveling wave current is

ISSN:0048-6604    

DOI:10.1029/95RS03105

年代:1996

数据来源: WILEY

摘要:

Refractometric sounding of the Earth's atmosphere using a multiorbit system of Global Positioning System (GPS) and low‐Earth orbit (LEO) satellites is modeled on the basis of data from the model of global atmospheric circulation (ECHAM3). The errors of the tomographic reconstruction of meteorological fields are investigated because of insufficient measured refractometric data. For a system of 100 LEO and 18 GPS satellites the estimates of errors of geopotential and temperature is obtained, which are 3—5 m and 0.3 K, respectively, in the pressure range 10—200

ISSN:0048-6604    

DOI:10.1029/95RS01353

年代:1996

数据来源: WILEY

摘要:

Large disparities in upper mesospheric winds measured with different radar techniques have been attributed earlier to the possibility that MF radars are sensitive to the phase speed of waves, rather than to true winds. An alternative explanation attributes these disparities to biases due to horizontal gradients in the wind field. These biases are common to the Doppler beam swinging (DBS), spaced antenna (SA), and imaging Doppler interferometry (IDI) methods and arise because of spatial filtering of the wind field by the beam configuration. We develop a theoretical model for intrinsic frequency and vertical wave number spectra of DBS wind component estimates that includes the effect of spatial filtering on an ensemble of gravity waves. The model shows a strong enhancement of frequency components above a knee, at ∼1 hour intrinsic period. A similar model for the SA experiment is inherently intractable. We suggest, instead, that the DBS model should also hold for the spectra of SA wind component estimates. The knee is clearly evident in recently observed SA wind component spectra, but is less pronounced and occurs at a frequency lower than the DBS model predicts. Better agreement is expected with a model refined to include the effect of Doppler shifts due to background winds. Occurrence of the knee in observed SA wind spectra implies that MF radars are sensitive to winds rather than to phase speed of atmospheric wave

ISSN:0048-6604    

DOI:10.1029/95RS02059

年代:1996

数据来源: WILEY

摘要:

We elsewhere suggest (Rastogi et al., this issue) that an analytic model for spectral distortion in the Doppler beam steering (DBS) horizontal wind estimates should also qualitatively hold for the imaging Doppler interferometry (IDI) and spaced antenna (SA) methods. Here we give further support for this suggestion through a statistical model for spectral distortion in the IDI horizontal wind estimates. In IDI the signals observed with a single or multiple receivers at ground are used to identify the locations and radial velocities of virtual atmospheric targets; wind components are then estimated by regression. The statistical model includes the effect of isotropic target configurations as a spatial filter on an isotropic ensemble of atmospheric gravity waves. It shows that the frequency spectrum of IDI horizontal wind estimates is enhanced beyond a knee, located at a ∼0.7‐hour period in the upper mesosphere, and then drops near the buoyancy frequency. The occurrence of the knee and its location is similar to the spectral distortion in the DBS horizontal wind estimates modelled earl

ISSN:0048-6604    

DOI:10.1029/95RS02058

年代:1996

数据来源: WILEY

摘要:

Simultaneous very long baseline interferometry (VLBI) and water vapor radiometer (WVR) measurements on a 21‐km baseline showed that calibration by WVRs removed a significant fraction of the effect of tropospheric delay fluctuations for these experiments. From comparison of the residual delay variations within scans and between scans, the total tropospheric contribution to the delay residuals for each of the three 5–20 hour sessions was estimated as 1%, 17%, and 10%, with the first value being very uncertain. The observed improvement in rms residual delay from WVR calibration during these three sessions was 4%, 16%, and 2%, respectively. The improvement is consistent with the estimated 2–3 mm path delay precision of current WVRs. The VLBI measurements of natural radio sources were conducted in April and May 1993 at Goldstone, California. Dual‐frequency (2.3 and 8.4 GHz) observations were employed to remove the effects of charged particles from the data. Measurements with copointed WVRs, located within 50 m of the axis of each antenna, were performed to test the ability of the WVRs to calibrate line‐of‐sight path delays. Factors which made WVR performance assessment difficult included the facts that (1) the level of tropospheric fluctuations was smaller than is typical for Goldstone during these experiments, and (2) VLBI delay variations on longer timescales (i.e., over multiple scans) contained uncalibrated instrumental effects (probably a result of slow temperature variations in the VLBI hardware) that were larger than the troposphe

ISSN:0048-6604    

DOI:10.1029/95RS02192

年代:1996

数据来源: WILEY