bluuebunny/crossref_metadata_2025 huggingface dataset:
Geoelectromagnetics charges on
Authors: Jiracek George R.
Published: February 2004

A continued resurgence of the use of electromagnetic (EM) methods in remarkably diverse geologic applications has occurred in the last four years (1991–1994). During this time EM methods in the United States have found a permanent place in major multidisciplinary continental dynamic projects, airborne EM has been successfully used by U.S. companies for diamond exploration in Canada's Northwest Territories, EM methods are now routinely used in environmental assessment, and renewed interest has occurred in seismoelectromagnetic monitoring. These activities have been accompanied by added emphasis on borehole EM, better on‐site processing and data presentation, improved inversion including fast (but lower resolution) imaging, and increased understanding of the possible causes of conductivity variations in the inaccessible regions of the Earth. This review is not meant to catalogue all EM geophysical efforts in the past four years. The emphasis will be on developments in the U.S.; however, the geophysics EM community is highly connected world‐wide. Many U.S. academic researchers and exploration companies have particularly close relations (e.g., joint research projects and exploration contracts) with Canadian and Mexican counterparts. Thus, it has been unavoidable, in fact necessary, to include references to many works published by our nearest North American neighbors. EM workshops held every two years under the sponsorship of the International Association of Geomagnetism and Aeronomy (IAGA) Working Group 1–3 strongly foster international cooperation. The workshop was held in Wellington, New Zealand in 1992 and it was in Brest, France in 1994. A pre‐meeting hands‐on, magnetotelluric (MT) data interpretation workshop was inaugurated prior to the Wellington meeting. All participants discussed analyses of common data sets distributed prior to the workshop. The success of this format prompted an equally successful activity in Cambridge, England before the Brest meeting.
The global circuit: Global thermometer, weather by‐product or climatic modulator?
Authors: Bering E. A.
Published: February 2004

This paper will review progress on the fair weather global circuit and its coupling to the upper atmosphere. The paper will confine its focus to work done by scientists working in the United States of America during the last four years. The global circuit is an old and familiar topic thought by many to have been well understood many years ago. The recent developments to be reviewed here suggest that this opinion is optimistic and possibly incorrect. Improvements in technology have permitted the deployment of new instruments with unprecedented capabilities that have produced data which have, in their turn, reopened a host of old questions and posed some fascinating new ones.
Dynamics of the core, geodynamo
Authors: Roberts Paul H.
Published: February 2004

“The mechanism for generating the geomagnetic field remains one of the central unsolved problems in geoscience.” So states the report on the National Geomagnetic Initiative (NGI) prepared by the U.S. Geodynamics Committee, et al [1993], with advice from the NGI Workshop held in Washington D.C. in March 1992. All analyses of the geomagnetic data point to the core as containing the source of the field and “The basic premise that virtually everyone accepts is that the Earth's magnetism is created by a self‐sustaining dynamo driven by fluid motions in Earth's core” (NGI, p.135). Dynamical questions at once arise, such as “What is the energy source driving those motions?” Jacobs [1953] proposed that the solid inner core (SIC) is the result of the freezing of the fluid outer core (FOC). Verhoogen [1961] noticed that the release of latent heat at the inner core boundary (ICB) during freezing would help drive thermal convection in the FOC, and Braginsky [1963] pointed out that the release of the light alloying elements during fractionation at the ICB would provide compositional buoyancy. These two sources suffice to supply the geodynamo with energy throughout geological time, even in the absence of dissolved radioactivity in the core [Braginsky and Roberts, 1994a; Kuang et al, 1994]. Stevenson [1991] argues that potential differences on the core‐mantle boundary (CMB) of electrochemical origin may be partially responsible for the geomagnetic field.
Seismic source modeling
Authors: Beroza Gregory C.
Published: February 2004

Seismic source modeling is taken to mean the modeling of seismograms to investigate kinematic properties of the earthquake source. Current trends such as the use of other data types (e.g. geodetic measurements of coseismic deformation) and the explicit consideration of source dynamics when modeling seismograms are beginning to blur this definition, but I will adhere to it in this review. I have divided seismic source modeling into teleseismic, regional, and near‐source modeling. Teleseismic modeling has the advantage of global coverage for earthquakes of magnitude 5.5 and larger, but does not provide much more than point‐source parameters. Regional‐distance modeling has the advantage of a more rapid assessment of earthquake source properties, but is complicated by relatively complex wave propagation. The coverage is also limited to areas of broadband regional networks ‐ typically continental areas of developed countries. Near‐source modeling has the potential for detailed imaging of source properties, but very few earthquakes occur in regions with sufficient instrumentation to allow this. I have also included a section on historical earthquakes because of the additional issues that seismologists face in working with pre‐WWSSN data and because of the important issues, such as earthquake recurrence, that require historical data. In addition to reviewing the progress in seismic source modeling during the last four years, I have also briefly reviewed some of the progress in understanding earthquake physics and hazards that has come about because of basic research in earthquake source modeling.
The iron hypothesis: Basic research meets environmental policy
Authors: Chisholm Sallie W.
Published: February 2004

In October 1993, an interdisciplinary group of scientists spread 445 kg of iron over a 64 km2 patch of surface ocean in the eastern equatorial Pacific. They monitored the patch for 9 days, comparing its biology and chemistry to the unfertilized surrounding waters. They found that the phytoplankton (microscopic plants which comprise the base of the food web in the oceans) were three times more productive in the patch than outside it. In other words, the cells were photosynthesizing—i.e., converting CO2 to organic carbon—at a significantly enhanced rate. The group concluded that iron was the limiting factor in this ecosystem, solving one of the more perplexing puzzles in biological oceanography in recent years. This event marked not only the first unenclosed ocean ecosystem experiment, but also the climax to a complex and fascinating evolution of ideas in biological oceanography. The climax was bittersweet, however, as John Martin, the designer of the experiment and originator of the hypothesis, died shortly before the expedition.
An introduction to the oceanography report
Authors: Weller Robert A.
Published: February 2004

This Oceanography Report marks a dramatic departure from past volumes. The past three reports included between 10 and 15 papers covering only physical and chemical oceanography. The papers were 15–20 pages in length and written as comprehensive summaries of the past four years' research in a specific area of either physical or chemical oceanography. The reference lists were extensive and complete. These papers provided excellent starting points for graduate students and oceanographers wishing to become current on the research being done in one of the areas covered.
An introduction to the Atmospheric Sciences Report
Authors: Nelson Stephan P.
Published: February 2004

The format of the U.S. National Report to the IUGG for the period 1991–1994 is substantially different from the previous report. Guidelines to the editors specified an increase in the number of articles and for the articles to be shorter with a more narrow focus than in previous years. Accordingly, the Atmospheric Sciences Section increased from twelve papers in the last report (including the separate atmospheric chemistry section) to seventeen. Additionally, considering the guidelines to authors with respect to page limitations, the articles could not be as comprehensive as in prior years and each author had to make a judgment as to which aspects of their subfield to pursue. Any limitations in the scope of their reviews are noted in their introductions. In one respect, the authors' tasks were somewhat easier in that their topics were more limited. This new format, however, also increased the possibility of overlaps or gaps between papers.
The geopause
Authors: Moore T. E. , Delcourt D. C.
Published: February 2004

Coupled to the Earth and protected by the geomagnetic field, terrestrial matter in the plasma state dominates a larger region of space than was suspected when the “space age” began, a region we refer to as the geosphere. Accelerated and heated by solar wind energy, this matter expands in size and increases in mass density in response to the Sun's ultraviolet spectrum, heliospheric conditions, and the occurrence of severe space storms. Such storms regularly damage spacecraft, interfere with communications, and trigger power grid interruptions or failures. They occur within the geopause region, that is, the volume defined by the limits of the instantaneous boundary between plasmas that are primarily heliospheric and geospheric. The geopause is analogous in some ways to the heliopause but also resembles the terrestrial air‐sea interface. It is the boundary layer across which the supersonically expanding solar plasma delivers momentum and energy to the terrestrial plasma and gas, exciting them into motion, “evaporating” them into space, and dissipating considerable amounts of power in thermal forms, while generating energetic particles through repeated storage and explosive release of electromagnetic energy. The intensity of the solar wind and the orientation of its magnetic field jointly control the strength of the coupling between solar and terrestrial plasmas and hence the occurrence of severe storms in the geopause region.
Seismology and the Test Ban: A new era
Authors: van der Vink Gregory E.
Published: February 2004

As many nations push for an indefinite extension of the Non‐Proliferation Treaty, international pressure for a Comprehensive Test Ban Treaty (CTBT) is simultaneously increasing. Advocates of a CTBT argue that a complete ban on nuclear testing by all nations will prevent the development of more advanced nuclear weapons, de‐emphasize the importance of nuclear weapons for national security, and reduce the discriminatory nature of the Non‐Proliferation Treaty regime. While it is generally recognized that nuclear testing is not a technical requirement for developing at least a simple fissile weapon, many argue that a CTBT will provide an unambiguous context in which other, more direct, restrictions can be implemented. In negotiating a CTBT, however, provisions must be made to ensure that the benefits of the treaty are not outweighed by undiscovered violations, should they occur. Because the development of such provisions requires value judgments as to the benefits of the treaty and the costs of undetected violations, verification has always been a sensitive issue for a CTBT [van der Vink and Paine, 1992]
Paleoclimatology and climate system dynamics
Authors: Overpeck Jonathan T.
Published: February 2004

The field of paleoclimatology, and more generally the broader field of “paleo‐science,” has seen substantial growth and increased recognition over the past decade. The prime motivation for these trends has been the need for a long‐term, or “paleo‐perspective” in understanding and assessing the environmental change that has been taking place and that will occur in the future. It is clear that climate varies substantially on all time scales, from seasons to millennia. It is also clear that the instrumental record of climate change is insufficient to observe and study how the climate system operates on time scales longer than a few decades, or under climatic forcing unlike that at present. The “paleo‐perspective” afforded by clever use of the paleoclimatic, paleoceanographic, and paleoecologic records can be tapped to put the present and future into the broad context of many realizations of past climate system dynamics. This paper reviews many of the important advances that have taken place recently in the field of paleoclimatology in a context illustrating the key role paleoclimatology plays in narrowing the uncertainties associated with predicting future change in the climate system. The American Geophysical Union has requested that this review focus on progress made by the U.S. scientific community, however many of the outstanding recent contributions in the field of paleoclimatology have been made by non‐U.S. scientists.
Basin‐scale hydrogeologic modeling
Authors: Person Mark , Raffensperger Jeff P. , Ge Shemin , Garven Grant
Published: February 2004

Mathematical modeling of coupled groundwater flow, heat transfer, and chemical mass transport at the sedimentary basin scale has been increasingly used by Earth scientists studying a wide range of geologic processes including the formation of excess pore pressures, infiltration‐driven metamorphism, heat flow anomalies, nuclear waste isolation, hydrothermal ore genesis, sediment diagenesis, basin tectonics, and petroleum generation and migration. These models have provided important insights into the rates and pathways of groundwater migration through basins, the relative importance of different driving mechanisms for fluid flow, and the nature of coupling between the hydraulic, thermal, chemical, and stress regimes. The mathematical descriptions of basin transport processes, the analytical and numerical solution methods employed, and the application of modeling to sedimentary basins around the world are the subject of this review paper. The special considerations made to represent coupled transport processes at the basin scale are emphasized. Future modeling efforts will probably utilize three‐dimensional descriptions of transport processes, incorporate greater information regarding natural geological heterogeneity, further explore coupled processes, and involve greater field applications.
Vertical velocities at Platteville, Colorado: An intercomparison of simultaneous measurements by the VHF and UHF profilers
Authors: McAfee J. R. , Gage K. S. , Strauch R. G.
Published: February 2004

Wind‐profiling Doppler radars measure the radial component of motion in the “clear atmosphere” along the radar beam. At lower VHF (e.g., 50 MHz) the backscattering process is highly anisotropic with maximum returns from stable layers near vertical incidence. By contrast, at UHF the backscattering process is very nearly isotropic. Because of these differences in scattering mechanism it is of interest to intercompare vertical velocities measured by collocated profilers, one operating at lower VHF and the other at UHF. An observational campaign was undertaken at Platteville, Colorado, to intercompare vertical motions measured by two collocated profilers. The VHF (50 MHz) wind profiler was operated in a vertical‐only mode interrupted every half hour for radio accoustic sounding system (RASS) measurements. The UHF (404 MHz) profiler was operated in low and high modes that are customary for the Demonstration Network of profilers in the central United States. Observations made by these two systems in October and November 1991 are intercompared. The overall agreement in vertical motions is quite good above 8 km with both profilers showing downward mean velocities in the lee of the Colorado Rocky Mountains as large as 6 cm s−1. Below 8 km the UHF velocities are systematically more downward than the VHF velocities. This systematic difference is attributed to the influence of hydrometeors on the UHF observations, as can be seen clearly in daily and hourly profiles.
Developments in UHF lower tropospheric wind profiling at NOAA's Aeronomy Laboratory
Authors: Carter D. A. , Gage K. S. , Ecklund W. L. , Angevine W. M. , Johnston P. E. , Riddle A. C. , Wilson J. , Williams C. R.
Published: February 2004

Developments in UHF profiling at 915 MHz are presented. Recent advances in UHF profiling are traced to early developments beginning about 8 years ago in the Aeronomy Laboratory at 915 MHz using microstrip antennas. This paper presents an overview of the architecture of the UHF profiler system as it has evolved over the past decade including the development of radio acoustic sounding system (RASS) capabilities. Hardware and software components are described and operational performance is summarized from experience gained from many installations, primarily from those in the tropics. Applications to wind profiling, boundary layer height determination, flux measurement, and precipitation profiling are considered.
First observations of equatorial spread F from Indian MST radar
Authors: Patra A. K. , Anandan V. K. , Rao P. B. , Jain A. R.
Published: February 2004

The newly established MST radar at Gadanki (13.5°N, 79.2°E) was operated on a few occasions in ionospheric field‐aligned backscatter mode to investigate the small‐scale (∼ 3 m) irregularities associated with the equatorial spread F. Observations are presented of sample Doppler spectra and height‐time variations of signal intensity and weighted mean Doppler velocity. The Doppler spectra are mostly of the composite type similar to that observed at Jicamarca and are quite variable both in height and time. The Doppler velocities of the 3‐m irregularities are found to be consistent in general with the slopes observed of the scattering structures in the height‐time‐intensity plots. The peak signal intensities are found to be as high as 40 dB above the mean noise level, and the peak Doppler velocities are around 70 m s−1.
Interpretation of VHF ST radar vertical echoes from in situ temperature sheet observations
Authors: Luce Hubert , Crochet Michel , Dalaudier Francis , Sidi Claude
Published: February 2004

The interpretation of the radar aspect sensitivity observed at VHF frequencies in the lower atmosphere is still a subject of controversy in the radar community. Indeed, scattering from anisotropic turbulence layers and partial reflection from stable thin horizontally stratified layers are generally proposed without leading to a definitive conclusion. A cause of this persistent discussion has been a lack of in situ high‐resolution observations which could identify and describe accurately the atmospheric structures at the origin of the radar aspect sensitivity. The Radars, Scidar and Balloons (RASCIBA 90) campaign (February‐March 1990, Aire sur l'Adour, France) was performed using simultaneously colocated VHF ST radars and balloon experiments. The objective was to obtain information about the small‐scale structures of the lower atmosphere and then to identify the origin of radar echoes. The main result was produced by the high vertical resolution (20 cm) temperature measurements. For the first time, very strong (positive) temperature gradients within thin layers were detected in the lower atmosphere (at least up to 27 km). Such an observation allowed then investigation of the partial reflection interpretation using available bifrequency radar measurements: a 45‐MHz radar for which typical 15‐dB vertical enhancement is observed just above the tropopause and a 72.5‐MHz radar for which an aspect sensitivity of the same order is presented for the first time at this frequency. A simple model considering flat and very extended sheets was assumed to estimate quantitatively the possible contribution of these temperature gradients to the VHF radar vertical power. Radar and reconstructed power reflection coefficient profiles are in good agreement in shape and in level showing that partial reflection from atmospheric sheets is an important and generally dominant process at vertical incidence. Furthermore, the effect of sheets of limited extent is investigated using simple theoretical considerations, and this model shows that the correction to the infinite sheet approximation is probably weak. The sheet distortion effect is also approximately evaluated applying Gaussian rough models. The power loss from the flat sheet case at VHF in the vertical direction is weak for a mean roughness height lower than a few tens of centimeters and for larger heights if the correlation length of the irregularities is larger than a few hundreds of meters. Sheet generation mechanisms are also briefly discussed using two previously published models: the viscosity wave model (Hocking et al., 1991) and “sheet and layer” model (Gossard et al., 1985). Neither of the models in their present forms seems to be able to account for the observations.
On the spaced antenna and imaging Doppler interferometer techniques
Authors: Vandepeer Brenton G. W. , Reid Iain M.
Published: February 2004

The theory developed by Briggs (1995) is extended to explain the measurement of a wind velocity equivalent to the spaced antenna apparent wind by imaging Doppler interferometer‐like (IDI‐like) interferometric analyses. The point is made that the presence of turbulence, or random change in the diffraction pattern on the ground, is directly responsible for the difference between the true and apparent velocities in the spaced antenna analysis, and also for the overestimation of the wind velocity by the IDI technique. An experiment is conducted to determine whether the “effective” scattering points resolved by IDI‐like analyses tend to lie upon the azimuth predicted by theory. This is found to be the case, even for unsmoothed cross spectra for which the theory was not explicitly derived. An averaging mechanism is proposed which may explain the extent to which the unsmoothed experimental data conform to the behavior predicted by theory.
Effective impedance of an irregular interface of media: Case of deterministic perturbations
Authors: Zhuck Nickolay P.
Published: February 2004

A rigorous equation is formulated and a small perturbation solution is found for the effective impedance operator of an irregular interface in an arbitrary linear stationary medium. Effective boundary conditions for an electromagnetic field are developed in the case of an arbitrarily shaped interface with slight deterministic roughness in a generally anisotropic inhomogeneous medium. Possible extensions of the results obtained are discussed.
Three‐dimensional satellite refractive tomography of the atmosphere: Numerical simulation
Authors: Gorbunov Mikhail E.
Published: February 2004

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 mbar.
The feasibility of radio sounding in the magnetosphere
Authors: Calvert W. , Benson R. F. , Carpenter D. L. , Fung S. F. , Gallagher D. L. , Green J. L. , Haines D. M. , Reiff P. H. , Reinisch B. W. , Smith M. F. , Taylor W. W. L.
Published: February 2004

A radio sounder outside the plasmasphere could provide nearly continuous remote density measurements of the magnetopause and plasmasphere, as well as other important density features elsewhere in this region. Using digital integration and tuned reception at frequencies from a few kilohertz to a few megahertz with 400‐m to 500‐m tip‐to‐tip dipole antennas and 10 W transmitter power, such a sounder would be capable of 10% density resolution and 500 to 1300 km spatial resolution in only a few minutes at distances of up to 4 RE. By providing such detailed observations of its principal density structures, such a sounder would then clearly revolutionize magnetospheric research.
Numerical simulation of scatterer positions in a very dense medium with an application to the two‐dimensional Born approximation
Authors: Siqueira P. R. , Sarabandi K. , Ulaby F. T.
Published: February 2004

Electromagnetic volume scattering theories for random media using a coherent field approach require some understanding of the statistical behavior of the locations of individual scatterers within the random medium. This knowledge manifests itself as the correlation function or pair distribution function, which is used to characterize the medium. This article presents a method for simulating the arrangements of particles of arbitrary shape within a very dense random medium, thus allowing for these functions to be calculated numerically. To demonstrate the significance of the Monte Carlo simulation, the problem of scattering from a two‐dimensional dense random medium is considered. Using a first‐order solution based on the Born approximation, it is shown that the approximate theoretically derived correlation function may lead to significant errors in the prediction of the backscatter response.
Monitoring saltwater injection using conductivity images obtained by electromagnetic cross‐hole measurements
Authors: Liu C. , Shen L. C. , Zhang Y. C.
Published: February 2004

An efficient noise‐tolerant, two‐dimensional conductivity‐imaging technique has been developed to invert the data measured by an electromagnetic (EM) cross‐hole tool. The method is successfully applied to image the field data measured by the Lawrence Livermore and Lawrence Berkeley National Laboratories at the Richmond Field Station at the University of California, Berkeley. The Richmond project aimed at monitoring the migration of salt water injected into a fresh water table. The cross‐hole distance is 20 to 26 m and a depth of 55 m is reached. A continuous wave EM system with an operating frequency of 18.5 kHz was used. A center injection well and four monitoring wells were drilled. After the first EM cross‐hole measurement between the center well and the four monitoring wells was made, 50,000 gallons (189,450 L) of salt water were injected from the center well at a depth of 26 to 30 m where a fresh water table is located. Another set of EM cross‐hole data was obtained after the saltwater injection. The migration of salt water can be clearly seen from the EM tomographs. The misfit between the measured data and computed data at the final iteration is less than 7% for all the conductivity tomographs reconstructed. The inversion method is a two‐dimensional full‐wave inversion process with an assumption of axial symmetry about the transmitter well. An iterative image reconstruction algorithm is used to reduce the computational complexity. Inversion is regularized by applying a maximum entropy constraint. A distorted Born approximation is employed in which a layered background is allowed. The inversion starts with an estimated homogeneous formation and inverts the homogeneous average conductivity first. Then, a layered conductivity image is constructed using the homogenous data as the initial value. Finally, a two‐dimensional conductivity image is reconstructed iteratively using the one‐dimensional image as the initial input. Conductivity tomographs obtained from Richmond site show that there is a high‐conductivity layer at the depth of 17 m with a conductivity approximately equal to 0.25 S/m, which is not shown in the resistivity logs. The formation around the transmitter well is a layered formation with relatively low conductivity. The average conductivity of the test field is approximately 0.05 S/m. The conductivity of the formation near the lower portion of the test field keeps a low value of less than 0.03 S/m. The success of the EM cross‐hole measurement and the inversion revealed the potential of using such a device to detect contaminated soil and monitor the migration of contaminants in the ground.
Spectral plasma lines at Sondrestrom
Authors: Showen Robert L.
Published: February 2004

A dynamical ionosphere is depicted using spectral analysis of the plasma line echo from the incoherent scatter radar at Sondrestrom, Greenland. F region plasma lines have been observed between 8.5 and 12 MHz, and E region plasma lines between 3 and 6 MHz. Accurate measurements of cutoff edges at the F layer peak and the E/F region valley have been made. Complexities in the plasma line spectra near the cutoff are related to microstructure within the radar interrogation volume. An observed asymmetry in the up‐ and downshifted cutoff frequencies may allow vertical electron velocity measurements and hence determination of field‐aligned currents. Variations in the cutoffs over time and during moving‐beam experiments have shown many passing disturbances, some of which are too small to detect using ion line returns. This technique can be applied to atmospheric gravity wave research and to the morphology of the complex high‐latitude ionosphere.
Wave propagation in moving chiral media: Fizeau's experiment revisited
Authors: Ben‐Shimol Yehuda , Censor Dan
Published: February 2004

The Fizeau experiment is discussed as a concrete example for investigating wave propagation in nonsimple moving media. Exact special relativistic formalism is used throughout, and first‐order approximations are developed from the exact forms. No Doppler frequency shifts occur to an observer in the laboratory frame of reference, because in Fizeau's experiment the moving fluid is contained within stationary boundaries. Consequently, only phase shifts are measurable. The results show that in order to measure the velocity effects from the interference fringes, one has to adequately modify the construction of the original Fizeau experiment. A relativistically exact model for first‐order in velocity was developed for the chiral medium giving a relatively simple formalism and enabling an easy solution to propagation and scattering of electromagnetic waves in the presence of moving chiral media.
Distortion of gravity wave spectra of imaging Doppler interferometry horizontal winds
Authors: Rastogi Prabhat K. , Kudeki Erhan
Published: February 2004

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 earlier.
Neural networks for several ionogram features
Authors: Hill J. N. , Koschmieder T. H.
Published: February 2004

Several demonstration neural networks will be described that identify certain characteristics of ionograms. The characteristics identified are ordinary and extraordinary F region critical frequency, F1 cusp frequency, sporadic E type C and type H cusp frequencies, and spread F. Accuracy for the neural networks varies, ranging from 50% to 90%. The ionograms come from a midlatitude ionosonde station.
Phase calibration of VHF spatial interferometry radars using stellar sources
Authors: Palmer R. D. , Vangal S. , Larsen M. F. , Fukao S. , Nakamura T. , Yamamoto M.
Published: February 2004

This work describes a method for phase calibration of spatial interferometry (SI) systems, which have recently seen increasing use in the wind profiling community. The various available techniques are inherently dependent on any phase imbalance in the system. The imbalance is caused by a path length difference between the multiple receivers, which is typically corrected at the time of construction. Aging of the system and other environmental changes can cause a modification in the path lengths, however. An obvious method for calibrating an SI system involves examining the phase differences between the receivers for a signal produced by a source with known location. For physically small systems, this can be accomplished by pointing the antenna beam toward a man‐made source, such as a simple transmitter with the same center frequency as the radar. For large MST (mesosphere/stratosphere/troposphere) radar systems, such as the Middle and Upper (MU) Atmosphere radar, this method is impractical. The present work describes the calibration of the MU radar using the radio star Cygnus A as a source of known location. As the radio star traverses the beam, the MU radar passively tracks it by measuring the phase differences of the cosmic noise signal received on spatially separated receivers. Since the location of Cygnus A is known from astronomical calculations, any phase imbalance can be compensated. Data from May 1992 show the usefulness of the method.
Asymptotic ray theory for transient diffusive electromagnetic fields
Authors: de Hoop Adrianus T. , Oristaglio Michael L. , Habashy Tarek M. , Torres‐Verdin Carlos
Published: February 2004

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 prospecting.
Physical‐mathematical model of the dynamics of rain attenuation based on rain rate time series and a two‐layer vertical structure of precipitation
Authors: Matricciani Emilio
Published: February 2004

We have developed and discussed the theory and applications of a physical‐mathematical model of the dynamics of rain attenuation and have tested it as a rain attenuation prediction model in slant paths. Other parameters, however, such as fade durations and rates of change of fades, can be calculated. The main physical input is the 1‐min rain rate time series of a site, which is converted to a rain rate space series along horizontal or slant paths by using an estimate of the storm translation speed v method known as “synthetic storm technique.” However, the long‐term predictions are found to be insensitive to v. The vertical structure of precipitation is modeled with two layers. The model was tested against the probability distributions of rain long‐term 11.6‐GHz attenuation collected at the three Italian stations (Fucino, Gera Lario, and Spino d'Adda) during the SIRIO propagation experiment (13 years of data) for which concurrent rain rate time series are available. In the outage probability range 10−1 to 5×10−3% defined the prediction error ε = ( Ap − Am) / Am (where Am and Ap are respectively, the measured and predicted rain attenuations, dB), <ε> = −10.6%, σ=7.6% and rms=13%. Compared to nine other well‐known prediction methods, the present model surmounts all of them in the three sites tested.
Scattering by a narrow groove in an impedance plane
Authors: Bindiganavale Sunil S. , Volakis John L.
Published: February 2004

An analysis of two‐dimensional scattering from a narrow groove in an impedance plane is presented. The groove is represented by an impedance surface and hence the problem reduces to that of scattering from an impedance strip in an otherwise uniform impedance plane. On the basis of this model, appropriate integral equations are constructed using a form of the impedance plane Green's functions involving rapidly convergent integrals. The integral equations are solved by introducing a single‐basis representation of the equivalent current on the narrow impedance insert. Both transverse electric and transverse magnetic polarizations are treated. The resulting solution is validated by comparison with results from the standard boundary integral method and a high‐frequency solution. It is found that the presented solution for narrow impedance inserts can be used in conjunction with the high‐frequency solution for the characterization of impedance inserts of any given width.
An electrothermodynamic model with distributed properties for effective permittivities of sea ice
Authors: Nghiem S. V. , Kwok R. , Kong J. A. , Shin R. T. , Arcone S. A. , Gow A. J.
Published: February 2004

This paper presents a model to calculate the temperature dependence of effective permittivities for sea ice, a heterogeneous medium containing multiphase scatterers. With the strong permittivity fluctuation approach the model accounts for the electrodynamic scattering effect together with the quasi‐static characteristics of multiple species and subspecies of inhomogeneities with distributed orientations, sizes, and shapes. Because of a preferential direction in the orientation distribution, the medium is effectively anisotropic. The size distribution is described with a probability density function in terms of normalized volumetric sizes. Scatterer shapes are nonuniform and have a general ellipsoidal form characterized by arbitrary axial ratios of correlation lengths which are related to physical geometries of the scatterers. In this formulation, sea ice consisting of solid ice, liquid brine, and gaseous inclusions is modeled to derive effective permittivities with thermodynamic phase redistribution and structural metamorphism. Theoretical results are in good agreement with experimental data at the C band frequency of 4.8 GHz for saline ice undergoing warming and cooling cycles. A competitive effect between the increase of liquid brine and the shape rounding of ellipsoidal scatterers at increasing temperatures explains the trend observed in measured data. Sensitivities of effective permittivities to structural and physical parameters characterizing sea ice are also studied.
Comparison of coherent and noncoherent polarimetric radar measurement techniques at 95 GHz
Authors: Mead James B. , Pazmany Andrew L. , Chang Paul S. , McIntosh Robert E.
Published: February 2004

Polarimetric radar measurements can be made either coherently, by measuring covariances of the various elements of the complex scattering matrix, or noncoherently, by measuring a series of scattered Stokes vectors associated with a set of transmit polarizations. The coherent approach can be difficult to apply at millimeter wavelengths, where rapid decorrelation of the scattered signal can cause errors in the measured covariance terms. The noncoherent method is not adversely affected by rapid decorrelation but requires a more complex transmitter to generate multiple polarizations. To compare these two techniques, a 95‐GHz polarimetric radar was recently modified to simultaneously measure the Mueller matrix using the coherent and noncoherent approaches. Backscatter measurements of foliage are presented showing the conditions under which the coherent measurements are in error. Finally, we show how to reduce motion‐induced errors associated with the coherent technique when sampling a rapidly decorrelating scene.
Quaternary deformation in the Eastern Pamirs, Tadzhikistan and Kyrgyzstan
Authors: Strecker M. R. , Frisch W. , Hamburger M. W. , Ratschbacher L. , Semiletkin S. , Zamoruyev A. , Sturchio N.
Published: February 2004

Active deformation in the eastern Pamir of Central Asia is concentrated on the margins of the orogen with minor deformation within the high terrain. Along the Trans‐Alai mountain front at the northern perimeter of the orogen, Quaternary thrusting is documented by uplifted pediments, now at >500 m above the piedmont, Holocene fault scarps, and large earthquakes with N to NW oriented P axes. Seismicity in the interior of the orogen outlines a N–S belt that includes normal faulting events with E–W oriented T axes. N–S striking, active normal faults in the interior Lake Karakul region are compatible with these earthquakes; they define an asymmetric graben with a master fault at the western basin margin. In the southern Pamirs, dextral strike‐slip faults root in the dextral Karakorum Fault, which bounds the Pamirs to the east. A mixture of dextral and reverse offsets totalling 135 m in Pleistocene terraces and 8 m in late Pleistocene/Holocene deposits demonstrates contemporary transpression, indicating average displacement rates of <1 mm/yr. The concentration of young thrusts along the Trans‐Alai, the northward migration of thrusting, and the scarcity of other large‐scale shortening features within the eastern Pamirs suggest that this part of the orogen moves northward en bloc and causes the progressive annihilation of the intermontane Alai Valley. Widespread dextral shear in the eastern Pamirs, both to the south and north of the extensional Karakul depression, and combined dextral strike‐slip and normal faulting in the Muji‐Tashgorgan graben of the Chinese Pamirs are interpreted as localized space accommodation phenomena, formed during progressive transfer of compressional deformation along a dextral strike‐slip deformation zone with extensional stepovers.
Internal fabric and strike‐slip emplacement of the Pan‐African granite of Solli Hills, northern Nigeria
Authors: Ferré E. , Gleizes G. , Bouchez J. L. , Nnabo P. N.
Published: February 2004

The Solli Hills pluton (250 km2) in northern Nigeria is an example of a Pan‐African granite emplaced circa 580 Ma ago in the Jos ‐ Adamawa block. Its micro structures and internal fabrics have been studied in order to help constrain the kinematics of the Pan‐African orogeny in this block. We used field observations and measurements, petrographic examinations, and measurements of the magnetic susceptibility and anisotropy of magnetic susceptibility of 124 regularly spaced stations. Most samples are ferromagnetic, i.e., belong to the magnetite‐series granites, and present predominantly magmatic microstructures. Particularly remarkable are the asymmetrical zoning of the petrographic types, the susceptibility magnitudes, and the planar ‐ linear magnetic fabrics. The magnetic foliations and lineations are very consistent in the pluton, around N 20°E, 60°W and N 205°E, 10°, respectively. Field and microscopic observations reveal that deformation occurred everywhere in the pluton and immediate country‐rock envelope during a dextral strike‐slip synplutonic episode. It is suggested that the Solli Hills pluton was emplaced in a late Pan‐African dextral tear‐fault system. The striking fabric asymmetry of the pluton strongly suggests a shear zone termination emplacement model.
Silurian plutonism in the Trinity terrane (Neoproterozoic and Ordovician), Klamath Mountains, California, United States
Authors: Wallin E. Timothy , Coleman Drew S. , Lindsley‐Griffin N. , Potter A. W.
Published: February 2004

New data indicate that the Trinity terrane of northern California is a polygenetic composite terrane rather than a single slice of oceanic lithosphere. We suggest approximately one third of the Trinity terrane consists of Silurian intrusive rocks that represent the roots of a previously unrecognized Silurian magmatic arc. Crosscutting relations and U‐Pb zircon isotopic data document at least one early Paleozoic deformation in the Trinity terrane of northern California. A ductile shear zone between Neoproterozoic metagabbro and Ordovician(?) harzburgite is intruded by the Upper Silurian China Mountain pluton. This evidence indicates a major early Paleozoic shear zone formed in the eastern Klamath Mountains after the Middle Ordovician but prior to Late Silurian plutonism.
Along‐strike Acadian structural variations in the Québec Appalachians: Consequence of a collision along an irregular margin
Authors: Malo Michel , Tremblay Alain , Kirkwood Donna , Cousineau Pierre
Published: February 2004

The Québec Appalachians have been shaped by two major orogenies: the Middle to Late Ordovician Taconian and the Middle Devonian Acadian. Thrust faults and nappe structures characterized the Taconian deformation throughout the Québec Appalachians, whereas structural styles pertaining to the Acadian orogeny differ from southern Québec to the Gaspé Peninsula. Structural analysis of Late Ordovician to Middle Devonian supracrustal rocks shows two different Acadian deformational regimes: strike‐slip tectonics in the Gaspé Peninsula and thrust or dip‐slip tectonics in southern Québec. In southern Québec, Acadian regional deformation produced folds which vary from upright to overturned and tight to isoclinal from the NW to the SE. Major faults (e.g., La Guadeloupe) are northwestward directed thrust faults marked by highly ductile shear zones outlined by mylonite. In the Gaspé Peninsula, structural trend is NE and major E–W dextral strike‐slip faults transect this trend. Folds are open and upright, inclined and tighter near the major faults where they have a clockwise rotation like the regional cleavage. Major faults (e.g., Grand Pabos) follow corridors delimiting high‐strain zones where the fabrics developed are indicative of a ductilebrittle regime of deformation. In the Témiscouata region, major structural features of both southern Québec and the Gaspé Peninsula are recognized. Structures of this region consist of ENE trending dextral strike‐slip faults and high‐angle WNW verging reverse faults, parallel to major NNE folds. These structures reflect the transition from purely horizontal movement along major strike‐slip faults of the Gaspé Peninsula to vertical movement along thrust faults in southern Québec. Acadian structural variations within the Québec Appalachians are interpreted in terms of a continental collision of Gondwana along the irregular margin of Laurentian and its Taconian accreted terranes.