bluuebunny/crossref_metadata_2025 huggingface dataset:
A method for improved microwave‐interferometer remote sensing of convective boundary layer turbulence using water vapor as a passive tracer
Authors: Jacobson Abram R. , Sramek Richard
Published: February 2004

Water vapor column irregularities within the lower troposphere's convective boundary layer constitute a nearly passive tracer of turbulent air motions and eddy convection. Remote sensing of turbulence in this layer from the ground is done with microwave radars and with lidars. Microwave interferometry is also sensitive to vapor column fluctuations via the known relative refractive delay of water vapor compared to dry air. However, interferometric measurements are intrinsically nonlocal, yielding only the baseline difference (the difference of measurements at two ends of an interferometer baseline) of vapor column, and thus are not amenable to classical data‐reduction methods for fluid turbulence, other than the equal‐time structure function. In order to remove this limitation of interferometry we have developed a method to estimate the time‐lagged cross covariance of vapor‐column fluctuations over two sites using interferometry data. This effectively circumvents the limitation to baseline‐differenced vapor column data at least in statistical second moments. The method is illustrated with 15‐GHz data taken by the very large array.
An explanation for the absence of sounder‐stimulated gyroharmonic resonances in the Io plasma torus by the Ulysses relaxation sounder
Authors: Benson R. F. , Fainberg J. , Hess R. A. , Osherovich V. A. , Stone R. G.
Published: February 2004

Plasma resonances were excited by the relaxation‐sounder component of the unified radio and plasma wave instrument (URAP) during the Ulysses encounter with Jupiter's Io plasma torus. The observed resonant spectra were quite different than expected, based on earlier published observations from topside sounders in the terrestrial ionosphere and relaxation sounders in the terrestrial magnetosphere, in that the URAP sounder spectra did not contain resonances at the harmonics of the electron gyrofrequency (which are commonly excited in the ionosphere and magnetosphere). Arguments are presented here to support the earlier suggestion that the absence of sounder‐excited gyroharmonic resonances in the Io plasma torus was due to the relatively long receiver dead time and to the inability to satisfy the required echo rendezvous conditions.
Cross correlations of fields scattered by horizontally anisotropic refractive index irregularities
Authors: Holloway Christopher L. , Doviak Richard J. , Cohn Stephen A.
Published: February 2004

In this paper we present an analytic expression for the cross‐correlation function of signals received by spaced antennas for scattering from an atmosphere with horizontally and/or vertically anisotropic scattering irregularities. We demonstrate that our expression is equivalent to the one Briggs [1984] obtained through heuristic arguments; we also show how the parameters in Briggs's expression are related to parameters that describe (1) the orientation, shape, and size of the scattering irregularities; (2) the turbulent flow; and (3) the radar. Finally, we introduce an alternative approach (different from Briggs's full correlation analysis) for estimating horizontal winds and turbulence.
Discrete inverse theory for 834‐Å ionospheric remote sensing
Authors: Picone J. M. , Meier R. R. , Kelley O. A. , Melendez‐Alvira D. J. , Dymond K. F. , McCoy R. P. , Buonsanto Michael J.
Published: February 2004

In the near future a number of global, multiyear, satellite‐borne ultraviolet remote sensing missions will scan or image the limb of the Earth to measure the O+ concentration within the ionospheric F layer. On the day side we will use discrete inverse theory (DIT) to retrieve vertical profiles of the O+ number density from measurements of the 834‐Å airglow. Here we describe the theoretical foundations and the components of the retrieval code, which computes both a maximum likelihood solution and the associated covariance matrix. New results include enhancement of the DIT formalism to distinguish between random and nonrandom (“systematic”) errors and comparisons of Millstone Hill incoherent scatter radar (ISR) data with generalized Chapman‐type representations of the O+ density profile. Of the candidates, the Chapman‐type profile with a constant‐gradient scale height provides the best fits to the ISR data.
Bandwidth occupancy profile of mid‐Atlantic HF band users
Authors: McNeal G. D.
Published: February 2004

Emerging technologies that use spread spectrum and frequency hopping signals may enable more efficient use of the high‐frequency (HF) band. High‐powered HF radar is an example of this technology where channels are used that appear to be unoccupied and the frequency hopping signal of the HF radar has been remarkably free from interfering with conventional spectrum occupants. Considering future application of HF systems of this type, it is of interest to examine the availability of free channels in the frequency range between 3 and 30 MHz for bandwidth (BW) availability and time duration. This paper discusses the characteristics of channels that are available for use in the HF spectrum based on spectrum measurements by the Navy's Relocatable Over‐the‐Horizon Radar (ROTHR) located in the southeast section of Virginia. In particular, the ROTHR data being analyzed are from the spectrum monitor (SM). The SM is a subsystem of the ROTHR which measures the power level in 3‐kHz intervals for frequencies between 5 and 28 MHz, the radar's operating frequency range. Measurements of the HF spectrum were analyzed for the fall and winter seasons at the radar's location. The analysis consisted of estimating the number of clear channels of various widths in selected megahertz bands over the ROTHR's operating frequency range. Characteristics of available channels are presented in various graphical forms which include clear channel availability, probability of at least one clear channel of a specified width, and duration of clear channels as a function of frequency. The Weibull distribution is proposed as an analytical model for investigating channel duration. Analysis results indicate which portion of the HF spectrum is most likely to have available bandwidths and what their expected durations are as a function of season and time of day.
Simulation studies of parametric decay processes associated with ionospheric stimulated radiation
Authors: Hussein A. A. , Scales W. A.
Published: February 2004

Parametric instability processes are thought to produce stimulated electromagnetic emissions during ionospheric heating experiments. The phenomenon is primarily attributed to plasma turbulence excited by the high‐frequency heater in the altitude region where the pump frequency ω0 is near the plasma upper hybrid frequency ωuh. In this study, parametric instability processes thought to produce ionospheric stimulated radiation are studied using electrostatic particle‐in‐cell simulation models. The emphasis in this work will be to consider the excitation and interactions between electrostatic waves generated by the pump wave. The simulation plasma is driven with a uniform oscillating electric field directed nearly perpendicular to the background geomagnetic field B to consider interactions when ωuh is near electron cyclotron harmonics nΩce. The pump frequency and amplitude are varied to consider the effects on the simulation electric field power spectrum. Also, the effect of varying the electron to ion temperature ratio Te/Ti is discussed and evaluated. The simulation results show richly structured sidebands upshifted and downshifted above and below the pump frequency, which are attributed to parametric instability processes. The relationship between the simulation results, the theoretical models, and the experimental spectrum are discussed.
Analytic solution for low‐frequency magnetic induction in an equatorially stratified sphere: Horizontal excitation
Authors: Dawson Trevor W.
Published: February 2004

This paper considers the analytical solution to the problem of low‐frequency induction, by quasi‐uniform magnetic fields parallel to the equatorial plane, in an equatorially stratified sphere having the particular conductivity distribution σ(ϕ) = σ0e−λ cos(ρϕ), where ρ ∈ {1,2} is a periodicity factor, σ0 is a conductivity amplitude factor, and λ > 0 is a dimensionless conductivity contrast parameter. This distribution has ρ conductivity maxima and minima as a function of equatorial angle. The resulting induced electric and current density fields are fully three‐dimensional and exhibit interesting, yet physically reasonable, behavior. Most noticeable are current vortices associated with each conductivity maximum. These solutions are supplementary to one previously published for the magnetic source vector perpendicular to the equatorial plane and so complete the solution for induction in the sphere by quasi‐uniform magnetic sources. The nature of the solution is illustrated for the case of both singly and doubly periodic conductivity distributions, for two orthogonal source directions.
First mesospheric observations using an imaging Doppler interferometer adaptation of the dynasonde at Halley, Antarctica
Authors: Jones G. O. L. , Charles K. , Jarvis M. J.
Published: February 2004

The digital ionospheric sounder, or dynasonde, located at the British Antarctic Survey's research station at Halley, Antarctica, has recently been operated as an imaging Doppler interferometer (IDI). The implementation of this mode of operation does not require any changes to the system hardware but involves a specially written sounding configuration and newly designed data analysis software. Using four independent antennas and two receivers, the signals returned from mesospheric altitudes are range‐gated and processed using Doppler sorting and spatial interferometry. A three‐dimensional (3‐D) image of the scattering locations can then be built up for a region stretching to over 30 degrees from the zenith. The sky map locations and Doppler velocities of the individual scattering points are used to fit a 3‐D velocity vector representing the motion of the neutral wind in the mesosphere. In this paper the radar hardware, sounding configuration, and data processing are described, and the relative merits of the IDI technique as applied at Halley are discussed. The characteristics of the scattering points are outlined, and initial neutral wind measurements are presented.
Remotely sensing cloud properties from microwave radiometric observations by using a modeled cloud database
Authors: d'Auria Giovanni , Marzano F. S. , Pierdicca N. , Nossai R. Pinna , Basili Patrizia , Ciotti Piero
Published: February 2004

As a first step for remote sensing cloud properties, a database of cloud genera has been established. This is derived from a microphysical model, and it considers the statistical profiles of four hydrometeor species for each cloud genus. From this database the corresponding radiative database is obtained making use of a radiative transfer model, so for each cloud genus the simulated microwave response at the special sensor microwave imager channels is found. The cloud and radiative databases allow the retrieval of the genera of the cloud and other relevant properties from satellite observations. An automatic cloud genus classifier has also been implemented. Several tests have been carried out, and the results are presented.
First‐order propagation in moving chiral media
Authors: Ben‐Shimol Yehuda , Censor Dan
Published: February 2004

The first‐order propagation (in v/c) of electromagnetic waves in moving isotropic chiral media is studied. The present analysis is based on Maxwell equations, Bohren decomposition, and Minkowski's relations taken to the first order in the velocity. Accordingly, the effect of acceleration on the electromagnetic fields is neglected, thus yielding a new model for solving problems involving moving chiral media in the presence of arbitrary irrotational velocity and stationary boundaries. This new model is demonstrated for a simple and constructive example, that is, transmission and reflection of a plane wave from a plane boundary separating a stationary simple medium and a moving chiral medium.
The two‐dimensional resolution kernel associated with retrieval of ionospheric and atmospheric refractivity profiles by Abelian inversion of radio occultation phase data
Authors: Ahmad Bilal , Tyler G. Leonard
Published: February 2004

Errors in radio occultation experiments associated with departures from sphericity can be analyzed on the basis of straight‐ray tomography in which (1) only one parallel projection of the object is available, (2) diffraction is neglected, and (3) inversion is achieved by assuming the object to be circularly symmetric. The straight‐line tomographic case and the actual curved ray paths in a spherically symmetric atmosphere are related by an analytic transformation. A previously unknown kernel function transforms the two‐dimensional (2‐D) atmospheric refractivity map from the plane of propagation to a 1‐D radial profile along the locus of ray periapsides. This function consists of a discrete, positive singularity plus an extended negative branch; the negative branch rises smoothly from negative infinity in the neighborhood of the positive singularity toward zero. Knowledge of the resolving kernel allows study of horizontal and vertical resolution in a rigorous manner. Spherically symmetric structures are reproduced exactly with the kernel; processing of band‐limited projection data results in a modified kernel of finite vertical resolving power. In general, the horizontal extent of significant contributions, including levels of 10% of the peak, is of the order 2√4.6rH centered on the locus of periapsides, where r is the radius of the point in consideration and H is the refractivity scale height; the vertical extent of significant contributions extends from approximately one sampling distance below r to a few sampling distances above it. We confirm the expected result that a refractivity structure of radial thickness Δr will be retrieved properly only if its circumferential extent is approximately 2√2rΔr or greater centered on the periapsis point. In addition, a positive refractivity structure limited in circumferential extent introduces artifacts of negative refractivity in the retrieved profile. The artifacts first appear at the projected altitude onto the vertical of the structure's circumferential edges and continue downward. The error represented by such negative refractivity artifacts is diminished somewhat by the general exponential increase in refractivity with decreasing altitude of a real atmosphere. Thus the relative error depends on the local scale height and the circumferential extent of the refractivity structure.
High‐resolution studies of atmosphere‐ionosphere coupling at Arecibo Observatory, Puerto Rico
Authors: Djuth F. T. , Sulzer M. P. , Elder J. H. , Wickwar V. B.
Published: February 2004

Very accurate measurements of electron density can be made at Arecibo Observatory, Puerto Rico, by applying the coded long‐pulse (CLP) radar technique [Sulzer, 1986a] to plasma line echoes from daytime photoelectrons [Djuth et al., 1994]. In the lower thermosphere above Arecibo, background neutral waves couple to the ionospheric plasma, typically yielding ∼1–3% electron density “imprints” of the waves. These imprints are present in all observations made to date; they are decisively detected at 30–60 standard deviations above the “noise level” imposed by the measurement technique. Complementary analysis and modeling efforts provide strong evidence that these fluctuations are caused by internal gravity waves. Properties of the neutral waves such as their period and vertical wavelength are closely mirrored by the electron density fluctuations. Frequency spectra of the fluctuations exhibit a high‐frequency cutoff consistent with calculated values of the Brunt‐Väisälä frequency. Vertical half wavelengths are typically in the range 2–25 km between 115‐ and 160‐km altitude, and the corresponding phase velocities are always directed downward. Some waves have vertical wavelengths short enough to be quenched by kinematic viscosity. In general, the observed electron density imprints are relatively “clean” in that their vertical wavelength spectrum is characteristically narrow‐banded. It is estimated that perturbations in the horizontal wind field as small as 2–4 m/s can give rise to the observed electron density fluctuations. However, the required wind speed can be significantly greater depending on the orientation of the neutral wave's horizontal wave vector relative to the geomagnetic field. Limited observations with extended altitude coverage indicate that wave imprints can be detected at thermospheric heights as high as 500 km.
Stochastic inversion in ionospheric radiotomography
Authors: Nygrén Tuomo , Markkanen Markku , Lehtinen Markku , Tereshchenko Evgeny D. , Khudukon Boris Z.
Published: February 2004

The stochastic inversion method in ionospheric radiotomography is reviewed with a special emphasis on regularization used in the inversion process. Regularization is used both for preventing vigorous point‐to‐point oscillations and for controlling the peak altitude and thickness of the inversion result. The latter usually means importing a priori information on the layer height and thickness to the solver. In this paper it is pointed out that due to the curvature of the Earth and of the ionosphere, the measurements contain some information on the ionospheric altitude and the profile shape even in the case of a purely horizontally stratified layer. If this information could be used in choosing an appropriate regularization, no additional information would be needed. Simulation tests are presented which indicate that the altitude of a horizontally stratified layer can be determined with a reasonable accuracy without any a priori information. An attempt is also made to use the data for determining the shape of a proper regularization profile. Although some success is achieved in this effort, it is concluded that available a priori information, for example, ionosonde or incoherent scatter measurements, should be used in choosing the regularization profile. The ideas are tested with true data obtained from difference Doppler measurements carried out in Scandinavia, and the results are compared with simultaneous observations made by the European incoherent scatter radar. The comparison shows a reasonable agreement, although clear discrepancies also occur, for instance, in the shape of the bottomside profile.
A model‐independent algorithm for ionospheric tomography: 1. Theory and tests
Authors: Fehmers G. C. , Kamp L. P. J. , Sluijter F. W. , Spoelstra T. A. T.
Published: February 2004

This paper presents a model‐independent algorithm for tomography of the ionosphere. Prior knowledge consists of the following pieces of information: electron density cannot be negative, the ionosphere is basically smooth and stratified, and electron density is low at high (≳700 km) and low (≲100 km) altitudes. Tests based on simulated measurements show that the method recovers the latitudinal structure well, whereas the vertical structure is recovered with moderate success: the estimated height of the layer of maximum electron density may be as much as 90 km in error. Because of the imposed smoothness the method tends to underestimate the peak in electron density by as much as one third in unfavorable cases.
A model‐independent algorithm for ionospheric tomography: 2. Experimental results
Authors: Fehmers G. C. , Kamp L. P. J. , Sluijter F. W. , Spoelstra T. A. T.
Published: February 2004

This paper presents the results of an ionospheric tomography experiment over midlatitude Europe. The campaign yielded 539 reconstructions over a 45‐day period in spring 1995. Special attention is paid to the performance of the reconstruction algorithm, specifically by comparison with ionosonde data. Tomography and soundings tend to agree on electron density, but they do not agree on the height of maximum electron density: tomography seems to overestimate Hmax. Whereas tomography finds a daily oscillation in Hmax between 250 km (daytime) and 450 km (nighttime), the soundings indicate an oscillation between 200 and 250 km.
Systematic analyses of the large‐scale topography and structure across the Dead Sea Rift
Authors: Wdowinski Shimon , Zilberman Ezra
Published: February 2004

The Dead Sea Rift (DSR) is one of the deepest continental depressions on the Earth's surface and is the best example of a continental rift lying along a transform plate boundary (the Dead Sea Transform). We systematically analyze the large‐scale topography, structure, and morphology across the central part of the DSR between Lake Kinneret and the Gulf of Elat and show a distinct asymmetrical topographic pattern across the rift axis. The topography analysis uses a Digital Terrain Model (DTM) of Israel and adjacent areas to plot a series of 64 profiles perpendicular to the rift axis. The profiles show that the eastern side is topographically higher than the western side and that its overall shape resembles an uplifted shoulder; the lower western side resembles an arch. This analysis also reveals along‐strike variations in the topography that allow us to subdivide the central DSR into five segments of similar topography. The large‐scale structure across the DSR is investigated by a series of 10 geological cross sections drawn perpendicular to the rift axis along the five segments. On the basis of the stratigraphic record and the geological history of the region, we identify a regional marker (Top Eocene Sequence) to trace the rift‐related structure. This marker shows that the structure parallels the topographic asymmetry across the rift axis: the rift's eastern margin is uplifted toward the axis, whereas the rift's western margin is downflexed toward the axis and defines a wide asymmetrical monocline. Our analyses indicate that (1) the large‐scale asymmetry across the DSR reflects a wide half‐graben structure (30–60 km wide), (2) the rift's eastern margin reflects broad regional uplift along the rift, and (3) the western side arching is a subsidary structure that follows the main rift structure.
Numerical modeling of trishear fault propagation folding
Authors: Hardy Stuart , Ford Mary
Published: February 2004

In contrast to kink band migration modeling methods, trishear numerical models produce fault propagation folds with smooth profiles and rounded hinges. Modeled fold hinges tighten and converge downward, within a triangular zone of distributed deformation which is focused on the fault tip. Such features have been reported from field studies and are also seen in analogue models of compressional deformation. However, apart from its initial application to Laramide folds, little quantitative work has been undertaken on trishear fault propagation folding in other settings. In addition, no study has been undertaken into the growth strata which might be associated with such structures. This paper uses an equivalent velocity description of the geometric model of trishear, together with models of erosion and sedimentation, to investigate trishear fault propagation folding of both pregrowth and growth strata. The trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement. The models show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata. When total slip on a structure is high, the model predicts overturned pregrowth and growth strata. During the initial stages of deformation, beds in the forelimb thicken but later thin when they become steep or overturned. The effect of variations in fault propagation to slip ratios on two‐dimensional finite strain in the models is assessed by the use of initially circular strain markers. High fault propagation to slip (p/s) ratios lead to narrow zones of high finite strain, while lower p/s ratios lead to more ductile deformation and broader zones of high strain. In all cases, hanging wall anticlines and footwall synclines originate as early ductile folds which are later cut by the propagating fault. Modeled structures are compared with natural examples.
Latest Neoproterozoic basin inversion of the Beardmore Group, central Transantarctic Mountains, Antarctica
Authors: Goodge John W.
Published: February 2004

Structural and age relationships in Beardmore Group rocks in the central Transantarctic Mountains of Antarctica indicate that they experienced a single deformation in latest Neoproterozoic to early Paleozoic time. New structural data contrast with earlier suggestions that Beardmore rocks record two orogenic deformations, one of the early Paleozoic Ross orogeny and a distinct earlier tectonic event of presumed Neoproterozoic age referred to as the Beardmore orogeny. In the Nimrod Glacier area, Beardmore metasedimentary rocks contain only a single set of geometrically related regional structures associated with the development of upright, large‐ and small‐scale flexural‐slip folds. Deformation of Beardmore strata involved west directed contraction of modest regional strain at relatively high crustal levels. Existing ages of detrital zircons from the Cobham and Goldie formations constrain Beardmore Group deposition to be younger than ∼600 Ma. This is significantly younger than previous age estimates and suggests that Beardmore deposition may be closely linked to a latest Neoproterozoic East Antarctic rift margin. The lack of structural evidence for polyphase deformation and the relatively young depositional age for the Beardmore Group thus raises the question of a temporally and/or technically unique Beardmore orogeny. Here I suggest that Beardmore shortening may be related to tectonic inversion of East Antarctic marginal‐basin strata because of localized compression during proto‐Pacific seafloor spreading. Basin inversion is but one stage in a protracted Ross tectonic cycle of rifting, tectonic inversion, subduction initiation, and development of a mature convergent continental margin during latest Neoproterozoic and early Paleozoic time. The term “Beardmore orogeny” has little meaning as an event of orogenic status, and it should be abandoned. Recognition of this latest Neoproterozoic history reinforces the view that the broader Ross orogeny was not a single event but rather was a long‐lived postrifting tectonic process along the East Antarctic margin of Gondwanaland.
Lithospheric strength and intraplate seismicity in the New Madrid seismic zone
Authors: Liu Lanbo , Zoback Mark D.
Published: February 2004

A simple hypothesis is proposed to explain the occurrence of localized zones of tectonic deformation and seismicity within intraplate regions subjected to relatively uniform far‐field tectonic stresses. In most intraplate regions (especially continental shield areas and old oceanic basins), temperatures in the lower crust and upper mantle are quite low so that the upper mantle is cold and strong. In these regions, significant lithospheric deformation does not occur because the cumulative strength of the lithosphere far exceeds the magnitude of plate‐driving forces. If lower crust and upper mantle temperatures are relatively high, however, plate‐driving forces are largely supported by the upper crust because the lower crust and upper mantle are relatively weak. In this case, the regions can deform relatively rapidly because the cumulative strength of the lithosphere is comparable in magnitude to that of the forces acting on the lithosphere. In this paper, we apply this hypothesis to the New Madrid seismic zone and the surrounding central and eastern United States. Within the seismic zone, the heat flow appears to be slightly elevated (about 60 mW/m2) relative to the background regional value of 45 mW/m2. Calculated crustal geotherms and laboratory‐derived ductile flow laws suggest that the lower crust and upper mantle are sufficiently weak within the seismic zone that intraplate stresses are largely transmitted through the upper crust and deformation can occur at relatively rapid rates for this intraplate area. In marked contrast, in the surrounding area where the heat flow is relatively low, cumulative lithospheric strength appears to far exceed the plate‐driving force, and the tectonic stress is carried in both the crust and upper mantle. Thus the marked contrast in seismicity between the seismic zone and the surrounding area appears largely because of heat flow and whether or not the lower crust and upper mantle support an appreciable fraction of the plate‐driving forces.
Transect across the northwestern Grenville orogen, Georgian Bay, Ontario: Polystage convergence and extension in the lower orogenic crust
Authors: Culshaw N. G. , Jamieson R. A. , Ketchum J. W. F. , Wodicka N. , Corrigan D. , Reynolds P. H.
Published: February 2004

The Grenville orogenic cycle, between ∼ 1190 and 980 Ma, involved accretion of magmatic arcs and/or continental terranes to the Laurentian craton. A transect across the western Central Gneiss Belt, Georgian Bay, Ontario, which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20–30 km, offers some insights on the thermal and mechanical behavior of the lower crust during the development of the Grenville orogen. Prior to Grenvillian metamorphism, this part of Laurentia consisted largely of Meso‐proterozoic (∼ 1450 Ma) granitoid orthogneisses, granulites, and subordinate mafic and supracrustal rocks. Grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed (∼ 1160 Ma) Parry Sound domain over the craton sometime between 1120 Ma and 1080 Ma. This stage of transport was followed by out‐of‐sequence thrusting and further convergence along successively deeper, foreland‐propagating ductile thrust zones. A major episode of extension at ∼ 1020 Ma resulted in southeast directed transport of allochthonous rocks along the midcrustal Shawanaga shear zone. The final stage of convergence involved deformation and metamorphism in the Grenville Front Tectonic Zone at ∼ 1000–980 Ma. Peak metamorphism along most of the transect at 1065–1045 Ma followed initial transport of allochthonous rocks over the craton by 15–35 m.y. Regional cooling, which postdated peak metamorphism by >70 m.y., was probably delayed by the combined effects of late‐stage extension and convergence. Transport of allochthons at least 100 km over the craton was accomplished along a weak, migmatitic decollement; further propagation of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement. Extensional deformation was associated with distributed ductile flow, the formation of regional transverse folds with axes parallel to the stretching direction, and reactivation of the allochthon‐parautochthon thrust boundary as an extensional decollement. The extensional lower crustal flow was likely the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the Central Gneiss Belt.
Shallow structure of the Yadong‐Gulu rift, southern Tibet, from refraction analysis of Project INDEPTH common midpoint data
Authors: Cogan Michael J. , Nelson K. D. , Kidd W. S. F. , Wu Changde
Published: February 2004

Refracted arrivals on International Deep Profiling of Tibet and the Himalaya (INDEPTH) common midpoint (CMP) data provide insight into the shallow structure of the Yadong‐Gulu rift in southern Tibet. This is the first such information available for the Neogene‐Quaternary rifts on the Tibetan Plateau. Geologic cross sections across three separate valleys within the rift have been constructed from these data and available surface geological information. In each case, the valleys are floored by asymmetric, low‐velocity sedimentary fills ranging from ∼100 m to ∼1.5 km in thickness. Beneath all three, the basement appears to dip gently to the east (∼ 5°). Seismic reflection data from one of these valleys (Nyinzhong) show a ∼30° east dipping reflector within the basement beneath the valley that projects upward to the normal‐sense Nyainqentanglha detachment exposed in the range immediately to the west. Field investigations by INDEPTH personnel have additionally shown the existence of previously unrecognized normal‐sense shear zones bordering the other two valleys. The newly discovered shear zones are 200 to >500 m thick and in both instances are characterized by moderately to steeply west dipping mylonites with top‐to‐west shear sense. Metamorphic minerals within the shear zones indicate at least amphibolite grade metamorphism with increasing metamorphic grade down section. The seismic data, coupled with the new geologic information and geomorphologic data from Landsat imagery, suggest that the structure of the northern Yadong‐Gulu rift is comparable to other well‐documented rift systems in which steep range‐front normal faults terminate at, or sole into, moderately dipping normal‐sense detachments at depth that have undergone substantial footwall uplift and rotation during extension. This geometry implies that the extension of the Tibetan Plateau crust has been accommodated at depth by lateral flow within the middle crust. It also suggests that E‐W extension of the plateau may have been substantially greater than has generally been presumed.
Bacteriophage and microsphere transport in saturated porous media: Forced‐gradient experiment at Borden, Ontario
Authors: Bales Roger C. , Li Shimin , Yeh T.‐C. Jim , Lenczewski Melissa E. , Gerba Charles P.
Published: February 2004

A two‐well forced‐gradient experiment involving virus and microsphere transport was carried out in a sandy aquifer in Borden, Ontario, Canada. Virus traveled at least a few meters in the experiment, but virus concentrations at observation points 1 and 2.54 m away from the injection well were a small fraction of those injected. A simplified planar radial advection‐dispersion equation with constant dispersivity, coupled with equilibrium and reversible first‐order mass transfer, was found to be adequate to simulate the attachment and transport process. During the experiment a short‐duration injection of high‐pH water was also made, which caused detachment of previously attached viruses. For simulating this detachment and associated transport, the same transport and mass‐ transfer equations were used; but all rate parameters were varied as groundwater pH changed from 7.4 to 8.4 and then back to 7.4. The physicochemical parameters obtained from fitting breakthrough curves at one sampling well were used to predict those at another well downstream. However, laboratory‐determined parameters overpredicted colloid removal. The predicted pattern and timing of biocolloid breakthrough was in agreement with observations, though the data showed a more‐disperse breakthrough than expected from modeling. Though clearly not an equilibrium process, retardation involving a dynamic steady state between attachment and detachment was nevertheless a major determinant of transport versus retention of virus in this field experiment.
Properties of exact and approximate traveling wave solutions for transport with nonlinear and nonequilibrium sorption
Authors: Simon Werner , Reichert Peter , Hinz Christoph
Published: February 2004

Nonlinear sorption leads to the existence of moving concentration fronts of substances transported in porous media that do not change shape. The cause for the existence of such traveling wave fronts is a balance between the self‐sharpening effect of nonlinear sorption and the spreading effects of dispersion and sorption kinetics. While analytical solutions for the transport of nonlinearly sorbing substances are usually not available, the asymptotic front speed and the shape of traveling waves can in some situations be determined analytically. This is done by deriving a traveling wave equation in a moving coordinate system. In the case of simultaneous linear equilibrium and nonlinear nonequilibrium sorption, this traveling wave equation is a second‐order differential equation. Because this equation cannot be solved analytically for typical nonlinear sorption isotherms, the second‐order term is usually neglected. In this paper the significance of the second‐order term is discussed for a simple piecewise linear sorption isotherm that allows the analytical solution of both the full and the simplified differential equations. These analytical solutions make it possible to calculate the approximation error as a function of flow and isotherm parameters and thus to localize domains where the error may be significant. In addition, the analytical solutions are used to discuss the identifiability of model parameters from asymptotic front shape data.
Discrete modeling of water and pesticide movement in soil
Authors: Vollmayr Hans , Kleint Friedemann , Schüürmann Gerrit
Published: February 2004

Simulations of water and pesticide movement are presented for the unsaturated zone. Water movement is studied with a two‐dimensional Monte Carlo technique, where particles hop between the sites of a square lattice according to probabilistic rules. We developed this lattice gas method for two‐dimensional simulations of water flow in inhomogeneous media. Generalization to three dimensions is straightforward. The dynamics of the pesticide is simulated by particle tracking. After presenting the underlying discrete algorithms we emphasize calibration of the system. A soil is divided into homogeneous horizons, and our model is compared with the conventional one‐dimensional model PESTLA. We observe good agreement for both water content profile and pesticide distribution. The random elements in our algorithms cause moderate fluctuations in the results. As an example of two‐dimensional simulation, spatiotemporal pesticide patterns under a ridge are compared with the distribution under a flat soil surface.
Changes in soil structure caused by the installation of time domain reflectometry probes and their influence on the measurement of soil moisture
Authors: Rothe A. , Weis W. , Kreutzer K. , Matthies D. , Hess U. , Ansorge B.
Published: February 2004

The time domain reflectometry technique for measuring the volumetric soil water content is highly sensitive to installation effects because the sampling volume is more heavily weighted close to the transmission line elements. To investigate changes in soil structure caused by the installation and its influence on the measurement, field and laboratory experiments covering different probe dimensions, soil types, and installation techniques were performed. The results indicate that merely pushing the probes into the soil entails a significant reduction of the measured water content up to 0.10 cm3/cm3. The effect is strongest close to saturation since the large pores are affected most. The degree of compression differed strongly according to the varying compressibility of the investigated soils. It was smaller for the smaller probe indicating a more favorable relationship between sampling volume and compression. The compression effect could be minimized using a drill to remove the soil for the probes. X ray‐computed tomography was used to visualize the installation effects and to measure the density distribution around the probes.
Experimental investigation of mobile armoring development
Authors: Marion Andrea , Fraccarollo Luigi
Published: February 2004

Vertical sorting of bed material in rivers has received increasing attention, but very little experimental data are available so far. A recently developed nondestructive sampling technique allows the determination of bed material composition over vertical surfaces of the bed. This is particularly interesting for determining particle arrangement on the surface of and inside bed forms. Experiments were carried out using bimodal and trimodal sediment mixtures in two recirculating flumes. The development of surface coarsening was detected and measured at various stages of the process. Variations in the initial bed material composition brought very different responses in the bed geometry. The mobile armoring process was also strongly affected by bed material composition. The experimental procedures described in this work indicate a possible path to be followed for further experimental research on armoring processes.
A modified surface‐resistance approach for representing bare‐soil evaporation: Wind tunnel experiments under various atmospheric conditions
Authors: Yamanaka Tsutomu , Takeda Atsushi , Sugita Fumi
Published: February 2004

A physically based (i.e., nonempirical) representation of surface‐moisture availability is proposed, and its applicability is investigated. This method is based on the surface‐resistance approaches, and it uses the depth of evaporating surface rather than the water content of the surface soil as the determining factor of surface‐moisture availability. A simple energy‐balance model including this representation is developed and tested against wind tunnel experiments under various atmospheric conditions. This model can estimate not only the latent heat flux but also the depth of the evaporating surface simultaneously by solving the inverse problem of energy balance at both the soil surface and the evaporating surface. It was found that the depth of the evaporating surface and the latent heat flux estimated by the model agreed well with those observed. The agreements were commonly found out under different atmospheric conditions. The only limitation of this representation is that it is not valid under conditions of drastic change in the radiation input, owing to the influence of transient phase transition of water in the dry surface layer. The main advantage of the approach proposed is that it can determine the surface moisture availability on the basis of the basic properties of soils instead of empirical fitting, although further investigations on its practical use are needed.
Two‐ and three‐parameter calibrations of time domain reflectometry for soil moisture measurement
Authors: Yu C. , Warrick A. W. , Conklin M. H. , Young M. H. , Zreda M.
Published: February 2004

Time domain reflectometry (TDR) is widely used to measure and monitor soil water. The commonly used calibration curve is the third‐degree “universal polynomial” of Topp et al. [1980]. The most common refinement is calibration to a specific soil but still using four parameters (coefficients) from fitting a third‐degree polynomial. Here we demonstrate that a three‐parameter expression,
)where the three parameters a, b, and α are determined by fitting water content θυ to the dielectric coefficient Ka). This form is consistent with the well‐known mixing model. For an isotropic soil with homogeneous water distribution this expression is further simplified to two parameters by taking α=0.5. When α is 0.5, its calibration is equivalent to the linear calibration between θυ and the travel time along the waveguide. In addition, the simple three‐parameter expression can be easily inverted without losing accuracy with regard to the original calibration. The TDR calibration expressed in a three‐parameter form not only achieves a good fit but also conveys a physical connotation.
Fault permeability near Yucca Mountain
Authors: Bredehoeft John D.
Published: February 2004

A deep Paleozoic carbonate aquifer is of special concern for the proposed high‐level nuclear repository at Yucca Mountain, Nevada. It provides a potential pathway for groundwater transport of contaminants from the repository back to the biosphere. The map of groundwater temperature at the water table has been interpreted by several investigators to indicate upwelling groundwater along major fault zones both east and west of Yucca Mountain. Of particular concern is the permeability of these fault zones. The deep carbonate aquifer has been penetrated by only one test hole in the vicinity of Yucca Mountain: UE‐25pl. In this borehole the hydraulic head in the deep carbonate aquifer was 20 m higher than head in the overlying tuff aquifer. Observations in UE‐25pl show good Earth tide water‐level fluctuations in the deep carbonate aquifer. Both the large 2‐cm amplitude of the tidal signal and the fact that it is in phase with the tidal potential indicate that the deep aquifer is well confined by a layer of low hydraulic conductivity. An estimate of the maximum hydraulic permeability of a fault zone that penetrates the confining is made by simulating the tidal response. Given a maximum permeability and the head difference, one can calculate the upward flow from the deep carbonate aquifer into the overlying tuffs. This analysis supports the interpretation of upward flow from the deep carbonate to the overlying tuffs. This is a favorable condition for the proposed repository. The upward potential for flow from the deep carbonate aquifer protects it from the downward movement of contaminants. Further deep drilling is necessary to confirm that the high head persists beneath the entire proposed repository. Nothing should be done either through construction of the repository or through groundwater development to reduce heads in the carbonate aquifer; the higher heads protect the carbonate aquifer.
Relating nutrient discharges from watersheds to land use and streamflow variability
Authors: Jordan Thomas E. , Correll David L. , Weller Donald E.
Published: February 2004

During a 1‐year period we measured discharges of water, suspended solids, and nutrients from 27 watersheds having differing proportions of cropland in the Piedmont and Coastal Plain provinces of the Chesapeake Bay drainage. Annual flow‐weighted mean concentrations of nitrate and organic N and C in stream water correlated with the relative proportions of base flow and storm flow. As the proportion of base flow increased, the concentration of nitrate increased and the concentrations of organic N and C decreased. This suggests that discharge of nitrate is promoted by groundwater flow but discharges of organic N and C are promoted by surface runoff. Concentrations of N species also increased as the proportion of cropland increased. We developed a statistical model that predicts concentrations of N species from the proportions of cropland and base flow. P concentrations did not correlate with cropland or base flow but correlated with the concentration of suspended solids, which differed among watersheds.
Stochastic analysis of one‐dimensional transport of kinetically adsorbing solutes in chemically heterogeneous aquifers
Authors: Espinoza Carlos , Valocchi Albert J.
Published: February 2004

A small perturbation approach is used to analyze the impact of chemical heterogeneity on the one‐dimensional transport of a pollutant that undergoes linear kinetic adsorption. We make an important simplifying assumption that the aquifer is physically homogeneous but chemically heterogeneous. The aquifer is assumed to be comprised of two distinct zones: reactive and nonreactive; a Bernoulli random process is used to characterize the spatial distribution of reactive zones along the aquifer. We develop analytical solutions to study the distribution of the ensemble mean, standard deviation, and coefficient of variation of the dissolved concentration after an instantaneous injection of contaminant. In addition, numerical solutions based on a Monte Carlo approach are used to determine the validity of the analytical solutions. Finally, an analysis involving temporal and spatial moments is used to derive expressions for the large‐scale effective parameters (velocity and dispersion) that capture the impact of chemical heterogeneity. Temporal moment analysis provides closed‐form analytical expressions for the asymptotic effective velocity and dispersion, while spatial moment analysis explains the effect of chemical heterogeneity on the preasymptotic value of these effective parameters. A key result from our analysis shows that chemical heterogeneity creates “pseudokinetic” or “macrokinetic” conditions characterized by a time‐dependent effective retardation coefficient even when the local equilibrium assumption is invoked.