bluuebunny/crossref_metadata_2025 huggingface dataset:
Method of moments analysis of electromagnetic scattering from a general three‐dimensional dielectric target embedded in a multilayered medium
Authors: He Jiangqi , Yu Tiejun , Geng Norbert , Carin Lawrence
Published: September 2002

The method of moments (MOM) is applied to the problem of electromagnetic scattering from general three‐dimensional dielectric targets in an arbitrary multilayered environment. The dyadic multilayered Green's function is computed via the method of complex images, and the Galerkin MOM solution is effected by employing triangular patch basis functions. Several example frequency and time domain results are presented, with application to radar‐based sensing of plastic land mines.
Terrain topography measurement using multipass polarimetric synthetic aperture radar data
Authors: Schuler Dale L. , Lee Jong‐Sen , Ainsworth Thomas L. , Grunes Mitchell R.
Published: September 2002

A method has been investigated for the measurement of topography using airborne fully polarimetric synthetic aperture radar (SAR) data. Terrain slopes in both the range and azimuthal directions have been estimated using multipass flight geometries. Using these slope values, the Poisson equation was then solved to create a Digital Elevation Model (DEM) of the terrain topography. The method measures polarimetric orientation angles which are then converted into terrain slopes in the azimuthal direction. The conversion of these orientation angles into terrain slopes requires additional knowledge of the radar look angle and the range direction terrain slopes. The solution for slopes is, therefore, a problem coupled between the range and azimuthal directions. For specialized multipass flight geometries these orthogonal terrain slopes are solved for, and maps of terrain slopes are produced. In particular, the processing of two‐pass orthogonal and two‐pass antiparallel (headings Θ and Θ+ 180°) NASA ‐ Jet Propulsion Laboratory airborne SAR data sets has been carried out for an area in central California. When orthogonal slopes are derived using either of these data sets, a digital elevation model may be generated. The L band, polarimetric SAR (POLSAR) DEM created by this solution is compared to a coregistered C band, interferometric SAR (IFSAR) DEM. Similar comparisons are made for terrain slopes in the azimuthal ‐ range directions which are generated by the POLSAR and IFSAR elevation data. The polarimetric SAR, operating from an aircraft, or satellite, in a strip‐mapping mode, is capable of measuring terrain topography for large areas provided that phase‐preserving fully polarimetric data are taken. Polarimetric SAR data are also widely used for studies of crop classification, surface roughness, biomass density, and soil moisture content. All of these studies are adversely affected by scattering changes attributable to topography. The technique investigated here potentially offers a means to correct for these effects by making simultaneous, coregistered estimates of local orthogonal terrain slopes.
A comparison of radome‐ and astrodome‐enclosed large radio telescopes at millimeter wavelengths: The Large Millimeter Telescope
Authors: Olmi Luca , Mauskopf Philip D.
Published: September 2002

We present a systematic comparison of the main figures of merit for an open‐air radio telescope and two different types of enclosed antennas: (1) an ordinary radome, with a metal space frame providing the required mechanical rigidity and a dielectric membrane, and (2) an “astrodome,” i.e., a corotating rigid dome with a large window covered by a tensile membrane structure. The analysis is limited to submillimeter and millimeter wavelengths and large (≳30 m) antenna/enclosure systems, where the window tensile structure is very unlikely to be removable and is supported by either a metal space frame or cable networks. As compared with previous studies of this type, here we concentrate on the specific effects that these large metallic support structures have on sensitive astronomical observations. In particular, we critically discuss how the wind‐induced random motions of the metal space frame can limit the sensitivity of continuum observations, as a result of fluctuating shadowing and spillover effects combined with various beam‐chopping techniques. Using the Large Millimeter Telescope as a benchmark, we provide baselines for future projects where a similar comparison is needed.
Sampling representation of electromagnetic fields over three‐dimensional domains
Authors: Bucci O. M. , Gennarelli C. , Riccio G. , Savarese C.
Published: September 2002

A fast, accurate, and stable three‐dimensional interpolation algorithm of central type is proposed to reconstruct the electromagnetic field in any point of a given volume external to an arbitrary source. It is based on a new sampling representation which allows us to drastically reduce the number of needed data when compared with conventional λ/2 sampling, thus significantly lowering measurement time and storage requirements when characterizing field distributions in electrically large domains. Moreover, for large sources it is also computationally convenient with respect to modal expansions or source integration methods. The efficiency and stability of the developed interpolation algorithm are assessed by means of numerical tests.
Fast inhomogeneous plane wave algorithm for the fast analysis of two‐dimensional scattering problems
Authors: Hu Bin , Chew Weng Cho , Michielssen Eric , Zhao Junsheng
Published: September 2002

A novel algorithm, the fast inhomogeneous plane wave algorithm (FIPWA), has been developed to accelerate the solution of integral equations pertinent to the analysis of the scattering from two‐dimensional perfect electric conducting surfaces. Unlike the fast steepest descent path algorithm, the proposed technique directly interpolates the far‐field pattern of the source group and matches it along a modified steepest descent path. A novel approach, which results in a diagonal translator with built‐in interpolation coefficients, is proposed. The computational complexity per matrix‐vector multiplication of a two‐level implementation of the proposed FIPWA is O(N4/3) and the multilevel implementation further reduces the complexity to O(NlogN), where N is the number of unknowns in the discretized integral equation. It is shown that this technique outperforms the previously developed fast methods such as the fast mulitpole method and the ray‐propagation fast multipole algorithm.
Field and source decomposition and Green dyadics for a class of nonreciprocal bianisotropic media
Authors: Olyslager F. , Lindell I. V.
Published: September 2002

A class of nonreciprocal bianisotropic media, known to allow a decomposition of electromagnetic fields into two individual parts with certain restrictions on polarization defined in terms of two arbitrary vectors, is studied. The field decomposition principle is extended to electromagnetic sources. It is shown that the medium in the two partial problems can be replaced by simpler equivalent media and that an analytic Green dyadic expression can be formulated for each of the two equivalent media filling the whole space. Thus the decomposition makes it possible to find the field of an elementary source in the original medium by integrating the field of the partial sources.
Fast integral methods for conformai antenna and array modeling in conjunction with hybrid finite element formulations
Authors: Volakis John L. , Eibert Thomas F. , Sertel Kubilay
Published: September 2002

Fast integral methods are used to improve the efficiency of hybrid finite element formulations for conformal antenna and array modeling. We consider here cavity‐backed configurations recessed in planar and curved ground planes as well as infinite periodic structures with boundary integral (BI) terminations on the top and bottom bounding surfaces. Volume tessellation is based on triangular prismatic elements which are well suited for layered structures and still give the required modeling flexibility for irregular antenna and array elements. For planar BI terminations of finite and infinite arrays the adaptive integral method is used to achieve O(N log N) computational complexity in evaluating the matrix‐vector products within the iterative solver. In the case of curved mesh truncations for finite arrays the fast multipole method is applied to obtain O(N1.5) complexity for the evaluation of the matrix‐vector products. Advantages and disadvantages of these methods as they relate to different applications are discussed, and numerical results are provided.
Sunrise effects on VLF signals propagating over a long north‐south path
Authors: Clilverd Mark A. , Thomson Neil R. , Rodger Craig J.
Published: September 2002

We present a detailed study of the times of amplitude minima observed on the 12‐Mm path from NAA (24 kHz, 1 MW, Cutler, Maine) to Faraday, Antarctica, during the period 1990–1995. (NAA is a naval transmitter call sign.) This study represents the first account of the effect of the sunrise terminator when it is parallel to a propagation path at some times of the year. Since the NAA‐Faraday path is within 3° of the north‐south meridian, parallel orientation happens close to the equinoxes, while the maximum angle of incidence occurs during the solstices. During the solstices the terminator takes a significant length of time to cross the entire propagation path, so modal conversion effects are observed over a range of hours. During the equinoxes, however, the leading edge of the night‐day transition region crosses the whole propagation path within 20 min. The interpretation of the timing of minima is consistent with modal conversion taking place as the sunrise terminator crosses the NAA‐Faraday transmission path at specific, consistent locations. The timing of minima is remarkably consistent from year to year. Long wave propagation modeling is used to show that the location of nightside minima at an altitude of 45–75 km in the subionospheric waveguide represents the location of the sunrise terminator on the great circle path when dayside minima occur.
Temporal and spatial forecasting of ionospheric critical frequency using neural networks
Authors: Kumluca Ayca , Tulunay Ersin , Topalli Ihsan , Tulunay Yurdanur
Published: September 2002

The ionospheric critical frequency, ƒoF2, is forecast 1 hour in advance by using artificial neural networks. The value ƒoF2 at the time instant k of the day is designated by ƒ(k). The inputs used for the neural network are the time of day; the day of year; season information; past observations ƒoF2, the first difference Δ1 (k) = ƒ(k) = ƒ(k − 1); the second difference Δ2 (k) = Δ1 (k) − Δ1 (k − 1); the relative difference RΔ(k) = Δ1(k)/ƒ(k); geomagnetic indices Kp, ap, Dst, sunspot number, and solar 10.7‐cm radio flux; and the solar wind magnetic field components By and Bz. This paper gives a new method, and it is the first application of neural networks for modeling both temporal and spatial dependencies. In order to understand the physical characteristics of the process and determine how important a particular input is, a test which shows the relative significance of inputs to the neural networks is performed at the output. The performance of a neural network is measured by considering errors. For the errors to be more meaningful, training and test times and times for comparison with other results are selected from the same solar activity period. Among the various neural network structures, the best configuration is found to be the one with one hidden layer with five hidden neurons, giving an absolute overall error of 5.88%, or 0.432 MHz.
Vertical resolution approaching 100m for GPS occultations of the Earth's atmosphere
Authors: Mortensen M. D. , Linfield R. P. , Kursinski E. R.
Published: September 2002

Vertical resolution is a fundamental aspect of any atmospheric remote sensing technique. Occultations using the Global Positioning System (GPS) signals provide inherently high vertical resolution. The phase coherence of the GPS signals allows improvement beyond the resolution limit set by Fresnel diffraction and the viewing geometry. We have characterized this improvement by simulating the forward propagation of GPS signals through the atmosphere to the GPS receiver. The signals are then back‐propagated to a position closer to the atmosphere, thereby reducing the effects of diffraction. Comparison between the direct and the back‐propagated results indicates that the resolution is improved by a factor of several, down to ∼100m.
On the predictability of f0F2 using neural networks
Authors: Poole Allon W. V. , McKinnell Lee‐Anne
Published: September 2002

Recently, there has been much interest in the use of neural networks (NNs) in ionospheric prediction models [Williscroft and Poole, 1996; Altinay et al., 1997]. This paper is divided into two parts. The first part presents an extension of the work of Williscroft and Poole [1996], in which a NN is trained to use nonionospheric geophysical parameters representing time, season, solar cycle, and magnetic activity to estimate f0F2. The NN is trained with data from two sunspot cycles at the midlatitude station of Grahamstown (26.5°E, 33.3°S, geographic) and predicts f0F2 for various combinations of the input parameters. It is further shown how the squared errors between f0F2 estimated from the NN and the measured values are themselves functions of the input parameters, and it is demonstrated how a second NN can be trained to predict the squared error, and hence the rms error, thus providing a measure of the uncertainty in the estimation. These uncertainties lie in the range 0.4–0.9 MHz, a considerable improvement on current non‐NN‐based models. In the second part, the input data to the net are expanded to include recent measured values of f0F2, which leads to a further improvement in the estimation of future values. We conclude that the inclusion of this ionospheric information in the input data is only justified for prediction times up to 4–5 hours ahead, whereafter a knowledge of the most recent values of f0F2 does not improve the prediction significantly, and the nonionospheric parameters described in the first part are adequate.
Analysis of radiation from two separate circular cylindrical waveguides by the method of moments
Authors: Cooray F. R. , Bird T. S.
Published: September 2002

A method of moments (MOM) technique is described for analyzing the radiation from two parallel perfectly conducting circular cylindrical waveguides whose apertures may be noncoplanar. By means of an equivalence principle and the appropriate boundary conditions, a set of integral equations is obtained with surface currents as the unknowns. These integral equations are solved by the MOM to yield the surface currents, from which the radiation patterns for the two waveguides are calculated. Numerical results are presented for two uniform waveguides and for two stepped waveguides. To validate the analysis, these results are compared with corresponding results obtained from other numerical methods and also with practical measurements.
Vertical structure of vegetated land surfaces from interferometric and polarimetric radar
Authors: Treuhaft Robert N. , Siqueira Paul R.
Published: September 2002

This paper describes the estimation of parameters characterizing the vertical structure of vegetated land surfaces, from combined interferometric and polarimetric radar data. Physical models expressing radar observations in terms of parameters describing vegetated land surfaces are the foundation for parameter estimation techniques. Defining a general complex cross correlation enables the unified development of models for interferometry and polarimetry, including polarimetric interferometry. Three simple physical models in this paper express this complex cross correlation in terms of vegetation parameters: (1) a randomly oriented volume, (2) a randomly oriented volume with a ground return, and (3) an oriented volume. For the first two models the parameters include vegetation height, extinction coefficient, underlying topography, and another parameter depending on ground electrical properties and roughness. For the oriented volume, additional parameters depend on the refractivity, extinction coefficients, and backscattering characteristics of waves propagating along eigenpolarizations of the vegetation volume. The above models show that the interferometric cross‐correlation amplitude and the polarimetric {HHHH/VVVV} ratio both change by about 1% per meter of vegetation height change, for experimental conditions typical of airborne and spaceborne interferometric radars. These vertical‐structure sensitivities prompt a parameter estimation demonstration with two‐baseline TOPSAR interferometric and zero‐baseline polarimetric data from the Boreal Ecosystem‐Atmosphere Study (BOREAS) Southern Study Area in Prince Albert National Park, Saskatchewan, Canada. The demonstrations show the feasibility of measuring vegetation height to better than 4.2 m, underlying topography to better than 6.5 m, and the ratio of ground‐to‐volume power to better than 10%, using interferometry and polarimetry, coupled with parameter‐constraining assumptions, concerning the degree of surface roughness. This paper suggests that single‐baseline and multibaseline fully polarimetric interferometry have the potential to obviate the need for such assumptions, thereby making parameter estimation more robust, accurate, and realistic.
Tectonic evolution of the Superior Boundary Zone from coincident seismic reflection and magnetotelluric profiles
Authors: White D. J. , Jones A. G. , Lucas S. B. , Hajnal Z.
Published: September 2002

The Superior Boundary Zone (SBZ) forms the northwestern margin of the Archean Superior craton and constitutes a tectonic foreland of the ∼1.8 Ga Trans‐Hudson Orogen. The Superior Boundary Fault (SBF) separates the SBZ from the adjacent Reindeer Zone, a collage of Paleoproterozoic juvenile intraoceanic rocks. Lithoprobe seismic reflection and magnetotelluric data were acquired along two profiles crossing the SBZ in an attempt to better constrain the deformation and crustal geometry resulting from Trans‐Hudson orogeny. Analysis and interpretation of spatially coincident regional seismic and magnetotelluric data acquired along the southern 200 km profile indicate the following: (1) the Reindeer Zone accretionary collage forms an east dipping, eastward steepening, crustal‐scale tectonic stack of moderately conductive rocks near the SBZ. (2) The SBZ is characterized at shallow depths (<5–6 km) by steep to moderately east dipping reflectivity that is associated with the limbs of third generation folds (F3 and D3) and east‐side‐up shear zones. At greater depth, the SBZ crust is highly resistive and is contiguous to the east with resistive crust beneath the Superior craton proper. (3) The SBF is recognized in the subsurface as an abrupt resistivity contrast between the Reindeer Zone and the SBZ, extending subvertically to 15 km depth. (4) Moderately conductive rocks of the Reindeer Zone extend eastward for 30 km beneath the SBZ at depths of 15–45 km. Seismic reflection data from a second crossing located 100 km NE along strike indicate a similar crustal structure with some notable exceptions: (1) The SBF is recognized in the subsurface as a truncation of interpreted collisional fabrics and extends subvertically to ∼30 km depth. (2) There is no compelling evidence for the eastward continuation of Reindeer Zone lower crust beneath the SBZ. To explain the present‐day SBZ crustal structure, we propose that the nature of the SBZ evolved over an ∼200 Myr convergent margin history from a lower plate collisional thrust belt setting at >1.88‐1.81 Ga, through lithospheric delamination at ∼1.82‐1.80 Ga to a steep transpressive plate boundary at 1.80‐1.72 Ga.
Uplift and convergence along the Himalayan Frontal Thrust of India
Authors: Wesnousky Steven G. , Kumar Senthil , Mohindra R. , Thakur V. C.
Published: September 2002

Along the Himalayan thrust front in northwestern India, terrace deposits exposed 20 to 30 m above modern stream level are interpreted to have been uplifted by displacement on the underlying Himalayan Frontal Thrust. A radiocarbon age limits the age of the terrace to ≤1665±215 calendar BC (≤3663±215 radiocarbon years before present), yielding a vertical uplift rate of ≥6.9±1.8 mm/yr. In combination with published studies constraining the dip of the Himalayan Frontal Thrust fault to about 30° in the study area, the observed uplift rate equates to horizontal shortening across the Himalayan Frontal Thrust of ≥11.9±3.1 mm/yr and the slip rate of the Himalayan Frontal Thrust of ≥13.8±3.6 mm/yr. This is similar to previously reported rate estimates along the Himalayan arc based on displacement of older Plio‐Miocene age rocks, or the much shorter records of geodesy and historical seismicity. The similarity is consistent with the idea that convergence across the Himalayan front has occurred at a relatively steady rate through time. The seismic expression of this deformation includes several great (M∼8) historical earthquakes which, due to lack of surface rupture during those events, have been attributed to their occurrence on blind thrusts. Yet, the occurrence of a possible fault scarp in the field area indicates that past earthquakes have been sufficiently large to rupture to the surface and produce coseismic scarps. These observations suggest a potential for earthquakes along the Himalayan Frontal Thrust larger than those observed historically.
Timing and magnitude of rotations in the frontal thrust systems of southwestern Sicily
Authors: Speranza F. , Maniscalco R. , Mattei M. , Di Stefano A. , Butler R. W. H. , Funiciello R.
Published: September 2002

We report new paleomagnetic and anisotropy of magnetic susceptibility (AMS) results from upper Tortonian to middle Pleistocene sediments which were deposited upon and adjacent to active thrust structures in southwestern Sicily. The data show that the Plio‐Pleistocene sediments from the Belice and Menfi basins (covering the Saccense shelf limestones) underwent any internal shortening after the early Pleistocene (Santernian), as well as any net rotation. Sediments around this area (which overlie basinal Meso‐Cenozoic successions) record systematic rotations: one upper Tortonian site to the west is ∼30° counterclockwise rotated, while to the east, lower Pliocene to middle lower Pleistocene sites within the Gela Nappe domain show 25° to 56° clockwise (CW) rotations. These data show that the ductile basinal sediments were bent and rotated around the rigid Saccense carbonates during the thin‐skinned southward propagation of the orogenic front. We document here that the coastal sediments from the southwestern Gela Nappe underwent both a post middle early Pleistocene ∼30°CW rotation and a post middle Pleistocene E–W to ESE–WNW flattening (revealed by AMS). Our data then constrain to the late Pleistocene‐Holocene the age of the last shortening episode occurring in the southwestern Gela Nappe front. Pleistocene rotations of similar amount also characterize the Sicanian domain, implying that it was incorporated in the Gela Nappe wedge during the recentmost episodes of deformation. This evidence allows us to better understand the very large (up to 114°) post Mesozoic rotations reported by Channell et al. [1980, 1990] for the Sicanian limestones, as related to both Miocene (or older?) deformational episodes and the Plio‐Pleistocene evolution of the Gela Nappe.
Structure of the Castillo granite, Southwest Spain: Variscan deformation of a late Cadomian pluton
Authors: Eguíluz L. , Apraiz A. , Ábalos B.
Published: September 2002

A geometrical reconstruction of the 500 Ma old Castillo granite pluton (SW Iberia) is completed on the basis of structural and geophysical (rock magnetism) techniques. The pluton is intrusive into latest Proterozoic‐earliest Cambrian metasediments and conforms a tabular intrusion 6 km in diameter and 1.7 km thick that was emplaced at a depth of 10 km. Its magnetic fabric reveals that the strike of moderately to steeply dipping magmatic flow planes forms a high angle to the regional tectonic trends. Magnetic foliations and associated moderately to gently plunging magnetic lineations represent magmatic flow planes and directions. The internal anisotropy of the granite together with the structure shown by the country rocks attest the lateral propagation of the pluton and its latter inflation. The pluton's root zone would correspond to a likely thin, subvertical feeder structure initiated near the orientation of regional σ1 at the time of emplacement. During the Variscan orogeny the pluton was tilted and underwent localized brittle‐ductile strain in relation to shear zone deformation in the footwall of a major ductile thrust. Tilting permits the observation and study of a vertical profile of the intrusion. Localized deformation caused superposition of tectonic zonations on the magmatic ones, a reactivation of the basal contact of the pluton, and dismemberment from its root. This and other granitoid plutons of similar age emplaced at a similar depth constrained the creation of crustal mechanical heterogeneity and anisotropy. This controlled the site of pluton emplacement, the nucleation of a major ductile thrust, and localization of deformation and tectonic displacements along the pluton margins during later orogenic reactivation.
Paleozoic rocks of northern Chukotka Peninsula, Russian Far East: Implications for the tectonics of the Arctic region
Authors: Natal'in Boris A. , Amato Jeffrey M. , Toro Jaime , Wright James E.
Published: September 2002

Paleozoic rocks exposed across the northern flank of the mid‐Cretaceous to Late Cretaceous Koolen metamorphic dome make up two structurally superimposed tectonic units: (1) weakly deformed Ordovician to Lower Devonian shallow marine carbonates of the Chegitun unit which formed on a stable shelf and (2) strongly deformed and metamorphosed Devonian to Lower Carboniferous phyllites, limestones, and andesite tuffs of the Tanatap unit. Trace element geochemistry, Nd isotopic data, and textural evidence suggest that the Tanatap tuffs are differentiated calc‐alkaline volcanic rocks possibly derived from a magmatic arc. We interpret the associated sedimentary facies as indicative of deposition in a basinal setting, probably a back arc basin. Orthogneisses in the core of the Koolen dome yielded a Devonian (between ∼369 and ∼375 Ma) U‐Pb zircon age which is similar to the ages of the Tanatap tuffs as well as granitic plutons formed within a Devonian active continental margin of northern Alaska. The stratigraphy of the Chegitun unit is similar to that of the Novosibirsk carbonate platform which overlies the Late Precambrian Bennett‐Barrovia block. The basement of the block is exposed in Chukotka where ortogneiss in the Chegitun River valley yielded Late Proterozoic (∼650 to 550 Ma) U‐Pb ages. These two tectonic units form the shelf of the Chukchi and East Siberian Seas and may continue into northern Alaska as the Hammond subterrane. The deep‐water Tanatap unit can be traced along the southern boundary of the Bennett‐Barrovia block from the Novosibirsk Islands to northern Alaska This basin was paired with a Devonian magmatic arc that existed farther to the south. The northern margin of the Bennett‐Barrovia block collided with North America in the Late Silurian to Early Devonian. In Chukotka, during Middle to Late Carboniferous time the reconstructed Devonian arc‐trench system at the southern edge of the Bennett‐Barrovia block collided with an unknown continental object, fragments of which now occur to the south of the South Anyui suture. Triassic to Cretaceous deformation strongly modified the Paleozoic units. Our results provide new constraints on the geometry and Paleozoic history of the Chukotka‐Arctic Alaska block, the essential element involved in the opening of the Canada basin.
Tectonic entrapment and its role in the evolution of continental lithosphere: An example from the Precambrian of western Canada
Authors: Ross Gerald M. , Eaton David W. , Boerner David E. , Miles Warner
Published: September 2002

New geophysical data acquired over the buried crystalline basement of western Canada provide constraints on the history of tectonic assembly of the western Canadian Shield in the interval 1.75–1.85 Ga. Specifically, these data provide new perspectives on the evolution of an Archean continental fragment (Heame province) that was trapped in a tectonic “vise” between coeval orogenic belts that dipped beneath the Hearne province. The Trans‐Hudson orogen developed along the southeastern margin of the Hearne province as a series of ocean floor, oceanic arc, and arc marginal basins were telescoped and thrust obliquely beneath the Hearne. Along the northwest edge of the Hearne, collapse and subduction of a narrow marginal basin, now marked by the subsurface extension of the Snowbird Tectonic Zone, led to formation of magmatic arc and collision of older Proterozoic terranes. The Hearne province itself is characterized by regional granulite‐grade metamorphism and evidence of extensive and pervasive partial melting of the crust. The internal character of the Hearne province seen on crustal seismic reflection profiles is that of a crustal‐scale structural fan with reflection fabrics that verge toward the bounding orogens. The deformation of the Hearne is predominantly of Paleoproterozoic age and constitutes a thorough reworking of this formerly Archean crustal domain over a distance of more than 600 km across strike. Entrapment and thermal weakening of the Hearne resulted from mechanical coupling of inferred buoyant subduction‐collision zones and removal of or modification of Archean lithospheric mantle that may have originally formed the keel to the Hearne. Long‐period magnetotelluric profiles show that anomalously conductive mantle lithosphere underlies the present day Hearne province, which is attributed to metasomatic modification of the subcontinental lithosphere following collisional thickening and delamination/convective removal of thickened lithosphere in the Proterozoic. Tectonic entrapment, as illustrated by the Proterozoic structural and thermal evolution of the Hearne province crust and subcontinental mantle, may be an example of the lithospheric consequences of opposing collisional polarity during assembly of continents.
Terranes or Cambrian polar wander: New data from the Scott Glacier area, Transantarctic Mountains, Antarctica
Authors: Grunow Anne M. , Encarnación John
Published: September 2002

We present new paleomagnetic and isotopic data from the Transantarctic Mountains in East Antarctica that constrain the paleogeographic position of this region during the late Early and Middle Cambrian. Two new poles have been determined from volcanic and granitic rocks in the Scott Glacier area. The first pole is from the Wyatt and Ackerman Formations (∼525 Ma) and the Mount Paine tonalite (40°E, 1°N, A95= 6° (error circle at the 95% confidence level), N=11 VGPs). The second pole is from the Zanuck granite (36°E, 7°S, A95= 9°, N=9 VGPs). These poles differ from the Gondwana early Paleozoic reference poles and could indicate that the Scott Glacier area (and by geologic correlation, north Victoria Land, eastern Australia, and West Antarctica?) was part of a terrane accreted to East Gondwana in the Cambrian. Another possibility is that these new poles support rapid apparent polar wander in the late Early Cambrian and Middle Cambrian. This apparent polar wander event could be related to rapid plate motions or to a true polar wander event. Lastly, there may have been a local vertical axis rotation of the Scott Glacier area if the Wyatt pole is compared with the African Ntonya pole.
Tectonic evolution of a greenstone sequence in northern Zimbabwe: sequential early stacking and pluton diapirism
Authors: Jelsma Hielke A. , Dirks Paul H. G. M.
Published: September 2002

Structural‐metamorphic relationships in the 2.71–2.64 Ga Harare‐Shamva greenstone belt in northern Zimbabwe demonstrate that the Archean stratigraphy of the Zimbabwe craton can not be regarded as a coherent and nonrepetitive, autochthonous succession of (ultra)mafic and felsic greenstones. The greenstone belt shows an early set of amphibolite‐grade D1/M1 structures and associated synkinematic metamorphic assemblages that were formed during an episode of layer‐parallel shearing at about 2.64 Ga. This resulted in sequential west directed imbricate stacking and recumbent folding of distinct tectonostratigraphic greenstone sequences and includes the development of a syntectonic sedimentary succession. D2/M2 fabrics, strain patterns, and contact aureoles overprint D1 geometries and are related to the diapiric emplacement of the Chinamora batholith. The structural data indicate that granite‐gneiss terrains can be subdivided into two types: (1) structurally emplaced terrains (the Madziwa gneiss terrain) and (2) diapiric domes (the Chinamora batholith). The history of the Harare‐Shamva greenstone belt indicates that the late Archean (2.7–2.6 Ga) evolution of the Zimbabwe craton must allow for accretion of diverse crustal fragments including oceanic and back arc mafic crust, volcanic arc felsic crust, continental crust, and related sedimentary sequences. The process of accretion was probably diachronous across the craton and may have involved concomitant diapirism, strike‐slip faulting, magma intrusion, and sedimentation.
The Pamir‐Hindu Kush seismic zone as a strain marker for flow in the upper mantle
Authors: Pavlis Gary L. , Das Shamita
Published: September 2002

We introduce a new hypothesis to explain the unusual pattern of intermediate‐depth earthquakes in the Pamir‐Hindu Kush region of central Asia. In our model the seismicity of the zone is defined by a remnant piece of oceanic crust that became neutrally buoyant and now hangs in the mantle underneath the center of the active mountain belt. We interpret an abrupt cutoff in seismicity shallower than 70 km under the Pamir as evidence that the slab has become decoupled from the surface deformation, and we suggest that the remnant slab has acted as a strain marker for flow in the upper mantle. The contortion of the slab can be explained by superimposed vertical and lateral simple shear related to the western edge of the Indian Plate. This model requires relatively low strain rates of the order of 10−15 s−1, consistent with strain rates estimated from earthquake moment release in Benioff zones. In contrast, the double‐facing subduction zone model applied by a number of previous authors requires strain rates of the order of 10−13 s−1, which are much higher than inferred strain rates in subducting slabs. The results are important for understanding mantle dynamics, providing a unique way to measure strain in the upper mantle. The implied flow field also has significant implications for the kinematics of the entire collisional plate boundary in Tibet and the Tien Shan.
Head and flux variability in heterogeneous unsaturated soils under transient flow conditions
Authors: Ferrante Marco , Yeh T.‐C. Jim
Published: September 2002

A numerical model for the analysis of uncertainty propagation in flow through unsaturated soils is developed. This model is based on a first‐order Taylor series expansion of the discretized Richards equation. Soil hydrologic properties (the saturated hydraulic conductivity and the pore size distribution parameter) are assumed to be stochastic processes in space. The surface boundary conditions can be considered as deterministic variables in time or stochastic time series. Spectral analysis and Monte Carlo simulations were used to verify this numerical model for flow under both steady and transient conditions. The model is then used to examine the effect of uncertainty in boundary conditions and the effect of heterogeneity on the pressure head and flux variance profiles at various times for one‐dimensional vertical flow cases. Dependence of pressure head variance on the flow conditions (drying or wetting) is examined. On the basis of the analysis it is found that the propagation of the head variance is similar to that of the concentration variance for solute transport in saturated aquifers. The head variance is proportional to the mean pressure gradient, and thus large head variances are associated with the wetting and the drying front of a moisture pulse. The peak head variance is smaller at the wetting front than it is at the drying front. This difference is attributed to the difference in the magnitude of mean hydraulic gradient and should not necessarily be interpreted as a hysteresis effect. In addition, it is shown how the variance of the flux of a moisture pulse increases with depth.
Pumping tests in heterogeneous aquifers: An analytical study of what can be obtained from their interpretation using Jacob's Method
Authors: Sánchez‐Vila Xavier , Meier Peter M. , Carrera Jesús
Published: September 2002

Interpretation of pumping tests to estimate hydraulic parameter values is typically based on the assumption of aquifer homogeneity. The applicability of the traditional methods of interpretation in real aquifers can be questioned, since the evaluation of the drawdown curves observed at different locations in a single test may not result in one consistent set of hydraulic parameters. Thus most hydrogeologists tend to look at estimated transmissivities (T) as some average property of the medium, while estimated storativities (S) are disregarded in some cases, particularly when they are obtained from data measured at the pumping well. An analytical study of drawdown under radially convergent flow toward a single point in heterogeneous aquifers shows that large time drawdown values form a straight line on a drawdown versus log time plot. Jacob's method consists of obtaining estimates for T and S from the slope and intercept of this line. We find that even in a heterogeneous field, these estimates provide valuable information about the aquifer. Estimated T values for different observation points tend to converge to a single value, which corresponds to the effective T derived under parallel flow conditions. Estimated storativities, however, display higher variability, but the geometric mean of the Sest values can be used as an unbiased estimator of the actual S. Thus it appears that although Jacob's method was originally derived for homogeneous media, it can provide valuable information in real aquifers.
On the sensitivity of Craig and Gordon Model for the estimation of the isotopic composition of lake evaporates
Authors: Kumar Bhishm , Nachiappan R. P.
Published: September 2002

The effect of errors in the estimated isotopic composition of atmospheric moisture by δa − δp equilibrium assumption method on the isotopic composition of lake evaporate (δE) estimated using the Craig and Gordon linear resistance model has been studied. It has been found that the propagated error in estimated δE increases with normalized relative humidity (h) but is less significant for h up to 0.5. The propagated error becomes unrealistic for h above 0.8. Since h is the major factor influencing the propagated error, an error in h would result in large uncertainty in the estimated δE. Simple analytical expressions for determining the propagated errors have been presented. The study also reveals that for the isotopic enrichment of a water body to continue, the difference between its isotopic composition and that of atmospheric air (Δla) should be greater than the equilibrium enrichment factor (ϵ*,). Using the relationship between Δla and ϵ*, the maximum isotopic enrichment that a water body can achieve at different temperatures can be deduced.
Geochemical and boron, strontium, and oxygen isotopic constraints on the origin of the salinity in groundwater from the Mediterranean Coast of Israel
Authors: Vengosh Avner , Spivack Arthur J. , Artzi Yohanan , Ayalon Avner
Published: September 2002

In order to identify the origin of the salinity and formation of saline plumes in the central part of the Mediterranean coastal aquifer of Israel, we determined the elemental and boron, strontium, and oxygen isotopic compositions of fresh and brackish groundwater (Cl up to 1500 mg/L). We distinguish between two key anthropogenic sources: (1) sewage effluents used for irrigation with high Na/Cl, SO4/Cl, and B/C1 ratios and low Br/Cl ratios relative to seawater ratios, low δ11B values (0–10‰) and high δ18O values (>−4‰); and (2) imported water from the Sea of Galilee that is artificially recharged to the aquifer with high Br/Cl (3 × 10−3) and δ18O values (−1‰) and a low 87Sr/86Sr ratio of 0.70753. The brackish groundwater from the saline plumes have relatively low Na/Cl ratios (0.5–0.8) and high Ca/Mg, Mg/Cl, and Ca/(SO4 + HCO3) (>1) ratios relative to seawater ratios; marine SO4/Cl and Br/Cl ratios; δ11B values of 24.8–49.9‰; δ18O of −2.95‰ to −4.73‰ and 87Sr/86Sr ratios of 0.708275–0.708532. The composition of most of the investigated groundwater from the saline plumes differs from those of the anthropogenic sources, imported water, fresh uncontaminated groundwater (87Sr/86Sr of 0.70866, δ11B of 20–30‰), and saline water from the adjacent Eocene aquitard. Only in areas of artificial recharge does local groundwater have high Br/Cl and δ18O values that are typical to the Sea of Galilee. The linear relationships between chloride and most of the ions, including B and Sr, the relatively high δ11B (>30‰) and low δ18)O (<−4‰) values, and the chemical signature of the saline plumes (e.g., marine Br/Cl and SO4/Cl ratios), suggest that (1) mixing processes control the chemical composition of the brackish water within the aquifer, and (2) the saline postulated end‐member has a chemical composition that resembles modified seawater with a marine and higher δ11B values, and a 87Sr/86Sr ratio of <0.7083. We propose that most of the salinization phenomena and the formation of saline plumes in the inner parts of the coastal aquifer are derived from upconing of underlying natural saline water bodies and enhanced by overexploitation and draw‐down of the overlying fresh groundwater.
A k‐nearest‐neighbor simulator for daily precipitation and other weather variables
Authors: Rajagopalan Balaji , Lall Upmanu
Published: September 2002

A multivariate, nonparametric time series simulation method is provided to generate random sequences of daily weather variables that “honor” the statistical properties of the historical data of the same weather variables at the site. A vector of weather variables (solar radiation, maximum temperature, minimum temperature, average dew point temperature, average wind speed, and precipitation) on a day of interest is resampled from the historical data by conditioning on the vector of the same variables (feature vector) on the preceding day. The resampling is done from the k nearest neighbors in state space of the feature vector using a weight function. This approach is equivalent to a nonparametric approximation of a multivariate, lag 1 Markov process. It does not require prior assumptions as to the form of the joint probability density function of the variables. An application of the resampling scheme with 30 years of daily weather data at Salt Lake City, Utah, is provided. Results are compared with those from the application of a multivariate autoregressive model similar to that of Richardson [1981].
Quantifying diffusive mass transfer in fractured shale bedrock
Authors: Jardine P. M. , Sanford W. E. , Gwo J. P. , Reedy O. C. , Hicks D. S. , Riggs J. S. , Bailey W. B.
Published: September 2002

A significant limitation in defining remediation needs at contaminated sites often results from an insufficient understanding of the transport processes that control contaminant migration. The objectives of this research were to help resolve this dilemma by providing an improved understanding of contaminant transport processes in highly structured, heterogeneous subsurface environments that are complicated by fracture flow and matrix diffusion. Our approach involved a unique long‐term, steady state natural gradient injection of multiple tracers with different diffusion coefficients (Br, He, Ne) into a fracture zone of a contaminated shale bedrock. The spatial and temporal distribution of the tracers was monitored for 550 days using an array of groundwater sampling wells instrumented within a fast flowing fracture regime and a slow flowing matrix regime. The tracers were transported preferentially along strike‐parallel fractures, with a significant portion of the tracer plumes migrating slowly into the bedrock matrix. Movement into the matrix was controlled by concentration gradients established between preferential flow paths and the adjacent rock matrix. Observed differences in tracer mobility into the matrix were found to be a function of their free‐water molecular diffusion coefficients. The multiple tracer technique confirmed that matrix diffusion was a significant process that contributed to the overall physical nonequilibrium that controlled contaminant transport in the shale bedrock. The experimental observations were consistent with numerical simulations of the multitracer breakthrough curves using a simple fracture flow model. The simulated results also demonstrated the significance of contaminant diffusion into the bedrock matrix. The multiple tracer technique and ability to monitor the fracture and matrix regimes provided the necessary experimental constraints for the accurate numerical quantification of the diffusive mass transfer process. The experimental and numerical results of the tracer study were also consistent with indigenous contaminant discharge concentrations within the fracture and matrix regimes of the field site. These findings suggest that the secondary source contribution of the bedrock matrix to the total off‐site transport of contaminants is relatively large and potentially long‐lived.
Scaling hydraulic properties of a macroporous soil
Authors: Mohanty Binayak P.
Published: September 2002

Macroporous soils exhibit significant differences in their hydraulic properties for different pore domains. Multimodal hydraulic functions may be used to describe the characteristics of multiporosity media. I investigated the usefulness of scaling to describe the spatial variability of hydraulic conductivity (K(−h)) functions of a macroporous soil in Las Nutrias, New Mexico. Piecewise‐continuous hydraulic conductivity functions suitable for macroporous soils in conjunction with a hybrid similar media‐functional normalization scaling approach were used. Results showed that gravity‐dominated flow and the related hydraulic conductivity (K(minus;h) functions of the macropore region are more readily scalable than capillary‐dominated flow properties of the mesopore and micropore regions. A possible reason for this behavior is that gravity‐dominated flow in the larger pores is mostly influenced by the pore diameter which remains more uniform as compared to tortuous mesopores and micropores with variable neck and body sizes along the pore length.