bluuebunny/crossref_metadata_2025 huggingface dataset:
Planetary boundary layer and precipitation studies using lower atmospheric wind profiler over tropical India
Authors: Krishna Reddy K. , Kozu Toshiaki , Ohno Yuichi , Nakamura Kenji , Higuchi Atsushi , Madhu Chandra Reddy K. , Anandan V. K. , Srinivasulu P. , Jain A. R. , Rao P. B. , Ranga Rao R. , Viswanathan G. , Narayana Rao D.
Published: October 2002

For the first time in India, an L‐band (1357.5 MHz) lower atmospheric wind profiler (LAWP) has been installed and successfully operated at Gadanki, India, since September 1997. The first results of the accuracy can be given on the basis of about 24‐day intercomparisons between LAWP and mesosphere‐stratosphere‐troposphere radar data. The root‐mean‐square differences (RMS deviation) have been found to range between 1.18 m/s and 1.6 m/s for the wind speed. The two wind profilers compliment each other quite well, considering both the availability and the reliability of the wind measurements. Statistics of the data availability can be shown based on 775 days of data in low mode and about 532 days of data in high mode. The 80% availability of the LAWP was determined with 3.6‐km wind measurements in low mode and 5‐km wind measurements in high mode. LAWP observations show well‐marked planetary boundary layer diurnal variation on clear sunny days. We found that with a few exceptions the drier period has a higher boundary layer compared with the wet period, indicating that in the wet season, most of the net solar radiation evaporated moisture rather than heating the surface and therefore contributed little to buoyant forcing. We classified precipitating clouds into three types: convective, transition, and stratiform. Diurnal and seasonal variation of the occurrence of precipitating cloud systems shows that the precipitation primarily occurs in the afternoon and the convective and transition clouds are most frequent in the summer monsoon, while the occurrence of stratiform clouds is predominant in the winter monsoon.
Study of a precipitating cloud system using Chung‐Li VHF radar
Authors: Krishna Reddy K. , Shun‐Peng Shih , Chu Yen Hsyang
Published: October 2002

In this paper, Chung‐Li VHF radar returns from hydrometeors and reflectivity fluctuations associated with “showery” precipitating cloud systems are studied in detail. The space antenna drift method is applied to three rain gauges in the vicinity of the radar to obtain the drift velocity of the rain cells during the passage of the precipitation cloud systems. The drift velocity of the rain cells is found to be 13.4 m/s and moving in the southwest to northeast direction, and these results are in good agreement with the simultaneous observations of the horizontal wind velocity observed with the radar. VHF radar can see the frontal structure so clearly because of its sensitivity to thermal stratification in the atmosphere. A composite analysis of the turbulence and precipitation echo intensity and vertical air velocity indicates that the vertical air velocity plays a vital role for the occurrence of showery precipitation. The mean raindrop diameters are estimated from the air motion adjusted Doppler velocities resolved by the radar. Observational results show that the mean raindrop sizes of 0.5–3.5 mm are primarily responsible for the showery precipitation. The ambient air motions and turbulent broadening effects are discussed before and during the passage of the precipitation event. The analysis suggests that the beam‐broadening effect needs to be considered if the information of the drop size distribution is to be estimated from the observed Doppler spectral width.
High‐order high‐frequency solutions of rough surface scattering problems
Authors: Bruno Oscar P. , Sei Alain , Caponi Maria
Published: October 2002

A new method is introduced for the solution of problems of scattering by rough surfaces in the high‐frequency regime. It is shown that high‐order summations of expansions in inverse powers of the wave number can be used within an integral equation framework to produce highly accurate results for surfaces and wavelengths of interest in applications. Our algorithm is based on systematic use and manipulation of certain Taylor‐Fourier series representations and explicit asymptotic expansions of oscillatory integrals. Results with machine precision accuracy are presented which were obtained from computations involving expansions of order as high as 20.
Irregular terrain wave propagation by a Fourier split‐step wide‐angle parabolic wave equation technique for linearly bridged knife‐edges
Authors: Eibert Thomas F.
Published: October 2002

A novel Fourier split‐step algorithm for the solution of the parabolic wave equation is proposed. The derivations are based on a two‐dimensional linearly bridged knife‐edge terrain model that considers ideally conducting as well as dielectric lossy ground together with a dielectric lossy layer as macroscopic representation of vegetation and buildings. The boundary condition at the terrain interface is formulated in the spectral domain, and it is shown that waves with a very wide angular spectrum with respect to the horizontal can accurately be modeled as long as the dominant portion of energy propagates above the terrain. Validation results are given for an ideally conducting as well as lossy dielectric wedge and an ideally conducting rounded obstacle. Also, real‐world propagation curves show good agreement with measured data.
Solar cycle dependence of polar cap patch activity
Authors: Dandekar B. S.
Published: October 2002

Ionospheric foF2 data from two stations, Sondrestromfjord and Qaanaq, Greenland, for 4 years corresponding to four different levels of sunspot activity (sunspot number (SSN) = 150 ± 50, 125 ± 50, 40 ± 20, and 10 ± 10) are used to study the dependence of occurrence and intensity of polar cap patches on solar cycle activity. At high sunspot activity, on the average, Sondrestromfjord sees six patches per day, and Qaanaaq sees eight patches per day. At low sunspot activity the number drops to three per day at both stations. It is found that on a daily basis the polar cap patch activity is independent of the sunspot number for weaker patches (≥3 MHz) but reduces with sunspot number for stronger patches (≥6 MHz). Only at high sunspot activity, stronger patches are seen for 20% of the time. The duration of polar cap patches reduces from 50% at high sunspot activity (SSN = 150) to 15% at low sunspot activity (SSN = 10). It exhibits a seasonal dependence with a maximum in winter months. The diurnal maximum in polar cap patch activity at both stations occurs close to local magnetic noontime. Assuming that polar cap patches are generated at high latitude (Sondrestromfjord) and drift to polar latitude (Qaanaaq), only about 30% of patches seen at Qaanaaq can be accounted for as originating at Sondrestromfjord. A reduction in patch activity with reduction in sunspot number is directly related to a reduction in the strength of the source: the tongue of ionization. The polar cap patches exhibit a weak dependence on the interplanetary magnetic activity index Kp. The distribution of the interplanetary magnetic field (IMF) for the patch occurrence very much resembles that of bx, by, bz of the IMF database. Over the solar cycle all these changes cover more than an order of magnitude of range. The effect of polar cap patch activity on communications operating at high‐latitude regions is briefly discussed.
Vertical profiling of atmospheric refractivity from ground‐based GPS
Authors: Lowry Anthony R. , Rocken Chris , Sokolovskiy Sergey V. , Anderson Kenneth D.
Published: October 2002

Atmospheric refractivity is typically estimated in situ from radiosonde measurements, which are expensive and may undersample the spatial and temporal variability of weather phenomena. We estimate refractivity structure near San Diego, California, using ray propagation models to fit measured GPS tropospheric delays in a least squares metric. We evaluate the potential and the limitations of ground‐based GPS measurements for characterizing atmospheric refractivity, and we compare refractivity structure estimated from GPS sensing with that measured by nearby radiosondes. The results suggest that ground‐based GPS provides significant constraint of inhomogeneous atmospheric refractivity, despite certain fundamental limitations of ground‐based measurements. Radiosondes typically are launched just a few times daily. Consequently, estimates of temporally and spatially varying refractivity that assimilate GPS delays could substantially improve over estimates using radiosonde data alone.
A direction‐sensitive model of atmospheric noise and its application to the analysis of HF receiving antennas
Authors: Coleman C. J.
Published: October 2002

Global maps of lightning occurrence are combined with ray tracing propagation calculations to form a direction‐sensitive model of atmospheric noise. The model suggests a very complex directional behavior that can vary strongly with location, time, season, sunspot number, and frequency. It is shown that the directional variability of noise, when coupled with the directional variability of antenna gain, can lead to marked changes in noise outcome between different antennas. The implication of directional varying noise for the optimum choice of receiver antenna is explored.
A new algorithm for high‐quality ionogram generation and analysis
Authors: Arıkan Feza , Arıkan Orhan , Salous Sana
Published: October 2002

Standard digital ionograms that are generated by fast Fourier transform or autoregressive modeling suffer from high interference levels due to other users of the HF channel which produce artifacts and distortion, hence complicating automatic processing and information extraction. In this paper, a new method is proposed to obtain high‐quality ionograms of the desired layer reflections and automatically extract important information such as critical frequencies. Following the standard procedures, two sets of periodograms are obtained by using rectangular and Blackman windows. These two periodograms are filtered and fused utilizing an automatic edge‐detection‐based time‐frequency detector. The fused ionogram provides sharp description of the layer reflections with very low sidelobe structure (ringing). The performance of this new ionogram algorithm is tested using chirp sounder data collected from an oblique midlatitude path. It is observed that the presented algorithm is highly successful in obtaining robust and sharp ionograms free of artifacts. Furthermore, a new algorithm is proposed for automated computation of dispersion and critical frequencies of the magnetoionic components detected on the ionogram. Since efficient signal‐processing algorithms are utilized, the proposed method can be implemented in real time.
Extraction of the surface waves and waveguide modes of the Green's function in layered anisotropic dielectrics between two ground planes
Authors: Song Ben , Hong Wei
Published: October 2002

In layered lossless dielectrics between two ground planes there exist the complicated waveguide modes as well as the surface waves in the electromagnetic field induced by a horizontal electric dipole. All these modes can be characterized by the rational parts with the real poles of the spectral domain vector and scalar potentials. The accurate extraction of these modes plays an important role in the evaluation of the Green's function in spatial domain. In our previous work [Song and Hong, 2002] a new algorithm based on rational approximation is presented for the extraction of surface waves and waveguide modes of the Green's function for layered isotropic dielectrics. In this paper, we extend the algorithm to the more complex case of layered anisotropic dielectrics; this algorithm can accurately extract all the real poles and their residues simultaneously. Thus we can get all the surface waves and waveguide modes, which can greatly help in the calculation of the spatial domain Green's function. The numerical results demonstrate the accuracy and efficiency of the proposed method.
Scattering from a buried circular cylinder illuminated by a three‐dimensional source
Authors: Hansen Thorkild B. , Meincke Peter
Published: October 2002

We employ plane and cylindrical wave expansions with the fast Fourier transform to solve scattering problems involving a circular cylinder buried in soil. The illumination is provided by a three‐dimensional source located in air above ground. Plane wave expansions describe transmitted and reflected fields at the air‐soil interface, and cylindrical wave expansions describe the fields scattered by the cylinder. The two types of expansions are joined by employing explicit expressions that relate cylindrical and plane waves. We neglect multiple interactions between the cylinder and the interface. The cylinder and soil can have frequency‐dependent permittivities and conductivities. With the formulas cast in a special form the scattered fields can be computed rapidly for fixed‐offset configurations in which the location of the source is different for each observation point. Fixed offset is the most commonly used transmitter‐receiver configuration for ground‐penetrating radar (GPR). Numerical simulations involving time domain fields and fixed‐offset configurations determine the radar responses of various types of pipes and conductive soils encountered in GPR.
A novel microwave beacon for coastal navigation
Authors: King Ronold W. P.
Published: October 2002

The remarkable properties of a unique resonant circular array are applied to provide a microwave beacon to supplement warning flashing lights, lighthouses, and bells in coastal shipping. Following a description of the structure and properties of the array of antennas, the design of a special form of the array is explained, and its radiation field is described. This consists of 90 very sharp spikes between 90 equally sharp nulls. In the suggested array, the single driven element is excited at a microwave frequency. By rotating the array a sequence of microwave pulses is radiated that can be made equal to the frequency of flashes from any flashing light.
Incoherent scatter spectral bandwidth and its applications
Authors: Zhou Qihou H.
Published: October 2002

We derive an analytical expression of equivalent incoherent scatter spectral bandwidth and discuss its practical and interpretative usefulness at different altitude ranges. In the region above 120 km, where collision can be neglected, the equivalent bandwidth is a simple function of electron and ion temperature, ion mass, and ion composition. In the collision‐dominated D region, equivalent bandwidth can be used to conveniently assess the effect of negative ions. As an example to illustrate the usefulness of the equivalent bandwidth, we also present nighttime incoherent scatter E region temperature measurement, which has not been reported previously for middle and low latitudes.
Studies of 19.5 GHz sky radiometric temperature: Measurements and applications
Authors: Shih Shuen‐Peng , Chu Yen‐Hsyang
Published: October 2002

Using a single‐frequency ground‐based microwave radiometer with steerable horn antenna at frequency 19.5 GHz, we measure sky radiometric temperature over the Taiwan area during the period May 1997 to September 1999. Statistics indicate that for more than 80% of the time, in the absence of surface precipitation, the brightness temperature is above 27 K. Different elevation angle observations show that the median brightness temperature increases with decreasing elevation angle from about 30 K at zenith to about 128 K at 80° off‐zenith angle. The opacity (or gaseous attenuation) is also estimated from measured brightness temperature. We found that the elevation angle variation of the median value of opacity follows exactly the secant law, while that of opacity with other probability does not. The mean radiated temperature Tm over the Taiwan area is also calculated using 5 years of radiosonde data, showing that Tm is a function of elevation angle and its value at 10° elevation angle is greater than that at zenith by 0.5–1.3%. A method of retrieving integrated precipitable water vapor content (IPWV) from the observed brightness temperature measured by a single‐frequency radiometer at 19.5 GHz is developed and examined. Comparing the retrieved and measured IPWVs shows a good agreement between them, validating the method proposed in this article. The correlation between 19.5 GHz radiometric opacity and 52 MHz wind profiler backscatter from clear‐air sky is also analyzed. The result shows that the correlation will be negative if the vertical distribution of the wind profiler backscatter is characterized by salient layered structures. Therefore great caution should be taken when radiometric measurements are incorporated with wind profiler backscatter to retrieve the humidity profile.
Method of moments solution of volume integral equations using parametric geometry modeling
Authors: Sertel Kubilay , Volakis John L.
Published: October 2002

A higher‐order geometry modeling technique is presented for the solution of volume integral equations (VIEs) involving arbitrarily shaped inhomogeneous dielectric structures. Conformal basis functions defined in curved hexahedral finite elements are used to develop a moment method solution of the VIE. Results are compared with available analytical solutions, and the method is shown to deliver higher accuracy using coarser meshes as compared to partial differential equation solvers.
Transverse electric and transverse magnetic Green's function for coherent and incoherent propagation over a finitely conducting rough surface
Authors: Ishimaru Akira , Rockway John D. , Kuga Yasuo , Lee Seung‐Woo
Published: October 2002

This paper presents wave propagation over a finitely conducting half‐space whose surface is bounded by a rough surface of small RMS height. An electric or magnetic current source is used to excite the rough surface, and the Green's function for transverse electric (TE) and transverse magnetic wave propagation is obtained. The appearance of roughness at the boundary produces both a coherent (mean) and incoherent (fluctuating) field distribution, which is obtained from Dyson's equation and Bethe‐Salpeter's equation, respectively. The coherent Green's function for vertical polarization exhibits similar characteristics to the Sommerfeld dipole problem where the Zenneck wave pole is modified by roughness. The incoherent field generated by rough surfaces is obtained for both vertical and horizontal polarization, and the conventional cross section per unit length of the rough surface is modified to include the effects of surface roughness. For angles near grazing, a low‐grazing‐angle cross section is obtained by evaluating the Bethe‐Salpeter's equation with the Sommerfeld solution. Finally, the coherent and incoherent intensity for the TE rough surface Green's function is obtained and compared to Monte Carlo simulations.
A comparison of rain attenuation and drop size distributions measured in Chilbolton and Singapore
Authors: Åsen Walther , Gibbins Chris J.
Published: October 2002

Attenuation of radio waves caused by precipitation, especially in the form of rain, is considered to be the limiting factor for new communication systems that will exploit the radio wave spectrum at frequencies higher than about 30 GHz. Over the last 40 years, much effort has gone into theoretical studies characterizing rain in terms of statistical drop size distributions (DSDs), the shapes and velocity dependence of raindrops, and the calculation of raindrop extinction cross sections. This paper focuses on specific data sets and data processing and different ways of viewing DSDs that may help in quantifying some of the important parameters in radio wave propagation from experimental data. Values for the coefficients k and α in the relationship for specific rain attenuation γ = kRα are presented, together with rain‐rate‐dependent parameters for fits of DSDs to standard statistical distributions. These are based on data from Chilbolton, England, and from Singapore. The distributions measured at Chilbolton and Singapore are very different, which strongly suggests that drop size distributions differ under different climatic conditions. Comparisons are also presented of attenuations calculated with values of k and α determined from the DSDs and values found from logarithmic regression between simultaneous rain rate and attenuation measurements at 57, 97, 135, and 210 GHz at Chilbolton. This paper gives a strong indication that the International Telecommunication Union Radiocommunication Sector model for specific rain attenuation is inadequate at frequencies higher than about 70 GHz.
Finite element analysis of two‐dimensional EM scattering via Padé approximation for complex permittivity
Authors: Abu‐Zaid Naser A. , Tosun Haluk
Published: October 2002

We present a computationally efficient algorithm which combines the finite element method with Padé approximation. The combination is used to solve the problem of transverse magnetic and transverse electric scattering from homogeneous lossy dielectric cylinders over a continuous range of the complex permittivity variable by requiring the finite element solution only at a single complex permittivity value. The proposed method is based on (1) assuming a power series expansion for the unknown solution vector, the excitation vector, and the system matrix, (2) substituting this into the system matrix equation, and (3) finding the recursion relation for the solution vectors.
An ionospheric error model for time difference of arrival applications
Authors: Prag A. B. , Brinkman D. G.
Published: October 2002

The geolocation accuracy of satellite‐borne time difference of arrival systems is limited by the uncompensated differential delay experienced by each pair of satellite‐to‐ground paths. We have measured the ionospheric component of this differential delay using two‐frequency GPS receivers at seven ground sites; one in the high‐latitude region, four at midlatitudes, and two in the equatorial region. The measurements were expressed as differential total electron content (ΔTEC) so that they could be used at frequencies other than the 1.6 and 1.2 GHz GPS values and be readily compared with ionospheric models. Three models were used to account for the measured ΔTEC values: a climatological model (RIBG) and two modified versions of Jet Propulsion Laboratory's GIM model (a postanalysis and a near‐real‐time implementation using a subset of reporting stations). All three models reduced the ΔTEC differences significantly. The postanalysis GIM model performed better than the other two in the tails of the residual error distribution, but there was no clear difference among the three in about 75% of the cases. However, when the data were binned according to local time and angle between the two rays, both GIM models were more accurate than RIBG between about 0600 and 1900 LT at angles greater than about 50°.
Evidence for 3650–3600 Ma assembly of the northern end of the Itsaq Gneiss Complex, Greenland: Implication for early Archaean tectonics
Authors: Nutman Allen P. , Friend Clark R. L. , Bennett Vickie C.
Published: October 2002

The world's most extensive, well preserved, early Archaean (>3500 Ma) structures occur in the northern end of the 3850–3560 Ma Itsaq Gneiss Complex, southern west Greenland, an area which includes the 3800–3700 Ma Isua supracrustal belt. These provide a unique opportunity to study early tectonics. The area is dominated by ∼3800–3650 Ma orthogneisses containing migmatized inclusions of ≥3800 Ma metavolcanic, sedimentary, ultramafic, and gabbroic rocks. The thickest and most extensive of these inclusions forms the southern side of the Isua supracrustal belt. Also present are panels of less migmatized 3710–3690 Ma volcanic and sedimentary rocks, in the northern side of the Isua supracrustal belt, and probably also in the Tussaap supracrustal belt in the south of the area. Combined mapping and geochronology has identified mylonites which were active before regional early Archaean folding and 3580–3560 Ma metamorphism. Sensitive high‐resolution ion microprobe dating of premylonitization, synmylonitization, and postmylonitization granite sheets indicates some mylonites were active between 3650 and 3600 Ma. During this period it is proposed that large horizontal motions and interleaving occurred, results of which included juxtaposition of supracrustal sequences in the Isua supracrustal belt with different depositional ages. The tectonic interleaving was most likely due to horizontal motions in the vicinity of plate margins. The new combined field observations and geochronology support hypotheses that some form of plate tectonics at least contributed to early Archaean sialic crustal development.
Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from40Ar/39Ar and (U‐Th)/He thermochronology
Authors: Kirby Eric , Reiners Peter W. , Krol Michael A. , Whipple Kelin X. , Hodges Kip V. , Farley Kenneth A. , Tang Wenqing , Chen Zhiliang
Published: October 2002

High topography in central Asia is perhaps the most fundamental expression of the Cenozoic Indo‐Asian collision, yet an understanding of the timing and rates of development of the Tibetan Plateau remains elusive. Here we investigate the Cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the Sichuan Basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed. Temperature‐time paths inferred from40Ar/39Ar thermochronology of biotite, multiple diffusion domain modeling of alkali feldspar40Ar release spectra, and (U‐Th)/He thermochronology of zircon and apatite imply that rocks at the present‐day topographic front of the plateau underwent slow cooling (<1°C/m.y.) from Jurassic times until the late Miocene or early Pliocene. The regional extent and consistency of thermal histories during this time period suggest the presence of a stable thermal structure and imply that regional denudation rates were low (<0.1 mm/yr for nominal continental geotherms). Beginning in the late Miocene or early Pliocene, these samples experienced a pronounced cooling event (>30°–50°C/m.y.) coincident with exhumation from inferred depths of ∼8–10 km, at denudation rates of 1–2 mm/yr. Samples from the interior of the plateau continued to cool relatively slowly during the same time period (∼3°C/m.y.), suggesting limited exhumation (1–2 km). However, these samples record a slight increase in cooling rate (from <1 to ∼3°C/m.y.) at some time during the middle Tertiary; the tectonic significance of this change remains uncertain. Regardless, late Cenozoic denudation in this region appears to have been markedly heterogeneous, with the highest rates of exhumation focused at the topographic front of the plateau margin. We infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable Sichuan Basin and thus marks the onset of deformation related to the development of the Tibetan Plateau in this region. The present margin of the plateau adjacent to and north of the Sichuan Basin is apparently no older than the late Miocene or early Pliocene (∼5–12 Ma).
Revisiting the paleomagnetism of the 1.476 Ga St. Francois Mountains igneous province, Missouri
Authors: Meert Joseph G. , Stuckey William
Published: October 2002

A paleomagnetic investigation of the St. Francois Mountains igneous province in southeastern Missouri provides a key 1476 ± 16 Ma paleomagnetic pole for Laurentia. The pole (13.2°S, 219.0°E; dp = 4.7°, dm = 8.0°) is considered primary on the basis of positive conglomerate, inverse baked contact, and fold tests. An analysis of 1470–1430 Ma poles from Laurentia highlights key differences between poles obtained from the Belt Supergroup, Electra Lake gabbro, and cratonic North America. Paleolatitudes based on the Lower Belt Supergroup poles are enigmatic, as two previous studies yielded a difference of 10°. Our new pole, combined with an analysis of previous results, favors the higher latitude interpretation for the Lower Belt Supergroup. Paleolatitudes from the younger Belt rocks indicate lower latitudes than coeval rocks from elsewhere in Laurentia for which there has been no adequate explanation. A comparison of the St. Francois Mountain pole with similar‐age poles from Baltica, Siberia and Australia allow first‐order tests of proposed continental configurations. Paleomagnetic data from Australia are compatible with proposed Rodinia reconstructions, whereas paleomagnetic data from Baltica are not. We are unable to rigorously test the alternative suggestion that places Siberia against the western margin of Laurentia due in part to large errors associated with Siberian paleomagnetic data.
Granite fabrics and regional‐scale strain partitioning in the Seridó belt (Borborema Province, NE Brazil)
Authors: Archanjo Carlos J. , Trindade Ricardo I.F. , Bouchez Jean Luc , Ernesto Marcia
Published: October 2002

Fabrics of the Brasiliano‐age granitoid plutons of the Seridó belt (northeastern Brazil) were organized following the partitioned regional strain field which shaped larger areas of the basement and its metapelitic cover. Nine plutons located at the central and western portions of the belt have been investigated by means of anisotropy of magnetic susceptibility. These plutons, emplaced between 580 and 575 Ma, are mainly potassic calk‐alcalic magnetite‐bearing monzogranites. The presence of primary coarse‐grained magnetite as the main magnetic mineral is indicated by optical and electronic microscopy, magnetic susceptibilities, hysteresis ratios, and thermomagnetic curves. The plutons show a strong internal coherence of magnetic fabric, with foliation poles distributed around a zone axis parallel to the gently plunging mean lineation. The magnetic fabric was used to trace the strain partitioning along the belt. In the central transpressive shear belt the magnetic lineations are parallel to north trending regional stretching. On the other hand, in the western basement block a voluminous granitic magmatism was emplaced as a consequence of a transtensional/extensional deformation. Lineations in these granites trend in a NE direction, which is parallel to the transport, inferred from kinematic indicators along the shear zones. A crustal‐scale east trending dextral shear zone connects both the transtensional and transpressional domains.
Mechanisms and timing of exhumation of collision‐related metamorphic rocks, southern Brooks Range, Alaska: Insights from 40Ar/39Ar thermochronology
Authors: Vogl James J. , Calvert Andrew T. , Gans Phillip B.
Published: October 2002

New 40Ar/39Ar thermochronologic data document the spatial distribution of cooling ages and rates across a metamorphic culmination in the south central Brooks Range, providing important constraints on the timing and processes responsible for exhumation of deep‐seated metamorphic rocks. The data indicate widespread episodic exhumation‐related cooling, with short‐lived events in the mid‐Albian to early Cenomanian and Paleocene to Eocene. Hornblende ages indicate that cooling from the metamorphic peak began at 105–103 Ma across the epidote‐amphibolite facies core of the culmination. Mica ages, however, increase southward from ∼90 Ma to ∼100 Ma, indicating that average cooling rates increased southward. Thermal modeling suggests that Albian cooling is the result of a short‐lived exhumation event that ended by the early Cenomanian, and that exhumation rates were between ∼1 and 8 mm/yr, with higher rates toward the south. The southward increase in cooling/exhumation rates is attributed to a southward increase in the component of tectonic exhumation by normal faulting on the south flank of the range. Rapid erosion, which is recorded by thick molasse deposits in flanking basins that are coeval with rapid cooling, may have been aided by surface uplift in the footwall of these faults. The northward younging of mica ages may not be explained by a simple core complex model involving metamorphic rocks being progressively dragged out from beneath an extensional fault. The overall spatial distribution of metamorphic zones and cooling ages/rates is best explained by a two‐step process: (1) a brief episode of extensional faulting producing a regional northward tilt with superimposed doming during the Albian‐Cenomanian and (2) relative uplift of the core during renewed Tertiary contraction, which enhanced the domal geometry. Diffusion‐domain modeling of K‐feldspar data indicates a period of little or no cooling that spanned much of the Late Cretaceous followed by rapid increase in cooling rates in the Paleocene to Eocene. Increased Tertiary cooling rates are attributed to renewed contraction leading to surface uplift and erosion.
Thermal consequences of a subduction boundary jump: A numerical model for generating subduction‐related clockwise pressure‐temperature paths
Authors: Aoya Mutsuki , Uehara Shin‐ichi , Wallis Simon R.
Published: October 2002

Clockwise pressure‐temperature (P‐T) paths with a temperature increase after peak P have been reported from a number of subduction‐related metamorphic belts around the world. However, an adequate tectonic interpretation of these paths has been lacking. One of the principal difficulties is the lack of a sufficient heat source to account for the post peak P temperature rise. However, two‐dimensional thermal calculations incorporating accretion of a thick metamorphic unit in a subduction zone reveal that, in general, an accreted sheet with a width of several kilometers experiences a post accretion heating without having to appeal to any special internal heat source. The heating effect is due to the spatial shift of the subduction boundary, or subduction boundary jump. This heating can account for the generation of subduction‐related clockwise P‐T paths within thick metamorphic units. For such units the styles of clockwise P‐T paths vary considerably and can be used as a general indicator of the exhumation rate by considering the balance between conduction and advection during exhumation. In contrast, a thin unit with a width of ≅1 km tends to record a nonclockwise P‐T path regardless of its exhumation rate. This is because the thinness decreases the potential both for post accretion heating and for maintaining the higher achieved temperature during exhumation. In addition, a subduction boundary jump can cause a post accretion transition of metamorphic P‐T conditions from high to low P‐T ratios, and it may therefore be incorrect to use peak T metamorphic conditions as an indicator of the geotherm along the coeval subduction boundary.
Caltepec fault zone: An Early Permian dextral transpressional boundary between the Proterozoic Oaxacan and Paleozoic Acatlán complexes, southern Mexico, and regional tectonic implications
Authors: Elías‐Herrera Mariano , Ortega‐Gutiérrez Fernando
Published: October 2002

The tectonic boundary between the Grenville‐age Oaxacan and Paleozoic Acatlán crystalline complexes in southern Mexico, named the Caltepec fault zone (CFZ), is characterized for the first time as a dextral transpressional, NNW trending and ENE dipping ductile fault zone of Early Permian age. The complexes are welded by a syntectonic magmatic epidote‐bearing granite along the entire length of the CFZ. From east to west, the 2–6 km wide CFZ consists of disrupted and retrograded banded gneisses of the Oaxacan complex, quartz‐feldspar mylonite, and the syntectonic magmatic epidote‐bearing Cozahuico granite (CZG) with huge xenoliths (up to several kilometers long and up to 600 m wide) of the Proterozoic gneisses, thrust westward over metasedimentary tectonites of the Acatlán complex. The CZG shows magmatic fabrics that represent a transition to solid‐state deformation characterized by subvertical foliation, subhorizontal NNE and SSE dipping mineral stretching lineation and dextral kinematics. The megaxenoliths underwent partial melting developing banded migmatites with layers of epidote‐bearing granitic neosome. The parallelism of fabrics in these anatexitic rocks and in the enclosing deformed granite suggests that ductile deformation, migmatization of xenolithic gneisses, and granite emplacement along the CFZ were coeval. The neosome yielded a U‐Pb zircon concordant age of 275.6 ± 1 Ma probably dating the peak of the tectonothermal event. We interpret the CFZ as a major terrane boundary accommodating transpressional interaction between the Acatlán and Oaxaquia blocks, which were amalgamated in an oblique convergent setting by Early Permian time, as the leading edge of Gondwana impinged onto the southern margin of Laurentia along the Marathon‐Ouachita suture to form Pangea.
Stress fields associated with the growth of a large shield volcano (La Palma, Canary Islands)
Authors: Fernández Carlos , de la Nuez Julio , Casillas Ramón , García Navarro Encarnación
Published: October 2002

Measurements of dyke orientations and fault slip data have been taken in 39 stations located in the northern part of the island of La Palma (Canary Archipelago). These structures affect the materials belonging to the submarine growth stage and to the lower units of the subaerial shield volcano stage. Four sets of dykes have been distinguished (Groups I, II, III‐1, and III‐2). The two younger dyke groups (III‐1 and III‐2), with highly variable strikes, are clearly related to the subaerial growth stage. Most of the measured faults are normal. Field relationships and the use of stress inversion methods allowed us to separate three different stress tensors. Tensor 2 is associated with the dykes of Group III‐1. Given the spatial distribution of the stations with faults of tensor 2 and dykes of its coeval Group III‐1, it has been possible to obtain a map of trajectories of the maximum (SH) and minimum (Sh) compressive stress axes in the horizontal. Comparison between this map and the results of theoretical models suggests that the stress field active during, at least, the early stages of growth of the shield volcano in La Palma (from near 1.7 Myr to, at least, 1.2 Myr ago), includes a compressive radial (volcanic) component plus a regional tensile stress of a tectonic origin. This second component might be the imprint in the Atlantic oceanic lithosphere of the Cenozoic convergence between the African and Eurasian plates.
The Swiss Alps and their peripheral foreland basin: Stratigraphic response to deep crustal processes
Authors: Pfiffner O. A. , Schlunegger F. , Buiter S. J. H.
Published: October 2002

This paper gives a synoptic view of the Cenozoic evolution of the Swiss Alps and their northern foreland basin. In this orogen, deep crustal processes (subduction, nappe stacking, underplating, and exhumation) are intimately linked with surface processes (surface uplift, erosion, basin formation, and basin‐axis migration). Within the foreland basin the spatial pattern of subsidence and alluvial fan construction suggests that an increase in flexure of the foreland plate and the creation of relief in the orogen migrated from east to west in the course of collision. In the orogen itself, crustal thickening involved lower crust of the Adriatic margin in the east and the European margin in the west. Exhumation of upper crustal units occurred earlier in the east as compared with the west. An Adriatic mantle wedge (the Ivrea body) and its associated wedge of lower crustal material are identified as an extra lithospheric load which contributed to downward flexure of the European plate. As a result of enhanced subsidence of the foreland plate, relief was generated presumably in order to adjust to critical taper geometry. It appears, therefore, that the westward motion of the Adriatic wedge ultimately caused the contemporaneous westward propagation of the location of enhanced rates of alluvial fan construction. Coeval strike‐slip and N‐S convergence juxtaposed the Adriatic wedge sequentially to different European upper crustal units which resulted in different styles of crustal structure and evolution along strike within the orogen.
Thermohydrologic behavior at an underground nuclear waste repository
Authors: Buscheck T. A. , Rosenberg N. D. , Gansemer J. , Sun Y.
Published: October 2002

We present a multiscale model that simulates coupled thermal and hydrological behavior driven by radioactive decay heat from a potential nuclear waste repository at Yucca Mountain, Nevada. We use this model to evaluate repository performance for different designs with respect to major thermal design goals (e.g., keeping waste packages dry). A locally boiling or globally boiling design uses rock dry out to create dry (low relative humidity (RH)) conditions around waste packages for a long period of time. A subboiling design eliminates boiling in the host rock (possibly reducing uncertainty) but is less effective at maintaining dry conditions. The addition of backfill increases the duration of RH reduction significantly without added boiling in the host rock but at the cost of higher waste package temperatures. The interaction between engineering design variables and natural system factors that affect thermohydrologic behavior is highly nonlinear. As a consequence, designs that differ by seemingly small details can result in markedly different thermohydrologic behavior and consequently repository performance, whereas with regards to other details, large differences may have little or no effect.
Modeling transport of a mixture of natural organic molecules: Effects of dynamic competitive sorption from particle to aquifer scale
Authors: van de Weerd H. , van Riemsdijk W. H. , Leijnse A.
Published: October 2002

Natural organic matter (NOM) can act as a carrier for contaminants. Therefore it is of great importance to understand its adsorption/desorption and transport behavior. NOM is a mixture of molecules varying from simple small molecules like citric acid to complicated large molecules like humic acid. To simulate sorption and transport of NOM in aquifer material, we used a previously developed model (NOMADS) describing the dynamic competitive sorption of NOM fractions. We calibrated NOMADS using independent batch adsorption data and incorporated it in a transport code. Sorption and transport of NOM in laboratory column experiments and a field experiment were well simulated using the calibrated model, indicating that the process descriptions used are valid over a wide range of temporal and spatial scales and mass‐to‐volume ratios. Simulation results provided insights into the influence of pore water velocity and NOM concentration history on the shape of breakthrough curves of NOM fractions. The heterogeneity of NOM appears to be essential to understanding its adsorption and transport behavior.
A stochastic model of hydraulic variations within stream channels
Authors: Stewardson Michael J. , McMahon Thomas A.
Published: October 2002

This paper presents a stochastic model of the joint depth and velocity probability distribution in stream channels. Model input is restricted to simple measures of channel geometry. Hydraulic distributions are quantified using two independent hydraulic variables, each a function of velocity and depth, and varying either longitudinally or laterally within a stream channel. The stochastic model provides a useful tool for exploring relations between fine‐scale hydraulic variations and channel geometry. One application is modeling fine‐scale hydraulic habitat conditions in stream channels.
Effects of soil heterogeneity on steady state soil water pressure head under a surface line source
Authors: Zhang Z. Fred , Parkin Gary W. , Kachanoski R. Gary , Smith James E.
Published: October 2002

There are numerous analytical solutions available for flow in unsaturated homogeneous porous media. In this paper, the stream tube model for one‐dimensional water movement is extended to two‐dimensional (2‐D) water movement from a line source as the stream plane model. As well, new solutions are derived to predict the mean and variance of pressure head of water movement under a surface line source in heterogeneous soil using the perturbation method with first‐order approximation (PM1) and with second‐order approximation (PM2). A variance expression was also developed based on the spectral relationship presented by Yeh et al. [1985a]. The new solutions were tested using the 2‐D stream plane model with parameters A = ln(α) and Y = ln(KS) and measurements from field experiments. Results show that the mean of steady state pressure head below the line source is not only a function of the mean parameter values but also a function of the variances of A and Y and the linear cross‐correlation coefficient (ρ) between A and Y. The PM2 model can predict the mean pressure head accurately in heterogeneous soils at any level of correlation between A and Y, except when both the soil variability and ρ are high. The pressure head variance estimation based on the PM1 model predicts the measured variance well only when both the soil variability and ρ are low. The field experimental results show that both the PM1 and the spectral models give reasonable predictions of the pressure head variance. Both the measured and predicted values of the variance of pressure head using the two models increase with the depth of soil. Both models show that the variance of pressure head decreases as the source strength increases, but on average, the pressure head variance was underestimated by both models.
Conditional first‐order second‐moment method and its application to the quantification of uncertainty in groundwater modeling
Authors: Kunstmann Harald , Kinzelbach Wolfgang , Siegfried Tobias
Published: October 2002

Decision making in water resources management usually requires the quantification of uncertainties. Monte Carlo techniques are suited for this analysis but imply a huge computational effort. An alternative and computationally efficient approach is the first‐order second‐moment (FOSM) method which directly propagates parameter uncertainty into the result. We apply the FOSM method to both the groundwater flow and solute transport equations. It is shown how conditioning on the basis of measured heads and/or concentrations yields the “principle of interdependent uncertainty” that correlates the uncertainties of feasible hydraulic conductivities and recharge rates. The method is used to compute the uncertainty of steady state heads and of steady state solute concentrations. It is illustrated by an application to the Palla Road Aquifer in semiarid Botswana, for which the quantification of the uncertainty range of groundwater recharge is of prime interest. The uncertainty bounds obtained by the FOSM method correspond well with the results obtained by the Monte Carlo method. The FOSM method, however, is much more advantageous with respect to computational efficiency. It is shown that at the planned abstraction rate the probability of exceeding the natural replenishment of the Palla Road Aquifer by overpumping is 30%.
Using a lognormal diffusion process to forecast river flows
Authors: Lefebvre Mario
Published: October 2002

The logarithm of the flow of a river in Quebec, Canada, is modeled as a Gaussian diffusion process. Using this model, forecasts of the river flow are made for up to 7 days ahead. The forecasts are found to be much more accurate for 1 day ahead than those produced by a deterministic model called PREVIS. The stochastic model also outperforms PREVIS for 2 days ahead forecasts and is comparable to PREVIS for 3 days ahead forecasts. In general, PREVIS is more accurate than the stochastic model proposed from 4 days ahead forecasts. Various possible improvements of the stochastic model are considered, including the incorporation of exogenous variables and the use of linear regression. The best forecasts are actually obtained by taking the average value of the forecasts produced by the stochastic model and by PREVIS. Finally, confidence interval for the forecasted values are also provided.