bluuebunny/crossref_metadata_2025 huggingface dataset:
The paleoclimatic record provided by eolian deposition in the deep sea: The geologic history of wind
Authors: Rea David K.
Published: February 2004

The mineral component of pelagic sediment is brought to the deep sea by transport in the wind. Extraction and analysis of this dust allows estimation of the past aridity of the eolian source region, via flux determinations, and of the intensity of the transporting winds, from grain size data. These two parameters, the grain size and mass flux of dust, vary independently. There are three significant sources of modern dust, eastern and central Asia, northwest Africa, and Arabia, all in the northern hemisphere. As the rainfall associated with the Intertropical Convergence Zone is an effective barrier to southerly transport of dust, the northern hemisphere is an order of magnitude more “dusty” than is the southern, an asymmetry that has characterized most of the Cenozoic. Eolian flux data show that most of the northern hemisphere was more arid during glacial maxima, with 3 to 5 times as much dust transported during glacial stages than during interglacials; only northwestern South America varied in the opposite sense. The periodicity of Quaternary variation in both eolian flux and eolian grain size data is strongly influenced by the Milankovitch cycles of orbital variability. Wind intensities vary on a shorter time scale than the general 100‐kyr cycles of glaciation and general aridity. Eolian grain sizes display forcing at precessional (19 and 23 kyr), tilt (41 kyr), and at approximately 30 kyr periodicities. As a result the generalization that winds are uniformly stronger during glacial times is not valid. Whole‐Cenozoic records show that the largest change in dust flux, an order of magnitude increase, occurred in the northern hemisphere and reflects continental drying associated with the late Pliocene onset of northern hemisphere glaciation. Southern hemisphere eolian records show no sign of paleoclimatic changes in the late Pliocene. The most important change in Cenozoic atmospheric circulation was a severalfold reduction in wind intensity that occurred at the time of the Paleocene‐Eocene boundary. Before then, latest Cretaceous and Paleocene winds were essentially as strong as those of the late Cenozoic. This shift appears to be one of several climatic responses to a change in the nature and amount of global heat transport at about 55 Ma.
Measurement of the electromagnetic properties of chiral composite materials in the 8–40 GHz range
Authors: Varadan V. V. , Ro R. , Varadan V. K.
Published: February 2004

Materials described by the constitutive equations D = εE + βε ∇ × E and B = μH + βμ ∇ × H lack inversion symmetry due to chirality or handedness in their microstructure. In this paper, we describe the first direct measurements of the material properties ε, μ, and β for microwave chiral composite materials prepared in our laboratory in the frequency range of 8–40 GHz. We have used both reflection and transmission measurements for normally incident, linearly polarized plane waves in a specially designed free‐space facility using spot focusing antennas. Different concentrations of the chiral inclusions in the form of miniature metallic springs that are left‐handed only, right‐handed only or equally mixed (racemic) have been studied. It is shown that ε and μ are comparable for all three samples at a given concentration, but β has equal but opposite values for the left‐ and right‐handed samples, whereas it is nearly zero for the equichiral sample. The values of β thus obtained are compared to estimated values for suspensions of naturally occurring chiral molecules. The accuracy of the measurements is assessed by using the experimental procedure and inversion algorithm for standard materials like quartz. A new technique involving time domain repines has been used to remove the ambiguity that is usually encountered in the inversion of measured S parameters.
Theoretical study of polar cap arcs: Time‐dependent model and its applications
Authors: Zhu L. , Sojka J. J. , Schunk R. W. , Crain D. J.
Published: February 2004

A time‐dependent theoretical model of polar cap arcs developed during the Coupling, Energetics, and Dynamics of Atmospheric Regions/High‐Latitude Plasma Structures (CEDAR/HLPS) campaigns in the past two years is briefly described. In the model the electrodynamics of the polar cap arcs are treated self‐consistently in the frame of the coupled magnetosphere‐ionosphere system. The preliminary simulation results of the temporal evolution and spatial structure of the polar cap arcs for both winter and summer conditions are presented. The model can be used to conduct both the model‐observation study of specific features of the polar cap arcs and the quantitative theoretical study of typical or average features of the arcs. Several potential specific applications of the model to the polar cap arc phenomenon are also discussed in the paper.
On the origin of quasi‐periodic radar backscatter from midlatitude sporadic E
Authors: Tsunoda Roland T. , Fukao Shoichiro , Yamamoto Mamoru
Published: February 2004

We describe a newly discovered polarization process that appears to be induced by an atmospheric gravity wave (AGW) when it altitude‐modulates a sporadic E (ES) layer in the nighttime, midlatitude ionosphere. This large‐scale polarization process appears capable of accounting for three as yet unexplained features found in radar backscatter from field‐aligned irregularities in ES layers: (1) kilometer‐scale, wavelike variations in the mean Doppler velocity, (2) mean Doppler velocities much larger than background ionospheric motion, and (3) quasi‐periodic patterns in backscatter power when plotted as a function of range and time. We show that the polarization electric field develops as a result of the altitude modulation and that its properties are similar to those of the AGW. The novel feature in this process is the spatial modulation that is produced in the field line‐integrated Pedersen conductivity by the variation in the ion‐neutral collision frequency associated with the varying altitude of the ES layer. The resultant electric field together with the altitude‐modulated ES layer then drive the production of secondary plasma waves via the gradient‐drift instability. The three puzzling features are shown to be associated with the characteristics of these secondary waves.
Influence of nonhomogeneous Earth on the rms phase error and beam‐pointing errors of large, sparse high‐frequency receiving arrays
Authors: Weiner M. M.
Published: February 2004

The performance of ground‐based high‐frequency (HF) receiving arrays is reduced when the array elements have electrically small ground planes. The array rms phase error and beam‐pointing errors, caused by multipath rays reflected from a nonhomogeneous Earth, are determined for a sparse array of elements that are modeled as Hertzian dipoles in close proximity to Earth with no ground planes. Numerical results are presented for cases of randomly distributed and systematically distributed Earth nonhomogeneities where one‐half of vertically polarized array elements are located in proximity to one type of Earth and the remaining half are located in proximity to a second type of Earth. The maximum rms phase errors, for the cases examined, are 18° and 9° for randomly distributed and systematically distributed nonhomogeneities, respectively. The maximum beampointing errors are 0 and 0.3 beam widths for randomly distributed and systematically distributed nonhomogeneities, respectively.
Estimates of the scattering cross section of equatorial electrojet irregularities over the 3‐ to 7‐MHz frequency band
Authors: Kudeki Erhan , Wright J. W.
Published: February 2004

Esq echo intensities recorded by a dynasonde system operated at Huancayo, Peru, are used to estimate the scattering cross section of equatorial electrojet plasma irregularities in the 3 to 7 MHz frequency band. The scattering radar cross section of vertically propagating ∼20‐m scale electrojet irregularities is estimated as ∼3×10−7 m−1 over a period exhibiting ƒ0E values of 2.7 to 2.9 MHz. The cross‐section variation with Bragg wavenumber k is found to be characterized with a power law index of approximately −3 over the HF band.
Static image principle for two conducting anisotropic half‐spaces with similar transverse anisotropies
Authors: Lindell I. V. , Ermutlu M. E. , Nikoskinen K. I. , Eloranta E. H.
Published: February 2004

Direct‐current image principle, previously developed for an anisotropic half space bounded by a perfectly electrically or magnetically conducting plane or an impedance surface, is extended to problems with two anisotropic half‐spaces. The anisotropies of the two spaces are restricted to be similar in the transverse plane, in which case the image expressions can be solved analytically. In fact, only in this case, for a point current source, there exists a point image source for reflected fields and a point image source for transmitted fields. The reflection and transmission image amplitudes are seen to be obey simple expressions similar to those of two isotropic media with effective isotropic conductivities. However, the locations of the images do not coincide with those of the isotropic theory, it is seen that in general the reflection image is shifted laterally and the transmission image both laterally and axially from those of the isotropic case.
Representation of propagation mode fading for the midlatitude wide bandwidth HF channel
Authors: Rifkin Robert , Bello Phillip A.
Published: February 2004

MITRE has made measurements of the wide bandwidth (1‐MHz bandwidth) impulse response of HF propagation modes for midlatitude paths in the United States. These paths included near‐vertical incidence, 2000 and 3000 km lengths, and primarily supported one‐hop F layer propagation. On the basis of these measurements, this paper proposes a stochastic representation for the path loss fluctuation of propagation modes observed for 15‐min time intervals. When expressed in decibels, this path loss is modeled as a normally distributed autoregressive process of low order. Estimates of the parameters for this model are extracted from the data for a variety of transmission conditions. A model order of 1 is suggested for link modeling.
Fast integral equation solutions using geometrical‐theory‐of‐diffraction‐like matrices
Authors: Canning Francis X.
Published: February 2004

Frequency domain integral equations allow high‐accuracy direct solvers, while often requiring excessive computational power for large problems. This contrasts with high‐frequency methods such as the geometrical theory of diffraction (GTD), which allow efficient solution of large problems, though often with less accuracy. Use of a special type of expansion functions in integral equations results in the impedance matrix localization method, which localizes the information in the impedance matrix to clumps of large numbers that are analogous to the terms of GTD. Fast numerical methods are developed for calculating these large matrix elements. Then the analogy to GTD is strengthened by developing an integral equation based on sources which radiate only exterior to a closed body. This is a new kind of combined source equation. The peaks (i.e., clumps of large numbers) are now restricted to near the diagonal for a convex body. This both allows a faster matrix solution and further strengthens the analogy to GTD. The local structure of the resulting matrix, its inverse, and the method of finding the inverse are found to be closely analogous to the associated high‐frequency method quantities. This suggests the possibility of deriving numerical GTD coefficients.
Simultaneous nonlinear reconstruction of two‐dimensional permittivity and conductivity
Authors: Habashy Tarek M. , Oristaglio Michael L. , de Hoop Adrianus T.
Published: February 2004

A new inversion algorithm for the simultaneous reconstruction of permittivity and conductivity recasts the nonlinear inversion as the solution of a coupled set of linear equations. The algorithm is iterative and proceeds through the minimization of two cost functions. At the initial step the data are matched through the reconstruction of the radiating or minimum norm scattering currents; subsequent steps refine the nonradiating scattering currents and the material properties inside the scatterer. Two types of basis functions are constructed for the nonradiating currents: “invisible” (global) basis functions, which are appropriate for discrete measurements and nonradiating (local) basis functions, which are useful in studying the limit of continuous measurements. Reconstructions of square cylinders from multiple source receiver measurements at a single frequency show that the method can handle large contrasts in material properties.
Early Triassic paleomagnetic data from the Dolomites (Italy)
Authors: Channell J. E. T. , Doglioni C.
Published: February 2004

Prefolding high unblocking temperature magnetization components have been resolved from 28 sites of red siltstones from the Lower Triassic Werfen Formation in the Dolomites (Southern Alps). Site mean declinations are more scattered than site mean inclinations, indicating small‐scale relative rotation among thrust sheets, particularly in the eastern part of the Dolomites affected by “Dinaric” (Paleogene) thrusting. Two sites from close to the Valsugana Line indicate large‐scale (∼80°) anticlockwise rotation in this zone of Alpine sinistral transpression. The overall mean direction (number of sites (N) = 26, Declination = 331°, Inclination = 13°, α95 = 6.6°), excluding sites close to the Valsugana Line, gives a pole position (Latitude = 42°N, Longitude = 233°E, dp/dm = 3.4°/6.8°) which lies close to published Early Permian poles from the Southern Alps and to Permian and Early Triassic poles for Africa, implying very little net rotation of the Dolomites relative to Africa since Permian time.
Postnappe stacking extension and exhumation of high‐pressure/low‐temperature rocks in the island of Crete, Greece
Authors: Fassoulas Charalampos , Kilias Adamantios , Mountrakis Demosthenis
Published: February 2004

The island of Crete consists largely of nappes of contrasting lithologies and metamorphism that were stacked southward during an Oligocene‐early Miocene N‐S compression, with the lower nappes undergoing a high‐pressure/low‐temperature (HP/LT) metamorphism. This was followed by a N‐S Miocene crustal extension that caused thinning of the nappes and uplift of the HP/LT metamorphic rocks. Ductile conditions took place in the lower nappes associated with a retrograde greenschist metamorphism, while semiductile to brittle conditions took place in the upper nappes. A major normal detachment fault separates the lower and the upper nappes. The quartz texture analyses and the symmetry of structures indicate bulk coaxial deformation, while the results of strain analysis suggest both constrictional and flattening strains. A younger NE‐SW compression affected both the thinned nappe pile and the late Miocene‐Pliocene sedimentary basins. Finally, a Pliocene NE‐SW extension led to further uplift and exhumation of the HP/LT metamorphic rocks. This cyclic tectonometamorphic process of alternate compression and extension took place during the migration of the Hellenic orogenic belt toward the most external units, including successive tectonic events.
Reassessment of Archean crustal development in the Barberton Mountain Land, South Africa, based on U‐Pb dating
Authors: Kamo S. L. , Davis D. W.
Published: February 2004

This paper presents U‐Pb ages for 23 samples from the Barberton Mountain Land, an Archean granite‐greenstone terrain in the Kaapvaal craton of South Africa. A reexamination of the geology of this area in the light of previous U‐Pb dating and the new ages indicates that much of its deformation can probably be accounted for by a single short‐lived compressional event within the time span 3230–3215 Ma which was synchronous with deposition of the Moodies Group and the upper part of the Fig Tree Group sediments. This event may have been responsible for thrusting of much older sequences in the Onverwacht Group, including slivers of 3.54 Ga old gneiss, circa 3.47 Ga old komatiitic and tholeiitic sequences, and circa 3.45 Ga old felsic sequences. Key constraints on the timing of deformation are previously determined 3227–3225 Ma ages for detrital zircons in thrusted units of the Fig Tree Group; a 3229 +4/−3 Ma age for a deformed porphyry which is pre‐ or synthrusting; a previously determined 3227 ± 3 Ma age for an undeformed porphyry in an inferred thrust fault; 3227 ± 1 Ma ages for both the Kaap Valley pluton and an ignimbrite in the Stolzburg syncline, both of which predate regional upright folding; and a 3216 ± 2 Ma age for the Dalmein pluton which appears to cut regional, upright NE‐SW trending folds in the Kromberg syncline, part of the final phase of deformation associated with regional compression. Areally extensive granitoid sheets and syenogranitic plutons were subsequently emplaced into the Barberton region at about 3105 Ma. The linear configuration of the syenogranitic plutons, which are also aligned with the gabbroic Usushwana complex as well as the geometry of the granitoid sheets, suggests that the circa 3105 Ma magmatism was caused by incipient rifting around a NW‐SE trending crustal fracture. This episode was probably coeval with a late regional transtensional phase of deformation recorded in the greenstone belt. Some of the above interpretations are in conflict with previous models for early development of the Barberton greenstone belt. A 3352 ± 6 Ma age from a metagabbro which is comagmatic with a proposed ultramafic sheeted dike complex is over 100 m.y. younger than the probable age of volcanic rocks which host the dikes. This conflicts with the suggestion that much of the Onverwacht Group comprises an ophiolitic section, obducted in an intraoceanic setting at circa 3.45 Ga, that partially melted to form trondhjemitic magmas which intruded along active thrust faults. Present data suggest that, despite its antiquity and the diversity of ages and lithologies found within it, the Barberton greenstone belt can generally be understood in terms of actualistic tectonic processes at convergent margins.
Late Paleozoic transcurrent tectonic assembly of the central Appalachian Piedmont
Authors: Valentino David W. , Gates Alexander E. , Glover Lynn
Published: February 2004

Recent investigations in south‐eastern Pennsylvania and northern Maryland have demonstrated a major anastomosing strike‐slip shear system. The Pleasant Grove‐Huntingdon Valley shear system emerges from beneath the coastal plain cover at Trenton, New Jersey, and extends to the area west of Baltimore, Maryland, where it is overlain by the Culpepper Mesozoic rift basin. The sense of offset across this system is dextral. In the Susquehanna River region and north of the shear zone, the rocks of the Octoraro Formation contain evidence for two metamorphisms and deformations prior to strike‐slip shearing, whereas south of the shear zone the Peters Creek Formation contains evidence for only one. The discordance in metamorphic and deformational history across the shear zone suggests the now juxtaposed rocks originated in different parts of the orogen. Although conclusive ages for the strike‐slip deformation do not exist at this time, the timing of deformation is loosely constrained where the shear system crosscuts known Taconian structures in the Piedmont. Comparison of deformation style with other regions in the Appalachian suggests the Pleasant Grove‐Huntingdon Valley shear system is related to Alleghanian transcurrent tectonics in the Piedmont. Palinspastic reconstruction of the Pleasant Grove‐Huntingdon Valley shear system reveals fundamental problems in current tectonic models for the central Appalachian Piedmont. A minimum of 150 km of dextral offset is proposed for the Pleasant Grove‐Huntingdon Valley shear system based on reconstruction of the Cambrian‐Ordovician shelf edge between northern Maryland and southeastern New York. Displacement of this magnitude can account for the previously proposed tectonic models that portray a failed Iapetan rift block and microcontinent that contains the Baltimore Grenvillian massifs. Even though a history of early orthogonal collision is preserved within discrete structural blocks, transcurrent shearing has greatly influenced the distribution of those blocks. Models not including the strike‐slip component of tectonic assembly need serious reconsideration, as evidence grows that the magnitude of orogen‐parallel displacement is equal to or larger than the orthogonal component.
Active arc‐continent collision: Earthquakes, gravity anomalies, and fault kinematics in the Huon‐Finisterre collision zone, Papua New Guinea
Authors: Abers Geoffrey A. , McCaffrey Robert
Published: February 2004

The Huon‐Finisterre island arc terrane is actively colliding with the north edge of the Australian continent. The collision provides a rare opportunity to study continental accretion while it occurs. We examine the geometry and kinematics of the collision by comparing earthquake source parameters to surface fault geometries and plate motions, and we constrain the forces active in the collision by comparing topographic loads to gravity anomalies. Waveform inversion is used to constrain focal mechanisms for 21 shallow earthquakes that occurred between 1966 and 1992 (seismic moment 1017 to 3 × 1020 N m). Twelve earthquakes show thrust faulting at 22–37 km depth. The largest thrust events are on the north side of the Huon Peninsula and are consistent with slip on the Ramu‐Markham thrust fault zone, the northeast dipping thrust fault system that bounds the Huon‐Finisterre terrane. Thus much of the terrane's crust but little of its mantle is presently being added to the Australian continent. The large thrust earthquakes also reveal a plausible mechanism for the uplift of Pleistocene coral terraces on the north side of the Huon Peninsula. Bouguer gravity anomalies are too negative to allow simple regional compensation of topography and require large additional downward forces to depress the lower plate beneath the Huon Peninsula. With such forces, plate configurations are found that are consistent with observed gravity and basin geometry. Other earthquakes give evidence of deformation above and below the Ramu‐Markham thrust system. Four thrust events, 22–27 km depth directly below the Ramu‐Markham fault outcrop, are too deep to be part of a planar Ramu‐Markham thrust system and may connect to the north dipping Highlands thrust system farther south. Two large strike‐slip faulting earthquakes and their aftershocks, in 1970 and 1987, show faulting within the upper plate of the thrust system. The inferred fault planes show slip vectors parallel to those on nearby thrust faults, and may represent small offsets in the overriding plate. These faults, along with small normal‐faulting earthquakes beneath the Huon‐Finisterre ranges and a 25° along‐strike rotation of slip vectors, demonstrate the presence of along‐strike extension of the accreting terrane and along‐strike compression of the lower plate.
Rifting, erosion, and uplift history of the Recôncavo‐Tucano‐Jatobá Rift, northeast Brazil
Authors: Magnavita Luciano P. , Davison Ian , Kusznir Nick J.
Published: February 2004

The Recôncavo‐Tucano‐Jatobá (RTJ) Rift and many other smaller sedimentary basins in northeast Brazil formed during South Atlantic rifting and were subsequently uplifted and exhumed so that Albian marine sediments are now located up to 800 m above sea level some 400 km inland from the Atlantic margin. Local erosion caused by footwall uplift and regional erosion, probably resulting from magmatic underplating and uplift, has removed a large part of the thermal sag phase sediments from the RTJ Rift. The flexural cantilever model, incorporating the flexural isostatic response to simple‐shear faulting in the upper crust and pure‐shear necking of the lower crust and upper mantle, can explain the main geometrical features observed in the RTJ Rift without resorting to lithosphere scale detachments. The model has also been used to estimate the amount of uplift and erosion of the faulted rift flanks. Rift flank erosion can produce uplift and erosion across the whole RTJ Rift, thus explaining the postrift unconformity which preceded deposition of the Aptian age Marizal Formation. The largest uplift is predicted to occur at the edges of the basin and may explain the anomalously shallow depth to onset of oil generation at the basin margins deduced from vitrinite reflectance data. The model also predicts that maximum burial of much of the basin fill in the RTJ Rift occurred at the end of rifting and not during the postrift infill, as is usually the case. The amount of observed coarse conglomeratic detritus in the Recôncavo subbasin suggests that about 25% of the eroded footwall detritus was deposited and preserved in the adjacent hanging wall half‐graben, with the rest being transported more distally as finer material.
Exhumation and tilting of the western metamorphic belt of the Coast orogen in southern southeastern Alaska
Authors: Cook R. D. , Crawford M. L.
Published: February 2004

The uplift history of the western metamorphic belt in southern southeastern Alaska can be traced using emplacement depths and cooling ages of mid‐Cretaceous plutons which intruded the metamorphic belt during the period 101 Ma to 89 Ma. During the period 101 Ma to 94 Ma the rocks of the metamorphic belt now at the surface were buried to a depth of approximately 30 km. Pressure data indicate that by 89 Ma the western portion of the metamorphic belt was at a shallower crustal level (∼23 km within the study area) while the eastern side remained at a depth of approximately 30 km. Cooling curves constructed from minerals in mid‐Cretaceous plutonic rocks indicate that the western side of the metamorphic belt cooled earlier than the eastern side; an observation consistent with the inferred uplift history. Southwestward transport of metamorphic belt rocks within the hanging walls of thrust and high‐angle faults may have provided one mechanism for uplift during mid‐ to late‐Cretaceous time. The eastern portion of the western metamorphic belt cooled rapidly at ∼55 Ma, probably in response to a period of rapid uplift. The Coast shear zone, an Early Tertiary high‐angle fault with west‐side‐up displacement, bounds the east side of the metamorphic belt. We suggest Tertiary uplift occurred by tilting of the metamorphic belt due to movement along this fault.
Extension in high‐grade terranes of the southern Omineca Belt, British Columbia: Evidence from paleomagnetism
Authors: Wingate M. T. D. , Irving E.
Published: February 2004

Paleomagnetic observations are used to construct a map of structural tilts in metamorphic and plutonic rocks of the southern Omineca Belt in southeastern British Columbia. During Eocene extension, midcrustal rocks were exhumed in footwalls of low to moderate angle normal faults that dip east and west in the eastern and western parts of the study area, respectively. Tilts (averaging 30° to 40°) are consistent over large areas and generally antithetic to the major faults. In the central part of the study area, however, the eastern half of the Coryell Pluton is untilted, and its western half, in the hanging wall of the east dipping Kettle River Fault, is tilted down‐to‐ the‐east, in the opposite sense to that expected. North of the pluton, these eastward tilted rocks are juxtaposed against westward tilted rocks. To explain the anomalous tilts, a major west dipping Eocene normal fault (Arrow Lake Fault) is inferred, and a revised kinematic history for the area proposed. Observations of magnetic polarity (86% normal), laboratory unblocking temperatures, and isotopic age data suggest that magnetizations in the west were acquired during chron 23n, and in the east, in chron 23n and/or chron 24n. Unless extension postdated the acquisition of magnetization, tilts would not be observed; thus it is argued that extension in the west occurred after 50.6 Ma, and in the east between 51.6 (possibly 53.3) and 50.6 Ma. These time intervals do not correspond to any known global plate reorganization event, indicating that the last phase of Eocene extension in the southern Omineca Belt was governed by regional rather than global processes.
Paleozoic ophiolitic assemblages within the southern New England orogen of eastern Australia: Implications for growth of the Gondwana margin
Authors: Aitchison Jonathan C. , Blake M. Clark , Flood Peter G. , Jayko Angela S.
Published: February 2004

Several ophiolitic assemblages occur in the southern New England orogen. The development of these rocks and their relations to the rest of the orogen have major implications for the tectonic evolution of eastern Gondwana. A major, narrow but elongate belt of Early Cambrian supra‐subduction zone ophiolite crops out along and near the Peel‐Manning Fault System and is juxtaposed against younger arc and subduction complex terranes. No pre‐Permian links with the rest of the New England orogen have been established for this terrane. It may represent portions of Lachlan Fold Belt basement which underlies younger, westward overthrust New England terranes, and has been diapirically emplaced at higher crustal levels as serpentinite‐matrix melange. Middle to Late Devonian ophiolitic rocks in the Yarras Complex comprise basement to the Birpai subterrane and represent a crustal cross section through a rifted island arc. Correlatives of this terrane also occur within the more extensive Gamilaroi terrane to the west of which deeper crustal levels are not exposed. The various components of serpentinite‐matrix melange in the Ngamba terrane at Port Macquarie superficially appear to represent a dismembered ophiolite association. However, the various components of the melange exhibit a wide range of ages, metamorphism, and tectonic affinities, rendering a genetically related origin unlikely. This terrane includes fragments of ocean floor accreted into a Late Devonian subduction complex, which was later affected by Early Carboniferous forearc serpentinite diapirism and high Mg series magmatism. Zircon inheritance in Triassic dikes, which intrude the melange attest to the development or later emplacement of this forearc region over an older Lachlan Fold Belt basement. Ultramafic rocks of the Bundjalung terrane in the east of the New England region probably formed at deep levels in an intraoceanic island arc and are intruded by boninitic dikes. The tectonic development of the NEO was significantly more complicated than has been suggested in earlier published models. Periodic accretion of island arc systems, some of which are now represented by supra‐subduction zone ophiolites, to the eastern margin of Gondwana suggests multiple phases of subduction with the possibility of polarity reversals throughout the history of accretion. Lateral accretion was not the only means by which Gondwana continental crust grew, and there was considerable postaccretion continentward overthrusting of younger terranes.
Optimization of groundwater remediation using artificial neural networks with parallel solute transport modeling
Authors: Rogers Leah L. , Dowla Farid U.
Published: February 2004

A new approach to nonlinear groundwater management methodology is presented which optimizes aquifer remediation with the aid of artificial neural networks (ANNs). The methodology allows solute transport simulations, usually the main computational component of management models, to be run in parallel. The ANN technology, inspired by neurobiological theories of massive interconnection and parallelism, has been successfully applied to a variety of optimization problems. In this new approach, optimal management solutions are found by (1) first training an ANN to predict the outcome of the flow and transport code, and (2) then using the trained ANN to search through many pumping realizations to find an optimal one for successful remediation. The behavior of complex groundwater scenarios with spatially variable transport parameters and multiple contaminant plumes is simulated with a two‐dimensional hybrid finite‐difference/finite‐element flow and transport code. The flow and transport code develops the set of examples upon which the network is trained. The input of the ANN characterizes the different realizations of pumping, with each input indicating the pumping level of a well. The output is capable of characterizing the objectives and constraints of the optimization, such as attainment of regulatory goals, value of cost functions and cleanup time, and mass of contaminant removal. The supervised learning algorithm of back propagation was used to train the network. The conjugate gradient method and weight elimination procedures are used to speed convergence and improve performance, respectively. Once trained, the ANN begins a search through various realizations of pumping patterns to determine whether or not they will be successful. The search is directed by a simple genetic algorithm. The resulting management solutions are consistent with those resulting from a more conventional optimization technique, which combines solute transport modeling and nonlinear programming with a quasi‐Newton search. The results suggest that the ANN approach has the following advantages over the conventional technique for the test remediations: more independence of the flow and transport code from the optimization, greater influence of hydrogeologic insight, and less computational burden due to the potential for parallel processing of the flow and transport simulations and the ability to “recycle” these simulations. The ANN performance was observed upon variation of the problem formulation, network architecture, and learning algorithm.
Mass flux statistics of kinetically sorbing solute in heterogeneous aquifers: Analytical solution and comparison with simulations
Authors: Selroos Jan‐Olof , Cvetkovic Vladimir
Published: February 2004

Variability in hydraulic properties and various mechanisms resulting in rate‐limited mass transfer influence field scale solute transport in aquifers. In this study analytical expressions are presented for the expected value and variance of the mass flux for a solute undergoing a nonequilibrium sorption reaction controlled by first‐order kinetics. The uncertainty in the mass flux, expressed through the variance, stems from finite solute injection volumes compared to the hydraulic conductivity integral scale. Two‐dimensional numerical simulations are performed under statistically identical conditions to the ones assumed in the derivation of the analytical results. Two cases of variability in the log hydraulic conductivity are considered (σY2 = 0.1 and σY2 = 1.0). The analytical model and the numerical simulations yield comparable predictions of the mass flux statistics for both variability cases. Comparing conservative and kinetically sorbing solute an apparent effect of sorption kinetics is to lower the discrepancy between the analytical model and the simulations due to increased spreading of the solute resulting from rate‐limiting mass transfer.
Recovering the release history of a groundwater contaminant
Authors: Skaggs Todd H. , Kabala Z. J.
Published: February 2004

Finding the history of a groundwater contaminant plume from measurements of its current spatial distribution is an ill‐posed problem and, consequently, its solution is extremely sensitive to errors in the input data. In this paper, Tikhonov regularization (Tikhonov and Arsenin, 1977) is used in numerical experiments to recover the release history of a plume that has originated from a known, single site. The recovered release history is then used to reconstruct the plume evolution history. The method is found to be insensitive to round off errors, but its accuracy is affected by plume measurement errors, the extent to which the plume has dissipated, and, to a lesser degree, the accuracy of the transport parameter estimates. A regularization approach may be effective at finding a plume history when there are adequate data.
Approximating the optimal groundwater pumping policy in a multiaquifer stochastic conjunctive use setting
Authors: Provencher Bill , Burt Oscar
Published: February 2004

This paper presents two methods for approximating the optimal groundwater pumping policy for several interrelated aquifers in a stochastic setting that also involves conjunctive use of surface water. The first method employs a policy iteration dynamic programming (DP) algorithm where the value function is estimated by Monte Carlo simulation combined with curve‐fitting techniques. The second method uses a Taylor series approximation to the functional equation of DP which reduces the problem, for a given observed state, to solving a system of equations equal in number to the aquifers. The methods are compared using a four‐state variable, stochastic dynamic programming model of Madera County, California. The two methods yield nearly identical estimates of the optimal pumping policy, as well as the steady state pumping depth, suggesting that either method can be used in similar applications.
Bare soil surface resistance to evaporation by vapor diffusion under semiarid conditions
Authors: van de Griend Adriaan A. , Owe Manfred
Published: February 2004

Based on Kohsiek's fast air circulation chamber, a method has been developed to measure the surface resistance to vapor diffusion in a drying topsoil. This resistance is important to estimate evaporation from bare soils using an aerodynamic resistance formulation. Measurements were done for a fine sandy loam during a dry down after artificial wetting. Surface resistance started to increase at a moisture content of 15% by volume in the 1‐cm top layer, which is 50% of its moisture content at field capacity. Calculations of the aerodynamic resistance were corrected for stability and were used to isolate the real surface resistance from the bulk resistance. Resistances could be modeled as a function of the top 1 cm soil moisture and varied between approximately 10 s/m for a wet and several thousand seconds per meter for a dry top layer. The measurements demonstrated a very pronounced diurnal course due to drying of the very top layer during the day and recovery of the moisture profile during nighttime hours.
Stochastic analysis of solute transport in partially saturated heterogeneous soil: 2. Prediction of solute spreading and breakthrough
Authors: Russo David , Zaidel Jacob , Laufer Asher
Published: February 2004

The applicability of results of Lagrangian‐stochastic analyses of vadosezone transport [Russo, 1993a, b] to realistic situations is investigated using results of detailed numerical simulations of transport in a hypothetical, yet realistic heterogeneous, partially saturated soil, obtained in the first companion paper (Russo et al., this issue) for both quasi steady state and transient, nonmonotonic flows. For both flow regimes, lower mean water saturation and longer travel time are shown to increase solute spreading, while lower water saturation and smaller travel distance are shown to increase the skewing of mean solute breakthrough curves. Solute plumes associated with the latter flow regimes, however, exhibit less spreading in the longitudinal direction and more spreading in the transverse direction, while the respective breakthrough curves are less skewed and less erratic, as compared with solute spreading and breakthrough associated with the former flow regimes. Components of the time‐dependent displacement covariance tensor, X and expected solute flux through a given horizontal control plane 〈s〉, based on the Lagrangian‐stochastic analyses, compared favorably with estimates of X and 〈s〉 obtained from the simulated transport under quasi steady state, essentially unidirectional flows, but failed to predict estimates of X and 〈s〉 obtained from the simulated transport under transient, nonmonotonic, multidirectional flows. The latter can be predicted by a particle‐tracking method [Rubin, 1990] that allows for deviation of the solute particles from their mean path, provided that the pertinent flow regime is quantifiable in terms of an appropriate velocity covariance function.
One‐dimensional stochastic analysis in leaky aquifers subject to random leakage
Authors: Hantush Mohamed M. , Mariño Miguel A.
Published: February 2004

The problem of one‐dimensional groundwater flow in a homogeneous leaky aquifer is analyzed within a stochastic framework. The stochastic analysis is straightforward and exact; it is based on a closed‐form solution obtained using the Laplace transform method. The solution relates linearly a stochastic perturbation of the aquifer head to uncertainty in the initial aquifer head and stochastic fluctuations of the water table in an overlying phreatic aquifer. Autocorrelation functions for the aquifer head, the leakage flow, and the base flow are developed for two cases of the water table fluctuations, namely, delta‐correlation fluctuations (i.e., white noise) and fully correlated fluctuations. The analysis clearly demonstrates that the stochasticity of the aquifer hydraulic response is highly controlled by the geohydrology of the problem: the geometry given by aquifer length and the hydraulic properties given by the leakage factor, aquifer diffusivity, and aquitard leakance. The results show that the temporal variance of aquifer head induced by a random initial head persists longer for larger values of the dimensionless leakage factor. The smaller the dimensionless leakage factor, the greater the variance of aquifer head, leakage, and base flow. Asymptotic results obtained for large times reveal that (1) water table fluctuations that are white noise induce a cumulative leakage whose variance is asymptotically linear in time, and quadratic in time if the fluctuations are fully correlated: (2) the variance of the aquitard leakage flux is asymptotically equal to the variance of its time average if the fluctuations of the water table are fully correlated in time; (3) cumulative base flow and cumulative leakage have identical autocorrelation function when water table fluctuations are fully correlated in time; and (4) the attenuating characteristic of aquifer flow systems, often observed in the spectral domain, is evident in the case of highly variable water table fluctuations (white noise); the base flow has a finite variance when the leakage flux has an infinite variance.
An experimental investigation of nonaqueous phase liquid dissolution in saturated subsurface systems: Transient mass transfer rates
Authors: Powers Susan E. , Abriola Linda M. , Weber Walter J.
Published: February 2004

This work focuses on the experimental measurement and mathematical modeling of processes affecting the dissolution of nonaqueous phase liquids (NAPLs) entrapped in sandy porous media. Results of a series of laboratory‐scale one‐dimensional column dissolution experiments indicate that the length of time required to dissolve NAPLs and substantially reduce aqueous phase effluent concentrations is many times greater than predicted by equilibrium calculations. Experimental measurements clearly show an influence of both grain size and grain size distribution on the evolution of effluent concentrations. The longer cleaning times associated with coarse or graded media are attributed to the larger and more amorphous NAPL blobs associated with these media. A general correlation for transient dissolution rates is proposed which incorporates porous medium properties, Reynolds number, and volumetric fraction of NAPL. The model is calibrated with results from styrene dissolution experiments and is shown to adequately predict trichloroethylene dissolution rates in the same sandy media over the period of time required to dissolve the NAPL.
Hydrologic processes controlling sulfate mobility in a small forested watershed
Authors: Huntington T. G. , Hooper R. P. , Aulenbach B. T.
Published: February 2004

Hydrologic controls on sulfate mobility were investigated in a forested catchment in the Georgia Piedmont using a watershed mass balance approach. Variations in annual sulfate export were governed primarily by differences in runoff rather than by differences in sulfate deposition or in total annual precipitation. However, 2 years with similar total runoff had substantially different sulfate export. A residual analysis indicated that a shift in the concentration‐discharge relationship accounted for 72% of the difference in sulfate export and that a change in the pattern of discharge accounted for the remainder of the difference. Stream water sulfate concentrations reflected past hydrologic conditions. Concentrations at the same discharge were higher following an extended dry period than following average periods. The elevation in stream water sulfate concentrations following dry periods persisted for several months. The influence of rainfall patterns on sulfate export underscores the need for long‐term records to adequately characterize the acidification status of the watershed and to understand trends in water quality.
A physically based model for the topographic control on shallow landsliding
Authors: Montgomery David R. , Dietrich William E.
Published: February 2004

A model for the topographic influence on shallow landslide initiation is developed by coupling digital terrain data with near‐surface through flow and slope stability models. The hydrologic model TOPOG (O'Loughlin, 1986) predicts the degree of soil saturation in response to a steady state rainfall for topographic elements defined by the intersection of contours and flow tube boundaries. The slope stability component uses this relative soil saturation to analyze the stability of each topographic element for the case of cohesionless soils of spatially constant thickness and saturated conductivity. The steady state rainfall predicted to cause instability in each topographic element provides a measure of the relative potential for shallow landsliding. The spatial distribution of critical rainfall values is compared with landslide locations mapped from aerial photographs and in the field for three study basins where high‐resolution digital elevation data are available: Tennessee Valley in Marin County, California; Mettman Ridge in the Oregon Coast Range; and Split Creek on the Olympic Peninsula, Washington. Model predictions in each of these areas are consistent with spatial patterns of observed landslide scars, although hydrologic complexities not accounted for in the model (e.g., spatial variability of soil properties and bedrock flow) control specific sites and timing of debris flow initiation within areas of similar topographic control.