bluuebunny/crossref_metadata_2025 huggingface dataset:
Postdepositional oxic degradation of alkenones: Implications for the measurement of palaeo sea surface temperatures
Authors: Hoefs Marcel J. L. , Versteegh Gerard J. M. , Rijpstra W. Irene C. , de Leeuw Jan W. , Damsté Jaap S. Sinninghe
Published: February 2004

Free and “bound” long‐chain alkenones (C37∶2 and C37∶3) in oxidized and unoxidized sections of four organic matter‐rich Pliocene and Miocene Madeira Abyssal Plain turbidites (one from Ocean Drilling Program site 951B and three from site 952A) were analyzed to determine the effect of severe post depositional oxidation on the value of U37k′. The profiles of both alkenones across the redox boundary show a preferential degradation of the C37∶3 compared to the C37∶2 compound. Because of the high initial U37k′ values and the way of calculating the U37k′ this degradation hardly influences the U37k′ profiles. However, for lower U37k′ values, measured selective degradation would increase U37k′ up to 0.17 units, equivalent to 5°C. For most of the U37k′ band‐width, much smaller degradation already increases U37k′ beyond the analytical error (0.017 units). Consequently, for interpreting the U37k′ record in terms of past sea surface temperatures, selective degradation needs serious consideration.
Testing the effects of tropical temperature, productivity, and mixed‐layer depth on foraminiferal transfer functions
Authors: Watkins James M. , Mix Alan C.
Published: February 2004

Statistical transfer functions relate living planktonic foraminiferal species of the central equatorial Pacific to measured sea surface temperature, integrated primary productivity, and mixed‐layer depth. The faunal estimates successfully reconstruct latitudinal patterns observed in both warm (El Niño, February–March 1992) and cool (La Niña, August–September 1992) seasonal settings. Predictions of mixed‐layer depth appear to be unbiased by temperature or productivity in our data set but tend to underestimate deep mixed layers. Interactions between productivity and temperature, perhaps through their common influence on respiration and growth rates, bias foraminiferal transfer functions for both properties. Paleoceanographic estimates may be improved by accounting for such biological processes that translate the environment into a faunal response preserved in the geologic record.
How do radiolarian sediment assemblages represent surface ocean ecology in the central equatorial Pacific?
Authors: Welling Leigh A. , Pisias Nicklas G.
Published: February 2004

Ever since Q‐mode factor analysis has been applied to microfossil data, investigators have attempted to determine whether statistical groupings of species in deep‐sea sediments are ecologically meaningful. We address this question for radiolaria using living, death, and sediment assemblages from the equatorial Pacific. Of the seven sediment assemblages identified from a basin‐wide analysis, four are important in the tropics. Eastern boundary and transitional sediment assemblages identify ecological environments similar to living assemblages. The western Pacific sediment assemblage does not appear to represent a natural grouping of species. The dominant, tropical sediment end‐member represents a combination of warm water species from subtropical and western Pacific environments. Of the two the predominant signal in tropical sediments is that of the western Pacific. The implication of a sediment bias toward warm water species may be due to the lack of an important cold water indicator species, Lophophaena hispida, in the sediment analysis.
Foraminiferal magnesium in Globeriginoides sacculifer as a paleotemperature proxy
Authors: Hastings David W. , Russell Ann D. , Emerson Steven R.
Published: February 2004

Foraminiferal magnesium shows increasing promise as a paleothermometer, but the accuracy and precision are limited by biases introduced by partial dissolution, salinity variations, Mg‐rich gametogenic calcite, and contaminant phases. We improved cleaning methods and reduced errors introduced by partial dissolution by sampling from well‐preserved cores in the equatorial Atlantic and the Caribbean Sea with different dissolution histories. All cores reveal a synchronous 25% increase in Mg/Ca from the stage 2/3 boundary to the Holocene core top, indicating that dissolution is not a controlling factor. Modern temperatures estimated from core top Mg/Ca are 24.5°–25.0°C, equal to mean annual water temperatures at 50–100 m. We estimate that sea surface temperature increased by 2.6°C (±1.3) from the last glacial maximum to the Holocene. Holocene values were comparable to those during isotope stage 5e. Our data indicate that biases from contaminant phases and partial dissolution can be reduced. This paleothermometer holds promise if uncertainties introduced by salinity variations and gametogenic calcite can be constrained.
An effect of dissolved nutrient concentrations on alkenone‐based temperature estimates
Authors: Epstein Bonnie L. , D'Hondt Steven , Quinn James G , Zhang Jiaping , Hargraves Paul E.
Published: February 2004

The ratio of 37‐carbon diunsaturated to diunsaturated and triunsaturated alkenones (UK′37) produced by some haptophytes is widely used as a proxy for past sea surface temperatures. However, our isothermal culturing experiments withEmiliania huxleyiclone CCMP372 show UK′37values to also vary with nutrient availability and cell division rate. These results provide a reasonable explanation for large isothermal variation in UK′37values of single coccolithophorid strains grown in culture. They also suggest that alkenone‐based estimates of past sea surface temperatures may have been influenced by dissolved nutrient concentrations as well as by temperature.
Fluid flow in accretionary prisms: Evidence for focused, time‐variable discharge
Authors: Carson Bobb , Screaton Elizabeth J.
Published: February 2004

Accretionary prisms are wedges of saturated sediment that are subject to intense deformation as a result of lithosphere convergence. Compressive stress and rapid burial of the accreted deposits result in sediment compaction and mineral dehydration. These latter processes, in conjunction with fermentation or thermal maturation of entrained organic matter, yield hydrocarbon‐bearing pore fluids that are expelled from the prism. Regional fluxes of heat and a number of dissolved chemical species, most notably carbon, are controlled by the advective expulsion of the pore waters. Numerical modeling, observation and monitoring of flow patterns and rates, and recent in situ hydrogeological tests quantify the conditions that control rates of fluid flow. Dispersed, intergranular flow (10−8 to 10−11 m/s), controlled by the vertical permeability of the prism (10−14 to 10−20 m²), is limited by low‐permeability lithologies and seems not to vary much from margin to margin. Focused flow (10−1 to 10−8 m/s) above the décollement is controlled by fault zones or sedimentary intrusions (diapiric structures). At low‐fluid pressures, fault zone permeability may be similar to that of adjacent wall rock, but as fluid pressure increases from hydrostatic (λ* = 0) to near lithostatic levels (λ* ≈ 1.0), fault zones dilate, and (fracture) permeability increases by 2–4 orders of magnitude (10−10 to 10−16 m²). Similarly, mud volcanoes and diapirs provide high‐permeability fluid conduits to the sediment‐water interface. As a result, faults and intrusions become primary flow paths and support surface vents at which syntectonic deposits (carbonate and gas hydrates) accumulate and chemosynthetic organisms cluster. Models of thermal and chemical anomalies and epigenetic deposits indicate that flow is temporally variable. That conclusion has been quantified by extended (1–10 months) seafloor and borehole experiments that measured temperature anomalies associated with flow events. On the Cascadia prism, flow (estimated velocity ∼3 × 10−5 m/s; 950 m/yr), confined to a thrust fault that cuts upward through the prism, has brought thermogenic hydrocarbons from depths >1.5 km to the surface within the last 400 years.
An improved interpretation of VHF oblique radar echoes by a direct balloon C2n estimation using a horizontal pair of sensors
Authors: Luce Hubert , Crochet Michel , Dalaudier Francis , Sidi Claude
Published: February 2004

Successful comparisons between VHF oblique (15°) radar echo profiles and reconstructed ones deduced from high‐resolution temperature balloon measurements were presented by Luce et al. [1996]. The method was based on the evaluation of the three‐dimensional isotropic refractive index spectrum at the Bragg wavelength from the available temperature profiles (“spectral method”). However, the isotropic hypothesis is questionable, especially in regions where temperature sheets [Dalaudier et al., 1994] are observed. Indeed, the associated anisotropic temperature fluctuations should contribute to the one‐dimensional vertical temperature spectrum at small scales. In the present paper, another method, less sensitive to anisotropic contaminations, is used. This method is based on an estimation of the temperature structure constant C2T from variances of horizontal differences of temperature measured by two high‐resolution sensors 1 m apart horizontally (“horizontal variance method”). It is shown that the main differences between the C2T estimations obtained from the two methods are mainly observed at high resolution in stable regions where few turbulent fluctuations in the temperature field are observed. However, the two methods give approximately equivalent results at the radar range resolution (600 m) and the quality of the comparisons with the radar observations is slightly improved with the horizontal variance method. In order to demonstrate the full advantages of this technique, the use of radars with more powerful capabilities is suggested for future investigations.
Development of an external interferometer for meteor wind observation attached to the MU radar
Authors: Nakamura Takuji , Tsuda Toshitaka , Tsutsumi Masaki
Published: February 2004

An external meteor detection system for the middle and upper atmosphere (MU) radar, Shigaraki, Japan, has been developed in order to monitor the wind velocities at 80–100 km. The characteristics of the MU radar system and the observations are closely inspected, and we design a three‐channel interferometer system using three independent single antennas. The data‐taking system was carefully designed, considering fast data rate, short interpulse period, wide data bits (19 bit), and fast sampling rate (1 million samples/s), and all the logics are assembled by programmable logic devices. This external interferometer was operated since March 1995 during the Doppler beam swinging observation by the MU radar, and the first results are presented. Meteor echo rate was around 2000/day, 1000 of which were usable for postprocessing, i.e., wind measurement. This echo rate is greater than that of the meteor radar system in Jakarta. Wind velocities were determined with time‐height resolutions of 1 hour × 2 km, which showed reasonable agreement with the wind velocities determined from the turbulent echoes in the mesosphere. We present the capability of an external interferometry system attached to the mesosphere‐stratosphere‐troposphere/stratosphere‐troposphere radar as a meteor wind observation system.
Ionospheric tomography via iterative cross‐entropy minimization
Authors: Kuklinski Walter S.
Published: February 2004

The problem of reconstructing ionospheric electron density from ground‐based receiver to satellite total electron content (TEC) measurements was formulated as an underdetermined discrete linear inverse problem. The fact that electron density, TEC, and the ray path distances which relate them are all positive, coupled with the sparse nature of the equations relating TEC and electron density, has been used to implement a computationally efficient ionospheric tomography reconstruction algorithm. The algorithm uses an iterative cross‐entropy optimization technique, where the Kullback‐Leibler distance is used to define a functional that is minimized using an alternating projection iterative method. Using PIM (parameterized ionospheric model) generated data as a nonnegative prior estimate of the electron density, both maximum entropy and minimum cross‐entropy reconstructions have been produced. The high quality of these reconstructions, coupled with the computational efficiency of this algorithm, indicates the potential utility of this technique for real‐time ionospheric tomography.
Statistical characteristics of the noise power spectral density in UHF and VHF wind profilers
Authors: Petitdidier Monique , Sy Amadou , Garrouste Axel , Delcourt Jean
Published: February 2004

In the first part of the paper, from a large set of spectrum data, obtained with the UHF/VHF (INSU/METEO) radars during the FRONTS87 and PYREX experiments, statistical characteristics of the noise power spectral density and behavior of its mean are pointed out. For each range gate and various sets of number of coherent and incoherent integrations, histograms of the noise power spectral density are computed using data from different parts of the campaign. The probability density function, so computed, is asymmetrical for low number of incoherent integrations and tends toward a Gaussian shape as the incoherent integration number increases. This histogram behavior is accounted for by a statistical approach, which assumes that the input noise is a random variable normally distributed and has a white spectrum for UHF as well as for VHF. In the second part of this paper, the results obtained by two different methods are compared and are explained by the shape of the probability density function. In the third part of this paper, it is shown that the mean value of the noise power spectral density is a very sensitive parameter for spectrum quality control. Some examples of its behavior as a function of the range gate, the observing direction, and the total power received are given and interpreted; these examples show how the noise mean power density can reveal malfunctioning of the radar, environmental perturbation, or variation due to the sky noise.
Average received signal power after two‐way propagation through ionized turbulence
Authors: Knepp Dennis L. , Brown William A.
Published: February 2004

In this paper we consider propagation through a structured ionosphere separating a monostatic or bistatic radar and its target. A new analytic calculation of the correlation function of the received, demodulated signal is presented. This result is a function of the locations of the transmit and receive apertures and of time during the duration of the received pulse. Strong scattering is assumed where the one‐way scintillation index is unity; under this assumption the parabolic wave equation can be solved using the quadratic approximation for the phase structure‐function; this facilitates tractable expressions for the mutual coherence function (MCF) for both one‐ and two‐way propagation. A new expression is given for the two‐position, two‐time, two‐frequency MCF for spherical wave propagation that allows simultaneous description of all effects of the ionospheric propagation channel (or transmission medium) on a propagating radar signal. To obtain useful and tractable results, the transmitted signal is assumed to be a chirp (linear FM) pulse, a waveform used by most long‐range defense radars. The result for the correlation function of the received signal is a function of the basic parameters that describe wave propagation through the ionospheric structure, including the channel decorrelation distance, decorrelation time, and coherence bandwidth. Both the two‐way enhancement in average power and aperture averaging effects are explicitly included in this derivation, and new results are presented that relate these effects to the characteristics of the propagation channel. The results are useful for propagation under conditions of saturated scintillation, for example HF through UHF radar in the natural equatorial environment and HF through gigahertz‐band radars that are required to operate in a disturbed propagation environment (i.e., in chemical releases or nuclear detonations).
Ionospheric irregularities: The role of the equatorial ionization anomaly
Authors: Rama Rao P. V. S. , Jayachandran P. T. , Ram P. Sri
Published: February 2004

The availability of similar ionosondes at four locations in India covering the regions from the equator to the northern crest of the equatorial ionization anomaly (EIA) has provided a unique opportunity to study the role of the EIA and related processes in the occurrence of spread F. The study is conducted during equinoctial months of high solar activity period (Rz=145), when the probability of occurrence of spread F is maximum. This study also deals with vertical movement of the F layer due to both electric field and neutral winds and the occurrence of spread F. The present study shows that a well‐developed anomaly beyond the latitude of 18°N is one of the base‐level conditions for the generation of spread F. The total integrated vertical movement is the controlling factor for the onset of spread F and not the peak value of h′F alone. There seems to be no threshold value in the altitude of the F layer (h′F) for the occurrence of spread F at all stations. The direction and magnitude of meridional winds that are crucial in the generation of spread. F showed higher values of poleward wind on non‐spread‐F days, indicating that the irregularity growth is suppressed due to the poleward wind.
Resistive treatment to reduce edge diffraction from large wedge‐shaped objects and planar antennas
Authors: Otero Michael F. , Rojas Roberto G.
Published: February 2004

The application of a tapered resistive card to reduce the EM scattering from the edge of an impedance wedge is presented. The problem is two‐dimensional, where the incident field is polarized transverse magnetic or electric to the axis of the wedge, and the resistive card is infinitesimally thin and is attached to the vertex of the wedge. An integral equation is formulated to solve for the scattering from the structure which utilizes a specialized Green's function for the impedance wedge. The integral equation is then solved using a method of moments technique. Since the presence of the wedge is taken into account by the Green's function, unknowns need to be placed only over the region of the resistive card and not along the boundaries of the wedge, thus reducing the size of the moment method matrix. Numerical optimization techniques are then employed to determine the resistance profiles for the cards which yield designs that minimize the scattered field over broad spatial regions and over broad frequency bands as well. The resulting designs for the resistive cards have therefore been optimized to yield maximum performance for scattering reduction. The application of this work to the reduction of edge diffraction from the edges of the ground plane of a microstrip antenna is also discussed, and measured as well as calculated results are presented.
Calibration and experimental results of a two‐dimensional interferometric radiometer laboratory prototype
Authors: Camps A. , Torres F. , Corbella I. , Bará J. , Soler X.
Published: February 2004

In recent years, Earth observation by means of aperture synthesis radiometry has received special attention by some space agencies as a possible solution to achieve high radiometric accuracy and spatial resolution at low microwave frequencies (L band), where the apparent brightness temperature is much more sensitive to soil moisture and sea surface salinity. This paper presents the characterization and calibration procedure, as well as some synthetic images measured with an X band experimental Y‐shaped Synthetic Aperture Interferometric Radiometer prototype developed at the Polytechnic University of Catalonia. The instrument is composed of a single pair of antennas that can be moved along the arms of an Y structure to synthesize a set of baselines. An experimental procedure is proposed to evaluate and then calibrate offset, inphase, quadrature, and amplitude errors generated by receivers and correlators.
Scattering from relatively flat surfaces using the adaptive integral method
Authors: Anastassiu Hristos T. , Smelyanskiy Mikhail , Bindiganavale Sunil , Volakis John L.
Published: February 2004

The adaptive integral method is applied to electromagnetic scattering from relatively flat, perfectly conducting surfaces. It is demonstrated that for nearly flat scatterers, as is the case with corrugated geometries and rough surfaces, the memory requirements and complexity of the technique are extremely low. Furthermore, for such geometries the algorithm can be effectively parallelized without excessive effort. Several numerical results are included, proving the accuracy and computational efficiency of the method.
Cylindrical and conical shrouds for the reduction of ground noise of paraboloidal antennas
Authors: Landecker T. L. , Smegal R. J. , McKinley J. M.
Published: February 2004

The spillover noise of a reflector antenna can be reduced by attaching a shroud extending forward from the edge of the reflector; the shroud prevents ground radiation from entering the feed. Symmetrical paraboloidal antennas of diameter 40 wavelengths, equipped with cylindrical and conical shrouds, are analyzed using the method of moments. A cylindrical shroud, parallel to the reflector axis, may reduce antenna noise, but it raises the sidelobe level in the front hemisphere substantially and can also reduce antenna gain. These drawbacks can be overcome by using a conical shroud, flared outward. Such a shroud reduces the spillover lobes in the back hemisphere, thus lowering the antenna noise temperature, but generates a conical sidelobe in the front hemisphere. The peak level of this sidelobe can be reduced by building the shroud using two cones of different flare angles or by curving its cross section. The decrease in noise temperature, as well as the location and level of the conical sidelobes in the front hemisphere, can be predicted to useful accuracy using geometrical optics. The addition of a shroud increases the level of cross polarization near the main beam. However, this effect is reduced if the sharp corner where the shroud joins the reflector rim is replaced by a smooth transition. The level of cross polarization is then at a level comparable to that produced by scattering from feed‐support struts.
Fourier transform analysis of dielectric‐filled edge‐slot antenna
Authors: Park Jong K. , Eom Hyo J.
Published: February 2004

We revisit the radiation characteristics of a dielectric‐filled edge‐slot (DFES) antenna, which is made by circumferentially removing the outer conductor of a coaxial line. The Fourier transform and the mode‐matching technique are utilized to obtain a new solution in a fast converging series. A dominant‐mode solution for the reflected power and the radiation field is given in a simple form. The numerical investigation is performed to show the behavior of the antenna input impedance and the radiation pattern.
Sea‐air and boundary layer temperatures measured by a scanning 5‐mm‐wavelength radiometer: Recent results
Authors: Westwater Ed R. , Han Yong , Irisov Vladimir G. , Leuskiy Vladimir Y. , Trokhimovski Yuri G. , Fairall Christopher W. , Jessup Andrew T.
Published: February 2004

A scanning 5‐mm‐wavelength (60 GHz) radiometer was deployed during two recent experiments. The first was in September–October in 1995 when the Environmental Technology Laboratory conducted the Coastal Ocean Probing Experiment off the Oregon coast. During this experiment the scanning radiometer was deployed on the Floating Instrument Platform, a research vessel operated by the Scripps Institution of Oceanography. Ground truth for the radiometer was provided by two independent in situ and infrared measurements of the ocean surface temperature; ground truth for air temperature was also provided by two in situ measurements at 0.5 and 12 m above the ocean surface. The second experiment was during September 10–30, 1996, at the Department of Energy's Atmospheric Radiation Measurement Program's Cloud and Radiation Testbed site near Lamont, Oklahoma. Here ground truth for derived quantities consisted of 3‐hourly radiosonde releases and measurements at 25 and 60 m from a meteorological tower. The results of the experiments indicate that the scanning radiometer yields accuracies of the order of 0.4°C rms for sea‐air temperature difference and accuracies in temperature of better than 0.7°C rms at altitudes below 60 m.
An improved interferometer design for use with meteor radars
Authors: Jones J. , Webster A. R. , Hocking W. K.
Published: February 2004

The measurement of the directions of radio meteors with an interferometric system is beset by two problems: (1) The ambiguity in the measured directions for antennas spaced by more than λ/2 and (2) the effects of mutual impedance when the antennas are spaced at λ/2 and less to avoid these ambiguities. In this paper we discuss the effects of mutual impedance between spaced antennas and describe an interferometer which both minimizes these effects and avoids the ambiguities associated with spacings larger than λ/2. We have modeled a version of this design numerically and show that under ideal conditions an interferometer of total span 4.5λ can yield directions accurate to about 0.3° with a signal‐to‐noise ratio of 20 dB. Finally, we have tested the design with observations from the 1996 Geminid and 1997 Quadrant meteor showers and find that even without a ground plane, the interferometer provides unambiguous directions to an accuracy of the order of 1.5°.
Flexural uplift of rift flanks in central Greece
Authors: Poulimenos George , Doutsos Theodor
Published: February 2004

Uplifts, with elevations of up to 2000 m and short wavelengths (30‐35 km), flank three major grabens in central Greece: the Rio, Sparta, and Atalanti grabens. They are bordered on their landward sides by narrow basins oriented parallel to the graben axes: the Manesi and Trichonis basins at the Rio graben and Copais and Istiea basins at the Atalanti graben. The flexural origin of these uplift profiles is investigated here by using thin, broken plate models. It is demonstrated that the observed topography of the graben flanks is consistent with the upward deflection of elastic and viscoelastic plates in response to upward directed forces applied at the graben flanks. In order to evaluate to applicability of each model, their predictions are checked against the observations. The elastic predictive modeling fits well with the observed flexural wavelengths and the flanking seismicity. However, it fails to predict the graben widths and the inferred elastic layer thickness. In contrast, the viscoelastic model successfully explains the graben widths and the “back” basins as flexural hinterland basins and matches the seismological data and the time constraints of rifting. It is therefore suggested that flexural uplift with viscoelastic relaxation accounts for the building of the graben flanks. The invoked viscoelastic models constrain the effective elastic thickness of the plates at 10 km for the Rio graben and 15 km for the Sparta and Atalanti grabens, suggesting a low‐viscosity lower crust. Furthermore, they predict low rates of tectonic uplift of the order of 0.1 mm a−1for the Atalanti graben, intermediate rates of 0.24–0.37 mm a−1for the Rio graben, and high rates of 0.7–0.9 mm a−1for the Sparta graben. The latter are quite possibly overestimated since napping events, capable of producing high local relief, are traced normal to the modeled profiles.
Lithospheric weakening during “retroforeland” basin formation: Tectonic evolution of the central South Alpine foredeep
Authors: Bertotti Giovanni , Picotti Vincenzo , Cloetingh Sierd
Published: February 2004

Beginning with the Late Cretaceous, convergence between Europe and Adria caused the southward subduction of the European plate beneath Adria. Contractional movements at high crustal levels were partly accommodated by southward thrusting (Southern Alps) causing flexure of the loaded Adriatic plate and the formation of the Late Cretaceous to Late Miocene South Alpine foredeep. With respect to the Alpine subduction system, the South Alpine foredeep is a retroforeland basin. In the Late Paleogene, dip values of the base foredeep varied between 3° and 5°. The shallower dips were found along profiles crossing domains which had undergone extension during previous Late Triassic to Middle Jurassic rifting. With ongoing convergence, the base foredeep along the entire basin steepened and dip values in the Tortonian are of about 7°–8°. The increase in dip goes hand in hand with an increase in the curvature of the loaded plate. This suggests a progressive weakening of the flexed plate. Modeled effective elastic thickness (Te) values derived from the analysis of plate curvatures decrease from 15–20 km in the Late Paleogene to <5 km in the Tortonian. Preconvergence values obtained from modeling studies were even higher, in the 24–27 km range. A practically complete decoupling of the upper/middle crust from its mantle substratum enabled the increase in curvature of the hinge zone. The progressive weakening of the South Alpine lithosphere around the bulge zone is correlated with the its “upper plate” position with respect to the Alpine subduction system.
New reflections on the structure and evolution of the Makkovikian ‐ Ketilidian Orogen in Labrador and southern Greenland
Authors: Kerr Andrew , Hall Jeremy , Wardle Richard J. , Gower Charles F. , Ryan Bruce
Published: February 2004

Marine seismic reflection profiles across the Early Proterozoic Makkovikian ‐ Ketilidian Orogen in the Labrador Sea region suggest that it is a doubly vergent, asymmetric orogenic belt, comparable in width to younger collisional orogens. A southeast dipping reflector package is correlated with on‐land shear zones that mark the southeastern limit of exposed reworked Archean crust and is also associated with a cryptic isotopic boundary in granites, which documents a transition from “ancient” to “juvenile” basement. This boundary is interpreted as a suture, along which juvenile Proterozoic crust has been juxtaposed over (thrust over ?) the Archean craton. A major synorogenic to postorogenic plutonic terrane to the southeast has a poorly reflective upper crust but shows strong subhorizontal reflectivity in the lower crust and Moho regions. The southeastern part of the profile correlates with metasedimentary terranes of the Ketilidian Belt in Greenland and contains low‐angle, northwest dipping, reflector packages suggestive of large‐scale crustal imbrication by thrusting. At least two broad zones of reflectivity at mantle depths (up to 16 s) are also recognized. One dips northwest below the Archean craton, but the most widespread mantle reflectivity is southeast dipping in opposition to the dominant fabric in the overlying crust. These contrasting crustal and subcrustal reflectivity patterns define a geometric “focus” beneath the orogenic belt and may provide information about subduction polarity during its development. The doubly vergent reflectivity pattern resembles images from possibly correlative Precambrian orogenic belts such as the Penokean and Svecofennian and also younger “small collisional orogens” such as the Appalachian and Pyrenean belts. It also broadly resembles some results of geodynamic crustal deformation models based on detachment and underthrusting of mantle lithosphere following collision and the squeezing of “weak” zones between rigid bounding blocks. A speculative multistage model for the belt incorporates initial northward subduction beneath the craton, shifting to later southward subduction, followed by oblique accretion of a composite arc terrane and juvenile (?) continental block. The accreted, hotter, juvenile, Proterozoic crust behaved differently than the stable, cool, Archean crust and experienced subhorizontal shearing in lower crustal and Moho regions, associated with southeast directed imbrication of the middle and upper crust by thrusting. However, the present reflectivity pattern of the orogen may also include elements related to post collisional extensional collapse.
Two‐dimensional crustal structures of Taiwan from gravity data
Authors: Yen Horng‐Yuan , Yeh Yih‐Hsiung , Wu Francis T.
Published: February 2004

To delineate the tectonic character of Taiwan, an island‐wide gravity survey of Taiwan was conducted between 1980 and 1987. The Bouguer anomaly map shows that, in general, isogals trend NNE in consonance with the overall structural trend of the island. With seismic and other geophysical data as constraints, the subsurface density structures were modeled along three profiles across the major structural trends. The gravity data are consistent with average continental Moho depths of 26 km in the Coastal Plain and the Western Foothills, 28 km underneath the Coastal Range in eastern Taiwan, and 33 km under the Central Range. A lack of significant correlation of the Bouguer anomalies with the topography implies dynamic, rather than isostatic, support of topography in the Taiwan region.
A novel method for determining residence times of river and lake sediments based on disequilibrium in the thorium decay series
Authors: Olley Jon M. , Roberts Richard G. , Murray Andrew S.
Published: February 2004

Sediment fluxes in rivers have proved difficult to measure or model at timescales relevant to land and water resource management decisions. Determining transport rates and residence times is complicated by the intermittent movement and temporary storage of sediments in natural streams. This paper presents a new tool for estimating the residence times of river‐borne sediments, based on the observed disequilibria between the naturally occurring radionuclides 228Ra and 232Th in the thorium decay series. Disequilibrium between concentrations of 228Ra (half‐life 5.75 years) and its long‐lived parent 232Th has been observed in marine environments and in terrestrial freshwaters but has not been previously reported in fluvial sediments or soils. Here we report 228Ra excesses in sandy stream and silty lake sediments from southeastern Australia. The excess is developed in hillslope soils in the catchment headwaters and begins to decay back to secular equilibrium once the sediment is finally removed from contact with the soil water. The short half‐life of 228Ra enables this process to be used to investigate fluvial sediment transfers, and alluvial and lacustrine sediment storages, over the past 30 years. When combined with indicators of sediment source, such as 226Ra/232Th and 230Th/232Th ratios, observation of this excess should provide a useful new tool for examining the residence and transit times of bed load and suspended load sediments in streams and lakes.
Magnetic resonance imaging of paramagnetic tracers in porous media: Quantification of flow and transport parameters
Authors: Greiner Andrea , Schreiber Wolfgang , Brix Gunnar , Kinzelbach Wolfgang
Published: February 2004

Experiments in model soil columns made of 0.5‐mm glass beads demonstrated that the convective and dispersive transport in porous media can be visualized and transport parameters can be quantified by using magnetic resonance imaging (MRI). Three‐dimensional information of water or tracer distribution in columns was obtained at different times in slices of 10 mm thickness and with an image plane resolution of 2.34 mm. The total mass of tracer in the columns and the pore water velocities determined by MRI were found to be consistent with the input values and with those derived from the outflow breakthrough curves (OBC). The dispersivity for a homogeneous medium was found to be on the order of half the grain diameter increasing by a factor of 4 in a heterogeneous medium, which is consistent with literature values. The dispersivities derived from the OBC were 3 times higher owing to the flow pattern inside the columns which we identified and took into account in our MRI analysis but not in the standard one‐dimensional analysis of the OBC. This apparatus‐induced dispersion could be demonstrated explicitly and visually.
Measurement of hydraulic properties in deep lake sediments using a tethered pore pressure probe: Applications in the Hamilton Harbour, western Lake Ontario
Authors: Harvey F. Edwin , Rudolph David L. , Frape Shaun K.
Published: February 2004

Estimates of groundwater seepage flux in lake bottom sediments require knowledge of the hydraulic gradient at the sediment‐surface water interface and the hydraulic conductivity of the lake‐bottom materials. In deep waters, in situ measurement of these parameters can be accomplished through the use of piezometer probes lowered and monitored remotely from a surface vessel. In this research work a new tethered piezometer probe was developed and tested for use in collecting hydraulic property data in deep‐lake bottom sediments. The probe uses a variable‐reluctance transducer to measure the differential sediment pore pressure between two ports spaced 100 cm apart. The dissipation of pore pressure transients that develop during rapid emplacement of the probe were extrapolated in time to estimate equilibrium hydraulic gradients. In addition, various data analysis techniques were evaluated for determining sediment hydraulic conductivity and specific storage through interpretation of the pore‐pressure dissipation data. The probe was used to estimate groundwater seepage in the bottom sediments of the Hamilton Harbour, at the western end of Lake Ontario. Upward gradients were measured at nine locations within the harbor ranging from 0.010 to 0.425 and a downward gradient of −0.015 was recorded at one site along the harbor's eastern boundary. Hydraulic conductivities determined from pore‐pressure dissipation over time ranged from 6.9 × 10−9to 4.8 × 10−7 m/s. Specific storage values ranged from 0.08 to 0.19 m−1. Calculated average linear seepage velocities ranged from 4.3 × 10−8 to −8.5 × 10−9 m/s. The groundwater contribution to the harbor through the deeper, fine‐grained sediments was estimated to be 9.1 × 10−2 m3/s, or 2.9 × 106 m3/yr. This represents approximately 1.0% of the harbor basin's total volume, 15% of precipitation's contribution, 1.2% of the contribution of surface inflows (excluding the Burlington ship canal) and 0.22% of the total surface outflow passing through the Burlington shipping canal, which connects the harbor to Lake Ontario.
Correcting laboratory‐measured moisture retention data for gravels
Authors: Khaleel R. , Relyea J. F.
Published: February 2004

Vadose zone soils in desert environments often contain a high gravel fraction (>2 mm size). Laboratory measurements of moisture retention are typically made on the fine fraction ∥<2 mm size); the measurements are then corrected for field conditions by accounting for the gravel fraction in the sample. A frequently used correction procedure for gravelly soils is tested. A total of 23 samples, containing gravel fractions up to 80% (by weight), were analyzed in the laboratory. Retention data on <2 mm size fraction were obtained and compared with measurements on bulk (soil and gravel) field samples. Results indicate that for the relatively wet moisture regime considered in this study, retention data for gravelly soils can be estimated reasonably well by correcting the pressure cell measurements for the field bulk density and the fraction of fines present in the field samples. Further work is needed on testing the correction procedure for the dry moisture regime.
Sensitivity of channel network planform laws and the question of topologic randomness
Authors: Costa‐Cabral Mariza C. , Burges Stephen J.
Published: February 2004

The random topology model approximately predicts all orientation‐free empirical laws of channel network planform. Explanation of the model's success has crucial significance for research direction. Shreve [1975] proposed the explanation that random factors dominate network development. An alternative explanation is that these geomorphologie laws are insensitive to topologic variability. Kirchner [1993] tested the sensitivity of the Horton ratios to particular types of topologic distribution. We extend the work of Kirchner to all orientation‐free empirical laws of channel network planform. We test the sensitivity of the topologic analog of each law using non‐topologically random network test samples. One type of sample is obtained with the stochastic variable parameter “Q model” of network growth, used as a test example, where the probability of tributary development on interior and exterior links is allowed to vary. We show that the coefficients of all channel network planform laws vary with model parameter Q. It is possible that these laws are sensitive also to other models of network growth. We conclude that the success of the random model in approximately predicting geomorphologie laws may not be due to the insensitivity of these laws and is a result that remains unexplained.
Inferring regional surface fluxes from convective boundary layer characteristics in a Sahelian environment
Authors: Lhomme J.‐P. , Monteny B. , Bessemoulin P.
Published: February 2004

Regional scale fluxes of sensible heat, latent heat, and CO2 are estimated by means of a method which involves the heat and mass budget of the convective boundary layer (CBL). This method is based upon an integrated form of the conservation equation for a given scalar and is used with radiosounding data collected during several clear days of measurements of the Hydrological Atmospheric Pilot Experiment in the Sahel (HAPEX‐Sahel) at the end of the rainy season and the beginning of the dry season in 1992. The regional estimates of sensible heat flux provide plausible results. For latent heat flux, only 4 days out of 8 give satisfactory agreement, and the introduction of a correction for advection does not improve the agreement for the rest of the days. For CO2 flux calculation the method used links regional fluxes to concentrations measured within the surface layer. The estimates obtained for the CO2 uptake at a regional scale differ from those measured on a fallow savannah, but the overall trend (a decrease of the CO2 uptake due to soil water depletion and vegetation drying out) is respected. Our general conclusion is that the CBL budget method can work to estimate sensible heat flux, but it is very hazardous to estimate latent heat flux and CO2 uptake.
Household willingness to pay for improved rural water supplies: A comparison of four sites
Authors: Piper Steven , Martin Wade E.
Published: February 2004

The estimated willingness to pay for improved rural water supplies at four sites in the western United States ranges from $4.43 to $17.29 per household per month. These results are similar to other rural water supply studies. The modeling results and comparison with previous studies indicate that the contingent valuation method can provide useful estimates of rural water supply benefits. This analysis provides information useful to private and public officials who must determine where limited funds should be spent for water supply improvements. Water suppliers and government analysts who do not have the resources to complete site specific valuation studies can use general benefit criteria and site specific factors presented in the analysis to target areas for water supply improvements. The range of benefits presented in the analysis can be used to help determine an acceptable level of costs for a proposed water supply project.
Micromodel visualization and quantification of solute transport in porous media
Authors: Corapcioglu Yavuz M. , Chowdhury Sabina , Roosevelt Sharon E.
Published: February 2004

Microscopic visualization can provide valuable information to enhance the understanding of pore‐scale transport phenomena. Micromodels are artificial models of porous media that can be used to simulate transport processes at the pore scale. Previous micromodel studies focused mostly on pore‐scale observation of immiscible fluids. In this study we observed and modeled solute transport in a horizontally mounted micromodel with a regular geometry of orthogonal channels. Dye solutions were injected as tracers at a constant flow rate to observe the solute transport with time. Images were video recorded and processed with an image analyzer to obtain the concentration contours of the solute front. Solute transport was also observed at point, pore, and REV scales through a microscope. Following the determination of micromodel porosity and hydraulic conductivity, we presented a methodology to calculate micromodel dispersivity based on REV‐scale breakthrough curves measured at three different points along the micromodel centerline. Experimental values were compared with the numerical results of an advective‐dispersive solute transport model which treats the micromodel as a macroscale porous medium. Furthermore, we employed a two‐dimensional discrete fracture model in which the network of channels is treated as a set of mutually orthogonal fractures in a nonporous medium. The simulation results compared favorably with experimental observations processed by an image analyzer.
LH moments for statistical analysis of extreme events
Authors: Wang Q. J.
Published: February 2004

Statistical analysis of extremes is often conducted for predicting large return period events. In this paper, LH moments, a generalization of L moments, that are based on linear combinations of higher‐order statistics are introduced for characterizing the upper part of distributions and larger events in data. Using LH moments reduces undesirable influences that small sample events may have on the estimation of large return period events. The method of LH moments is formulated for the generalized extreme value (GEV) distribution along with two detailed examples. Monte Carlo simulations are conducted to illustrate the performance of using the method of LH moments to fit the GEV distribution to both GEV and non‐GEV samples. LH moment diagrams are constructed using annual maximum flow data for 107 streams worldwide to examine the ability of the GEV distribution to model those series.
Seasonal generalized exponential probability models with application to interstorm and storm durations
Authors: Lambert Martin , Kuczera George
Published: February 2004

A simple generalization of the exponential probability model is presented which provides a flexible method for identifying and fitting probability distributions. When coupled with likelihood‐based estimation, this approach enables a parsimonious description of seasonal dependence using harmonic functions. A case study involving modeling of interstorm and storm durations for several Australian state capitals illustrates practical issues in calibration and identification. Because the rainfall data were stored in a binned format consisting of rainfall depths accumulated over fixed time intervals, the start and end times of storms were known only to the resolution of the fixed time interval. A likelihood function is developed which properly makes use of such information. Likelihood ratio statistics along with monthly distribution plots are used to select the number of harmonics necessary to model seasonal dependence. It is shown for Melbourne that a single harmonic adequately describes seasonal dependence, thereby reducing the number of parameters from 48 to 6.
flow in fractured media: A modified renormalization method
Authors: Gavrilenko Pierre , Guéguen Yves
Published: February 2004

We present a methodology to account for fluid flow from microscales to macroscales in fractured media. Renormalization group (RG) theory coupled to a percolation approach provides a convenient tool to allow dealing with the complex connectivity of fracture systems and with the intricate relationship between the various scales involved and the observation scale. This method also provides us with a way to describe scale dependence of permeability (K). Two‐ and three‐dimensional calculations can be performed for broad distributions of both conductances and fracture lengths. Here we introduce what we call a “modified renormalization” method. This method is then tentatively applied to the upper crust where many scales of fractures are present. The results are discussed relative to the 3‐ to 4‐orders‐of‐magnitude increase in permeability which has been reported by some authors from sample scale to regional scale, and we examine how our model could be used to predict possible scaling laws.