bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantitative estimation of Tropical Rainfall Mapping Mission precipitation radar signals from ground‐based polarimetric radar observations
Authors: Bolen Steven M. , Chandrasekar V.
Published: May 2003

The Tropical Rainfall Mapping Mission (TRMM) is the first mission dedicated to measuring rainfall from space using radar. The precipitation radar (PR) is one of several instruments aboard the TRMM satellite that is operating in a nearly circular orbit with nominal altitude of 350 km, inclination of 35°, and period of 91.5 min. The PR is a single‐frequency Ku‐band instrument that is designed to yield information about the vertical storm structure so as to gain insight into the intensity and distribution of rainfall. Attenuation effects on PR measurements, however, can be significant and as high as 10–15 dB. This can seriously impair the accuracy of rain rate retrieval algorithms derived from PR signal returns. Quantitative estimation of PR attenuation is made along the PR beam via ground‐based polarimetric observations to validate attenuation correction procedures used by the PR. The reflectivity (Zh) at horizontal polarization and specific differential phase (Kdp) are found along the beam from S‐band ground radar measurements, and theoretical modeling is used to determine the expected specific attenuation (k) along the space‐Earth path at Ku‐band frequency from these measurements. A theoretical k‐Kdp relationship is determined for rain when Kdp ≥ 0.5°/km, and a power law relationship, k = a Zhb, is determined for light rain and other types of hydrometers encountered along the path. After alignment and resolution volume matching is made between ground and PR measurements, the two‐way path‐integrated attenuation (PIA) is calculated along the PR propagation path by integrating the specific attenuation along the path. The PR reflectivity derived after removing the PIA is also compared against ground radar observations.
Air temperature profile and air/sea temperature difference measurements by infrared and microwave scanning radiometers
Authors: Cimini D. , Shaw J. A. , Westwater E. R. , Han Y. , Irisov V. , Leuski V. , Churnside J. H.
Published: May 2003

A system of two scanning radiometers has been developed by the National Oceanic and Atmospheric Administration/Environmental Technology Laboratory and deployed on the NOAA R/V Ronald H. Brown during the Nauru99 cruise in the tropical western Pacific in June and July 1999. The system is composed of a high‐quality temperature sensor and two independent, vertically scanning radiometers, measuring atmospheric and oceanic emission in the microwave (MW), and infrared (IR) regions. Both radiometers measure emission from a uniformly mixed atmospheric gas: oxygen for MW (60 GHz) and carbon dioxide for IR (14.2 μm). The high atmospheric absorption at these frequencies allows one calibration point from the horizontal atmospheric view using the in situ temperature sensor measurements as a reference. The signal at all other scan angles is scaled relative to that at the horizontal, resulting in a differential technique that is independent of calibration offset. This technique provides continuous and accurate estimates of boundary layer air temperature profile and air/sea temperature difference. The main advantage of this technique is that the water skin temperature can be measured at different optical depths without disturbing the skin layer (magnitude order of microns). We first compare radiometric data collected during the experiment with simulations obtained by atmospheric and oceanic radiative transfer models. We then use statistical inversion techniques to estimate air temperature profiles from upward looking measurements, based on an a priori data set of about 1500 ship‐based radiosonde observations. For the “well‐posed” problem of air/sea temperature difference estimation, we apply a physical retrieval algorithm to the downward looking measurements, accounting for air attenuation and sea surface roughness. Then we show retrieval results and evaluate the achieved accuracy. Finally, we compare radiometric estimates with in situ measurements, discussing similarities and discrepancies.
Accuracy of cloud liquid water path from ground‐based microwave radiometry 2. Sensor accuracy and synergy
Authors: Crewell Susanne , Löhnert Ulrich
Published: May 2003

The influence of microwave radiometer accuracy on retrieved cloud liquid water path (LWP) was investigated. Sensor accuracy was assumed to be the sum of the relative (i.e., Gaussian noise) and the absolute accuracies of brightness temperatures. When statistical algorithms are developed the assumed noise should be as close as possible to the real measurements in order to avoid artifacts in the retrieved LWP distribution. Typical offset errors of 1 K in brightness temperatures can produce mean LWP errors of more than 30 g m−2 for a two‐channel radiometer retrieval, although positively correlated brightness temperature offsets in both channels reduce this error to 16 g m−2. Large improvements in LWP retrieval accuracy of about 50% can be achieved by adding a 90‐GHz channel to the two‐channel retrieval. The inclusion of additional measurements, like cloud base height from a lidar ceilometer and cloud base temperature from an infrared radiometer, is invaluable in detecting cloud free scenes allowing an indirect evaluation of LWP accuracy in clear sky cases. This method was used to evaluate LWP retrieval algorithms based on different gas absorption models. Using two months of measurements, the Liebe 93 model provided the best results when the 90‐GHz channel was incorporated into the standard two‐channel retrievals.
Fundamental experiment of a polarization‐rotating reflector of Van Atta array design for polarimetric calibration
Authors: Fujita Masaharu , Okubo Hisashi , Fujino Yoshiyuki , Tanaka Masato
Published: May 2003

Polarization‐rotating reflectors are important as a reference target for polarimetric calibration of a synthetic aperture radar. A polarization‐rotating retrodirective reflector of Van Atta array design has already been proposed by one of the authors. This paper describes the result of an experimental verification of the proposed polarization‐rotating retrodirective reflector. The experimental system consists of two subarrays each composed of 2 × 2 dual‐polarized elements for horizontal and vertical polarizations working at 2.4‐GHz frequency band. In the experiment, two subarrays are arranged side by side, and the respective orthogonal polarization ports of the subarrays are connected with lines of the same electrical length to form a one‐dimensional polarization‐rotating Van Atta array. To increase the radar cross section, amplifiers are inserted into the lines. The reflection pattern of the reflector is measured in a radio anechoic chamber by rotating the reflector around the axis perpendicular to the array normal direction. From the experiment, a wider angular response of the proposed reflector in the horizontal direction, which is provided by the retrodirective characteristics of the Van Atta array, than the conventional array reflector of the same element number as the proposed reflector is verified in addition to its polarization‐rotating capability introduced by the dual‐polarized antenna elements.
Spatial and temporal fluctuations of atmospheric microwave emission
Authors: Kutuza Boris G.
Published: May 2003

Characteristics of spatial and temporal fluctuations of atmospheric absorption and emission at millimeter and centimeter wavelengths are presented. Theoretical relations and experimental data are given on structure functions of absorption and brightness temperature at wavelengths of 0.8 and 1.35 cm. It is shown that in cloudless weather the intensity of fluctuations at these wavelengths is determined mainly by random fluctuations of the humidity field. Under cloudy conditions the intensity of fluctuations of microwave atmospheric radiation grows, with stratiform and cumuliform clouds differently affecting the character of the space‐time dependency of fluctuations.
Accuracy of cloud liquid water path from ground‐based microwave radiometry 1. Dependency on cloud model statistics
Authors: Löhnert Ulrich , Crewell Susanne
Published: May 2003

This paper investigates the influence of cloud model statistics on the accuracy of statistical multiple‐frequency liquid water path (LWP) retrievals for a ground‐based microwave radiometer. Statistical algorithms were developed from a radiosonde data set in which clouds were modeled by using a relative humidity threshold and a modified adiabatic assumption. Evaluation of the algorithms was then performed by applying the algorithms to four data sets in which clouds were generated in different ways (i.e., threshold method, gradient method, and cloud microphysical model). While classical two‐channel algorithms, in this case using frequencies at 22.985 and 28.235 GHz, do not show a significant dependency on the cloud model, the inclusion of an additional 50‐GHz channel can introduce significant systematic errors. The addition of a 90‐GHz frequency to the two‐channel algorithm leads to a larger increase in LWP accuracy than in case of the 50‐GHz channel and is less sensitive to the choice of cloud model. A drizzle case from the cloud microphysical model shows no significant loss of accuracy for the microwave radiometer algorithms, in contrast to simple cloud radar retrievals of liquid water. In case of rain, however, the results deteriorate when the total liquid water path is larger than 700 g m−2.
Differential‐frequency Doppler weather radar: Theory and experiment
Authors: Meneghini R. , Bidwell S. W. , Liao L. , Rincon R. , Heymsfield G. M.
Published: May 2003

To move toward spaceborne weather radars that can be deployed routinely as part of an instrument set consisting of passive and active sensors requires the development of smaller, lighter‐weight radars. At the same time, the addition of a second frequency and an upgrade to Doppler capability are essential to retrieve information on the drop size distribution (DSD), vertical air motion, and storm dynamics. One approach to the problem is to use a single broadband transmitter‐receiver and antenna where two narrowband frequencies are spaced apart by 7–10%. Use of Ka‐band frequencies (26.5–40 GHz) provides adequate spatial resolution with a relatively small antenna. Moreover, the differential reflectivity and mean Doppler signals in this band are directly related to the median mass diameter of the snow and raindrop size distributions. We present in the paper theoretical calculations of the differential reflectivity and Doppler for several frequency pairs including those proposed for the Global Precipitation Mission (GPM) at 13.6 and 35 GHz. Measurements from a zenith‐directed radar operated at 9.1 and 10 GHz are used to investigate the qualitative characteristics of the differential signals. Disdrometer data taken at the surface, just below the radar, show that the differential signals are related to characteristics of the raindrop size distribution. The stability of the DSD estimation procedure is tested using a simulation. The results indicate that reasonably stable estimates of the particle size distribution are feasible with a [31.5 GHz, 35 GHz] combination as long as a large number of independent samples are obtained.
Laboratory polarimetric measurements of microwave emission from capillary waves
Authors: Trokhimovski Y. G. , Kuzmin A. V. , Pospelov M. N. , Irisov V. G. , Sadovsky I. N.
Published: May 2003

The results of polarimetric microwave measurements of thermal emission from a water surface with artificial periodic structure are presented. The goal of the experiment is to verify the results of a model describing polarized microwave emission from gravity‐capillary waves on a sea surface. Periodic undulations of water surface were produced in a tank by a set of parallel nylon threads raised slightly above the mean surface. Due to surface tension, the water surface assumes a periodic profile; the amplitude of the roughness was changed by moving the frame with the threads up and down. The measurements of the three Stokes parameters of the emission were carried out with the Ka‐band polarimeter. The experimental setup allowed measurements to be made over a wide range of view angles (from near‐nadir to 70°). The tank rotation allowed variation of the azimuthal angle between the periodic structure and the observation plane of more than 300°. The experimental data show the existence of the resonant peak in the Stokes parameters predicted by the model. The amplitude of the peak reached 10 K. Also, drastic contrasts were registered in the azimuth dependencies of the Stokes parameters. This result is evidence of the important role that short gravity‐capillary waves play in sea‐surface polarized microwave emission.
A novel L‐band polarimetric radiometer featuring subharmonic sampling
Authors: Rotbøll Jesper , Søbjærg Sten Schmidl , Skou Niels
Published: May 2003

A novel L‐band radiometer trading analog components for digital circuits has been designed, built and operated. It is a fully polarimetric radiometer of the correlation type, and it is based on the subharmonic sampling principle in which the L‐band signal is directly sampled by a fast A to D converter at a frequency well below L‐band. Overall stability has been a design driver, as the instrument is intended for airborne measurements of polarimetric sea signatures.
Faraday rotation and L band oceanographic measurements
Authors: Skou Niels
Published: May 2003

Spaceborne radiometric measurements of the L band brightness temperature over the oceans make it possible to estimate sea surface salinity. However, Faraday rotation in the ionosphere disturbs the signals and must be corrected. Two different ways of assessing the disturbance directly from the radiometric measurements, and hence enabling a correction for the effect, are being discussed. Also, a method, which aims at circumventing the problem by using the first Stokes parameter in the salinity retrieval, is being discussed.
Crosswind ocean radar backscatter and two‐scale scattering model at low grazing angles
Authors: Smirnov A. V. , Fuks I. M. , Naugolnykh K. A.
Published: May 2003

A two‐scale model of rough surface in a crosswind ocean radar illumination is considered. Model characteristics such as polarization and azimuthal dependence are illustrated through numerical calculations. Predictions of the model are compared with rare field measurements made in a crosswind environment. Measurements were obtained at a 2° grazing angle. These data, collected by X‐ and Ka‐band (9.3 and 34.6 GHz) radars under crosswind during the 1995 Coastal Ocean Probing Experiment (COPE) off the Oregon Coast, indicate that the backscatter intensity of local “sea‐spikes” measured with horizontal transmit‐and‐receive polarization (HH) reaches or exceeds that measured with the vertical transmit‐and‐receive polarization (VV). We suggest that for low grazing angles and in a crosswind environment, a radar geometrical condition can occur where some HH‐spikes can be dominated by vertically polarized scattered radiation due to cross tilt of a long wave, when its local slope becomes greater than 45°.
Design and performance of a high‐stability water vapor radiometer
Authors: Tanner Alan B. , Riley A. Lance
Published: May 2003

The design of two new high‐stability microwave water vapor radiometers is presented along with a performance evaluation. The radiometers operate next to a spacecraft tracking station at NASA's Goldstone facility in California, where they will be used to calibrate tropospheric path delay fluctuations during an upcoming gravity‐wave search experiment (GWE) involving the Cassini spacecraft. Observing frequencies of the radiometers are 22.2, 23.8, and 31.4 GHz, and the antenna beam width is 1°. The instruments are room temperature Dicke radiometers with additive noise injection for gain calibration. Design highlights include: (1) a practical temperature control system capable of stabilizing the entire receiver to a few millikelvin from day to night; (2) redundant noise diode injection circuits with 30 ppm RF power stability; and (3) a voice coil actuated waveguide vane attenuator which is used as a high‐performance Dicke switch. Performance of the radiometers is evaluated from intercomparisons of the two radiometers and from continuous tip curve calibrations spanning nearly 1 year. Structure function analysis of the intercomparison data indicates that the brightness temperature stability of these radiometers is better than 0.01 K on 1000–10,000 s timescales. Analysis of tip curve calibrations indicates RMS errors of ∼0.05 K on 30‐day timescales and 0.15 K on 1‐year timescales.
Radar reflectivity calibration using differential propagation phase measurement
Authors: Vivekanandan J. , Zhang Guifu , Ellis Scott M. , Rajopadhyaya D. , Avery Susan K.
Published: May 2003

A method for calibrating radar reflectivity using polarization radar measurements in rain is described. Accurate calibration of radar reflectivity is essential for obtaining reliable rain rate estimation. In the case of polarization radar, rain rate can be independently estimated using power and phase measurements. Thus phase measurement can be estimated from power measurements. Comparison of the direct estimate of propagation phase (Φm) measurement, which is unaffected by absolute calibration of the radar system, with the estimated propagation phase (Φe) from power measurements is the basis for the calibration method. Polarization measurements such as reflectivity (Z), differential reflectivity (ZDR), and differential propagation phase (ΦDP) are sensitive to drop size distribution (DSD) and mean drop shape. It is important to devise a calibration technique relatively unperturbed by changes in DSD and drop shape. Statistical fluctuation in Φe is derived to estimate the accuracy of the calibration procedure, raindrop shape, and attenuation. The proposed method is applied for calibrating reflectivity measurements of the National Center for Atmospheric Research (NCAR) S‐band polarization radar (S‐Pol) in midlatitude, subtropical, and tropical rain events.
Wave‐optical simulation of the oblique HF radio field
Authors: Hocke Klemens , Igarashi Kiyoshi
Published: May 2003

A wave‐optical simulation of the oblique HF radio field is carried out by means of the multiple phase screen (MPS) technique which is frequently used for study of atmospheric effects on the propagation of electromagnetic waves. Contrary to previous studies on ionospheric HF radio propagation, the simulation is carried out for a cone of HF radio waves transmitted with elevation angles 0‐35° from a point source at the ground. The ionospheric phase screens are approximately orthogonal to the Earth's surface, and the curvature of the Earth is considered. The HF radio field is determined from the surface to the topside ionosphere in two dimensions (x,z) by using ionospheric phase path screens with a z‐resolution of 10 m and a screen spacing of 0.5 km in x‐direction. Reflection of radio waves at the Earth's surface is implemented for the MPS simulation by means of an artificial decrease of the refractive index at heights below the surface (surface acts like an ionospheric layer in this rough approximation). The results are checked by means of ray tracing. The wave‐optical simulation is applied for description of focusing and defocusing of oblique HF radio waves around the main focus in the F1‐region and along the caustics where radio scintillations of high spatial frequencies are found due to interference and diffraction effects. One example is presented for the impact of a sporadic E layer on the HF radio field. Angular spectra of received radio signals are calculated at different locations of the HF radio field.
High‐order solution for the electromagnetic scattering by inhomogeneous dielectric bodies
Authors: Gedney Stephen D. , Lu Cai‐Cheng
Published: May 2003

A high‐order method of moment solution with quadrature‐point‐based sampling is presented for the solution of the volume electric field integral equation for the scattering of inhomogeneous dielectric bodies. The proposed scheme efficiently allows for the material profile to be inhomogeneous within a curvilinear cell. It is demonstrated that the method leads to exponential convergence in both the radar cross section (RCS) and the volume current density. It is also demonstrated that the method can be more efficient than surface field integral equation formulations for thin‐material scattering.
Solution of the aliasing and least squares problems of spaced antenna interferometric measurements using lattice methods
Authors: Harris C. R.
Published: May 2003

A method for solving the aliasing and least squares problems of spaced antenna interferometric measurements is presented. The method uses the Hermite normal form of an integer matrix together with LLL basis reduction and allows for the automatic analysis of antenna arrays and pulse patterns.
Built‐up terrain wave propagation by Fourier split‐step parabolic wave equation‐ray optical techniques
Authors: Eibert Thomas F.
Published: May 2003

Fourier split‐step (FSS) solutions of the parabolic wave equation (PWE) represent wave fields in terms of plane wave decompositions. However, those field solutions are usually only valid in the air space above built‐up terrain, whereas field predictions for modern wireless systems often require knowledge of the fields on a street level. Since FSS PWE solutions with large step sizes are not applicable for field computations between irregular scattering obstacles such as buildings, this problem is overcome by a two‐step approach combining the FSS solution of the PWE with ray optical techniques to compute the fields at ground level in wooded and urbanized areas. To account for the great variety of propagation effects in a statistical sense, direct rays, reflected rays, diffracted rays and attenuated rays at typical receiver locations are included into the considerations. Comparisons to a wide variety of measured data show that this two‐step approach produces better results than state of the art semiempirical field prediction techniques.
About the effects of instrument errors in interferometric radiometry
Authors: Anterrieu Eric , Waldteufel Philippe , Caudal Gérard
Published: May 2003

The SMOS mission is a European Space Agency (ESA) project aimed at global monitoring of surface soil moisture and sea surface salinity from radiometric L‐band observations. The radiometer onboard SMOS uses a 2‐D synthetic aperture concept in order to achieve satisfactory spatial resolution performances for a minimal cost in terms of payload mass and volume. As the satellite moves ahead, every area on Earth's surface is seen at a variety of incidence angles. This multiangular capability is used in the retrieval of geophysical parameters. A major issue for obtaining useful measurements of the surface salinity is the radiometric accuracy, since the overall dynamic range resulting from ocean salinity variations only extends over a few Kelvin. To improve instrument performances, it is foreseen that independent retrieved salinity estimates will be averaged over a suitable space/time domain. This should bring random uncertainties due to radiometric sensitivity down to around 0.1 Practical Salinity Scale (PSS). However, several systematic error sources are also present: biases arise from channel or baseline instrument errors and are superimposed to Gibbs oscillations generated through the reconstruction of brightness temperature fields from correlation products. It is thus of importance to assess to which extent these errors can be averaged out when building space/time averages of the retrieved salinity values. The presents study is a step toward addressing this issue.
MU radar spaced antenna observations with varying apertures: Scatterer and antenna contributions to the ground diffraction pattern
Authors: Hassenpflug G. , Rao P. B. , Yamamoto M. , Fukao S.
Published: May 2003

The 46.5 MHz MU radar located at Shigaraki, Japan (34.85°N, 136.10°E), is used to investigate the consistency of refractive index scale measurements utilizing a method based on Full Correlation Analysis (FCA) which includes compensation for the scale effects of transmitting and receiving antenna apertures. Horizontal correlation lengths of refractive index irregularities within the troposphere are determined from spaced antenna (SA) observations using six configurations with differently sized transmitting and receiving antennas. Results show that the algorithm effectively compensates for transmitting and receiving antenna aperture sizes. Estimates of turbulence structure scatterer correlation lengths and turbulence intensity are compared to background wind and shear from Doppler beam swinging (DBS) data to show how enhanced turbulence decreases the size of scatterers.
An efficient method to calculate surface currents on a PEC cylinder with flat end caps
Authors: Stevens Nobby , Martens Luc
Published: May 2003

In this paper, an efficient method is developed to model the scattering of electromagnetic waves by large three‐dimensional perfectly conducting (PEC) bodies of revolution with sharp edges. The magnetic field integral equation (MFIE) is implemented and applied to a finite solid cylinder, including the upper and lower end caps. Point matching is used to construct the interaction matrix. The numerical instabilities that generally occur to the current behavior near sharp edges are completely eliminated by the proper choice of special base functions near the edges. Not taking the singular behavior of the currents into account leads to a bad convergence or to incorrect results.
Estimation of dielectric slab permittivity using a flared coaxial line
Authors: Shin Dong H. , Eom Hyo J.
Published: May 2003

Estimation of dielectric slab permittivity is considered by using a flared coaxial line. A problem of reflection from a flared coaxial line that radiates into a dielectric slab with a flange is solved. A flared coaxial line is modeled with multiply stepped coaxial lines with different inner and outer conductors. A set of simultaneous equations for the modal coefficients is constituted based on the boundary conditions. Computations are performed to illustrate the reflection behavior in terms of the coaxial line geometry, frequency, and permittivity of a dielectric slab. Nomograms are developed to estimate the permittivity from the measured reflection coefficients. The utility of a flared coaxial line for the determination of slab permittivity is discussed.
Preface to special section: Remote Sensing of the Earth's Environment by Microwave Radiometers and Radar
Authors: Westwater Ed R. , Vivekanandan J.
Published: May 2003

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A new very high resolution interference rejection method with potential for radio astronomy applications
Authors: Minkoff J.
Published: May 2003

A scheme for interference rejection at an array is presented, in which the interference, represented as vectors, is removed by an operation in which none of the physical array characteristics such as beam width and resolution come into play. As a result, observation of signals of interest from weak sources, intact, unaffected by the presence of interfering signals from stronger sources arbitrarily close in angle to the weaker sources, is possible. The essential limit on this capability is determined solely by receiver noise. Rejection of up to M‐1 interfering sources with an array of M elements is accomplished by a single operation. The method seems especially well suited for radio‐astronomy applications.
Jurassic to Miocene magmatism and metamorphism in the Mogok metamorphic belt and the India‐Eurasia collision in Myanmar
Authors: Barley M. E. , Pickard A. L. , Zaw Khin , Rak P. , Doyle M. G.
Published: May 2003

Situated south of the eastern Himalayan syntaxis at the western margin of the Shan‐Thai terrane the high‐grade Mogok metamorphic belt (MMB) in Myanmar occupies a key position in the tectonic evolution of Southeast Asia. The first sensitive high‐resolution ion microprobe U‐Pb in zircon geochronology for the MMB shows that strongly deformed granitic orthogneisses near Mandalay contain Jurassic (∼170 Ma) zircons that have partly recrystallized during ∼43 Ma high‐grade metamorphism. A hornblende syenite from Mandalay Hill also contains Jurassic zircons with evidence of Eocene metamorphic recrystallization rimmed by thin zones of 30.9 ± 0.7 Ma magmatic zircon. The relative abundance of Jurassic zircons in these rocks is consistent with suggestions that southern Eurasia had an Andean‐type margin at that time. Mid‐Cretaceous to earliest Eocene (120 to 50 Ma) I‐type granitoids in the MMB, Myeik Archipelago, and Western Myanmar confirm that prior to the collision of India, an up to 200 km wide magmatic belt extended along the Eurasian margin from Pakistan to Sumatra. Metamorphic overgrowths to zircons in the orthogneiss near Mandalay date a period of Eocene (∼43 Ma) high‐grade metamorphism possibly during crustal thickening related to the initial collision between India and Eurasia (at 65 to 55 Ma). This was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 Ma. Similar syntectonic syenites and leucogranites intruded the Ailao Shan‐Red River shear belt in southern China and Vietnam during the Eocene‐Oligocene to Miocene, and the Wang Chao and Three Pagodas faults in northern Thailand (that most likely link with the MMB) were also active at this time. The complex history of Eocene to early Miocene metamorphism, deformation, and magmatism in the MMB provides evidence that it may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of India and resulting extrusion or rotation of Indochina.
Structural assemblage of high‐pressure mantle and crustal rocks in a subduction channel (Cabo Ortegal, NW Spain)
Authors: Ábalos B. , Puelles P. , Gil Ibarguchi J. I.
Published: May 2003

High‐pressure metamorphic fold‐nappes and ductile thrusts occur in the Cabo Ortegal upper allochthons. They involve arc‐root lithospheric mantle peridotites, high‐pressure volcanic‐arc granulites, normal mid‐oceanic ridge basalt eclogites and shallow crustal volcanosedimentary high‐pressure gneiss units. Confluence of units from such disparate tectonic settings occurred at an eo‐Hercynian convergent plate margin. Their internal deformation, juxtaposition, and high‐pressure synmetamorphic reworking resulted from kinematically simple but intense and polyphase ductile deformations. Fold nappes exhibit a remarkable sheath‐like form and contain mineral and stretching lineations defined by high‐pressure mineral assemblages subparallel to fold axes. We argue that these structures outline the internal organization of high‐pressure sheets at deep tectonic realms of an oblique subduction/collision orogenic channel.
Décollement depth versus accretionary prism dimension in the Apennines and the Barbados
Authors: Bigi Sabina , Lenci Federica , Doglioni Carlo , Moore J. Casey , Carminati Eugenio , Scrocca Davide
Published: May 2003

Along representative cross sections of the Apennines and the Northern Barbados accretionary prisms, we measured the area, the décollement depth, the angle α of the upper envelope and the angle β of the dip of the regional monocline. The continental sections of the Apennines accretionary prism have a deeper décollement than the oceanic sections of the Northern Barbados, 6–10 km depth and <1 km depth, respectively, because the sediment pile is thinner on the incoming Barbados plate and its denser oceanic structure is more easily subducted. Considering the frontal 50 km, the Apennines have an average cross‐sectional area of 500 km2 and the Northern Barbados Ridge of 100 km2. The total area is a function of the depth of the décollement plane. Therefore, at a given amount of subduction, the deeper the décollement depth is, the bigger the area of the wedge will result, assuming negligible compaction and erosion. As a consequence, the larger area/volume and higher elevation of the Apennines with respect to the Barbados is determined by the Apenninic deeper décollement. Despite these differences, the geometry of both décollements is, in some cases, comparable, in particular, close to the boundary between the crystalline crust and the sediment pile, where the main density and strength contrasts are concentrated. Variations in depth of the décollement occur moving along strike in both accretionary prisms. The geometry of the prisms is further controlled by the different values of α and β, their sum, and the distance of the accretionary prism relative to the subduction hinge.
Exhumation tectonics of the ultrahigh‐pressure metamorphic rocks in the Qinling orogen in east China: New petrological‐structural‐radiometric insights from the Shandong Peninsula
Authors: Faure Michel , Lin Wei , Monié Patrick , Le Breton Nicole , Poussineau Stéphane , Panis Dominique , Deloule Etienne
Published: May 2003

In eastern China, the Sulu area is recognized as the eastern extension of the Qinling‐Dabie Belt, which is famous for its ultrahigh‐pressure (UHP) metamorphism. Although numerous petrologic and geochemical works are available, structural data are still rare. This paper provides the first extensive study of bulk geometry and kinematic analysis of the Shandong Peninsula. The study area is divided into three tectonic areas by Cretaceous faults, namely, a southern UHP belt or Sulu area, a northern migmatite area, and an eastern eclogite and migmatite area or Weihai area. Conversely to the deeply entrenched idea that the later area belongs to the North China Belt, and the two others to the South China Block (SCB), we argue that all three areas are parts of the SCB. Structural, petrologic, 40Ar/39Ar, and U/Pb data comply with this new interpretation. In the North Shandong area, mafic granulites enclosed as blocks within gneissic migmatites do not significantly differ from the Sulu and Weihai eclogites which also experienced a granulite facies overprint before migmatization. The circa 210–200 Ma age of the main ductile deformation is related to an extensional event during the Triassic (or Indosinian) orogeny. This date corresponds to the temperature climax, but the time of the pressure peak, i.e., the real age of the UHP metamorphism is discussed.
Seepage area and rate of bedrock groundwater discharge at a granitic unchanneled hillslope
Authors: Uchida Taro , Asano Yuko , Ohte Nobuhito , Mizuyama Takahisa
Published: May 2003

Recent studies have demonstrated the importance of water movement through the bedrock in the rainfall‐runoff process on steep hillslopes. However, quantitative information on this process is still limited. The objective of this study was to address the following questions: (1) How large is the area where bedrock groundwater seeps into the soil layer, and (2) what is the rate of water flow out of the bedrock? To address these questions, detailed hydrological, hydrochemical, and thermal measurements were conducted at a forested steep unchanneled granitic concave slope in the Tanakami Mountains, central Japan. The relationship between the amplitude of annual soil temperature variation and the measurement depth showed that in a normal low‐flow period, the seepage area ranged between 14 and 21 m2 and the ratio of this area to that of the whole catchment was about 2.0%. In a drought period the seepage area ranged between 3.5 and 5.5 m2, and the ratio to the whole catchment was around 0.5%. The variation in the area of seepage was controlled both by the short‐term precipitation pattern during the preceding several weeks and by the long‐term pattern over several preceding months. A two‐component geochemical hydrograph separation indicated that the ratio of bedrock groundwater to streamflow was about 0.82 for the normal low‐flow periods and 0.90 for the drought period. The rate of flow out of the bedrock into the soil layer ranged from 0.5 to 3.3 m3 d−1. That is, although the seepage area was small (0.5–2.0% of the catchment), the contribution of bedrock groundwater was considerable (50–95% of streamflow).
Depth‐integrated modeling of suspended sediment transport
Authors: Bolla Pittaluga M. , Seminara G.
Published: May 2003

We derive a depth‐averaged model of suspended sediment transport. The development of the analysis leads us to revisit the asymptotic approach originally developed by Galappatti [1983], more recently generalized by >Wang [1992] and widely employed in commercial codes. We show that Galappatti's approach is formally incorrect as it differs from the formal asymptotic expansion of the exact solution. Moreover, the correct approach rather than leading to a differential equation for the depth‐averaged concentration actually provides higher order corrections for the leading order equilibrium approximation of the depth‐averaged concentration. Such corrections can be expressed in terms of spatial and temporal derivatives of the leading order solution. The latter picture is demonstrated on a model problem which is easily amenable to analytical treatment. On the basis of the formal asymptotic expansion of the exact solution we are then able to derive an analytical form for the flux of suspended sediment in slowly varying flows, which is suitable to applications to a variety of morphodynamic contexts including tidal and fluvial environments. An example of potential applications of the present approach is provided by examining the problem of suspended sediment transport due to a flood wave.
Hydrodynamic control of tracer retention in heterogeneous rock fractures
Authors: Cheng H. , Cvetkovic V. , Selroos J. O.
Published: May 2003

We investigate the statistical properties of a Lagrangian random variable β [T/L], which has been shown to quantify hydrodynamic impact on retention [Cvetkovic et al., 1999], using Monte Carlo simulations of flow and transport in a single fracture. The “local cubic law” of water flow is generalized to a power law Q ∼ bn, where Q is the flow rate, b is the half aperture, and n ≤ 3. Simulations of flow and particle transport are carried out assuming “local cubic law” (n = 3) and “local quadratic law” (n = 2), and for two typical flow configurations: uniform flow and radially converging flow. We find that β is related to τ as β ∼ τm, where m is dependent on the power n and the configuration of flow and transport. Simulation results for uniform flow indicate that for a small source section; as the source section increases, we have the convergence to β ∼ τ. For radially converging flow, we find β ∼ τ for a small source section and a convergence to β = const for an increasing source section. Simulation results for both flow configurations are consistent with the results for a homogeneous fracture. The results for a homogeneous fracture provide reasonable bounds for simulated β. The correlation between β and Q is relatively weak for all cases studied.
Impact of wettability alteration on two‐phase flow characteristics of sandstones: A quasi‐static description
Authors: Al‐Futaisi Ahmed , Patzek Tad W.
Published: May 2003

We describe a two‐phase pore network simulator of drainage and imbibition which integrates a realistic representation of pore connectivity and morphology, a quasi‐static description of fluid displacement mechanisms, and a sound representation of the wetting properties of a sedimentary rock and of their alteration. The simulator works with 3‐D disordered networks of cylindrical ducts with triangular, square, and circular cross sections obtained directly from the analysis of microfocused computed tomography (CT) images of rock samples. All pore‐level displacement mechanisms (piston type, snap off, and cooperative pore body filling) are considered with arbitrary receding and advancing contact angles. Bond invasion percolation description is used in primary drainage, while bond site invasion percolation with ordinary percolation on a dual network and compact cluster growth is used in secondary imbibition. In this paper, we resolve how to calculate the relative permeability of nonaqueous phase liquid (NAPL) in the quasi‐static approximation of imbibition and illustrate spatial distribution of the clusters of trapped NAPL using our generalization to disordered networks of the Hoshen‐Kopelman cluster‐labeling algorithm. To understand the impact of wettability alteration on the capillary pressure and relative permeability functions, we perform a series of drainage and imbibition simulations by changing the range of advancing contact angles. Our study indicates that in imbibition, transport properties of a permeable solid are quite sensitive to the hysteresis between the receding and advancing contact angle. This sensitivity reflects competition among the different displacement mechanisms, which shapes the relative permeabilities, capillary pressures, and the distribution of the trapped NAPL.
Factors controlling riffle‐scale hyporheic exchange flows and their seasonal changes in a gaining stream: A three‐dimensional groundwater flow model
Authors: Storey Richard G. , Howard Kenneth W. F. , Williams D. Dudley
Published: May 2003

Subsurface flow within a single riffle of a low‐gradient gravel bed stream was modeled in three dimensions using MODFLOW, a finite difference groundwater flow model. Model simulations showed that exchange flows can only occur in this low‐gradient, gaining stream because of a zone of alluvial sediment around the stream that has much higher permeability than the surrounding catchment (K = 10−4 m s−1, compared with K = 10−6 to 10−8 m s−1). The key factors controlling exchange flow within the alluvial zone were identified as the hydraulic conductivity of the alluvium, the hydraulic gradient between upstream and downstream ends of the riffle, and the flux of groundwater entering the alluvium from the sides and beneath. In the study riffle each of these factors changes with season, causing a reversal of flow paths in the alluvium and a reduction in exchange flows from about 0.2–0.5 m3 d−1 per meter stream length in summer to about 0.008–0.04 m3 d−1 per meter stream length during fall to spring. The model also revealed that exchange flows are up to twice as strong, but more variable, at the sides of the stream than near the center, and that vertical flow paths beneath the channel are more persistent under the range of conditions modeled than lateral flow paths into the banks.
Geomorphic controls on hyporheic exchange flow in mountain streams
Authors: Kasahara Tamao , Wondzell Steven M.
Published: May 2003

Hyporheic exchange flows were simulated using MODFLOW and MODPATH to estimate relative effects of channel morphologic features on the extent of the hyporheic zone, on hyporheic exchange flow, and on the residence time of stream water in the hyporheic zone. Four stream reaches were compared in order to examine the influence of stream size and channel constraint. Within stream reaches, the influence of pool‐step or pool‐riffle sequences, channel sinuosity, secondary channels, and channel splits was examined. Results showed that the way in which channel morphology controlled exchange flows differed with stream size and, in some cases, with channel constraint. Pool‐step sequences drove hyporheic exchange in the second‐order sites, creating exchange flows with relatively short residence times. Multiple features interacted to drive hyporheic exchange flow in the unconstrained fifth‐order site, where pool‐riffle sequences and a channel split created exchange flows with short residence times, whereas a secondary channel created exchange flows with long residence times. There was relatively little exchange flow in the bedrock‐constrained fifth‐order site. Groundwater flow models were effective in examining the morphologic features that controlled hyporheic exchange flow, and surface‐visible channel morphologic features controlled the development of the hyporheic zone in these mountain streams.
Lattice BGK simulations of macrodispersion in heterogeneous porous media
Authors: Talon L. , Martin J. , Rakotomalala N. , Salin D. , Yortsos Y. C.
Published: May 2003

We use the extended Darcy's law, which also accounts for the Brinkman correction, to study macrodispersion in a two‐dimensional (2‐D) porous medium. The former is necessary when permeability changes fast at a relatively small scale, and in general, it is a more complete description of flow in a heterogeneous medium. Lattice‐gas methods are ideally suited to simulate such flows. Simulations using a lattice BGK method and a small‐fluctuation approach are described for an isotropic, exponentially decaying correlation function of the permeability field. The analytical results contain the additional parameter Kl/λ2 (where Kl and λ are the typical permeability and velocity variation length, respectively), the sensitivity to which was studied. As expected, the contribution of the Brinkman effect is insignificant for typical field values of this parameter, in which case, the classical results are recovered. At larger values, for example, for heterogeneous media of a small correlation length, and possibly in laboratory applications, the Brinkman correction leads to a decrease in macrodispersivity, reflecting the smoothing effect of the Brinkman correction on the velocity field. Nonetheless, for practical values of the parameters, this reduction is no larger than 50% of the classical expression. The small fluctuation theory was found to be in good agreement with the simulations, provided that it was consistently applied (namely, by not mixing first‐order with second‐order expansions). The results also show that lattice‐gas simulations can be usefully employed to study macrodispersion in heterogeneous porous media.
Travel time moments for sorbing solutes in heterogeneous domains under nonuniform flow conditions
Authors: Sanchez‐Vila Xavier , Rubin Yoram
Published: May 2003

A methodology for evaluating the unconditional and conditional moments of travel time for a sorbing solute is presented. The approach is applicable for any flow configuration and for a wide range of mass transfer rate‐limited linear processes. The methodology is applicable to the general case of spatially variable hydrological and chemical parameters. The sorption model used to derive the temporal moments is that of a continuous distribution of mass rate coefficients [Haggerty and Gorelick, 1998]. Models such as instantaneous equilibrium, first‐order and two‐site sorption kinetics, among others, can be considered as particular cases of this general model. Using a deterministic approach, the low‐order moments of the breakthrough curves for reactive solutes can be obtained as a function of those for conservative tracers. Using a stochastic approach, the unconditional low‐order statistics of the travel time moments can be obtained. These moments depend on the statistics of two Lagrangian functions, the travel time for a conservative solute, and an integral of the variations of the chemical parameters weighted by the inverse local velocity along the trajectory. Finally, conditional temporal moments are derived. Moments can be conditioned to any type of information, hard or soft, hydraulic or geochemical. Conditioning is found to reduce uncertainty, characterized by a reduction in the variance of the travel time. The general results are particularized for both uniform in the mean and convergent flow conditions and for simple sorption models such as linear instantaneous equilibrium and first‐order kinetics. In all such cases, close‐form results, based on small perturbations expansions, are presented for the travel time moments.