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Attenuation Coefficient (attenuation + coefficient)
Selected AbstractsNumerical simulation of sediment-associated water quality processes for a Mississippi delta lakeECOHYDROLOGY, Issue 3 2009Xiaobo Chao Abstract Three major sediment-associated processes were presented to describe the effects of sediment on the water quality processes, including the effect of sediment on the light intensity for the growth of phytoplankton (PHYTO), the adsorption,desorption of nutrients by sediment and the release of nutrients from the bed sediment layer. A formula was generated from field measurements to calculate the light attenuation coefficient by considering the effects of concentrations of chlorophyll and suspended sediment (SS). The concentrations of adsorbed and dissolved nutrients because of adsorption,desorption were calculated using two formulas that were derived based on the Langmuir Equation. The release rates of nutrients from the bed sediment were calculated by considering the effects of the concentration gradient across the water-sediment interface, pH, temperature and dissolved oxygen (DO) concentration. Model algorithms describing the adsorption and desorption of nutrients from sediment particles as well as the release of nutrients from bed sediment were tested using experimental data. These sediment-associated water quality processes were included in a three-dimensional (3D) water quality model, CCHE3D_WQ, developed by the National Center for Computational Hydroscience and Engineering (NCCHE), to simulate the concentrations of PHYTO and nutrients in a shallow Mississippi Delta lake with special emphasis on sediment-related processes. The simulated concentration of PHYTO (as chlorophyll) and nutrients were generally in good agreement with field observations. This study shows that there are strong interactions between sediment-associated processes and water quality constituents. Copyright © 2009 John Wiley & Sons, Ltd. [source] Amplitude fluctuations due to diffraction and refraction in anisotropic random media: implications for seismic scattering attenuation estimatesGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2003T. M. Müller SUMMARY We calculate the variance of the log-amplitude within the Rytov approximation for plane waves propagating in weakly inhomogeneous and statistically anisotropic random media. Since there is a simple relation between the log-amplitude variance and the attenuation coefficient of seismic primaries in the weak wavefield fluctuation regime, we also obtain scattering attenuation estimates that additionally depend on the aspect ratio of longitudinal and transverse correlation scales of the inhomogeneities. These estimates can be useful for the statistical characterization of anisotropic, large-scale inhomogeneities (large compared with the wavelength of the probing pulse) in the Earth crust and mantle, such as fault zones. With the help of plane-wave-transmission numerical experiments using the finite-difference method we compute the log-amplitude variance as a function of the propagation distance and observe reasonable agreement with the analytical results. We discuss the implications of our results in the context of seismic scattering attenuation estimations. [source] Scattering behaviour at Merapi volcano (Java) revealed from an active seismic experimentGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2001Ulrich Wegler SUMMARY The seismic structure of the stratovolcano Merapi (Java, Indonesia) was studied using an active seismic experiment. Three 3 km long seismic profiles each consisting of up to 30 three-component seismometers with an interstation distance of 100 m were built up in an altitude range between 1000 and 2000 m above sea level. The detailed study of the seismic properties of the propagation media in active volcanic regions is important to understand the natural seismic signals used for eruption forecasting. The seismic experiment at Merapi therefore concentrates on the heterogeneous structure within a radius of 5 km from the active dome, where the sources of most of the natural volcanic seismic events are located. The cone of Merapi volcano consists of different materials changing on a small scale due to the layering of eruptive material. Additionally, the topography of the erosion valleys leads to an irregular deposition, which cannot be described by a simple 1-D layering. These inhomogeneities have a strong influence on seismic signals. The direct P and S waves are attenuated quickly and show only small amplitudes on seismograms. The energy lost from the direct waves, however, is not changed into heat but scattered and can be observed as seismic coda following the direct waves. The observed seismograms show a spindle-like amplitude increase after the direct P phase. This shape of the envelope can be explained by the diffusion model. According to this model there are so many strong inhomogeneities that the direct wave can be neglected and all energy is concentrated in multiple scattered waves. Besides the envelope, the coherence and polarization properties of the wavefield also indicate strong scattering. Only the first onset shows coherence over a station spacing of 100 m, whereas the late phases carrying the major part of the energy are mainly incoherent. The horizontal components of the seismograms have larger amplitudes than the vertical component, but within the horizontal plane the polarization is almost arbitrary, corresponding to waves arriving from scatterers located arbitrarily in space. As a result of the inversion using the diffusion model we obtain values of the S -wave scattering attenuation coefficient, ,s, and the S -wave intrinsic absorption coefficient, ,i. In the frequency range of 4,20 Hz used in this study the scattering attenuation is at least one order of magnitude larger than the intrinsic absorption (,s,,i). The mean free path of S waves is as low as 100 m (,s,1,100 m). The scattering coefficient is independent of frequency (,s,f0.0), whereas the coefficient of intrinsic attenuation increases with increasing frequency (,i,f1.6). The natural seismic signals at Merapi volcano show similar characteristics to the artificial shots. The first onsets have only small amplitudes and the energy maximum arrives delayed compared to the direct waves. Therefore, these signals appear to be strongly affected by multiple scattering also. [source] Effective elastic properties of randomly fractured soils: 3D numerical experimentsGEOPHYSICAL PROSPECTING, Issue 3 2004Erik H. Saenger ABSTRACT This paper is concerned with numerical tests of several rock physical relationships. The focus is on effective velocities and scattering attenuation in 3D fractured media. We apply the so-called rotated staggered finite-difference grid (RSG) technique for numerical experiments. Using this modified grid, it is possible to simulate the propagation of elastic waves in a 3D medium containing cracks, pores or free surfaces without applying explicit boundary conditions and without averaging the elastic moduli. We simulate the propagation of plane waves through a set of randomly cracked 3D media. In these numerical experiments we vary the number and the distribution of cracks. The synthetic results are compared with several (most popular) theories predicting the effective elastic properties of fractured materials. We find that, for randomly distributed and randomly orientated non-intersecting thin penny-shaped dry cracks, the numerical simulations of P- and S-wave velocities are in good agreement with the predictions of the self-consistent approximation. We observe similar results for fluid-filled cracks. The standard Gassmann equation cannot be applied to our 3D fractured media, although we have very low porosity in our models. This is explained by the absence of a connected porosity. There is only a slight difference in effective velocities between the cases of intersecting and non-intersecting cracks. This can be clearly demonstrated up to a crack density that is close to the connectivity percolation threshold. For crack densities beyond this threshold, we observe that the differential effective-medium (DEM) theory gives the best fit with numerical results for intersecting cracks. Additionally, it is shown that the scattering attenuation coefficient (of the mean field) predicted by the classical Hudson approach is in excellent agreement with our numerical results. [source] Modelling of diffraction from fibre texture gradients in thin polycrystalline filmsJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2007M. Birkholz Crystallographic textures in thin polycrystalline films typically exhibit a rotational symmetry, i.e. they occur as a fibre texture with the texture pole being orientated in the direction of the substrate normal. As a further characteristic of thin-film textures, it was often observed that the degree of preferred orientation increases with increasing thickness. It is shown in this work how a fibre texture gradient may be modelled in kinematical X-ray diffraction and which effects it has on the intensity mapping of the IHKL reflection, when the HKL pole is the fibre axis. A general expression for IHKL is derived for a depth-dependent fibre texture that is based on the finite Laplace transform of the texture distribution. The concept is outlined for the cosn, function to model the tilt-angle dependence of intensity, with the parameter n denoting the degree of texture. It is found that the measured intensity distribution sensitively depends on the ratio of texture gradient over X-ray attenuation coefficient. For particular cases, it is found that the maximum intensity may occur for non-zero tilt angles and thus arise at a different tilt angle from the pole of the fibre texture. [source] Computed tomographic measurements of thigh muscle cross-sectional area and attenuation coefficient predict hip fracture: The health, aging, and body composition studyJOURNAL OF BONE AND MINERAL RESEARCH, Issue 3 2010Thomas Lang Abstract Fatty infiltration of muscle, myosteatosis, increases with age and results in reduced muscle strength and function and increased fall risk. However, it is unknown if increased fatty infiltration of muscle predisposes to hip fracture. We measured the mean Hounsfield unit (HU) of the lean tissue within the midthigh muscle bundle (thigh muscle HU, an indicator of intramuscular fat), its cross-sectional area (CSA, a measure of muscle mass) by computed tomography (CT), bone mineral density (BMD) of the hip and total-body percent fat by dual X-ray absorptiometry (DXA), isokinetic leg extensor strength, and the Short Physical Performance Battery (SPPB) in 2941 white and black women and men aged 70 to 79 years. Sixty-three hip fractures were validated during 6.6 years of follow-up. Proportional hazards regression analysis was used to assess the relative risk (RR) of hip fracture across variations in thigh muscle attenuation, CSA, muscle strength, and physical function for hip fracture. In models adjusted by age, race, gender, body mass index, and percentage fat, decreased thigh muscle HU resulted in increased risk of hip fracture [RR/SD,=,1.58; 95% confidence interval (CI) 1.10,1.99], an association that continued to be significant after further adjustment for BMD. In models additionally adjusted by CSA, muscle strength, and SPPB score, decreased thigh muscle HU but none of the other muscle parameters continued to be associated with an increased risk of hip fracture (RR/SD,=,1.42; 95% CI 1.03,1.97). Decreased thigh muscle HU, a measure of fatty infiltration of muscle, is associated with increased risk of hip fracture and appears to account for the association between reduced muscle strength, physical performance, and muscle mass and risk of hip fracture. This characteristic captures a physical characteristic of muscle tissue that may have importance in hip fracture etiology. © 2010 American Society for Bone and Mineral Research [source] Ultrasonic Investigation of the Effect of Vegetable Shortening and Mixing Time on the Mechanical Properties of Bread DoughJOURNAL OF FOOD SCIENCE, Issue 9 2009K.L. Mehta ABSTRACT:, Mixing is a critical stage in breadmaking since it controls gluten development and nucleation of gas bubbles in the dough. Bubbles affect the rheology of the dough and largely govern the quality of the final product. This study used ultrasound (at a frequency where it is sensitive to the presence of bubbles) to nondestructively examine dough properties as a function of mixing time in doughs prepared from strong red spring wheat flour with various amounts of shortening (0%, 2%, 4%, 8% flour weight basis). The doughs were mixed for various times at atmospheric pressure or under vacuum (to minimize bubble nucleation). Ultrasonic velocity and attenuation (nominally at 50 kHz) were measured in the dough, and dough density was measured independently from specific gravity determinations. Ultrasonic velocity decreased substantially as mixing time increased (and more bubbles were entrained) for all doughs mixed in air; for example, in doughs made without shortening, velocity decreased from 165 to 105 ms,1, although superimposed on this overall decrease was a peak in velocity at optimum mixing time. Changes in attenuation coefficient due to the addition of shortening were evident in both air-mixed and vacuum-mixed doughs, suggesting that ultrasound was sensitive to changes in the properties of the dough matrix during dough development and to plasticization of the gluten polymers by the shortening. Due to its ability to probe the effect of mixing times and ingredients on dough properties, ultrasound has the potential to be deployed as an online quality control tool in the baking industry. [source] A Noncontact Ultrasound Approach for Mechanical Property Determination of CheesesJOURNAL OF FOOD SCIENCE, Issue 7 2003B.-K. Cho ABSTRACT: Noncontact ultrasound parameters, such as velocity and attenuation coefficient, were directly measured using air instability compensation transducers and correlated with Young's modulus, hardness, and toughness of different types of cheeses. Ultrasound velocity was highly correlated with the mechanical properties of cheeses whereas no significant relationship between the ultrasound energy attenuation coefficient and the mechanical properties were shown. The noncontact ultrasound technique demonstrated excellent performance for thickness measurement of cheeses with an accuracy of 99.98% (standard error = 0.089 mm). Results indicated that noncontact air instability compensation ultrasound technique has a good potential for nondestructive and accurate measurement of ultrasound parameters, thickness, and mechanical properties of food materials. [source] Measuring bubble, drop and particle sizes in multiphase systems with ultrasoundAICHE JOURNAL, Issue 11 2004A. H. G. Cents Abstract A technique is developed for measurement of bubble, droplet and particle-size distributions in multiphase systems, based on the propagation speed and attenuation of ultrasound. The measurement of the size distribution of the dispersed phase in multiphase systems was desired to analyze the mass-transfer mechanism in gas-liquid-liquid and gas-liquid-solid systems. To obtain this information, both the ultrasonic velocity and the attenuation coefficient of tone-burst signals are determined for a large frequency range (typically 100 kHz , 100 MHz). From these parameters, the size distributions and the volume fraction of the different dispersed phases can be determined using a scattering model. It was shown that the interfacial area can be determined very accurately, however, for the exact size distribution of the gas bubbles in the used size range (1,3 mm) an independent gas holdup determination is required. Experiments were performed in gas-liquid, solid-liquid, and gas-liquid-solid systems. The results showed good agreement of the particle-size distribution compared to a commercial laser-scattering analyzer, both with and without gas bubbles present. Furthermore, a good agreement between the scattering model and the experiments was found in the systems that contained gas bubbles, but these results should be validated using for instance, a digital camera technique. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2750,2762, 2004 [source] Lead,Natural rubber composites as gamma radiation shields.POLYMER COMPOSITES, Issue 4 2002II: High concentration Natural rubber composites were prepared by the incorporation of different lead concentration, namely: 100, 300, 500, 1000, 1500 and 2000 phr to be used as gamma radiation shields. A non-linear dependence of the attenuation coefficient upon the lead concentration was observed for both 60Co and 137Cs as gamma radiation sources. Sample containing the ultimate lead concentration (2000 phr) and thickness 1 mm was found to have 0.336 mm and 0.383 mm lead equivalent for 60Co and 137Cs respectively. Electrical and mechanical properties of the investigated composites were also measured. [source] Cross-calibration of X-ray µCT and MRX for tissue analysisPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2009H. Rahn Ferrofluids are being considered as an aid for local cancer treatments, such as Magnetic Drug Targeting (MDT) and Magnetic Hyperthermia (MHT). Both methods make use of the strong influence of a magnetic field on the nanoparticles, with the aim of treating the cancer locally while reducing, or even eliminating, the side effects that usually occur during conventional cancer treatments. Microcomputed tomography analysis has been performed on tumour tissue after MDT and MHT in order to examine the distribution of the magnetic nanoparticles within the tissue. The majority of the measurements has been performed in a laboratory based on a polychromatic X-ray source. The strong energy dependence of the attenuation coefficient and the occurrence of the so called beam hardening artefacts make the quantitative evaluation of data acquired with polychromatic tomography equipment very difficult. In this paper we present a cross-calibration method for magnetorelaxometry and polychromatic X-ray tomography for biological tissue samples enriched with magnetic nanoparticles. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Diverse optical characteristic of the prostate and light delivery system: implications for computer modelling of prostatic photodynamic therapyBJU INTERNATIONAL, Issue 9 2005Jerzy Jankun OBJECTIVE To explore the use of photodynamic therapy (PDT) as a minimally invasive form of treatment for organ-confined prostate cancer, for although there are several therapies, ablative treatments are associated with significant morbidity. MATERIALS AND METHODS Using the photosensitizer tin etiopurpurin, dogs were treated with interstitially placed laser fibres in an effort to validate PDT for treating prostate cancer. Earlier models assumed a uniform distribution of light output from a cylindrical fibre and a uniform attenuation coefficient throughout the prostate. Subsequent observations show that this model was too simple and that light radiance is not linear. To overcome under-treatment, a computer program to complement real-time fibre placement was developed. RESULTS As light radiance from interstitially placed laser fibres varies significantly from the commonly assumed ideal cylindrical emission, a predictive mathematical model of prostate PDT needs to consider the real emission. Also, the optical properties of the prostate, e.g. absorption and scattering of light, are anisotropic. Differences in the attenuation coefficient (combining absorption and scattering of light) also varied among different animals. Incorporating all these variables into a computer program produced a virtual model of the photo-ablated zone within ± 2 mm of that observed in animals. CONCLUSION PDT of the prostate is not trivial and should benefit from computer-aided methods as it is developed for clinical use. [source] Separation of intrinsic and scattering seismic attenuation in the Southern Apennine zone, ItalyGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2002F. Bianco Summary Scattered waves observed at the seismographs of the National Italy's seismic network have been used to investigate the intrinsic dissipation and scattering properties of the lithosphere under the Southern Apennines, Italy. First, we investigate the coda-Q properties, then we apply the MLTW analysis in the hypothesis of velocity and scattering coefficient constant with depth, and finally we interpret these results with the aid of numerical simulations in a medium with depth dependent velocity and scattering coefficient. Results obtained in the hypothesis of a uniform model show that a low scattering-Q,1 and a relatively higher intrinsic-Q,1 characterize the lithosphere of the Southern Apennines. Numerical simulations of the seismogram energy envelopes were performed hypothesizing a strongly scattering crust and trasparent upper mantle, both with reasonable intrinsic dissipation coefficients. In these symplifying assumptions the theoretical curves calculated for the homogeneous model fit to the synthetic envelopes with scattering attenuation coefficients always greater than the synthetic values. This results lead to the consideration that scattering-Q,1 obtained using MLTW analysis under the assumption of uniform medium are overestimated. The values of the scattering-Q,1 estimated for Apennines at low frequency (1,2 Hz) in the hypothesis of uniform medium are of the same order of those obtained in several areas around the world. The estimates obtained for frequencies ranging from 2 to 12 Hz are very low if compared with those obtained in the same hypothesis for other areas around the world. Coda Q,1 closely resembles intrinsic Q,1. [source] | ||||||||||||||||