611 Matching Results

Search Results

Advanced search parameters have been applied.

Convergent Validity of Variables Residualized By a Single Covariate: the Role of Correlated Error in Populations and Samples

Description: This study examined the bias and precision of four residualized variable validity estimates (C0, C1, C2, C3) across a number of study conditions. Validity estimates that considered measurement error, correlations among error scores, and correlations between error scores and true scores (C3) performed the best, yielding no estimates that were practically significantly different than their respective population parameters, across study conditions. Validity estimates that considered measurement error and correlations among error scores (C2) did a good job in yielding unbiased, valid, and precise results. Only in a select number of study conditions were C2 estimates unable to be computed or produced results that had sufficient variance to affect interpretation of results. Validity estimates based on observed scores (C0) fared well in producing valid, precise, and unbiased results. Validity estimates based on observed scores that were only corrected for measurement error (C1) performed the worst. Not only did they not reliably produce estimates even when the level of modeled correlated error was low, C1 produced values higher than the theoretical limit of 1.0 across a number of study conditions. Estimates based on C1 also produced the greatest number of conditions that were practically significantly different than their population parameters.
Date: May 2013
Creator: Nimon, Kim
Partner: UNT Libraries

A nonlinear theory for predicting the effects of unsteady laminar, turbulent, or transitional boundary layers on the attenuation of shock waves in a shock tube with experimental comparison

Description: Report presenting a modification of the linearized attenuation theory by using an unsteady compressible local skin-friction coefficient, a nonlinear approach, and the consideration of transition effects. Curves for predicting attenuation for shock pressure ratios up to 20 and a range of shock-tube Reynolds numbers are provided.
Date: September 1958
Creator: Trimpi, Robert L. & Cohen, Nathaniel B.
Partner: UNT Libraries Government Documents Department

High-Density Concrete for Gamma and Neutron Attenuation

Description: Report issued by the Oak Ridge National Laboratory discussing the development of a high-density concrete. As stated in the abstract, "the basic approach to high-density concrete mix design is reviewed to show that theoretical calculations for concrete mixes do give results that are reliable" (p. 1). This report includes tables, illustrations, and photographs.
Date: March 1965
Creator: Northup, T. E.
Partner: UNT Libraries Government Documents Department

Diffraction Effects in Neutron Attenuation Measurements

Description: All errors due to diffraction effects in a neutron attenuation experiment are computed. Also a special experiment to measure the forward intensity of diffracted neutrons from lead and copper is described, and the results given. These agree with the theoretical values.
Date: November 1, 1947
Creator: Sewell, D.C. & McMillan, E.M.
Partner: UNT Libraries Government Documents Department

Remedy Evaluation Framework for Inorganic, Non-Volatile Contaminants in the Vadose Zone

Description: Contaminants in the vadose zone may act as a potential long-term source of groundwater contamination and need to be considered in remedy evaluations. In many cases, remediation decisions for the vadose zone will need to be made all or in part based on projected impacts to groundwater. Because there are significant natural attenuation processes inherent in vadose zone contaminant transport, remediation in the vadose zone to protect groundwater is functionally a combination of natural attenuation and use of other remediation techniques, as needed, to mitigate contaminant flux to groundwater. Attenuation processes include both hydrobiogeochemical processes that serve to retain contaminants within porous media and physical processes that mitigate the rate of water flux. In particular, the physical processes controlling fluid flow in the vadose zone are quite different and generally have a more significant attenuation impact on contaminant transport relative to those within the groundwater system. A remedy evaluation framework is presented herein that uses an adaptation of the established EPA Monitored Natural Attenuation (MNA) evaluation approach and a conceptual model based approach focused on identifying and quantifying features and processes that control contaminant flux through the vadose zone. A key concept for this framework is to recognize that MNA will comprise some portion of all remedies in the vadose zone. Thus, structuring evaluation of vadose zone waste sites to use an MNA-based approach provides information necessary to either select MNA as the remedy, if appropriate, or to quantify how much additional attenuation would need to be induced by a remedial action (e.g., technologies considered in a feasibility study) to augment the natural attenuation processes and meet groundwater protection goals.
Date: May 1, 2013
Creator: Truex, Michael J. & Carroll, Kenneth C.
Partner: UNT Libraries Government Documents Department

Pressure diffusion waves in porous media

Description: Pressure diffusion wave in porous rocks are under consideration. The pressure diffusion mechanism can provide an explanation of the high attenuation of low-frequency signals in fluid-saturated rocks. Both single and dual porosity models are considered. In either case, the attenuation coefficient is a function of the frequency.
Date: April 8, 2003
Creator: Silin, Dmitry; Korneev, Valeri & Goloshubin, Gennady
Partner: UNT Libraries Government Documents Department

Microwave Attenuation Standards and Measurements

Description: Report discussing microwave attenuation measurement methods and standards. In addition, a relatively new and more precise way of representing and analyzing an attenuation measurement is presented. This in turn permits more rigorous definitions and error analyses than were previously possible. The referral of microwave attenuation measurements to standards operating at lower frequencies is discussed with particular attention to the errors in the referral processes as well as the errors in the standards themselves. Desirable characteristics are listed for attenuators which are suitable for calibration. Measurement methods are classified and described. Comments are made on the accuracy and convenience of various methods, and references are given which cover most of the basic and important research in this field. Key words: Microwave, attenuation, measurements, standards, tutorial.
Date: April 3, 1967
Creator: Beatty, Robert W.
Partner: UNT Libraries Government Documents Department

The Theory of the Optical Wedge Beam Splitter

Description: Report discussing optical wedge beam splitters and the basic theory for computing the ratio of the intensity of the incident beam to the intensity of any selected emerging beam and also for computing the direction of the emerging beam, assuming that the wedge angle, index of refraction, angle of incidence, and number of reflections are known.
Date: October 1974
Creator: Beers, Yardley
Partner: UNT Libraries Government Documents Department

Subsurface electromagnetic measurement through steel casing

Description: Numerical calculations show that useful information can be obtained in an electromagnetic crosswell survey where one of the wells is cased in steel. Our simple model is based on the assumption of an infinitely long uniform casing embedded in a homogeneous full space. Nevertheless the results indicate that if the pipe characteristics are independently known then the formation signal can be accurately recovered. This is best done at a single frequency where the pipe attenuation is modest. In fact we show that the optimal frequency for formation signal recovery is defined mainly by the pipe parameters and is largely independent of the formation conductivity.
Date: November 1998
Creator: Becker, A. B.; Wang, B. & Lee, K. H.
Partner: UNT Libraries Government Documents Department

The attenuation characteristics of four specially designed mufflers tested on a practical engine setup

Description: Attenuation characteristics of four different resonator mufflers were determined in both cold tests and engine field tests and compared with the theoretical calculations. These mufflers were specifically designed for a helicopter. Engine-exhaust sound pressures, temperatures, and noise levels from the helicopter were measured. The experimental muffler cold tests indicated close a agreement with theory, whereas the engine tests indicated some discrepancies. Test results show the usefulness of the theoretical equation used for predicting muffler attenuation characteristics.
Date: May 1953
Creator: Stokes, George M. & Davis, Don D., Jr.
Partner: UNT Libraries Government Documents Department

BIOGEOCHEMICAL GRADIENTS AS A FRAMEWORK FOR UNDERSTANDING WASTE SITE EVOLUTION

Description: The migration of biogeochemical gradients is a useful framework for understanding the evolution of biogeochemical conditions in groundwater at waste sites contaminated with metals and radionuclides. This understanding is critical to selecting sustainable remedies and evaluating sites for monitored natural attenuation, because most attenuation mechanisms are sensitive to geochemical conditions such as pH and redox potential. Knowledge of how gradients in these parameters evolve provides insights into the behavior of contaminants with time and guides characterization, remedy selection, and monitoring efforts. An example is a seepage basin site at the Savannah River Site in South Carolina where low-level acidic waste has seeped into groundwater. The remediation of this site relies, in part, on restoring the natural pH of the aquifer by injecting alkaline solutions. The remediation will continue until the pH up-flow of the treatment zone increases to an acceptable value. The time required to achieve this objective depends on the time it takes the trailing pH gradient, the gradient separating the plume from influxing natural groundwater, to reach the treatment zone. Predictions of this length of time will strongly influence long-term remedial decisions.
Date: October 17, 2008
Creator: Denham, M. & Karen Vangelas, K.
Partner: UNT Libraries Government Documents Department

THE SCENARIOS APPROACH TO ATTENUATION-BASED REMEDIES FOR INORGANIC AND RADIONUCLIDE CONTAMINANTS

Description: Guidance materials based on use of conceptual model scenarios were developed to assist evaluation and implementation of attenuation-based remedies for groundwater and vadose zones contaminated with inorganic and radionuclide contaminants. The Scenarios approach is intended to complement the comprehensive information provided in the US EPA's Technical Protocol for Monitored Natural Attenuation (MNA) of Inorganic Contaminants by providing additional information on site conceptual models and extending the evaluation to consideration of Enhanced Attenuation approaches. The conceptual models incorporate the notion of reactive facies, defined as units with hydrogeochemical properties that are different from surrounding units and that react with contaminants in distinct ways. The conceptual models also incorporate consideration of biogeochemical gradients, defined as boundaries between different geochemical conditions that have been induced by waste disposal or other natural phenomena. Gradients can change over time when geochemical conditions from one area migrate into another, potentially affecting contaminant mobility. A recognition of gradients allows the attenuation-affecting conditions of a site to be projected into the future. The Scenarios approach provides a stepwise process to identify an appropriate category of conceptual model and refine it for a specific site. Scenario materials provide links to pertinent sections in the EPA technical protocol and present information about contaminant mobility and important controlling mechanism for attenuation-based remedies based on the categories of conceptual models.
Date: August 4, 2011
Creator: Vangelas, K.; Rysz, M.; Truex, M.; Brady, P.; Newell, C. & Denham, M.
Partner: UNT Libraries Government Documents Department

Measurements of transient electromagnetic propagation through concrete and sand

Description: This is the final report of a one-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). If a beam-chopping system could be developed for the Los Alamos Meson Physics Facility low-energy beam line, there would be potential to operate the Los Alamos Neutron Scattering Center (LANSCE) at much higher power and duty factor and enable such operation with a radio-frequency quadrapole (RFQ) injector. This would greatly extend the capability of the facility. To accommodate LANSCE operation in the new configuration, a chopped beam must be created in the low-energy transport line before the RFQ. Chopping in this region has never been demonstrated and constitutes the major uncertainty of the proposal and determines the critical path for project completion. This study produces a better understanding of the physics involved in chopping an H-beam in a dilute plasma background, and in transporting a chopped H-beam through a neutralized or partially neutralized plasma channel, as well as an estimate for the optimum neutralization strategy for the beam chopping and transport between the ion source and the RFQ.
Date: September 1, 1996
Creator: Aurand, J.F.
Partner: UNT Libraries Government Documents Department

Exact Reconstruction From Uniformly Attenuated Helical Cone-Beam Projections in SPECT

Description: In recent years the development of cone-beam reconstruction algorithms has been an active research area in x-ray computed tomography (CT), and significant progress has been made in the advancement of algorithms. Theoretically exact and computationally efficient analytical algorithms can be found in the literature. However, in single photon emission computed tomography (SPECT), published cone-beam reconstruction algorithms are either approximate or involve iterative methods. The SPECT reconstruction problem is more complicated due to degradations in the imaging detection process, one of which is the effect of attenuation of gamma ray photons. Attenuation should be compensated for to obtain quantitative results. In this paper, an analytical reconstruction algorithm for uniformly attenuated cone-beam projection data is presented for SPECT imaging. The algorithm adopts the DBH method, a procedure consisting of differentiation and backprojection followed by a finite inverse cosh-weighted Hilbert transform. The significance of the proposed approach is that a selected region of interest can be reconstructed even with a detector with a reduced field of view. The algorithm is designed for a general trajectory. However, to validate the algorithm, a numerical study was performed using a helical trajectory. The implementation is efficient and the simulation result is promising.
Date: December 18, 2008
Creator: Gullberg, Grant T; Huang, Qiu; You, Jiangsheng & Zeng, Gengsheng L.
Partner: UNT Libraries Government Documents Department

Scanning ARM Cloud Radar Handbook

Description: The scanning ARM cloud radar (SACR) is a polarimetric Doppler radar consisting of three different radar designs based on operating frequency. These are designated as follows: (1) X-band SACR (X-SACR); (2) Ka-band SACR (Ka-SACR); and (3) W-band SACR (W-SACR). There are two SACRs on a single pedestal at each site where SACRs are deployed. The selection of the operating frequencies at each deployed site is predominantly determined by atmospheric attenuation at the site. Because RF attenuation increases with atmospheric water vapor content, ARM's Tropical Western Pacific (TWP) sites use the X-/Ka-band frequency pair. The Southern Great Plains (SGP) and North Slope of Alaska (NSA) sites field the Ka-/W-band frequency pair. One ARM Mobile Facility (AMF1) has a Ka/W-SACR and the other (AMF2) has a X/Ka-SACR.
Date: June 18, 2012
Creator: Widener, K; Bharadwaj, N & Johnson, K
Partner: UNT Libraries Government Documents Department

Diffraction Effects in Neutron Attenuation Measurements

Description: All errors due to diffraction effects in a neutron attenuation experiment are computed. Also a special experiment to measure the forward intensity of diffracted neutrons from lead and copper is described, and the results given. These agree with the theoretical values.
Date: November 1, 1947
Creator: McMillan, E.M. & Sewell, D.C.
Partner: UNT Libraries Government Documents Department

ENHANCEMENTS TO NATURAL ATTENUATION: SELECTED CASE STUDIES

Description: In 2003 the US Department of Energy (DOE) embarked on a project to explore an innovative approach to remediation of subsurface contaminant plumes that focused on introducing mechanisms for augmenting natural attenuation to achieve site closure. Termed enhanced attenuation (EA), this approach has drawn its inspiration from the concept of monitored natural attenuation (MNA).
Date: May 15, 2007
Creator: Vangelas, K; W. H. Albright, W; E. S. Becvar, E; C. H. Benson, C; T. O. Early, T; E. Hood, E et al.
Partner: UNT Libraries Government Documents Department

High Frequency Laser-Based Ultrasound

Description: To obtain micrometer resolution of materials using acoustics requires frequencies around 1 GHz. Attenuation of such frequencies is high, limiting the thickness of the parts that can be characterized. Although acoustic microscopes can operate up to several GHz in frequency, they are used primarily as a surface characterization tool. The use of a pulsed laser for acoustic generation allows generation directly in the part, eliminating the loss of energy associated with coupling the energy from a piezoelectric transducer to the part of interest. The use of pulsed laser acoustic generation in combination with optical detection is investigated for the non-contact characterization of materials with features that must be characterized to micrometer resolution.
Date: September 12, 2005
Creator: Huber, R; Chinn, D; Balogun, O & Murray, T
Partner: UNT Libraries Government Documents Department

Gas Detector LCLS Engineering Specifications Document

Description: There are two Gas Detectors, located upstream and downstream of the FEL attenuation materials, which provide a non-intrusive measure of the FEL pulse energy in the fundamental, in real-time, on a pulse-by-pulse basis. The FEL operators and the users will use this information to monitor the performance of the FEL and the Attenuator and to cross-calibrate other detectors. The Gas Detectors measure the FEL pulse energy by measuring the fluorescence induced in a small volume of N{sub 2} gas by the passage of the FEL.
Date: February 9, 2007
Creator: Hau-Riege, S
Partner: UNT Libraries Government Documents Department

Regional Body-Wave Attenuation Using a Coda Source Normalization Method: Application to MEDNET Records of Earthquakes in Italy

Description: We develop a new methodology to determine apparent attenuation for the regional seismic phases Pn, Pg, Sn, and Lg using coda-derived source spectra. The local-to-regional coda methodology (Mayeda, 1993; Mayeda and Walter, 1996; Mayeda et al., 2003) is a very stable way to obtain source spectra from sparse networks using as few as one station, even if direct waves are clipped. We develop a two-step process to isolate the frequency-dependent Q. First, we correct the observed direct wave amplitudes for an assumed geometrical spreading. Next, an apparent Q, combining path and site attenuation, is determined from the difference between the spreading-corrected amplitude and the independently determined source spectra derived from the coda methodology. We apply the technique to 50 earthquakes with magnitudes greater than 4.0 in central Italy as recorded by MEDNET broadband stations around the Mediterranean at local-to-regional distances. This is an ideal test region due to its high attenuation, complex propagation, and availability of many moderate sized earthquakes. We find that a power law attenuation of the form Q(f) = Q{sub 0}f{sup Y} fit all the phases quite well over the 0.5 to 8 Hz band. At most stations, the measured apparent Q values are quite repeatable from event to event. Finding the attenuation function in this manner guarantees a close match between inferred source spectra from direct waves and coda techniques. This is important if coda and direct wave amplitudes are to produce consistent seismic results.
Date: February 1, 2007
Creator: Walter, W R; Mayeda, K; Malagnini, L & Scognamiglio, L
Partner: UNT Libraries Government Documents Department