472 Matching Results

Search Results

Advanced search parameters have been applied.

Monitoring microbe-induced physical property changes using high-frequency acoustic waveform data: Toward the development of a microbial megascope

Description: A laboratory investigation was undertaken to determine the effect of microbe generated gas bubbles in controlled, saturated sediment columns utilizing a novel technique involving acoustic wave propagation. Specifically, the effect of denitrifying bacteria on saturated flow conditions was evaluated in light of the stimulated production of N{sub 2} gas and the resulting plugging of the pore throats. The propagation of high frequency acoustic waves through the sediment columns was used to locate those regions in the column where gas accumulation occurred. Over a period of six weeks, regions of gas accumulation resulted in the attenuation of acoustic wave energies with the decreases in amplitude typically greater than one order of magnitude.
Date: May 20, 2002
Creator: Williams, Kenneth Hurst
Partner: UNT Libraries Government Documents Department

3-D full waveform inversion of seismic data; Part I. Theory

Description: Full waveform inversion of seismic data is a challenging subject partly because of the lack of precise knowledge of the source. Since currently available approaches involve some form of approximations to the source, inversion results are subject to the quality and the choice of the source information used. A new full waveform inversion scheme has been introduced (Lee and Kim, 2003) using normalized wavefield for simple two-dimensional (2-D) scalar problems. The method does not require source information, so potential inversion errors due to source estimation may be eliminated. A gather of seismic traces is first Fourier-transformed into the frequency domain and a normalized wavefield is obtained for each trace in the frequency domain. Normalization is done with respect to the frequency response of a reference trace selected from the gather, so the complex-valued normalized wavefield is source-independent and dimensionless. The inversion algorithm minimizes misfits between measured normalized wavefield and numerically computed normalized wavefield. In this paper the full waveform inversion is extended to three-dimensional (3-D) problems.
Date: May 12, 2003
Creator: Lee, Ki Ha
Partner: UNT Libraries Government Documents Department

Source-independent full waveform inversion of seismic data

Description: A rigorous full waveform inversion of seismic data has been a challenging subject partly because of the lack of precise knowledge of the source. Since currently available approaches involve some form of approximations to the source, inversion results are subject to the quality and the choice of the source information used. We propose a new full waveform inversion methodology that does not involve source spectrum information. Thus potential inversion errors due to source estimation can be eliminated. A gather of seismic traces is first Fourier-transformed into the frequency domain and a normalized wavefield is obtained for each trace in the frequency domain. Normalization is done with respect to the frequency response of a reference trace selected from the gather, so the complex-valued normalized wavefield is dimensionless. The source spectrum is eliminated during the normalization procedure. With its source spectrum eliminated, the normalized wavefield allows us construction of an inversion algorithm without the source information. The inversion algorithm minimizes misfits between measured normalized wavefield and numerically computed normalized wavefield. The proposed approach has been successfully demonstrated using a simple two-dimensional scalar problem.
Date: March 20, 2002
Creator: Lee, Ki Ha & Kim, Hee Joon
Partner: UNT Libraries Government Documents Department

Numerical Calculation of the Spectrum of the Severe (1%) Lighting Current and Its First Derivative

Description: Recently, the direct-strike lighting environment for the stockpile-to-target sequence was updated [1]. In [1], the severe (1%) lightning current waveforms for first and subsequent return strokes are defined based on Heidler's waveform. This report presents numerical calculations of the spectra of those 1% lightning current waveforms and their first derivatives. First, the 1% lightning current models are repeated here for convenience. Then, the numerical method for calculating the spectra is presented and tested. The test uses a double-exponential waveform and its first derivative, which we fit to the previous 1% direct-strike lighting environment from [2]. Finally, the resulting spectra are given and are compared with those of the double-exponential waveform and its first derivative.
Date: February 12, 2010
Creator: Brown, C G; Ong, M M; Perkins, M P & Speer, R D
Partner: UNT Libraries Government Documents Department

Computational Complexity of Subspace Detectors and Matched Field Processing

Description: Subspace detectors implement a correlation type calculation on a continuous (network or array) data stream [Harris, 2006]. The difference between subspace detectors and correlators is that the former projects the data in a sliding observation window onto a basis of template waveforms that may have a dimension (d) greater than one, and the latter projects the data onto a single waveform template. A standard correlation detector can be considered to be a degenerate (d=1) form of a subspace detector. Figure 1 below shows a block diagram for the standard formulation of a subspace detector. The detector consists of multiple multichannel correlators operating on a continuous data stream. The correlation operations are performed with FFTs in an overlap-add approach that allows the stream to be processed in uniform, consecutive, contiguous blocks. Figure 1 is slightly misleading for a calculation of computational complexity, as it is possible, when treating all channels with the same weighting (as shown in the figure), to perform the indicated summations in the multichannel correlators before the inverse FFTs and to get by with a single inverse FFT and overlap add calculation per multichannel correlator. In what follows, we make this simplification.
Date: December 1, 2010
Creator: Harris, D B
Partner: UNT Libraries Government Documents Department

Spotforming with an array of ultra-wideband radio transmitters

Description: Ultra-wideband (UWB) array signal processing has the distinct advantage in that it is possible to illuminate or focus on ''spots'' at distant points in space, as opposed to just illuminating or steering at certain directions for narrowband array processing. The term ''spotforming'' is used to emphasize the property that point-focusing techniques with UWB waveforms can be viewed as a generalization of the well-known narrowband beamforming techniques. Because methods in spotforming can lead to powerful applications for UWB systems, in this paper we derive, simulate and experimentally verify UWB spot size as a function of frequency, bandwidth and array aperture.
Date: February 5, 2004
Creator: Dowla, F & Spiridon, A
Partner: UNT Libraries Government Documents Department

Up-conversion time microscope demonstrates 103x magnification of an ultrafast waveforms with 300 fs resolution

Description: We have demonstrated a system for the temporal expansion of arbitrarily shaped ultrafast optical waveforms based on the principle of temporal imaging. This system has demonstrated 103x magnification of an input signal with 300 fs resolution, thus allowing ultrafast phenomena to be recorded with slower conventional technology. The physics of temporal imaging work on a single shot basis, thus it is expected that this technology will lead to a new class of single transient recorders with ultrafast resolution.
Date: October 17, 1998
Creator: Bennett, C. V. & Kolner, B. H.
Partner: UNT Libraries Government Documents Department

Analysis of seismic events in and near Kuwait

Description: Seismic data for events in and around Kuwait were collected and analyzed. The authors estimated event moment, focal mechanism and depth by waveform modeling. Results showed that reliable seismic source parameters for events in and near Kuwait can be estimated from a single broadband three-component seismic station. This analysis will advance understanding of earthquake hazard in Kuwait.
Date: May 11, 1999
Creator: Harris, D B; Mayeda, K M; Rodgers, A J & Ruppert, S D
Partner: UNT Libraries Government Documents Department

The Response of Long-Span Bridges to Low Frequency, Near-Fault Earthquake Ground Motions

Description: Historical seismic hazard characterizations did not include earthquake ground motion waveforms at frequencies below approximately 0.2 Hz (5 seconds period). This resulted from limitations in early strong motion instrumentation and signal processing techniques, a lack of measurements in the near-field of major earthquakes and therefore no observational awareness, and a delayed understanding in the engineering community of the potential significance of these types of motions. In recent years, there is a growing recognition of the relevance of near-fault, low frequency motions, particularly for long-period structures such as large bridges. This paper describes a computationally based study of the effects of low frequency (long-period) near-fault motions on long-span bridge response. The importance of inclusion of these types of motions for long span cable supported bridges is demonstrated using actual measured broad-band, near-fault motions from large earthquakes.
Date: February 27, 2009
Creator: McCallen, David; Astaneh-Asl, A.; Larsen, S.C. & Hutchings, Larry
Partner: UNT Libraries Government Documents Department

Spherical Wave Propagation in a Nonlinear Elastic Medium

Description: Nonlinear propagation of spherical waves generated by a point-pressure source is considered for the cases of monochromatic and impulse primary waveforms. The nonlinear five-constant elastic theory advanced by Murnaghan is used where general equations of motion are put in the form of vector operators, which are independent of the coordinate system choice. The ratio of the nonlinear field component to the primary wave in the far field is proportional to ln(r) where r is a propagation distance. Near-field components of the primary field do not contribute to the far field of nonlinear component.
Date: July 1, 2009
Creator: Korneev, Valeri A.
Partner: UNT Libraries Government Documents Department

Seismic Waveform Characterization at LLNL: Analyst Guidelines and Issues

Description: In the first section of this paper we present an overview of general set of procedures that we have followed in seismic waveform analysis. In the second section we discuss a number of issues and complexities that we have encountered in analysis of events in the Middle East, North Africa, Europe, and parts of the European Arctic. To illustrate these complexities we can include examples of waveforms recorded over a variety of paths in these regions.
Date: November 1, 2001
Creator: Ryall, F & Schultz, C A
Partner: UNT Libraries Government Documents Department

A Multisensor system for the detection and characterization of UXOMM-0437

Description: A prototype active electromagnetic system has been developedfor detecting and characterizing UXO. The system employs two orthogonalvertical loop transmitters and a pair of horizontal loop transmittersspaced apart vertically by 0.7 m. Eight vertical field detectors aredeployed in the plane of each of the horizontal loops and are arranged tomeasure offset vertical gradients of the fields. The location andorientation of the three principal polarizabilities of a target can berecovered from a single position of the transmitter-receiver system.Further characterization of the target is obtained from the broadbandresponse. The system employs a bipolar half sine pulse train currentwaveform and the detectors are dB/dt induction coils designed to minimizethe transient response of the primary field pulse. The target transientis recovered in a 40 mu-sec to 1.0 msec window. The ground responseimposes an early time limit on the time window and system/ambient noiselimits the late time response. Nevertheless for practical transmittermoments and optimum receivers the size and the ratio of conductivity topermeability canbe accurately recovered. The prototype system hassuccessfully recovered the depths and polarizabilities of ellipsoidaltest targets.
Date: June 1, 2006
Creator: Gasperikova, Erika; Becker, A.; Morrison, H.F. & Smith, J.T.
Partner: UNT Libraries Government Documents Department

ADVANCED WAVEFORM SIMULATION FOR SEISMIC MONITORING EVENTS - REPORTING PERIOD AUGUST 1, 2007-OCTOBER 31, 2007

Description: We have developed a forward modeling technique to retrieve rupture characteristics of small earthquakes (3<M<5), including rupture propagation direction, fault dimension, and rupture speed.
Date: September 30, 2007
Creator: Helmberger, Donald V.; Tromp, Jeroen & Rodgers, Arthur J.
Partner: UNT Libraries Government Documents Department

Behavior of Repeating Earthquake Sequences in Central California and the Implications for Subsurface Fault Creep

Description: Repeating earthquakes (REs) are sequences of events that have nearly identical waveforms and are interpreted to represent fault asperities driven to failure by loading from aseismic creep on the surrounding fault surface at depth. We investigate the occurrence of these REs along faults in central California to determine which faults exhibit creep and the spatio-temporal distribution of this creep. At the juncture of the San Andreas and southern Calaveras-Paicines faults, both faults as well as a smaller secondary fault, the Quien Sabe fault, are observed to produce REs over the observation period of March 1984-May 2005. REs in this area reflect a heterogeneous creep distribution along the fault plane with significant variations in time. Cumulative slip over the observation period at individual sequence locations is determined to range from 5.5-58.2 cm on the San Andreas fault, 4.8-14.1 cm on the southern Calaveras-Paicines fault, and 4.9-24.8 cm on the Quien Sabe fault. Creep at depth appears to mimic the behaviors seen of creep on the surface in that evidence of steady slip, triggered slip, and episodic slip phenomena are also observed in the RE sequences. For comparison, we investigate the occurrence of REs west of the San Andreas fault within the southern Coast Range. Events within these RE sequences only occurred minutes to weeks apart from each other and then did not repeat again over the observation period, suggesting that REs in this area are not produced by steady aseismic creep of the surrounding fault surface.
Date: July 9, 2007
Creator: Templeton, D C; Nadeau, R & Burgmann, R
Partner: UNT Libraries Government Documents Department

Spotforming with an Array of Ultra-Wideband Radio Transmitters

Description: Ultra-wideband (UWB) array signal processing has the distinct advantage in that it is possible to illuminate or focus on ''spots'' at distant points in space, as opposed to just illuminating or steering at certain directions for narrowband array processing. The term ''spotforming'' is used to emphasize the property that point-focusing techniques with UWB waveforms can be viewed as a generalization of the well-known narrowband beamforming techniques. Because methods in spotforming can lead to powerful applications for UWB systems, in this paper we derive, simulate and experimentally verify UWB spot size as a function of frequency, bandwidth and array aperture.
Date: September 29, 2003
Creator: Dowla, F & Spiridon, A
Partner: UNT Libraries Government Documents Department

National Nuclear Security Administration Knowledge Base Core Table Schema Document

Description: The National Nuclear Security Administration is creating a Knowledge Base to store technical information to support the United States nuclear explosion monitoring mission. This document defines the core database tables that are used in the Knowledge Base. The purpose of this document is to present the ORACLE database tables in the NNSA Knowledge Base that on modifications to the CSS3.0 Database Schema developed in 1990. (Anderson et al., 1990). These modifications include additional columns to the affiliation table, an increase in the internal ORACLE format from 8 integers to 9 integers for thirteen IDs, and new primary and unique key definitions for six tables. It is intended to be used as a reference by researchers inside and outside of NNSA/DOE as they compile information to submit to the NNSA Knowledge Base. These ''core'' tables are separated into two groups. The Primary tables are dynamic and consist of information that can be used in automatic and interactive processing (e.g. arrivals, locations). The Lookup tables change infrequently and are used for auxiliary information used by the processing. In general, the information stored in the core tables consists of: arrivals; events, origins, associations of arrivals; magnitude information; station information (networks, site descriptions, instrument responses); pointers to waveform data; and comments pertaining to the information. This document is divided into four sections, the first being this introduction. Section two defines the sixteen tables that make up the core tables of the NNSA Knowledge Base database. Both internal (ORACLE) and external formats for the attributes are defined, along with a short description of each attribute. In addition, the primary, unique and foreign keys are defined. Section three of the document shows the relationships between the different tables by using entity-relationship diagrams. The last section, defines the columns or attributes of the various tables. Information that ...
Date: September 1, 2002
Creator: CARR,DORTHE B.
Partner: UNT Libraries Government Documents Department

Generation of time histories with a specified auto spectral density and probability density function

Description: It is recognized that some dynamic and noise environments are characterized by time histories which are not Gaussian. An example is high intensity acoustic noise. Another example is some transportation vibration. A better simulation of these environments can be generated if a zero mean non-Gaussian time history can be reproduced with a specified auto (or power) spectral density (ASD or PSD) and a specified probability density function (pdf). After the required time history is synthesized, the waveform can be used for simulation purposes. For example, modem waveform reproduction techniques can be used to reproduce the waveform on electrodynamic or electrohydraulic shakers. Or the waveforms can be used in digital simulations. A method is presented for the generation of realizations of zero mean non-Gaussian random time histories with a specified ASD, and pdf. First a Gaussian time history with the specified auto (or power) spectral density (ASD) is generated. A monotonic nonlinear function relating the Gaussian waveform to the desired realization is then established based on the Cumulative Distribution Function (CDF) of the desired waveform and the known CDF of a Gaussian waveform. The established function is used to transform the Gaussian waveform to a realization of the desired waveform. Since the transformation preserves the zero-crossings and peaks of the original Gaussian waveform, and does not introduce any substantial discontinuities, the ASD is not substantially changed. Several methods are available to generate a realization of a Gaussian distributed waveform with a known ASD. The method of Smallwood and Paez (1993) is an example. However, the generation of random noise with a specified ASD but with a non-Gaussian distribution is less well known.
Date: August 1, 1996
Creator: Smallwood, D.O.
Partner: UNT Libraries Government Documents Department

Preliminary regional magnitude in the Middle East Region using narrowband Lg coda envelopes

Description: Because many regional seismic discriminants are functions of magnitude, it is important to obtain a stable measurement especially for smaller events that will likely have very limited station coverage. We have collected and analyzed regional broad band waveforms from stations in the middle east region for the purpose of calibrating a stable regional magnitude scale that can be applied to events that are too small to detect teleseismically. Our approach is to obtain frequency-dependent empirical Greens function coda envelopes for narrow frequency bands that can be used to correct for gross path effects. We make the assumption that the moment-rate spectra are generally flat below{approximately}2 Hz for these events smaller than Mw{approximately}3.5. In a least squares sense, we obtain frequency-dependent corrections to the Lg coda measurements to fit the scalar moment estimates. These frequency-dependent corrections remove the effects of the S-to-Lg coda transfer function, thus correcting back to the S-wave source spectra. Due to the averaging nature of Lg coda waves we are then able to obtain a stable single-station estimate of the source spectra.To avoid regional biases we tie our coda envelope amplitude measurements to seismic moments obtained from long period 1-D waveform modeling for moderate sized earthquakes (A4w-3.5.- 4.5). Most importantly, we can now apply the same corrections to significantly smaller events that cannot be observed teleseismically. Our empirical approach takes into account scattering,absorption, and waveguide losses as well as frequency-dependent site effects.Moreover, the use of the coda envelope mitigates the undesirable effects of source anisotropy, random site interference, path variability, and directivity that plague direct wave measurements. This approach was successfully applied to other regions where it was observed that the coda-derived Mw estimates showed significantly smaller dependence on lateral path variation and source radiation anisotropy than the more conventional approaches such as mb(Pg),mb(Lg),and teleseismic mb.
Date: July 1, 1997
Creator: Mayeda, K. & Walter, W.R.
Partner: UNT Libraries Government Documents Department

Two-dimensional magnetic quantum oscillations observed in an organic metal

Description: The de Haas-van Alphen (dHvA) signal of the organic superconductor {beta}{double_prime}-(BEDT-TTF){sub 2}SF{sub 5}CH{sub 2}CF{sub 2}SO{sub 3} shows an inverse-sawtooth wave form which proves the existence of an ideal two-dimensional (2D) Fermi surface. The dHvA wave shape can almost perfectly be described by a 2D theory assuming a constant chemical potential. This either implies the existence of the predicted quasi-one-dimensional band with an exceptionally large density of states or the chemical potential may be pinned due to localized states near the Fermi energy.
Date: July 24, 2000
Creator: Hagel, J.; Wanka, S.; Wosnitza, J.; Balthes, E.; Schlueter, J. A.; Kini, A. M. et al.
Partner: UNT Libraries Government Documents Department

THE ROTATING MAGNETIC FIELD OSCILLATOR SYSTEM FOR CURRENT DRIVE IN THE TRANSLATION, CONFINEMENT AND SUSTAINMENT EXPERIMENT

Description: The experimental setup and test results for the {approximately}125 MW rotating magnetic field current drive system of the Translation, Confinement and Sustainment Experiment at the University of Washington are described. The oscillator system, constructed at Los Alamos National Laboratory, drives two tank circuits (15 kV{sub peak} potential, 8.5 kA{sub peak} maximum circulating current in each tank to date) operated 90{degree} out of phase to produce a 54 G rotating magnetic field with a frequency of 163 kHz ({omega} = 1.02{sup x} 10{sup {minus}6} s{sup {minus}1}). Programmable waveform generators control ''hot deck'' totem pole drivers that are used to control the grid of 12 Machlett 8618 magnetically beamed triode tubes. This setup allows the current to be turned on or off in less than 100 ns ({approximately}6{degree}). Both tank circuits are isolated from the current source by a 1:1 air core, transmission line transformer. Each tank circuit contains two saddle coils (combined inductance of 1.6 {micro}H) and radio frequency capacitors (580 nF). Test results are presented for three conditions: no external load, a resistive external load and a plasma load. A SPICE model of the oscillator system was created. Comparisons between this model and experimental data are given.
Date: December 1, 2000
Creator: Tobin, S. J.; Reass, W. A.; Schrank, L. S.; Wurden, G. A.; Guo, H. Y.; Hoffman, A. L. et al.
Partner: UNT Libraries Government Documents Department

Fatigue Properties of Type 316 LN Stainless Steels as a Function of Frequency and Waveform

Description: The low cycle fatigue behavior of type 316LN stainless steel was investigated in air and mercury at frequencies from 0.1 to 10 Hz. Cyclic stress ratios (R) of {minus}1 and 0.1 were used with sinusoidal, triangular and positive sawtooth wave forms. Mercury appears to reduce fatigue life at high stress amplitudes, but the endurance limit may be unaffected. Low frequency and mean stress decreased the fatigue endurance limit, but type of waveform did not appear to affect fatigue life under the conditions of these tests.
Date: January 30, 2001
Creator: DiStefano, J.R.
Partner: UNT Libraries Government Documents Department