scholarly journals Multi-mode analysis of Rayleigh-type Lg. Part 2. Application to southern California and the northwestern Sierra Nevada

1981 ◽  
Vol 71 (4) ◽  
pp. 985-1002
Author(s):  
Michel Cara ◽  
J. Bernard Minster ◽  
Ronan Le Bras
Keyword(s):  
Sierra Nevada ◽  
Southern California ◽  
Mode Conversion ◽  
Wave Train ◽  
Companion Paper ◽  
Mode Analysis ◽  
Time Frequency ◽  
Short Period ◽  
Multi Mode ◽  
Group Velocities

abstract The UC diagram technique described in the companion paper (Part 1), is applied to nine sets of Lg phases recorded through the CEDAR system in southern California, and two sets of Lg phases recorded along the northwestern margin of the Sierra Nevada. A clear image of the signal is obtained in time-frequency-wavenumber space, and we discuss in particular observations at 2.5-sec period, for events 200 to 300 km outside the profiles. From the gross features of UC diagrams we conclude that a representation of Lg as a single coherent multi-mode wave train is oversimplified in the case of southern California but is more appropriate for the Sierra block. In southern California, peaks observed at group velocities smaller than 3.2 km/sec are not predicted by realistic crustal models of the area, and are probably due to lateral heterogeneities effects such as mode conversion and multipathing. On the other hand, for group velocities between 3.2 and 3.6 km/sec, peaks observed in either area can generally be interpreted in terms of overtones excited at the source and propagating through spatially averaged structures, although care must be taken to monitor the stability of the algorithm on actual short-period records.

scholarly journals Estimation of Gravity Wave Momentum Flux and Phase Speeds from Quasi-Lagrangian Stratospheric Balloon Flights. Part I: Theory and Simulations

2008 ◽  
Vol 65 (10) ◽  
pp. 3042-3055 ◽  
Author(s):  
Gillian Boccara ◽  
Albert Hertzog ◽  
Robert A. Vincent ◽  
François Vial
Keyword(s):  
Gravity Wave ◽  
Wave Packets ◽  
Phase Speed ◽  
Companion Paper ◽  
Time Frequency ◽  
Long Duration ◽  
Short Period ◽  
Momentum Fluxes ◽  
Stratospheric Balloon ◽  
Wave Momentum

A methodology for estimating gravity wave characteristics from quasi-Lagrangian observations provided by long-duration, superpressure balloon flights in the stratosphere is reviewed. Wavelet analysis techniques are used to detect gravity wave packets in observations of pressure, temperature, and horizontal velocity. An emphasis is put on the estimation of gravity wave momentum fluxes and intrinsic phase speeds, which are generally poorly known on global scales in the atmosphere. The methodology is validated using Monte Carlo simulations of time series that mimic the balloon measurements, including the uncertainties associated with each of the meteorological parameters. While the azimuths of the wave propagation direction are accurately retrieved, the momentum fluxes are generally slightly underestimated, especially when wave packets overlap in the time–frequency domain, or for short-period waves. A proxy is derived to estimate by how much momentum fluxes are reduced by the analysis. Retrievals of intrinsic phase speeds are less accurate, especially for low phase speed waves. A companion paper (Part II) implements the methodology to observations gathered during the Vorcore campaign that took place in Antarctica between September 2005 and February 2006.

scholarly journals Multi-mode analysis of Rayleigh-type Lg. Part 1. Theory and applicability of the method

1981 ◽  
Vol 71 (4) ◽  
pp. 973-984 ◽  
Author(s):  
Michel Cara ◽  
J. Bernard Minster
Keyword(s):  
Linear Array ◽  
Random Phase ◽  
Mode Analysis ◽  
Phase Fluctuations ◽  
Time Frequency ◽  
Analysis Technique ◽  
Crustal Structures ◽  
Velocity Changes ◽  
Multi Mode ◽  
Lateral Heterogeneities

abstract Rayleigh-type Lg propagating in a laterally homogeneous continental crust can be synthesized by adding only a few overtones at periods greater than 2 sec. Under minimal assumptions, we show that wavenumber analysis of Lg recorded on a several hundred kilometers long linear array of 10 stations allow us to isolate the different overtones, providing a tool to study crustal structures and excitation of the overtones at the source. In this first paper, we use synthetic Lg seismograms to investigate the applicability of a time-frequency-wavenumber analysis technique (UC diagram algorithm) to realistic arrays of stations. The behavior of the algorithm in the presence of lateral heterogeneities is studied numerically by introducing either random or coherent phase perturbations. We find that (1) the method is tractable if random phase fluctuations from station to station are spread over less than half a cycle, and (2) coherent velocity changes between two halves of a profile are spatially averaged if they are too small to be resolved by the array.

Time-Frequency Channel Parameterization with Application to Multi-Mode Receivers

2009 ◽  
Vol E92-B (12) ◽  
pp. 3717-3725
Author(s):  
Thomas HUNZIKER ◽  
Ziyang JU ◽  
Dirk DAHLHAUS
Keyword(s):  
Frequency Channel ◽  
Time Frequency ◽  
Multi Mode

scholarly journals Water Level Sensing in a Steel Vessel Using A0 and Quasi-Scholte Waves

2017 ◽  
Vol 2017 ◽  
pp. 1-11 ◽  
Author(s):  
Peng Guo ◽  
Bo Deng ◽  
Xiang Lan ◽  
Kaili Zhang ◽  
Hongyuan Li ◽  
...  
Keyword(s):  
Water Level ◽  
Guided Waves ◽  
Mode Conversion ◽  
Experimental Studies ◽  
Low Frequency ◽  
Steel Vessel ◽  
Theoretical Predictions ◽  
Travelling Time ◽  
The Comparative Study ◽  
Group Velocities

This paper presents a water level sensing method using guided waves of A0 and quasi-Scholte modes. Theoretical, numerical, and experimental studies are performed to investigate the properties of both the A0 and quasi-Scholte modes. The comparative study of dispersion curves reveals that the plate with one side in water supports a quasi-Scholte mode besides Lamb modes. In addition, group velocities of A0 and quasi-Scholte modes are different. It is also found that the low-frequency A0 mode propagating in a free plate can convert to the quasi-Scholte mode when the plate has one side in water. Based on the velocity difference and mode conversion, a water level sensing method is developed. For the proof of concept, a laboratory experiment using a pitch-catch configuration with two piezoelectric transducers is designed for sensing water level in a steel vessel. The experimental results show that the travelling time between the two transducers linearly increases with the increase of water level and agree well with the theoretical predictions.

An earthquake-explosion reversed refraction line in the Peninsular Ranges of southern California and Baja California Norte

1982 ◽  
Vol 72 (4) ◽  
pp. 1195-1206
Author(s):  
F. Alejandro Nava ◽  
James N. Brune
Keyword(s):  
Sierra Nevada ◽  
High Velocity ◽  
Southern California ◽  
Baja California ◽  
Crustal Thickness ◽  
Arrival Times ◽  
Narrow Zone ◽  
Peninsular Ranges ◽  
Thick Crust

abstract An approximate reversed refraction profile has been obtained for the center of the Peninsular Ranges of southern California and Baja California Norte using arrival times from Corona blasts to obtain the NW-SE profile, and arrival times from the well-located Pino Solo earthquake of 17 July 1975 to obtain the reversing SE-NW profile. The results indicate a relatively high-velocity crust, with P velocities of 6.57 to 6.95 km/sec, similar to the high velocities found by Hadley and Kanamori (1979). A crustal thickness of about 40 km was found for the axis of the Peninsular Ranges, significantly greater than was found by Hadley and Kanamori (1979) for the average crustal thickness of the northern part of the province. This suggests that the thick crust may be confined to a relatively narrow zone along the axis of the province. The crustal thickness found here is approximately 10 km less than found for the deeper crust of the Sierra Nevada (Bateman and Eaton, 1967; Pakiser and Brune, 1980).

scholarly journals Automated detection and association of surface waves

1994 ◽  
Vol 37 (3) ◽  
Author(s):  
R. G. North ◽  
C. R. D. Woodgold
Keyword(s):  
Surface Waves ◽  
Group Velocity ◽  
Rayleigh Waves ◽  
Arrival Time ◽  
Narrow Band ◽  
Broad Band ◽  
Detection Algorithm ◽  
Automated Detection ◽  
Short Period ◽  
Group Velocities

An algorithm for the automatic detection and association of surface waves has been developed and tested over an 18 month interval on broad band data from the Yellowknife array (YKA). The detection algorithm uses a conventional STA/LTA scheme on data that have been narrow band filtered at 20 s periods and a test is then applied to identify dispersion. An average of 9 surface waves are detected daily using this technique. Beamforming is applied to determine the arrival azimuth; at a nonarray station this could be provided by poIarization analysis. The detected surface waves are associated daily with the events located by the short period array at Yellowknife, and later with the events listed in the USGS NEIC Monthly Summaries. Association requires matching both arrival time and azimuth of the Rayleigh waves. Regional calibration of group velocity and azimuth is required. . Large variations in both group velocity and azimuth corrections were found, as an example, signals from events in Fiji Tonga arrive with apparent group velocities of 2.9 3.5 krn/s and azimuths from 5 to + 40 degrees clockwise from true (great circle) azimuth, whereas signals from Kuriles Kamchatka have velocities of 2.4 2.9 km/s and azimuths off by 35 to 0 degrees. After applying the regional corrections, surface waves are considered associated if the arrival time matches to within 0.25 km/s in apparent group velocity and the azimuth is within 30 degrees of the median expected. Over the 18 month period studied, 32% of the automatically detected surface waves were associated with events located by the Yellowknife short period array, and 34% (1591) with NEIC events; there is about 70% overlap between the two sets of events. Had the automatic detections been reported to the USGS, YKA would have ranked second (after LZH) in terms of numbers of associated surface waves for the study period of April 1991 to September 1992.

Femtosecond pulse delivery using a chirped long-period grating of multi-mode fiber for mode conversion

2010 ◽  
Vol 57 (6) ◽  
pp. 485-491 ◽  
Author(s):  
J. Yao ◽  
L. Zhan ◽  
Y.X. Wang ◽  
H.G. Li ◽  
S.Y. Luo ◽  
...  
Keyword(s):  
Femtosecond Pulse ◽  
Mode Conversion ◽  
Long Period Grating ◽  
Long Period ◽  
Multi Mode

Analysis of Autoignition Chemistry in Aeroderivative Premixers At Engine Conditions

2021 ◽  
Author(s):  
Sandeep Jella ◽  
Gilles Bourque ◽  
Pierre Gauthier ◽  
Philippe Versailles ◽  
Jeffrey M. Bergthorson ◽  
...  
Keyword(s):  
Residence Time ◽  
Real Life ◽  
Precursor Chemistry ◽  
Mode Analysis ◽  
Safety Factors ◽  
Eddy Simulation ◽  
Reduced Mechanism ◽  
Short Period ◽  
Large Eddy ◽  
Pure Methane

Abstract The minimization of autoignition risk is critical to premixer design. Safety factors based on ignition delays of homogeneous mixtures, are generally used to guide the choice of a residence time for a given premixer. However, autoignition chemistry at aeroderivative conditions is fast (0.5-2 milliseconds) and can be initiated within typical premixer residence times. The analysis of what takes place in this short period involves the study of low-temperature precursor chemistry. By coupling the evolution of the Chemical Explosive Modes to turbulence, it is possible to obtain a measure of spatial autoignition risk where both chemical (e.g. ignition delay) and aerodynamic (e.g. local residence time) influences are unified. In this article, we describe a method that couples Large Eddy Simulation to newly developed, reduced autoignition chemical kinetics to study autoignition precursors in an example premixer representative of real life geometric complexity. A blend of pure methane and dimethyl ether (DME), a common fuel used for experimental autoignition studies, was transported using the reduced mechanism (38 species / 238 reactions) at engine conditions at increasing levels of DME concentration until exothermic autoignition kernels were formed. The Chemical Explosive Mode analysis closely follows the large thermochemical changes in the premixer as a function of DME concentration and identifies where the premixer is sensitive and flame anchoring is likely to occur.

Local Richter magnitude and total signal duration in southern California

1973 ◽  
Vol 63 (5) ◽  
pp. 1809-1827 ◽  
Author(s):  
Charles R. Real ◽  
Ta-Liang Teng
Keyword(s):  
Southern California ◽  
Epicentral Distance ◽  
Total Duration ◽  
Signal Duration ◽  
Local Magnitude ◽  
Total Signal ◽  
Source Effects ◽  
Short Period ◽  
Seismic Networks ◽  
Southern California Region

abstract Seismograms of 320 earthquakes (1,486 observations) from short-period seismometers occurring from January 1969 to April 1971 and 91 earthquakes (257 observations) during 1971 have been used to establish a relationship between total signal duration and the local Richter magnitude for the CIT and BHSN telemetered seismic networks in southern California. The data have been fitted using regression analysis to relationships of the form M τ = C 0 + C 1 log ⁡ τ + C 2 Δ M τ ≦ 3.8 M τ = C 0 + C 1 ( log ⁡ τ ) 2 + C 2 Δ M τ > 3.8 where τ is the total duration in seconds and Δ is the epicentral distance in kilometers. These relations explain up to 88 per cent (CIT) and 94 per cent (BHSN) of the variation in the data and yield magnitudes having standard deviations as low as 0.15 (CIT) and 0.14 (BHSN) magnitude units. It has been found that the local magnitude based on signal duration is relatively insensitive to variations in azimuth and source effects. In view of the limited distribution and low magnifiation of the Wood-Anderson torsion seismometer, and the previously recognized problems of “saturation” and instrument response associated with the amplitude technique, it is concluded that the method of duration applied to vertical short-period seismograph records will greatly improve the assignment of local magnitude to earthquakes in the southern California region.

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