Extracting Long-Period Surface Waves and Free Oscillations Using Ambient Noise Recorded by Global Distributed Superconducting Gravimeters

2020 ◽  
Vol 91 (4) ◽  
pp. 2234-2246
Author(s):  
Hang Li ◽  
Jianqiao Xu ◽  
Xiaodong Chen ◽  
Heping Sun ◽  
Miaomiao Zhang ◽  
...  
Keyword(s):  
Surface Waves ◽  
Group Velocity ◽  
Ambient Noise ◽  
Velocity Dispersion ◽  
Group Velocity Dispersion ◽  
Dispersion Curves ◽  
Free Oscillations ◽  
Long Period ◽  
Superconducting Gravimeters ◽  
Noise Data

Abstract Inversion of internal structure of the Earth using surface waves and free oscillations is a hot topic in seismological research nowadays. With the ambient noise data on seismically quiet days sourced from the gravity tidal observations of seven global distributed superconducting gravimeters (SGs) and the seismic observations for validation from three collocated STS-1 seismometers, long-period surface waves and background free oscillations are successfully extracted by the phase autocorrelation (PAC) method, respectively. Group-velocity dispersion curves at the frequency band of 2–7.5 mHz are extracted and compared with the theoretical values calculated with the preliminary reference Earth model. The comparison shows that the best observed values differ about ±2% from the corresponding theoretical results, and the extracted group velocities of the best SG are consistent with the result of the collocated STS-1 seismometer. The results indicate that reliable group-velocity dispersion curves can be measured with the ambient noise data from SGs. Furthermore, the fundamental frequency spherical free oscillations of 2–7 mHz are also clearly extracted using the same ambient noise data. The results in this study show that the SG, besides the seismometer, is proved to be another kind of instrument that can be used to observe long-period surface waves and free oscillations on seismically quiet days with a high degree of precision using the PAC method. It is worth mentioning that the PAC method is first and successfully introduced to analyze SG observations in our study.

Noncontact Evaluation of Defects in Thin Plate With Multimodes Lamb’s Waves and Wavelet Transform

2002 ◽  
Author(s):  
Morimasa Murase ◽  
Koichiro Kawashima
Keyword(s):  
Group Velocity ◽  
Thin Plate ◽  
Wavelet Transformation ◽  
Velocity Dispersion ◽  
Lamb Waves ◽  
Group Velocity Dispersion ◽  
Dispersion Curves ◽  
Contact Method ◽  
Intact Specimen ◽  
Time Frequency

Multimode’s Lamb waves in aluminum plates with various defects were excited by a Q-switched Nd:YAG laser. The Lamb waves past through the defects were received a laser interferometer. The received signals of the Lamb waves are processed by the wavelet transformation. The wavelet transformation is generally shown on the time-frequency domain. By dividing a propagation distance by the time, the group velocities are identified. In this way, group velocity dispersion maps of multimode’s Lamb waves are constructed with the received temporal signals. By changing the shape of the mother wavelet, Gabor function, we can identify the dispersion curves of the higher mode Lamb waves. The group velocity dispersion maps of a intact specimen agree well on theoretical dispersion curves of S0, A0, S1, A1, S2, A2, and A3 modes. The difference between the dispersion maps of the intact specimen and that with defects clearly visualizes the existence of defects. This non-contact method is effective for inspecting various defects in thin plate structures.

GULF COAST SURFACE WAVES

Geophysics ◽  
1953 ◽  
Vol 18 (1) ◽  
pp. 41-53 ◽  
Author(s):  
Lynn G. Howell ◽  
E. F. Neuenschwander ◽  
A. L. Pierson
Keyword(s):  
Rayleigh Wave ◽  
Surface Waves ◽  
Velocity Dispersion ◽  
Gulf Coast ◽  
Group Velocity Dispersion ◽  
Vertical Strain ◽  
Long Period ◽  
Short Period ◽  
Component Velocity ◽  
Wave Trains

Surface wave recordings were made with the following: a three‐component velocity seismometer, a long‐period displacement seismometer, six dynamic seismometers, an air‐actuated condenser microphone, and a vertical strain seismometer. Wave trains were recorded similar to those obtained by B. F. Howell in California. We have divided the surface waves into two trains instead of three. The early train seems to have properties of the M‐2 wave of Sezawa; the late train seems to be a Rayleigh wave. An air‐coupled wave is shown to be associated with the M‐2 wave. In the group velocity dispersion curve of the Rayleigh wave, the short‐period branch was found as predicted by theory as well as the usually observed long‐period branch. By making certain assumptions, the thickness of the top layer appears to be about 50 feet according to the theoretical curves of Kanai.

Lamb Wave-Based Damage Location Detection Method of Composite Plate

2011 ◽  
Vol 301-303 ◽  
pp. 1260-1266
Author(s):  
Li Shao Zhang ◽  
Huan Guo Chen ◽  
Jian Min Li ◽  
Li Tian
Keyword(s):  
Group Velocity ◽  
Lamb Wave ◽  
Composite Laminates ◽  
Composite Plate ◽  
Velocity Dispersion ◽  
Lamb Waves ◽  
Group Velocity Dispersion ◽  
Dispersion Curves ◽  
Damage Location ◽  
The Difference

To understand more about Lamb waves on composite laminates damage detection features, the Lamb wave group velocity dispersion curves are calculated and plotted by using dichotomy method in MATLAB. The signal parameters are chosen according to Group velocity dispersion curves. The dynamic response signals of the composite plate are obtained by finite element method. Damage location is calculated by the actual group velocity of Lamb wave and time of flight of the difference signal before and after damage.

scholarly journals Velocity structure of the Whataroa Valley using Ambient Noise Tomography

2021 ◽  
Author(s):  
◽  
Andy McNab
Keyword(s):  
Rayleigh Wave ◽  
Group Velocity ◽  
Shear Wave Velocity ◽  
Correlation Functions ◽  
Ambient Noise ◽  
Cross Correlation ◽  
Velocity Dispersion ◽  
Group Velocity Dispersion ◽  
Velocity Model ◽  
Ambient Noise Tomography

<p>This thesis applies ambient noise tomography to investigate the shallow structure of the Whataroa Valley. Ambient noise techniques are applied to continuous seismic recordings acquired on 158 geophones deployed during the Whataroa Active Source Seismic Experiment. Despite only having four days of data, a robust shear-wave velocity model is calculated using a phase-weighted stacking approach to improve the cross-correlation functions' signal-to-noise ratios, allowing for robust velocity measurements to be obtained between periods of 0.3 and 1.8\,s. This yields a database of 12,500 vertical component cross correlation functions and the corresponding Rayleigh wave phase and group velocity dispersion curves. Linearised straight-ray tomography is applied to phase and group velocity dispersion measurements at periods ranging from periods of 0.3 to 1.8\,s. The tomography reveals a velocity that decreases from the vicinity of the DFDP-2B borehole to the centre of the valley. This is interpreted to be the geologic basement deepening towards the centre of the valley. A Monte-Carlo inversion technique is used to jointly invert Rayleigh-wave phase and group velocity dispersion curves constructed from phase and group velocity tomography maps of successively higher periods. Linear interpolation of the resulting 1D shear-wave velocity profiles produces a pseudo-3D velocity model of the uppermost 1,000\,m of the Whataroa Valley. Using sharp increases in velocity to represent lithological change, we interpret two velocity contours at 1,150 and 1,250\,m/s as potential sediment-basement contacts. Depth isocontours of these velocities reveal that the basement deepens towards the centre of the valley, reaching a maximum depth of 400 or 600\,m for the 1,150 and 1,250\,m/s velocity contours respectively. These depths indicate strong glacial over-deepening and have implications for future drilling projects in the Whataroa Valley. A sharp velocity increase of 200\,m/s also occurs at 100\,m depth at the DFDP-2B borehole. We interpret this to be a change in sedimentary rock lithology from fluvial gravels to lacustrine silty sands, related to a change in sedimentary depositional environment.</p>

scholarly journals Effects of Adhesive Parameters on Dispersion Characteristics of Ultrasonic Guided Waves in Composite Pipes

2019 ◽  
Vol 2019 ◽  
pp. 1-10 ◽  
Author(s):  
Juanjuan Li ◽  
Yan Han
Keyword(s):  
Group Velocity ◽  
Guided Waves ◽  
Velocity Dispersion ◽  
Group Velocity Dispersion ◽  
Adhesive Layer ◽  
Dispersion Curves ◽  
Ultrasonic Guided Waves ◽  
Bonding Quality ◽  
Mode Identification ◽  
Composite Pipes

The aim of the investigation presented here was to understand how the viscosity parameters of an adhesive layer affect group velocity and attenuation of the double-layer adhered pipe. Various parameter combinations (attenuation of longitudinal wave and shear wave, pαL and qαT; thickness, d; and density, nρ) were utilized in order to generate different uncured degrees of the adhesive layer. In the frequency range 0∼500 kHz, the group velocity dispersion curves and attenuation dispersion curves were obtained from these models. Then, the group velocity and attenuation of the two commonly used modes, L0,2 and T0,1, were compared and analyzed. The results have shown that it is important to remark that little effect on group velocity was caused, and significant linear increases of attenuation occur with increase in q, d, and n. However, variable p had little effect on attenuation; more modes emerged when d increased or n decreased, causing difficulties on mode identification and signal processing. The numerical results provided a useful way to evaluate bonding quality by measuring the group velocity and attenuation in the pipelines.

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