Arrival-angle effects on two-receiver measurements of phase velocity

2019 ◽  
Vol 220 (3) ◽  
pp. 1838-1844
Author(s):  
Fabrizio Magrini ◽  
Giovanni Diaferia ◽  
Lapo Boschi ◽  
Fabio Cammarano
Keyword(s):  
Ambient Noise ◽  
Western Mediterranean ◽  
Propagation Path ◽  
Data Set ◽  
Arrival Angle ◽  
Phase Velocities ◽  
Angle Correction ◽  
Wide Range ◽  
Cross Correlations ◽  
Teleseismic Events

SUMMARY We compile a data set of Rayleigh-wave phase velocities between pairs of stations, based on teleseismic events located on the same great circle as the two stations. We validate our observations against dispersion estimates based on ambient-noise cross correlations at the same station pairs. Discrepancies between the results of the two methods can in principle be explained by deviations in the wave propagation path between earthquake and receivers, due to lateral heterogeneity in the Earth’s structure, but the latter effect has, so far, not been precisely quantified nor corrected for. We implement an algorithm to measure the arrival angle of earthquake-generated surface waves and correct the dispersion measurements accordingly. Application to a data set from the Central-Western Mediterranean shows that the arrival-angle correction almost entirely accounts for the discrepancy in question, decreasing significantly the velocity bias for a wide range of periods.

scholarly journals Prediction of Wide Range Two-Dimensional Refractivity Using an IDW Interpolation Method from High-Altitude Refractivity Data of Multiple Meteorological Observatories

2021 ◽  
Vol 11 (4) ◽  
pp. 1431
Author(s):  
Sungsik Wang ◽  
Tae Heung Lim ◽  
Kyoungsoo Oh ◽  
Chulhun Seo ◽  
Hosung Choo
Keyword(s):  
High Altitude ◽  
Korean Peninsula ◽  
Interpolation Method ◽  
Atmospheric Condition ◽  
Propagation Path ◽  
Two Dimensional ◽  
Data Set ◽  
Wide Range ◽  
Atmospheric Data ◽  
Terrain Surface

This article proposes a method for the prediction of wide range two-dimensional refractivity for synthetic aperture radar (SAR) applications, using an inverse distance weighted (IDW) interpolation of high-altitude radio refractivity data from multiple meteorological observatories. The radio refractivity is extracted from an atmospheric data set of twenty meteorological observatories around the Korean Peninsula along a given altitude. Then, from the sparse refractive data, the two-dimensional regional radio refractivity of the entire Korean Peninsula is derived using the IDW interpolation, in consideration of the curvature of the Earth. The refractivities of the four seasons in 2019 are derived at the locations of seven meteorological observatories within the Korean Peninsula, using the refractivity data from the other nineteen observatories. The atmospheric refractivities on 15 February 2019 are then evaluated across the entire Korean Peninsula, using the atmospheric data collected from the twenty meteorological observatories. We found that the proposed IDW interpolation has the lowest average, the lowest average root-mean-square error (RMSE) of ∇M (gradient of M), and more continuous results than other methods. To compare the resulting IDW refractivity interpolation for airborne SAR applications, all the propagation path losses across Pohang and Heuksando are obtained using the standard atmospheric condition of ∇M = 118 and the observation-based interpolated atmospheric conditions on 15 February 2019. On the terrain surface ranging from 90 km to 190 km, the average path losses in the standard and derived conditions are 179.7 dB and 182.1 dB, respectively. Finally, based on the air-to-ground scenario in the SAR application, two-dimensional illuminated field intensities on the terrain surface are illustrated.

Upper crustal structure at the KTB drilling site from ambient noise tomography

2020 ◽  
Author(s):  
Ehsan Qorbani ◽  
Irene Bianchi ◽  
Petr Kolínský ◽  
Dimitri Zigone ◽  
Götz Bokelmann
Keyword(s):  
Ambient Noise ◽  
Velocity Model ◽  
Ambient Noise Tomography ◽  
Data Set ◽  
Crustal Rocks ◽  
Velocity Models ◽  
Path Coverage ◽  
Passive Seismic ◽  
Cross Correlations ◽  
Drilling Site

<p>In this study, we show results from ambient noise tomography at the KTB drilling site, Germany. The Continental Deep Drilling Project, or ‘Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland’ (KTB) is at the northwestern edge of the Bohemian Massif and is located on the Variscan belt of Europe. During the KTB project crustal rocks have been drilled down to 9 km depth and several active seismic studies have been performed in the surrounding. The KTB area therefore presents an ideal test area for testing and verifying the potential resolution of passive seismic techniques. The aim of this study is to present a new shear-wave velocity model of the area while comparing the results to the previous velocity models and hints for anisotropy depicted by former passive and active seismological studies. We use a unique data set composed of two years of continuous data recorded at nine 3-component temporary stations installed from July 2012 to July 2014 located on top and vicinity of the drilling site. Moreover, we included a number of permanent stations in the region in order to improve the path coverage and density. Cross correlations of ambient noise are computed between the station pairs using all possible combination of three-component data. Dispersion curves of surface waves are extracted and are then inverted to obtain group velocity maps. We present here a new velocity model of the upper crust of the area, which shows velocity variations at short scales that correlate well with geology in the region.</p>

Reconciling phase velocities from ambient noise and earthquake-generated surface waves by accounting for arrival-angle effects

2020 ◽  
Author(s):  
Giovanni Diaferia ◽  
Fabrizio Cammarano ◽  
Lapo Boschi ◽  
Fabio Cammarano
Keyword(s):  
Surface Waves ◽  
Ambient Noise ◽  
Radial Component ◽  
Systematic Bias ◽  
Finite Frequency ◽  
Arrival Angle ◽  
Phase Velocities ◽  
Shear Wave Velocities ◽  
Lateral Inhomogeneity ◽  
Seismic Anomalies

<p>The shear-wave velocities structure at depth can be unraveled from ambient noise (AN) as well as from earthquake-generated (EQ) surface waves. While the first approach mostly provides information at crustal scale, earthquake-based surface waves sense deeper structures due to their lower frequency content. However, for periods between 20 and 40 s, where the two methods often overlap, a number of studies have shown that phase velocities from EQ surface waves are systematically higher (~1%) than those retrieved from AN. The reason for such systematic bias is still debated; finite-frequency effects, overtone contamination, and off-path propagation of surface waves due to structural inhomogeneities have all been invoked as possible explanations of the discrepancy in question.</p><p>We explore the validity of the latter hypothesis, by correcting Rayleigh-wave phase velocities for the effect of off-path arrivals at two stations. The deviation from the theoretical path is estimated by evaluating the resemblance of the vertical with the π/2-shifted radial component of the recorded seismograms. We developed a two-station algorithm implementing such a correction and tested it on a dataset of seismograms collected from more than 350 stations recording 443 earthquake events from 2005 to 2019. We demonstrate that by compensating for the arrival-angle effects, the discrepancy between the two methods is significantly reduced. This result suggests that the off-path propagation between epicenters and receivers due to lateral inhomogeneity in the Earth's structure explains most of the discrepancy between AN and EQ phase velocities previously reported in the literature. Such improvement in determining Rayleigh phase velocities will lead to more reliable seismic tomographies and enhanced interpretations of seismic anomalies in terms of thermo-chemical characteristics.</p>

scholarly journals 3-D shear wave velocity model of the lithosphere below the Sardinia–Corsica continental block based on Rayleigh-wave phase velocities

2019 ◽  
Vol 220 (3) ◽  
pp. 2119-2130 ◽  
Author(s):  
Fabrizio Magrini ◽  
Giovanni Diaferia ◽  
Islam Fadel ◽  
Fabio Cammarano ◽  
Mark van der Meijde ◽  
...  
Keyword(s):  
Rayleigh Wave ◽  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Velocity Model ◽  
Western Mediterranean ◽  
Crust And Upper Mantle ◽  
Phase Velocities ◽  
Continental Block ◽  
Wide Range

SUMMARY Rayleigh-wave dispersion curves from both ambient noise and teleseismic events allow us to provide the first high-resolution 3-D shear wave velocity (VS) model of the crust and upper mantle below the Sardinia–Corsica microplate, an important continental block for understanding the evolution of the central-western Mediterranean. For a wide range of periods (from 3 to ∼30 s), the phase velocities of the study area are systematically higher than those measured within the Italian peninsula, in agreement with a colder geotherm. Relative and absolute variations in the VS allow us to detect a very heterogeneous upper crust down to 8 km, as opposed to a relatively homogeneous middle and lower crust. The isosurface at 4.1 km s−1 is consistent with a rather flat Moho at a depth of 28.0 ± 1.8 km (2σ). The lithospheric mantle is relatively cold, and we constrain the thermal lithosphere–asthenosphere boundary at ∼100 km. We find our estimate consistent with a continental geotherm based on a surface heat flow of 60 mW m−2. Our results suggest that most of the lithosphere endured the complex history of deformation experienced by the study area and imply, in general, that deep tectonic processes do not easily destabilize the deeper portion of the continental lithosphere, despite leaving a clear surface signature.

scholarly journals Improving cross-correlations of ambient noise using an rms-ratio selection stacking method

2020 ◽  
Vol 222 (2) ◽  
pp. 989-1002
Author(s):  
Jinyun Xie ◽  
Yingjie Yang ◽  
Yinhe Luo
Keyword(s):  
Short Duration ◽  
Ambient Noise ◽  
Signal To Noise Ratio ◽  
Ambient Noise Tomography ◽  
Noise Sources ◽  
Time Duration ◽  
Crust And Upper Mantle ◽  
Phase Velocities ◽  
Noise Data ◽  
Cross Correlations

SUMMARY Stacking of ambient noise correlations is a crucial step to extract empirical Green's functions (EGFs) between station pairs. The traditional method is to linearly stack all short-duration cross-correlation functions (CCFs) over a long period of time to obtain final stacks. It requires at least several months of ambient noise data to obtain reliable phase velocities at periods of several to tens of seconds from CCFs. In this study, we develop a new stacking method named root-mean-square ratio selection stacking (RMSR_SS) to reduce the time duration required for the recovery of EGFs from ambient noise. In our RMSR_SS method, rather than stacking all short-duration CCFs, we first judge if each of the short-duration CCF constructively contributes to the recovery of EGFs or not. Then, we only stack those CCFs which constructively contribute to the convergence of EGFs. By applying our method to synthetic noise data, we demonstrate how our method works in enhancing the signal-to-noise ratio of CCFs by rejecting noise sources which do not positively contribute to the recovery of EGFs. Then, we apply our method to real noise data recorded in western USA. We show that reliable and accurate phase velocities can be measured from 15-d long ambient noise data using our RMSR_SS method. By applying our method to ambient noise tomography (ANT), we can reduce the deployment duration of seismic stations from several months or years to a few tens of days, significantly improving the efficiency of ANT in imaging crust and upper-mantle structures.

Seismic noise‐reduction techniques for use with vertical stacking: An empirical comparison

Geophysics ◽  
1987 ◽  
Vol 52 (3) ◽  
pp. 322-334 ◽  
Author(s):  
Simon L. Klemperer
Keyword(s):  
Noise Reduction ◽  
Ambient Noise ◽  
Automatic Gain Control ◽  
Gain Control ◽  
Noise Levels ◽  
Data Set ◽  
Reduction Techniques ◽  
Noise Rejection ◽  
Wide Range ◽  
Low Levels

A systematic comparison of a wide range of noise‐reduction techniques applied to a single data set collected for this purpose was attempted. The study includes a comparison of the relative benefits of noise‐reduction schemes for very different noise levels. The comparisons use computer simulations of field techniques for noise reduction as applied to deep crustal, 48-fold VIBROSEIS® data. The noise‐reduction techniques involve amplitude scaling (diversity stacking and automatic gain control), noise editing (the use of fixed‐gain and self‐updating noise‐rejection systems with and without buffered memory for recursive editing), and reduced numbers of recording bits (mantissa‐only and sign‐bit‐only formats). The effectiveness of each noise‐reduction procedure is assessed by a study of its effects on noise levels seen on source‐point gathers and on both true‐amplitude and amplitude‐balanced common‐midpoint (CMP) stacks. For low levels of ambient noise, true‐amplitude CMP stacks can be substantially improved by the appropriate noise‐reduction techniques, but CMP stacks incorporating gain control before CMP stacking show only minor improvements. In contrast, when a high level of ambient noise is present, both true‐amplitude and gain‐controlled CMP stacks can be greatly improved by appropriate noise‐reduction processing before vertical stacking. Of procedures involving zeroing of noise bursts, self‐updating noise‐rejection systems were more effective than fixed‐gain noise‐rejection systems for all the conditions simulated here. A system incorporating recursive editing techniques is still more effective at low levels of ambient noise, but this system tends to edit recorded traces too severely at very high noise levels. Mantissa‐only and sign‐bit‐only recording give very similar results, and show an effect comparable to that given by self‐updating editing systems. Diversity stacking produced significant noise reduction in all conditions studied, and may be the most widely applicable and most generally useful of the noise‐reduction methods studied here. ®Trade and service mark of Conoco Inc.

scholarly journals Helmholtz tomography of ambient noise surface wave data to estimate Scholte wave phase velocity at Valhall Life of the Field

Geophysics ◽  
2013 ◽  
Vol 78 (2) ◽  
pp. WA99-WA109 ◽  
Author(s):  
Aurélien Mordret ◽  
Nikolaï M. Shapiro ◽  
Satish S. Singh ◽  
Philippe Roux ◽  
Olav I. Barkved
Keyword(s):  
Phase Velocity ◽  
Ambient Noise ◽  
Waveform Inversion ◽  
Wave Model ◽  
P Wave ◽  
Data Set ◽  
Phase Velocities ◽  
Wave Phase ◽  
Wave Phase Velocity ◽  
Scholte Wave

We applied the Helmholtz tomography technique to 6.5 hours of continuous seismic noise record data set of the Valhall Life of Field network. This network, that has 2320 receivers, allows us to perform a multifrequency, high-resolution, ambient-noise Scholte wave phase velocity tomography at Valhall. First, we computed crosscorrelations between all possible pairs of receivers to convert every station into a virtual source recorded by all other receivers. Our next step was to measure phase traveltimes and spectral amplitudes at different periods from crosscorrelations between stations separated by distances between two and six wavelengths. This is done in a straightforward fashion in the Fourier domain. Then, we interpolated these measurements onto a regular grid and computed local gradients of traveltimes and local Laplacians of the amplitude to infer local phase velocities using a frequency dependent Eikonal equation. This procedure was repeated for all 2320 virtual sources and final phase velocities were estimated as statistical average from all these measurements at each grid points. The resulting phase velocities for periods between 0.65 and 1.6 s demonstrate a significant dispersion with an increase of the phase velocities at longer periods. Their lateral distribution is found in very good agreement with previous ambient noise tomography done at Valhall as well as with a full waveform inversion P-wave model computed from an active seismic data set. We put effort into assessing the spatial resolution of our tomography with checkerboard tests, and we discuss the influence of the interpolation methods on the quality of our final models.

Evaluating Uncertainties of Phase Velocity Measurements from Cross-Correlations of Ambient Seismic Noise

2020 ◽  
Vol 91 (3) ◽  
pp. 1717-1729
Author(s):  
Yinhe Luo ◽  
Yingjie Yang ◽  
Jinyun Xie ◽  
Xiaozhou Yang ◽  
Fengru Ren ◽  
...  
Keyword(s):  
Phase Velocity ◽  
Ambient Noise ◽  
Signal To Noise Ratio ◽  
The United States ◽  
Velocity Measurements ◽  
Phase Velocities ◽  
Phase Velocity Dispersion ◽  
Noise Data ◽  
Cross Correlations ◽  
Fitting In

Abstract Ambient-noise tomography (ANT) has become a well-established method to image the crust and uppermost mantle structures in the past 15 yr. Having a good estimate of uncertainties of phase velocity dispersion measurements in ANT is critical as they can guide the level of data fitting in tomography. However, to date, there are still no systemic studies to evaluate these uncertainties. In this study, we obtain cross correlations with different stacking durations from 17 yr of ambient-noise data recorded at 120 stations in the United States. We analyze the variations of signal-to-noise ratio (SNR) and phase velocities of cross correlations. We find that the uncertainties of phase velocities are affected by SNRs, interstation distances, and stacking durations. However, none of those three variables can be solely used as a proxy to estimate the uncertainties of phase velocity measurements. Based on our analysis, we graphically present empirical relations of uncertainties of phase velocity measurements as a function of SNR, interstation distance, and stacking duration. These relations can be employed as a guide to estimate phase velocity uncertainties in applications of ANT, assisting in evaluating the reliability of resulting models from ANT.

Rayleigh phase velocities in Southern California from beamforming short-duration ambient noise

2017 ◽  
Vol 211 (1) ◽  
pp. 450-454 ◽  
Author(s):  
Philippe Roux ◽  
Yehuda Ben-Zion
Keyword(s):  
Ambient Noise ◽  
Southern California ◽  
Adaptive Beamforming ◽  
Time Interval ◽  
Short Time Interval ◽  
Seismic Arrays ◽  
Phase Velocities ◽  
Correlation Processing ◽  
Cross Correlations ◽  
Point To Point

Abstract Beamforming of ambient noise recorded by regional arrays of seismometers is presented as an alternative imaging approach to cross-correlations between pairs of sensors. The method is used to obtain phase velocities and propagation directions of Rayleigh surface waves around the first and second microseism peaks in southern California. The derived velocity maps and propagation directions correlate with major geological structures and changes of the coastal shape in the region. The results are consistent with and complementary to those obtained using cross-correlations of long-duration data between pairs of sensors. Significant advantages of the presented high-resolution adaptive beamforming method over point-to-point noise cross-correlations are the short time interval of required data (hours to days compared to a year) and robust performance with directive (rather than omnidirectional) noise propagation. Given the recent trend toward dense and large seismic arrays at various scales, the combination of beamforming and noise-correlation processing may provide an optimal strategy for performing noise-based tomography.

Structure Guided Molecular Docking Assisted Alignment Dependent 3DQSAR Study on Steroidal Aromatase Inhibitors (SAIs) as Anti-breast Cancer Agents

2019 ◽  
Vol 16 (7) ◽  
pp. 808-817 ◽  
Author(s):  
Laxmi Banjare ◽  
Sant Kumar Verma ◽  
Akhlesh Kumar Jain ◽  
Suresh Thareja
Keyword(s):  
Breast Cancer ◽  
Molecular Docking ◽  
Aromatase Inhibitors ◽  
Direct Synthesis ◽  
3D Qsar ◽  
Chemotherapeutic Agents ◽  
Modeling Tools ◽  
Data Set ◽  
Wide Range ◽  
Steroidal Aromatase Inhibitors

Background: In spite of the availability of various treatment approaches including surgery, radiotherapy, and hormonal therapy, the steroidal aromatase inhibitors (SAIs) play a significant role as chemotherapeutic agents for the treatment of estrogen-dependent breast cancer with the benefit of reduced risk of recurrence. However, due to greater toxicity and side effects associated with currently available anti-breast cancer agents, there is emergent requirement to develop target-specific AIs with safer anti-breast cancer profile. Methods: It is challenging task to design target-specific and less toxic SAIs, though the molecular modeling tools viz. molecular docking simulations and QSAR have been continuing for more than two decades for the fast and efficient designing of novel, selective, potent and safe molecules against various biological targets to fight the number of dreaded diseases/disorders. In order to design novel and selective SAIs, structure guided molecular docking assisted alignment dependent 3D-QSAR studies was performed on a data set comprises of 22 molecules bearing steroidal scaffold with wide range of aromatase inhibitory activity. Results: 3D-QSAR model developed using molecular weighted (MW) extent alignment approach showed good statistical quality and predictive ability when compared to model developed using moments of inertia (MI) alignment approach. Conclusion: The explored binding interactions and generated pharmacophoric features (steric and electrostatic) of steroidal molecules could be exploited for further design, direct synthesis and development of new potential safer SAIs, that can be effective to reduce the mortality and morbidity associated with breast cancer.

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