scholarly journals Monte Carlo simulations of coupled body- and Rayleigh-wave multiple scattering in elastic media

2021 ◽  
Author(s):  
Zongbo Xu ◽  
Ludovic Margerin ◽  
T Dylan Mikesell
Keyword(s):  
Monte Carlo ◽  
Free Surface ◽  
Multiple Scattering ◽  
Energy Partitioning ◽  
The Body ◽  
Body Waves ◽  
Coda Waves ◽  
Elastic Media ◽  
Energy Evolution ◽  
Scattering Process

Summary Seismic coda waves are commonly used in estimation of subsurface Q values and monitoring subsurface changes. Coda waves mainly consist of multiply scattered body and surface waves. These two types of waves interact with each other in the multiple scattering process, which thus leads to a spatiotemporal evolution of the body- and surface-wave energies. One cannot characterize the evolution because one has not fully understood the multiple scattering of the two types of waves. Thus one commonly assumes only one type of waves exists or ignores their interaction while studying the coda waves. However, neglecting the interaction leads to an incorrect energy evolution of the two types of waves and consequently biases the Q estimation or interpretation of coda-wave changes for monitoring. To better understand the interaction between these waves during multiple scattering and to model the energy evolution correctly, we propose a Monte Carlo algorithm to model the multiple scattering process. We describe the physics of the scattering for the two types of waves and derive scattering properties like cross sections for perturbations in elastic properties (e.g. density, shear modulus and Lamé parameters). Our algorithm incorporates this knowledge and thus physically models the body- and surface-wave energy evolution in space and time. The energy partitioning ratios between surface and body waves provided by our algorithm match the theoretical prediction based on equipartition theory. In the equipartition state, our simulation results also match Lambert’s cosine law for body waves on the free surface. We discuss how the Rayleigh-to-body-wave scattering affects the energy partitioning ratios. Our algorithm provides a new tool to study multiple scattering and coda waves in elastic media with a free surface.

Sensitivity kernels for coda-wave interferometry in a three-dimensional scalar scattering media

2020 ◽  
Author(s):  
Andres Barajas ◽  
Ludovic Margerin ◽  
Michel Campillo
Keyword(s):  
Free Surface ◽  
Surface Waves ◽  
Mixed Boundary ◽  
Surrounding Medium ◽  
Body Waves ◽  
Coda Waves ◽  
Coda Wave ◽  
Scattering Media ◽  
The Impact ◽  
Coda Wave Interferometry

<p>The ambient seismic noise has proven to be a powerful tool to assess velocity changes within the ground using coda-wave interferometry (CWI). CWI is based on the analysis of small waveform changes in the coda of the signals. Localizing and imaging the source that generates changes can be done with the help of sensitivity kernels which contain information on how each part of the surrounding medium contributes to the overall waveform perturbation that is recorded at a receiver. Although progress has been made in the theory of sensitivity kernels in the case of a full elastic space,  the inclusion of a free surface has proven to be difficult. Indeed, the free surface couples body waves and surface waves, which affects the sensitivity of coda waves with respect to the full-space case. Furthermore, one expects the depth sensitivity of coda waves to be strongly dependent on the relative contribution of surface and body waves, which depends on the lapse-time, source-receiver distance and scattering properties of the medium. Using the Monte-Carlo method, we compute traveltime-sensitivity kernels in a 3D scalar problem that includes body and surface waves, based on a recent theoretical model that integrates both through a mixed boundary condition. From these results, we assess the impact of the depth of a velocity perturbation on the recorded signals at the surface. Our results will be compared with previous numerical approaches from the literature. </p>

scholarly journals Slithering CSF: Cerebrospinal Fluid Dynamics in the Stationary and Moving Viper Boa, Candoia aspera

Biology ◽  
2021 ◽  
Vol 10 (7) ◽  
pp. 672
Author(s):  
Bruce A. Young ◽  
Skye Greer ◽  
Michael Cramberg
Keyword(s):  
Cerebrospinal Fluid ◽  
Cardiac Cycle ◽  
Low Frequency ◽  
Mean Amplitude ◽  
The Body ◽  
Body Waves ◽  
Lower Amplitude ◽  
Lateral Undulation ◽  
Recovery From Anesthesia ◽  
The Relationship

In the viper boa (Candoia aspera), the cerebrospinal fluid (CSF) shows two stable overlapping patterns of pulsations: low-frequency (0.08 Hz) pulses with a mean amplitude of 4.1 mmHg that correspond to the ventilatory cycle, and higher-frequency (0.66 Hz) pulses with a mean amplitude of 1.2 mmHg that correspond to the cardiac cycle. Manual oscillations of anesthetized C. aspera induced propagating sinusoidal body waves. These waves resulted in a different pattern of CSF pulsations with frequencies corresponding to the displacement frequency of the body and with amplitudes greater than those of the cardiac or ventilatory cycles. After recovery from anesthesia, the snakes moved independently using lateral undulation and concertina locomotion. The episodes of lateral undulation produced similar influences on the CSF pressure as were observed during the manual oscillations, though the induced CSF pulsations were of lower amplitude during lateral undulation. No impact on the CSF was found while C. aspera was performing concertina locomotion. The relationship between the propagation of the body and the CSF pulsations suggests that the body movements produce an impulse on the spinal CSF.

Influence of Free Surface Environment on the Shear Zone in Metal Cutting

1966 ◽  
Vol 181 (1) ◽  
pp. 687-705 ◽  
Author(s):  
P. L. Barlow
Keyword(s):  
Free Surface ◽  
Carbon Tetrachloride ◽  
Strain Hardening ◽  
Shear Zone ◽  
Chemical Reaction ◽  
Cutting Forces ◽  
Metal Cutting ◽  
The Body ◽  
Surface Barrier ◽  
Rehbinder Effect

It has previously been suggested that the reduction in cutting forces obtained by the presence of fluids such as CCl4 on the backface or free surface of the forming chip was due to diffusion of the fluid into the body of the chip in the region of the shear zone. In the present work, experiments with carbon tetrachloride tagged with carbon-14 and with carbon tetrachloride tagged with chlorine-36 were performed with the object of assessing the extent of diffusion of lubricants into the chip when present on the free surface only. The results obtained disprove former hypotheses and suggest that the reduced cutting force is due solely to chemical reaction at the surface of the chip. Confirmation of the sensitivity of the surface of the deforming shear zone to change in surface condition was obtained by removing metal from this region by an electropolishing technique during slow speed cutting. By varying the electropolishing conditions increased or decreased cutting forces could be obtained. It is proposed that the result both of chemical reaction at the surface and of surface removal is to reduce the strain-hardening rate of the metal undergoing shear by reducing the surface barrier to the flow of dislocations out of the metal. The association of the surface reaction of carbon tetrachloride with a change in the strain-hardening characteristics of the metal in the shear zone leads to a classification of the backface phenomenon as a Rehbinder effect and enables this effect to be more closely defined than was hitherto possible. Evidence is also presented which indicates that the backface effect does not contribute to the reduction in cutting forces during rakeface lubrication and is therefore unimportant in practice where flood lubrication of the cutting region invariably occurs.

Monte Carlo simulations of the free surface of polymer melts

1994 ◽  
Vol 49 (17) ◽  
pp. 2899-2906 ◽  
Author(s):  
Sanat K. Kumar ◽  
Thomas P. Russell ◽  
Arvind Hariharan
Keyword(s):  
Monte Carlo ◽  
Free Surface ◽  
Monte Carlo Simulations ◽  
Polymer Melts

Boundaries Influence on the Flow Field Around an Oscillating Sphere

2013 ◽  
Author(s):  
Domenica Mirauda ◽  
Antonio Volpe Plantamura ◽  
Stefano Malavasi
Keyword(s):  
Free Surface ◽  
Flow Field ◽  
Boundary Conditions ◽  
Damping Ratio ◽  
Water Channel ◽  
Open Water ◽  
Free Surface Flows ◽  
The Body ◽  
Sphere Diameter ◽  
Oscillating Sphere

This work analyzes the effects of the interaction between an oscillating sphere and free surface flows through the reconstruction of the flow field around the body and the analysis of the displacements. The experiments were performed in an open water channel, where the sphere had three different boundary conditions in respect to the flow, defined as h* (the ratio between the distance of the sphere upper surface from the free surface and the sphere diameter). A quasi-symmetric condition at h* = 2, with the sphere equally distant from the free surface and the channel bottom, and two conditions of asymmetric bounded flow, one with the sphere located at a distance of 0.003m from the bottom at h* = 3.97 and the other with the sphere close to the free surface at h* = 0, were considered. The sphere was free to move in two directions, streamwise (x) and transverse to the flow (y), and was characterized by values of mass ratio, m* = 1.34 (ratio between the system mass and the displaced fluid mass), and damping ratio, ζ = 0.004. The comparison between the results of the analyzed boundary conditions has shown the strong influence of the free surface on the evolution of the vortex structures downstream the obstacle.

Seismic cone penetration test and seismic tomography in permafrost

2004 ◽  
Vol 41 (5) ◽  
pp. 796-813 ◽  
Author(s):  
Anne-Marie LeBlanc ◽  
Richard Fortier ◽  
Michel Allard ◽  
Calin Cosma ◽  
Sylvie Buteau
Keyword(s):  
Seismic Tomography ◽  
Dynamic Properties ◽  
Cone Penetration Test ◽  
The Body ◽  
Body Waves ◽  
Reconstruction Technique ◽  
Seismic Velocities ◽  
Penetration Test ◽  
Cone Penetration ◽  
Seismic Cone Penetration Test

Two high-resolution multi-offset vertical seismic profile (VSP) surveys were carried out in a permafrost mound near Umiujaq in northern Quebec, Canada, while performing seismic cone penetration tests (SCPT) to study the cryostratigraphy and assess the body waves velocities and the dynamic properties of warm permafrost. Penetrometer-mounted triaxial accelerometers were used as the VSP receivers, and a swept impact seismic technique (SIST) source generating both compressional and shear waves was moved near the surface following a cross configuration of 40 seismic shot-point locations surrounding each of the two SCPTs. The inversion of travel times based on a simultaneous iterative reconstruction technique (SIRT) provided tomographic images of the distribution of seismic velocities in permafrost. The Young's and shear moduli at low strains were then calculated from the seismic velocities and the permafrost density measured on core samples. The combination of multi-offset VSP survey, SCPT, SIST, and SIRT for tomographic imaging led to new insights in the dynamic properties of permafrost at temperatures close to 0 °C. The P- and S-wave velocities in permafrost vary from 2400 to 3200 m/s and from 900 to 1750 m/s, respectively, for a temperature range between –0.2 and –2.0 °C. The Young's modulus varies from 2.15 to 13.65 GPa, and the shear modulus varies from 1.00 to 4.75 GPa over the same range of temperature.Key words: permafrost, seismic cone penetration test, vertical seismic profiling, seismic tomography, dynamic properties.

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