An analysis of seismic scattering attenuation in a random elastic medium

Seismic attenuation has been inherent media characteristics in which an interesting topic of research, for it reflects the seismic waves propagate. There are many factors that cause seismic wave attenuation, such as geometry attenuation caused by energy dissipating during propagation, friction attenuation by relative sliding among rock grains, and scattering attenuation by rock heterogeneity. In this paper we study P-wave scattering attenuation in a random elastic medium by numerical simulations from a statistical point of view. A random elastic medium model is built based on general stochastic process theory. Then a staggered-grid pseudo-spectral method is used to simulate wave propagation. Scattering attenuation is estimated by the spectral ratio method based on virtual detector records. Random elastic media numerical scatter results with various heterogeneity levels show that the higher heterogeneous levels cause greater scattering attenuation. When the scatter sizes are smaller than a wave length, the larger scatters give a greater attenuation. Finally, we propose a method to evaluate fluid-flow attenuation in porous media. The fluid- flow attenuation is derived from total attenuation and scattering attenuation in random porous media and the attenuation is estimated quantitatively. Results show that in the real seismic frequency range when the heterogeneous scale is about 10^1 meters （less than one wave length）, scattering attenuation is larger than fluid-tlow attenuation in random porous media and scattering attenuation is the main factor of seismic attenuation in real heterogeneous porous media.

Influence of random shrinkage porosity on equivalent elastic modulus of casting： A statistical and numerical approach

Shrinkage porosity is a type of random distribution defects and exists in most large castings. Different from the periodic symmetry defects or certain distribution defects, shrinkage porosity presents a random "cloud-like" configuration, which brings difficulties in quantifying the effective performance of defected casting. In this paper, the influences of random shrinkage porosity on the equivalent elastic modulus of QT400-18 casting were studied by a numerical statistics approach. An improved random algorithm was applied into the lattice model to simulate the "cloud-like" morphology of shrinkage porosity. Then, a large number of numerical samples containing random levels of shrinkage were generated by the proposed algorithm. The stress concentration factor and equivalent elastic modulus of these numerical samples were calculated. Based on a statistical approach, the effects of shrinkage porosity's distribution characteristics, such as area fraction, shape, and relative location on the casting's equivalent mechanical properties were discussed respectively. It is shown that the approach with randomly distributed defects has better predictive capabilities than traditional methods. The following conclusions can be drawn from the statistical simulations:(1) the effective modulus decreases remarkably if the shrinkage porosity percent is greater than 1.5%;(2) the average Stress Concentration Factor(SCF) produced by shrinkage porosity is about 2.0;(3) the defect's length across the loading direction plays a more important role in the effective modulus than the length along the loading direction;(4) the surface defect perpendicular to loading direction reduces the mean modulus about 1.5% more than a defect of other position.

Effect of Shoushen granule on arterial elasticity in patients with carotid atherosclerosis:a clinical randomized controlled trial

OBJECTIVE: To investigate the effectiveness of Shoushen granule,Chinese herbal preparation,on carotid artery elasticity in patients with carotid atherosclerosis.METHODS: The total of 156 carotid atherosclerosis patients were randomly divided into the intervention group(83 cases,treated with Shoushen granule) and the control group(73 cases,treated with pravastatin). Brachial-ankle pulse wave velocity(ba PWV) and Ankle-Brachial Pressure Index(ABI)were measured by automated arteriosclerosis detector. The changes of common carotid artery intima-media thickness(IMT) and parameters of the carotid artery elasticity in patients,including stiffness parameter(β),pressure-strain elastic modulus(Ep),arterial compliance(Ac),augmentation index(AI),and pulse wave velocity β(PWVβ) were detected by Echo-Tracking(ET) technique before and after 24 week treatment. In the meantime,levels of blood lipid,and liver and renal function were measured respectively.RESULTS: After 24 weeks,ba PWV,IMT and parameters of the carotid artery elasticity(β,Ep,AI and PWVβ) were markedly decreased in intervention group compared with those of before treatment(P < 0.01),but the level of Ac was increased significantly(P < 0.01). And there were no significant differences compared with control group on the same period(P > 0.05).CONCLUSION: In this pilot study,it was demonstrated ET technology and automated arteriosclerosis detector could be used to evaluate carotid artery elasticity effectively,and the action of Shoushen granule on carotid atherosclerosis might be related to the regulation of carotid artery elasticity.