Monte Carlo Simulation of Stress‐Associated Scattering Attenuation from Laboratory Ultrasonic Measurements
Monte Carlo Simulation of Stress‐Associated Scattering Attenuation from Laboratory Ultrasonic Measurements
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DOI：
10.1785/0120130082
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发表时间：
2014-04
影响因子：
3
通讯作者：
Wei Wei-Wei;L. Fu
中科院分区：
文献类型：
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作者：
Wei Wei-Wei;L. Fu
由小尺度不均匀性散射的地震尾波包含关于介质应力变化的信息，因为材料的物理状态发生了变化。基于对圆柱形砂岩样品在不同应力下的超声测量，研究了应力变化对超声尾波衰减的影响，旨在表征其应力/频率依赖模式。考虑到实验室实验中从有限尺寸岩石样品中测量的超声尾波波形的复杂性，采用Monte Carlo模拟来合成超声包络，该包络考虑了多次散射和边界反射对尾波的影响。最佳的模拟参数，估计通过最小化之间的残差观测和合成的信封，表明所研究的岩石样品呈现中度非均质性。对于直达波和尾波S波，衰减和应力之间的关系是相似的，并且在约30-60 MPa的高效应力范围内保持稳定，应力敏感性较低。这两种类型波的增强衰减都发生在较低的有效应力下，但尾波衰减更快、更强，呈现出完全不同的应力非线性行为。尾波衰减在低于15 MPa的极低有效应力下急剧增加，因为岩石柔度增加，对高孔隙压力的敏感性比固有衰减大得多。该研究加深了我们对超声尾波衰减机制及其与应力和频率的标度关系的理解。
Abstract Seismic coda waves scattered by small‐scale heterogeneities contain information on stress changes of the medium, because of changes in the physical state of materials. Based on the ultrasonic measurements under different stresses for a cylindrical sandstone sample, we investigate the influence of stress changes on ultrasonic S ‐coda attenuation and aim to characterize its stress/frequency‐dependent pattern. Considering the complexity of ultrasonic coda waveforms measured from finite‐size rock samples in laboratory experiments, the Monte Carlo simulation is employed to synthesize ultrasonic envelopes, which act by incorporating the effect of multiple scatterings and boundary reflections on coda waves. The optimal simulation parameters, estimated by minimizing the residual between the observed and synthesized envelopes, indicate that the rock sample under study presents moderate heterogeneities. The relationship between attenuation and stress is similar for direct and coda S waves and remains stable in the range of high‐effective stresses around 30–60 MPa, with less stress sensitivity. Enhanced attenuation for both types of waves occurs at lower‐effective stresses, but with coda attenuation much faster and stronger, presenting a quite different nonlinear behavior with respect to stress. Coda attenuation increases drastically at extremely low‐effective stresses below 15 MPa because of the increase in rock compliance, showing much greater sensitivity to high‐pore pressure than intrinsic attenuation. This study improves our understanding of the mechanism of ultrasonic coda attenuation and its scaling dependence on stress and frequency.