In Situ Measurements of Damping Ratio Using Surface Waves

[402358]当地震波从基岩传播到地表时，控制其放大的一个重要土壤参数是土壤的滞回阻尼比。阻尼通常是在实验室测试的土样上测量的，但是当样品从地面上移除时，扰动对土壤性质的影响是不确定的。此外，实验室试验中使用的频率通常比地震工程中感兴趣的频率高得多。另一种方法是使用现场测试来原位测量阻尼比。然而，尽管现场测试具有优势，但报道的现场衰减测量相对较少。该项目的目标之一是开发和实施使用表面波进行阻尼测量的现场测试程序。与井间和井下技术相比，面波测量有几个优点，其中一个优点是它是非侵入性的。另一个目标是更好地了解实验室和现场测量之间经常观察到的阻尼值差异的原因。该项目包括三个任务：1。为了获取南卡罗来纳州艾肯萨凡纳河站点的表面波衰减数据，使用几种不同类型的输入运动（扫描正弦，随机和瞬态）来确定哪种运动提供了例如最佳的信噪比。衰减数据也将记录在几个不同的源接收器组合，以评估近场波的影响。2. 开发和评估以下算法：(i)估计粒子运动振幅谱，（ii）从观测到的表面波振幅计算衰减参数，以及（iii）反演衰减数据以获得阻尼比。在整个任务中，重点将放在量化每一步所涉及的不确定性上，目的不仅是确定最可能的阻尼值，而且是确定值的可能范围。3. 作为任务1和任务2的一部分，一旦制定了适当的现场程序和分析方法，并且为了将表面波阻尼值与现场钻孔方法和实验室测试获得的值进行比较，就可以在旧金山湾的金银岛国家岩土工程试验场和受北岭地震影响的洛杉矶地区的一个或多个地点进行表面波衰减测量。＊＊＊

9402358 Rix An important soil parameter that controls the amplification of seismic waves as they propagate from bedrock to the surface is the hysteretic damping ratio of the soil. Damping is usually measured on soil samples tested in the laboratory, but the effect of the disturbance on the soil properties when the sample is removed from the ground is uncertain. Moreover, the frequencies used in the laboratory tests are usually much higher than those of interest in earthquake engineering. An alternative approach is to use field tests to measure the damping ratio in situ. However, despite the advantages of in situ tests, there have been relatively few field attenuation measurements reported. One of the objectives of this project is to develop and implement an insitu test procedure for damping measurements using surface waves. Surface wave measurements offer several advantages compared to crosshole and downhole techniques, one being that they are non-invasive. Another objective is to understand better the causes of the frequently-observed differences in the damping values between laboratory and in situ measurements. The project involves three tasks: 1. To acquire surface wave attenuation data at sites within the Savannah River Site in Aiken, South Carolina, using several different types of input motion (swept-sine, random, and transient) to determine which one provides, for example, the best signal-to-noise ratio. Attenuation data will also be recorded at several different source-receiver combinations to evaluate the influence of near-field waves. 2. To develop and evaluate algorithms for: (i) estimating particle motion amplitude spectra, (ii) calculating attenuation parameters from observed surface wave amplitudes, and (iii) inverting attenuation data to obtain damping ratio. Throughout this task, emphasis will be placed on quantifying the uncertainties involved in each step, with the aim of establishing not only the most likely value of damping , but also the possible range of values. 3. Once appropriate field procedures and analysis methods have been developed as part of Tasks 1 and 2, and in order to compare surface wave damping values with values obtained from in situ borehole methods and laboratory tests, to perform surface wave measurements of attenuation at the Treasure Island National Geotechnical Experimentation Site in San Francisco Bay, and at one or more sites in the Los Angeles area affected by the Northridge earthquake. ***