声波与表层土壤水分含量变化的实验研究

为研究地面表层土壤中水分含量变化与震前记录到微震和极微震的关系，进行了声波与表层土壤含水量变化的实验．实验的目的是探讨震前微破裂引起的声波能否改变土壤表层的水分含量，从而建立起遥感图像中的热异常信息与孕震关系．通过实验发现，声波对土壤作用使得原本自然蒸发呈降低趋势的湿度值呈上升趋势，而土壤表面的温度呈下降趋势．      

岩石破裂与氡含量变化关系的实验研究

为探索氡含量的变化能否视作地震前兆,作者进行了岩石破裂与氡含量变化关系的模拟实验。实验结果表明,可以认为岩石破裂是引起地震之前氡含量异常的原因之一。文末,根据实验结果对在地震区某些实际观测中的问题进行了初步讨论。     

Evolution of the shear wave velocity during shaking modeled in centrifuge shaking table tests

Two in-flight shear wave velocity measurement systems were developed to perform the subsurface exploration of shear wave velocity in a centrifuge model. The bender elements test and the pre-shaking test used in the study provided reliable and consistent shear wave velocity profiles along the model depth before and after shaking in the centrifuge shaking table tests. In addition, the use of the bender elements measurement system particularly developed here allowed continuous examination of the evolution of shear wave velocity not only during and after the shaking periods in the small shaking events but also during the dissipation period of excess pore water pressure after liquefaction in the large shaking events. The test results showed that the shear wave velocity at different values of excess pore water pressure ratio varied as the effective mean stress to the power of 0.27, to a first approximation. Consequently, a relationship between the shear wave velocity evolution ratio and the excess pore water pressure ratio is proposed to evaluate the changes in shear wave velocity due to excess pore water generation and dissipation during shaking events. This relation will assist engineers in determining the shear stiffness reduction ratio at various r_u levels when a sand deposit is subjected to different levels of earthquake shaking.     

Acoustic parameters inversion and sediment properties in the Yellow River reservoir

The physical properties of silt in river reservoirs are important to river dynamics. Unfortunately, traditional techniques yield insufficient data. Based on porous media acoustic theory, we invert the acoustic parameters for the top river-bottom sediments. An explicit form of the acoustic reflection coefficient at the water–sediment interface is derived based on Biot’s theory. The choice of parameters in the Biot model is discussed and the relation between acoustic and geological parameters is studied, including that between the reflection coefficient and porosity and the attenuation coefficient and permeability. The attenuation coefficient of the sound wave in the sediments is obtained by analyzing the shift of the signal frequency. The acoustic reflection coefficient at the water–sediment interface is extracted from the sonar signal. Thus, an inversion method of the physical parameters of the riverbottom surface sediments is proposed. The results of an experiment at the Sanmenxia reservoir suggest that the estimated grain size is close to the actual data. This demonstrates the ability of the proposed method to determine the physical parameters of sediments and estimate the grain size.     

2004年印尼8.7级强震前昌黎台的远场电性异常特征探讨

分析了在2004年12月26日印尼8.7级地震前河北昌黎台记录到的自然电位及电场异常, 结果显示昌黎台自电位、 电场正常时段观测到的半日、 半月潮汐波周期变化在震前1个多月发生畸变, 周期消失, 变化幅度相应减小, 震后快速恢复, 且潮汐波记录比震前正常时段更加清晰。 该台地电阻率的观测结果显示起始下降变化比较平缓, 震前一周左右, 变化加剧, 出现多次阶变。 究其原因, 在强震发生前断层的蠕滑作用导致孔隙压力变化, 产生一种可以在岩石中传播的应变波, 由于台站下方岩土层对小应变信号具有很好的放大作用, 所以记录到了这种小幅度的应变波, 也就是远场的静态应变效应。 随着地震临近, 应力作用加强, 远程应变波引起台站下方水位、 水化学性质以及岩石孔隙度、 渗透率等的改变, 导致电阻率、 电场大幅度的阶变, 这反映了远场应变波的动态效应。 当然就异常幅度而言, 应变波的效应还嫌弱一些, 因此昌黎台异常变化可能还叠加了震源区激发的电磁波。     

震前地声与动物异常关系的研究

本文根据近十年来我国地声观测的某些结果, 分析了地声的时-空分布特点, 并估计了透入基岩面上和水面中的声压.同时与震前动物异常特点, 以及与人和某些动物的听觉和振动感受器的敏感阈值进行了比较.最后, 对唐山余震期间同时段内鱼和地声活动的观察结果, 进行了相关分析.其结果如下:1.地声和动物异常的时-空分布图式基本相似, 并可认为都是临震前兆.2.三级左右地震震中附近地区, 地声透入基岩面上和水面中的声压, 在150——250赫芝频段内, 分别为23——19分贝和90——86分贝.由此可见, 人耳是不可能感觉到的, 但是通常在地面上栖息的猫、狗、雉和穴居的鼠是可能觉察到的, 而骨鳔鱼类可能优先于这些陆生动物觉察到水中的地声信息.同时, 基岩面位移的垂直振幅约为10-7厘米.这虽比某些动物的振动感受器的阈振幅约低3个数量级.但可推测, 在强烈地震前的地面位移的垂直振幅可达到或超过微米量级.这就可能为这些动物的振动感受器觉察到.3.相距约20公里的鱼和地声活动观察点, 同期内的观察结果表明, 它们共同反映了同一地区内的唐山余震活动.同时, 鱼和地声活动的日频度序列之间具有一定的互相关性.      

回填土不确定性对场地地震反应的影响

选取某核电场地控制性钻孔的厚度、剪切波速、密度等实际勘探数据,通过改变回填土剪切波速,分析了回填土不确定性对场地地震动参数的影响。研究结果表明:在回填土层厚度不变和模型总厚度不变的情况下,地表的水平向峰值加速度随着回填土剪切波速的增大而减小,但水平向峰值加速度增幅逐渐减小;回填土剪切波速到达一定的波速就不再影响地表水平峰值加速度;随着回填土剪切波速的增加,整个反应谱的谱值都普遍减小。     

Soil macroporosity and infiltration characteristics of a forest podzol

Spatial distribution of soil macroporosity was determined for a forest podzol from tension infiltrometer measurements at the soil surface. Surface‐derived macroporosity values were compared with point infiltration characteristics obtained from soil water content and soil water chemistry measurements during an experimental irrigation, and with parameters of a kinematic wave model applied to soil water content data. Macroporosity estimated by the tension infiltrometer ranged from 0·00087 to 0·0219% of soil volume, and infiltration at these two sites was dominated by propagation of a well‐defined wetting front through the soil profile and bypass flow via soil macropores, respectively. Infiltration at sites with intermediate macroporosities reflected a combination of these two processes, although results were inconclusive at one site owing to lateral flow at the base of the soil profile. There was no agreement between macroporosities estimated by the tension infiltrometer and the kinematic wave model. The maximum soil conductance parameter within the profile at a site, however, was related directly to the surface‐derived macroporosity. The partial agreement between surface‐derived macroporosity estimates and point infiltration characteristics shown here supports the use of tension infiltrometry as a rapid, non‐destructive method of assessing spatial variations in the relative contribution of macropore flow to the infiltration process. Copyright © 2000 John Wiley & Sons, Ltd.     

拉萨地震台PET重力仪同震响应特征

从面波延迟时间、初动方向、波幅、同震持续时间等4个方面研究拉萨台PET重力仪的同震响应特征,结果表明：（1）同震响应的面波延迟时间与地震震中距离具有较好正相关性;（2）初动方向没有规律;（3）同震响应的波幅不仅与地震震级大小有关,还与震中距远近有关;（4）远震的同震波一般表现为脉冲形式,近震除了脉冲外,还有阶跃;（5）同震波的延续时间与震级没有明确的比例关系.这有助于对重力仪器进行深入的了解,可以为未来西藏地区监测台网布设方案提供参考数据.     

论地声序列在短临预报中的应用和意义

本文在研究宝坻地声序列特点的基础上,进一步探讨了地声序列在短临预报中的应用和意义。结果表明,宝坻地声现场观测结果与地震活动性和岩石力学实验结果非常一致。     

地下水位变化对地震地面运动的影响

本文以Biot提出的流体饱和多孔介质波动理论为基础, 建立了成层地基模型, 把地下水位以下的饱和土层用水饱和多孔介质模拟, 地下水位以上土层用气饱和多孔介质模拟. 通过研究入射平面简谐波在成层地基中的传播, 分析了地下水位变化对地震地面运动的影响. 结果表明: P波入射下, 当土体骨架相对刚度较小时, 地下水位变化对地表位移尤其是竖向地表位移幅值影响较大, 随着地下水位的下降, 竖向位移逐渐增加, 相对应的峰值频率逐渐减小; 当土体骨架相对刚度较大时, 地下水位变化对地面运动影响不大.      

震级频度与古登堡-里克特关系式偏离的前兆意义

地震震级频度的古登堡-里克特关系式(简称G-R关系式)一直是许多地震学问题研究的重要前提之一, 然而实际分布常常与这一关系式有一定的偏离。 文中利用已有的对含有凹凸体、 挤压雁列破裂及Ⅲ型剪切破裂等三种类型的岩石实验结果, 对接收到的声发射事件序列分别进行了分段地震震级频度的G-R关系式拟合, 重点考察了每一时间窗内表征实际地震震级频度与这一关系式偏离程度的线性拟合系数r值的变化。 研究结果表明, 含有凹凸体、 挤压雁列破裂的岩石声发射序列在加压－破裂过程中, 分段扫描分别计算得出的r值在标本失稳前出现数字下降的现象, 表明临近岩石失稳, 实际地震震级频度与G-R关系式偏离程度有加大的趋势; 而含Ⅲ型剪切破裂的岩石在标本失稳前, 声发射序列r值数字下降的现象不明显。 部分实际震例也表明, 在一些中强地震前也存在着这种偏离。 研究结果可能对利用区域地震活动资料探求部分类型的中强地震前可能存在的前兆信息具有一定的参考意义。     

Study of water tension differences in heterogeneous sandy soils using surface ERT

Herbaceous vegetation in the Sahel grows almost exclusively on sandy soils which preferentially retain water through infiltration and storage. The hydrological functioning of these sandy soils during rain cycles is unknown. One way to tackle this issue is to spatialize variations in water content but these are difficult to measure in the vadose zone. We investigated the use of Electrical Resistivity Tomography (ERT) as a technique for spatializing resistivity in a non-destructive manner in order to improve our knowledge of relevant hydrological processes. To achieve this, two approaches were examined. First, we focused on a possible link between water tension (which is much easier to measure in the field by point measurements than water content), and resistivity (spatialized with ERT). Second, because ERT is affected by solution non-uniqueness and reconstruction smoothing, we improved the accuracy of ERT inversion by comparing calculated solutions with in-situ resistivity measurements. We studied a natural microdune during a controlled field experiment with artificial sprinkling which reproduced typical rainfall cycles. We recorded temperature, water tension and resistivity within the microdune and applied surface ERT before and after the 3 rainfall cycles. Soil samples were collected after the experiment to determine soil physical characteristics. An experimental relationship between water tension and water content was also investigated. Our results showed that the raw relationship between calculated ERT resistivity and water tension measurements in sand is highly scattered because of significant spatial variations in porosity. An improved correlation was achieved by using resistivity ratio and water tension differences. The slope of the relationship depends on the soil solution conductivity, as predicted by Archie's law when salted water was used for the rain simulation. We found that determining the variations in electrical resistivity is a sensitive method for spatializing the differences in water tension which are directly linked with infiltration and evaporation/drainage processes in the vadose zone. However, three factors complicate the use of this approach. Firstly, the relation between water tension and water content is generally non-linear and dependent on the water content range. This could limit the use of our site-specific relations for spatializing water content with ERT through tension. Secondly, to achieve the necessary optimization of ERT inversion, we used destructive resistivity measurements in the soil, which renders ERT less attractive. Thirdly, we found that the calculated resistivity is not always accurate because of the smoothing involved in surface ERT data inversion. We conclude that further developments are needed into ERT image reconstruction before water tension (and water content) can be spatialized in heterogeneous sandy soils with the accuracy needed to routinely study their hydrological functioning.     

Canopy cover effects on local soil water dynamics in a tropical agroforestry system: Evaporation drives soil water isotopic enrichment

Despite the widely held assumption that trees negatively affect the local water budget in densely planted tree plantations, we still lack a clear understanding of the underlying processes by which canopy cover influences local soil water dynamics in more open, humid tropical ecosystems. In this study, we propose a new conceptual model that uses a combination of stable isotope and soil moisture measurements throughout the soil profile to assess potential mechanisms by which evaporation (of surface soil water and of canopy‐intercepted rainfall) affects the relationship between surface soil water isotopic enrichment (lc‐excess) and soil water content. Our conceptual model was derived from soil water data collected under deciduous and evergreen plants in a shade grown coffee agroforestry system in Costa Rica. Reduced soil moisture under shade trees during the “drier” season, coinciding when these trees were defoliated, was largely the result of increase soil water evaporation as indicated by the positive relationship between soil water content and lc‐excess of surface soil water. In contrast, the evergreen coffee shrubs had a higher leaf area index during the “drier” season, leading to enhanced rainfall interception and a negative relationship between lc‐excess and soil water content. During the wet season, there was no clear relationship between soil water content and between lc‐excess of surface soil water. Greater surface soil water under coffee during the dry season may, in part, explain greater preferential flow under coffee compared with under trees in conditions of low rainfall intensities. However, with increasing rainfall intensities during the wet season, there was no obvious difference in preferential flow between the two canopy covers. Results from this study indicate that our new conceptual model can be used to help disentangling the relative influence of canopy cover on local soil water isotopic composition and dynamics, yet also stresses the need for additional measurements to better resolve the underlying processes by which canopy structure influences local water dynamics.     

Dependence of borehole shear‐horizontal‐wave seismoelectric response on soil textures

The phenomenon of acoustic waves inducing electric fields in porous media is called the seismoelectric effect. Earlier investigators proposed the usage of seismoelectric effect for well logging. Soil texture has a strong influence on the coupled wave fields during shallow surface explorations. In this article, we study the borehole pure shear‐horizontal wave and the coupling transverse‐electric field (acoustic–electrical coupling wave fields) in the partially saturated soil. Combined with related theories, we expand the formation parameters to partially saturated forms and discuss the influence of soil texture conditions on the seismoelectric wave fields. The results show that the elastic and electrical properties of porous media are sensitive to water saturation. The compositions of the acoustic and electric fields for different soil textures do not change, but the waveforms differ. We also use the secant integral method to simulate the interface‐converted electromagnetic waves. The results show that interface response strength is greatly influenced by soil texture. In addition, considering the sensitivity of the inducing electric field to fluid salinity, we also simulate the time‐domain waveforms of electric field for different pore fluid salinity levels. The results show that as the salinity increases, the electric field amplitude decreases monotonically. The above conclusions have certain significance for the application of borehole shear wave and its coupled electric fields for resource exploration, saturation assessment and groundwater pollution monitoring.     

强地震引起的同震形变响应

以华北地区地倾斜、地应变数字化观测为基础,研究了形变同震响应的分布特征,给出了地震面波响应延迟时间、响应幅度、同震信号持续时间(在可分辨的范围内)等特征与地震震中距离及震级之间的关系,同时还探讨了短临形变前兆与未来地震之间可能存在的关系及地震短临预测面临的科学问题。     

面波测试在汶川强震区土体损伤调查中的应用

汶川地震后,确定地震对岩土体扰动的范围和损伤程度,是预测震后次生山地灾害演化趋势,以及制定道路修复加固工程对策的重要依据。面波勘察是通过获取剪切波速变化情况,推断土体扰动范围的快捷手段。本文在位于汶川地震强震区的213国道都汶段内的2个边坡工点和3个路堤工点,布设6个测段进行了高精度面波浅层地震勘查。结果表明：①9度和10度地震烈度区土体扰动的深度无明显差别;②在汶川地震强震区,人工填筑的路堤工程所造成的强扰动区深度不小于5 m,自然土体造成的强扰动区深度为0-5 m;③路堤工程表面填、挖交界处的损伤明显。上述结论可供灾后道路修复工程设计中确定边坡加固深度和路堤翻挖深度时参考。     

A review of rock mechanics studies in the United States pertinent to earthquake prediction

Premonitory phenomena such as dilatancy, creep, acoustic emission, and changes in seismic velocity and attenuation, electrical resistivity, magnetic moment, and gas emission, which occur before fracture of initially intact rock and before stick-slip on faults or between finely ground surfaces of rock, have been reviewed and discussed in relation to earthquake prediction. This review is restricted to the results of laboratory experiments that have been carried out in the United States of America.     

不连续岩石试件加载变形与破坏前后声发射波形的线性度分析

对单节理岩石试件加载变形破坏过程中的声发射波形数据使用时间线性度和空间线性度的算法进行了处理分析，研究了所得时间线性度（ｒ）和空间线性度（ａ）作为地震前兆指标的可能性。所得结果表明：时间线性度（ｒ）在不同节理角的岩石试件破坏前和破坏后，都呈现下降；而空间线性度（ａ）也表现为下降，且下降幅度更大一些。在不同节理角岩石变形过程的声发射事件中，最大声发射事件（破坏瞬间）发生之前ｒ和ａ都呈现下降的异常变化     

断层地震孕震的椭圆形区域及其前兆规律

首先运用岩石介质的损伤本构关系式和断层地震的简单力学模型，讨论了断层和围岩介质系统的相互作用及其失稳过程，通过差分计算，得到了与地震变形和声发射等有关的前兆序列曲线，然后根据实际断层带滑移与错动的分布规律，通过计算机模拟出断层地震的椭圆形孕震区并此解释了Ｐ波初动分布规律。