模拟退火方法在三维速度模型地震波走时反演中的应用

采用块状建模以及三角形拼接的界面描述方式,并通过立方体速度网格线性插值获得块体内部的速度分布。正演过程中采用逐段迭代射线追踪方法计算三维复杂地质模型中的射线走时,并采用模拟退火方法进行了三维模型中的地震波走时反演研究。模型测试结果表明,使用的射线追踪和走时反演算法有效。     

安宁河-则木河断裂带及周边地区Rayleigh波群速度背景噪声成像研究

安宁河-则木河断裂带位于川滇地块、巴颜喀拉地块和华南地块的交接部位,是川滇菱形块体的东部重要边界。利用布设在安宁河-则木河断裂带周边区域的西昌台阵和川西台阵均历时两年、共187个宽频带地震台站的垂直分量的背景噪声数据,采用噪声层析成像方法获得了这一区域4~20s的Rayleigh波群速度分布图像。与前人研究相比,本文结果的横向分辨率有明显改进,在安宁河-则木河断裂带可达20km左右,在其它区域可以达到20~40km。成像结果表明,安宁河-则木河断裂地区上地壳的速度结构存在明显横向不均匀性,速度分布特征与地表地质构造基本一致,不同周期的速度分布变化较小。盐源盆地、西昌盆地和四川盆地西南缘表现为低速异常。九龙附近和南部的德昌-盐边-巧家附近表现为高速异常,分别与出露的花岗岩体和峨眉山玄武岩有关。在安宁河断裂南段和则木河断裂北段能观测到断裂两侧的速度存在明显差异,其余断裂带两侧的速度对比不明显。贡嘎山附近的中上地壳表现为明显的低速异常,其东侧和西南侧高速体的阻挡,以及鲜水河-安宁河断裂带走向的变化,在贡嘎山区形成一个挤压弯曲段,使得川滇菱形块体的东南向水平运动转换为垂直于断裂的挤压作用和垂直隆升,导致了贡嘎山的快速隆起。     

Analysis of Anomalous Characteristics of Regional Crustal Deformation before the September 16th, 2019 Zhangye MS5.0 Earthquake, Gansu, China

To study the crustal movement in the vicinity of the epicenter before the Zhangye M_S5.0 earthquake in 2019, the characteristics of crustal deformation before the earthquake are discussed through the GPS velocity field analysis based on the CMONOC data observed from GPS. The baseline time series between two continuous GPS stations and the strain time series of an area among several stations are analyzed in the epicenter area. The resulting time series of baseline azimuth around the epicenter reflects that the energy of the fault in the northern margin of Qilian Mountain is accumulated continuously before 2017. Besides, the movement trend of azimuth slows down after 2017, indicating the stress accumulation on both sides of the seismogenic fault zone has reached a certain degree. The first shear strain and EW-direction linear strain in the epicentral area of the Zhangye M_S5.0 earthquake remain steady after 2017, and the surface strain rate decreases gradually after 2016. It is illustrated that there is an obvious deformation loss at the epicentral region three years before the earthquake, indicating that a certain degree of strain energy is accumulated in this area before the earthquake.     

Impact of user parameters in inversion velocity analysis

Migration velocity analysis is a method devoted to the evaluation of both reflectivity and background velocity models, associated with the high and low wavenumber components of the model, respectively. Inversion velocity analysis is one of its improved versions, leading to more stable background velocity updates. Still, the impact of the user parameters should be understood for an optimal update of the background velocity. We show that a sign reversal of the background velocity gradient could occur when the selected surface offset range or the space lag range is too small. We derive the theoretical limits and check their consistency through simulations in a simple model with a single interface. These guidelines determine the necessary ranges of surface offsets and space lags for a proper update of the background velocity model. We discuss their applicability on the Marmousi model. Artefacts in the retrieved background velocity model are observed when the guidelines are not satisfied.     

The effectiveness of a pseudo-inverse extended Born operator to handle lateral heterogeneity for imaging and velocity analysis applications

Wave equation–based migration velocity analysis techniques aim to construct a kinematically accurate velocity model for imaging or as an initial model for full waveform inversion applications. The most popular wave equation–based migration velocity analysis method is differential semblance optimization, where the velocity model is iteratively updated by minimizing the unfocused energy in an extended image volume. However, differential semblance optimization suffers from artefacts, courtesy of the adjoint operator used in imaging, leading to poor convergence. Recent findings show that true amplitude imaging plays a significant role in enhancing the differential semblance optimization's gradient and reducing the artefacts. Here, we focus on a pseudo-inverse operator to the horizontally extended Born as a true amplitude imaging operator. For laterally inhomogeneous models, the operator required a derivative with respect to a vertical shift. Extending the image vertically to evaluate such a derivative is costly and impractical. The inverse operator can be simplified in laterally homogeneous models. We derive an extension of the approach to apply the full inverse formula and evaluate the derivative efficiently. We simplified the implementation by applying the derivative to the imaging condition and utilize the relationship between the source and receiver wavefields and the vertical shift. Specifically, we verify the effectiveness of the approach using the Marmousi model and show that the term required for the lateral inhomogeneity treatment has a relatively small impact on the results for many cases. We then apply the operator in differential semblance optimization and invert for an accurate macro-velocity model, which can serve as an initial velocity model for full waveform inversion.     

Attenuation Characteristics of Earthquake Ground Motion for Large Volume Airgun

In order to further deepen the understanding of seismic wave propagation characteristics induced by the large volume airgun source, experimental data from multiple fixed excitation points in Fujian Province were used to obtain the equivalent single excitation high signal-to-noise ratio velocity and displacement records through linear stacking and simulation techniques. Then the peak displacements of different epicentral distances were used to calculate the equivalent magnitude of the airgun source excitation at different fixed excitation points so as to establish the attenuation relationship between equivalent magnitude,epicenter distance and velocity peak. Our results show that:① Within 270 km of epicentral distance,for the large-volume airgun's single shot,the peak velocity range is about 700-4 nm/s,and the peak displacement range is about 200. 0-0. 2 nm;② The equivalent magnitude of the P-wave from the airgun source with a total capacity of 8,000 in 3 is 0. 181-0. 760,and the equivalent magnitude of the S-wave is 0. 294-0. 832. By contrast,the equivalent magnitude of the P-wave from the airgun source with a total capacity of 12,000 in 3 is 0. 533-0. 896,and the equivalent magnitude of the S-wave is 0. 611-0. 946. The S-wave energy is greater than the P-wave energy, and the excitation efficiency varies greatly with different excitation environment;③ The peak velocity increases with the equivalent magnitude,and decreases with the epicentral distance. The vertical component of the P-wave peak velocity is the largest among those three components,while the S-wave has the smallest vertical component and similar horizontal components. Hence,our research can provide an important basis for the quantitative judgment of the seismic wave propagation distance using the airgun and the design of the observation system in deep exploration or monitoring with airgun.     

晋冀蒙交界地区最小一维速度模型联合反演

文章使用内蒙古地震台网记录到的晋冀蒙交界地区517个定位地震,在参考原始编目报告的基础上,用和达法拟合走时曲线删除错误数据,使用Kissling方法联合反演得到晋冀蒙交界地区最小一维P波速度模型和震源位置。结果显示,研究区P波速度随着深度增加而递增,无明显间断面,联合反演得到的震中位置较原始报告分布更集中,簇状分布更明显。     

利用断裂带首波分析甘孜—玉树断裂带拉张盆地结构特征

对断裂带及附近区域地层精细结构的描述是理解地震孕育和发生过程的基础.不同板块或块体边界在构造或区域应力作用下,常常会形成速度间断面和低速的断裂带,间断面和低速带的存在会产生特殊的断裂带地震波,比如断裂带首波和围陷波,并会影响地震的物理过程、破裂速度和破裂方向等.本文主要利用2010年4月14日M_W6.9玉树地震后布设的流动地震台站,对沿着甘孜—玉树断裂带传播的断裂带首波进行识别和分析.分析结果表明,在甘孜—玉树断裂带的不同区域均观测到了断裂带首波,在西段的结隆拉张盆地附近的3个台站沿断裂带界面的平均纵波速度差异值在5%～8%,而其他区域的平均速度差异为1%～3%.通过台站位置分布和断裂带首波特征关系,初步分析了断裂带拉张盆地的结构,结果显示结隆盆地的长度为～40km,宽度为5.35～5.97km,深度不超过5km,在地表浅层形成了一个低速区,分别与巴颜喀拉块体(NE)和羌塘块体(SW)产生了两个物性差异界面,但没有延伸到主震和余震区震源深度.我们的结果表明结合密集台阵资料,通过断裂带首波特征分析可以为断裂带精细结构及几何特征提供一种新的技术方法和途径.     

Input-output Balances of Nutrients and Plankton in a Flooded Area of the Lower Odra

The retention of particulate matter of the Odra River in flooded areas was estimated by determining suspended particulate matter (SPM) elimination and particle-bound nutrient retention in a polder area of the Lower Odra Valley national park. Water and suspended matter samples collected before, during, and after the 1997 summer flood at the inlet and the outlet of the investigated polder (Polder A/B close to Schwedt) offer the opportunity to balance the matter retention inside the floodplain. The maximum level of retained SPM (more than 80%) was calculated for the record flood of summer 1997, while in ordinary winter floods retention differs between 33% and 70%. Basic properties of the flowing particles like settling velocity, density, loss on ignition etc. change on their way through the polder area. In the investigated Polder A/B a retention of at least 50% of particle-bound phosphorus which was independent of the incoming suspended matter load from the Odra River was observed. The retention of particle-bound nitrogen and carbon varied seasonally to different extents. The presented balance demonstrates that particle-bound nutrient retention in polders is an important factor in the self-purification process of the river system. Some additional effort was done to study changes of plankton composition: during a joint field experiment in spring 1998, the authors determined biological parameters (abundance and biomass of phytoplankton and zooplankton) of water samples at polder inflow and outflow points.     

Experimental study on mechanism of sea-dike failure due to wave overtopping

This work, which was largely a fruit of China's national marine hazard mitigation service, explicitly reveals the major mechanism of sea-dike failure during wave overtopping. A large group of wave-flume experiments were conducted for sea dikes with varying geometric characteristics and pavement types. The erosion and slide of the landward slope due to the combined effect of normal hit and great shear from overtopping flows was identified the major trigger of the destabilization of sea dikes. Once the intermittent hydrodynamic load and swash caused any deformation (bump or dent) of the pavement layer, pavement fractions (slabs or rubble) on the slope started to be initiated and removed by the water. The erosion of the landward slope was then gradually aggravated followed by entire failure within a couple of minutes. Hence, the competent velocity would be helpful evaluate the failure risk if as well accounted in standards or criteria. However, the dike top was measured experiencing the largest hydrodynamic pressure with a certain cap while the force on the wall increased rapidly as the overtopping intensity approached the dike-failure threshold. The faster increase of the force on the wall than on the landward slope yielded the sequencing of loads reaching hypothetic limits before failure as: dike top – top-mounted wall – landward slope. Therefore, beside the slide failure, the fatigue damage due to the instantaneous hydrodynamic impact might be another mechanism of the dike failure, which did not appear in the experiment but should be kept in mind. Instead of the widely adopted tolerable overtopping rate, a 0.117–0.424 m³/(m s) range of overtopping discharge and a 10 m/s overtopping velocity for the failure risk of typical sea dikes along China's coastlines were suggested, which enables the possible failure risk prediction through empirical calculations. The failure overtopping rate was identified strongly dependent on the pavement material, the landward slope and the dike-mounted wall but showed little variation with the width of the dike top. The flat concrete pavement and gentle landward slopes are suggested for the dike design and construction. For given configurations and hydrodynamic conditions in the experiment, the dike without the wall experienced less overtopping volume than those with the 1-m top-mounted wall. Meanwhile, the remove of the wall increased the failure overtopping rate, which means a certain increase of the failure criterion. Thus, care must be taken to conclude that the dike-mounted wall seems not an entirely appropriate reinforcement for the stability and safety of coastal protections. This should be further checked and discussed by researchers and engineers in the future.