江西数字地震台网近震震级偏差与量规函数关系的研究

采用江西数字地震台网数字地震记录资料,选取自2007年10月正式运行至2009年12月所记录的138次ML≥1.5的地震事件,计算了各台站单台震级与台网平均震级的偏差、平均偏差和标准偏差,从震级偏差频次分布、量规函数、平均周期等因素对单台震级的影响上作了定量统计和分析。结果表明:①除九江、赣州、石城、都昌、高安5个台的震级与台网的平均震级偏差比较大外,其余台站均比较小;②目前所用量规函数在震中距小于40 km时偏小,大于330 km时偏大,给出了量规函数校正值,③平均周期小于0.17 s时,变得极不稳定,离散程度明显增大,这种情况应该是由于单台震中距超过300 km的样本数太少所导致的。     

Application of Spectral Decomposition to Detection of Fracture-Cavity Carbonate Reservoir Beds in the Tahe Oiifieid, Tarim Basin, NW China

Ordovician fracture-cavity carbonate reservoir beds are the major type of producing formations in the Tahe oilfield, Tarim Basin. The seismic responses of these beds clearly changes depending on the different distance of the fracture-cavity reservoir bed from the top of the section. The seismic reflection becomes weak or is absent when the fracture-cavity reservoir beds are less than 20 ms below the top Ordovician. The effect on top Ordovician reflection became weaker with deeper burial of fracture-cavity reservoir beds but the developed deep fracture-cavity reservoir beds caused stronger reflection in the interior of the Ordovician. This interior reflection can be divided into strong long-axis, irregular and bead string reflections, and was present 80 ms below the top Ordovician. Aimed at understanding reflection characteristics, the spectral decomposition technique, which uses frequency to ＂tune-in＂ bed thickness, was used to predict Ordovician fracture-cavity carbonate formations in the Tahe oilfield. Through finely adjusting the processing parameters of spectral decomposition, it was found that the slice at 30 Hz of the tuned data cube can best represent reservoir bed development. Two large N-S-trending strong reflection belts in the mid-western part of the study area along wells TK440- TK427-TK417B and in the eastern part along wells TK404-TK409 were observed distinctly on the 30 Hz slice and 4-D time-frequency data cube carving. A small N-S trending reflection belt in the southern part along wells T403-TK446B was also clearly identified. The predicted reservoir bed development area coincides with the fracture-cavities connection area confirmed by drilling pressure testing results. Deep karst cavities occur basically in three reservoir bed-development belts identified by the Ordovician interior strong reflection. Spectral decomposition proved to be a useful technique in identifying fracture-cavity reservoir beds.     

Feasibility of borehole ambient noise interferometry for permanent reservoir monitoring

The analysis of seismic ambient noise acquired during temporary or permanent microseismic monitoring campaigns (e.g., improved/enhanced oil recovery monitoring, surveillance of induced seismicity) is potentially well suited for time‐lapse studies based on seismic interferometry. No additional data acquisition required, ambient noise processing can be automatized to a high degree, and seismic interferometry is very sensitive to small medium changes. Thus there is an opportunity for detection and monitoring of velocity variations in a reservoir at negligible additional cost and effort. Data and results are presented from an ambient noise interferometry study applied to two wells in a producing oil field in Romania. Borehole microseismic monitoring on three component geophones was performed for four weeks, concurrent with a water‐flooding phase for improved oil recovery from a reservoir in ca. 1 km depth. Both low‐frequency (2 Hz–50 Hz) P‐ and S‐waves propagating through the vertical borehole arrays were reconstructed from ambient noise by the virtual source method. The obtained interferograms clearly indicate an origin of the ambient seismic energy from above the arrays, thus suggesting surface activities as sources. It is shown that ambient noise from time periods as short as 30 seconds is sufficient to obtain robust interferograms. Sonic log data confirm that the vertical and horizontal components comprise first arrivals of P‐wave and S‐waves, respectively. The consistency and high quality of the interferograms throughout the entire observation period further indicate that the high‐frequency part (up to 100 Hz) represents the scattered wave field. The temporal variation of apparent velocities based on first‐arrival times partly correlates with the water injection rate and occurrence of microseismic events. It is concluded that borehole ambient noise interferometry in production settings is a potentially useful method for permanent reservoir monitoring due to its high sensitivity and robustness.     

相控震源定向地震波信号分析

应用可控震源地震勘探,当环境噪声很强,采用组合震源工作仍不能满足信噪比要求时,引入能形成定向地震波的相控震源.由相控震源定向照明地震理论分析,主波束方向上3单元相控震源产生的反射地震波信号信噪比比单震源高6.53~9.54 dB,比组合震源高1.76~4.77dB.为研究相控地震实际效果,在同一测区进行了三种震源地震对比实验.由单炮地震记录和水平叠加时间剖面可知,相控震源反射波信号信噪比明显高于单震源情况,略高于组合震源情况.进一步对反射波信号功率谱特性做定量分析,得到如下结果：与单震源情况相比,相控震源使各道反射波信号信噪比提高了0.75~8.15 dB,平均提高3.65 dB;与组合震源情况相比,各道信噪比提高了0.93~3.17 dB,平均提高2.02 dB,实验结果与理论分析吻合.综上所述可知,基于相控震源的定向照明地震技术是可行的,可以有效提高地震信号的信噪比.     

九江-瑞昌序列震源位置及震源区速度结构的联合反演

采用震源位置和速度结构的联合反演方法确定2005年九江一瑞昌5.7级地震序列的分布和震源区的速度结构.结果显示九江一瑞昌地震序列存在NW330.方向和NE550方向的两组共轭分布,其中NW330.方向的优势分布更为明显;震源深度分布在3～18km的范围.地震主体破裂发生在上地壳-中地壳过渡层的P波速度低速区内,主震发生后余震向东南与西北方向两头扩展,其中往西北方向破裂时可能遇到了障碍体,积累的能量转向西南-北东方向继续破裂,引发了4.8级强余震.     

豫北地区地震活动性及其与区域地壳运动的关系

本根据地震地质和地震分布条带,把豫北地区从西至东分为弱,中,强三个区段。由于区域地壳深部主断裂和大震活动的牵动作用,是造成本地震活动的重要原因,因而从区域地壳块体活动背景上探讨了本区及邻近地区地震活动趋势,中还指出,豫北西部太行山区的林县－薄壁地震带,虽不具有强震构造背景,但小震年月频率多寡和震群活动,却能反映了大区域地壳应力场强弱,亦可作为判断区域性地震活动的指标。     

模拟地震波传播的三维逐元并行谱元法

高效地震波场正演模拟对于复杂模型中地震波传播与成像研究至关重要.本文在谱元法原理框架内,对已有逐元谱元法改进,提出一种新的逐元并行谱元法求解三维地震波运动方程,并得到地震波场.逐元并行谱元法的核心思想在于在单元上进行质量矩阵与解向量的乘积运算,并将此运算平均分配至每一个CPU计算核心,此处理有利提升谱元法的并行计算效率.同时,根据Gauss-Lobatto-Legendre (GLL)数值积分点与插值点重合的特点,将稠密单元刚度矩阵的存储转化成单元雅克比矩阵行列式的值及其逆的存储,大幅减少谱元法计算内存开销.此外,在模型边界上利用逐元并行谱元法求解二阶位移形式完美匹配层(PML)吸收边界条件,消除边界截断而引入的虚假反射.通过逐元并行谱元法得到的数值解与解析解对比,以及实际地震波场模拟,数值结果证实了逐元并行谱元法用于地震波场模拟的高效性.

     

地震非均匀度在华北地震中短期预报中的应用

利用空间扫描方法研究了华北地区中强以上地震前地震非均匀度GL值的中短期异常变化特征，结果表明在中强地震前1年左右，未来震中周围的GL值中期异常区通常出现明显的收缩或消失，一些地震临震前数月还再次出现GL值异常区，讨论了使用GL值进行地震中短期预报的有关问题。     

带阻尼的一维地震反射波成像

本文是文献［８］的继续，文中提出了地震勘探的带阻尼波动方程反演的新方法，应用此反演算法可以通过微机识别地层的波阻抗，这里把地层分成１００层，地震数据含有１０％的误差，且反演计算时间仅仅是正演计算时间的三倍．文献［１］－［４］的计算方法是基于波动方程的混合问题，而文献［８］所提出的反演算法是基于波动方程的哥西问题，许多例子充分说明后者更有效．     

Seismic structure of the Helan–Liupan–Ordos western margin tectonic belt in North-Central China and its geodynamic implications

We study high-resolution three-dimensional P-wave velocity (Vp) tomography and anisotropic structure of the crust and uppermost mantle under the Helan–Liupan–Ordos western margin tectonic belt in North-Central China using 13,506 high-quality P-wave arrival times from 2666 local earthquakes recorded by 87 seismic stations during 1980–2008. Our results show that prominent low-velocity (low-V) anomalies exist widely in the lower crust beneath the study region and the low-V zones extend to the uppermost mantle in some local areas, suggesting that the lower crust contains higher-temperature materials and fluids. The major fault zones, especially the large boundary faults of major tectonic units, are located at the edge portion of the low-V anomalies or transition zones between the low-V and high-V anomalies in the upper crust, whereas low-V anomalies are revealed in the lower crust under most of the faults. Most of large historical earthquakes are located in the boundary zones where P-wave velocity changes drastically in a short distance. Beneath the source zones of most of the large historical earthquakes, prominent low-V anomalies are visible in the lower crust. Significant P-wave azimuthal anisotropy is revealed in the study region, and the pattern of anisotropy in the upper crust is consistent with the surface geologic features. In the lower crust and uppermost mantle, the predominant fast velocity direction (FVD) is NNE–SSW under the Yinchuan Graben and NWW–SEE or NW–SE beneath the Corridor transitional zone, Qilian Orogenic Belt and Western Qinling Orogenic Belt, and the FVD is NE–SW under the eastern Qilian Orogenic Belt. The anisotropy in the lower crust may be caused by the lattice-preferred orientation of minerals, which may reflect the lower-crustal ductile flow with varied directions. The present results shed new light on the seismotectonics and geodynamic processes of the Qinghai–Tibetan Plateau and its northeastern margin.