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991.
Seismic imaging is an important step for imaging the subsurface structures of the Earth. One of the attractive domains for seismic imaging is explicit frequency–space (f – x) prestack depth migration. So far, this domain focused on migrating seismic data in acoustic media, but very little work assumed visco‐acoustic media. In reality, seismic exploration data amplitudes suffer from attenuation. To tackle the problem of attenuation, new operators are required, which compensates for it. We propose the weighted L 1 ‐error minimisation technique to design visco‐acoustic f – x wavefield extrapolators. The L 1 ‐error wavenumber responses provide superior extrapolator designs as compared with the previously designed equiripple L 4 ‐norm and L∞‐norm extrapolation wavenumber responses. To verify the new compensating designs, prestack depth migration is performed on the challenging Marmousi model dataset. A reference migrated section is obtained using non‐compensating f – x extrapolators on an acoustic dataset. Then, both compensating and non‐compensating extrapolators are applied to a visco‐acoustic dataset, and both migrated sections are then compared. The final images show that the proposed weighted L 1 ‐error method enhances the resolution and results in practically stable images. 相似文献
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993.
Deamplification of strong motion and the increase of the effective period of soil deposits are typical nonlinear effects; we seek them in SMART1-array data by applying the horizontal-to-vertical spectral ratio (HVSR) technique. The recordings, from four soil and one rock stations, represent 23 earthquakes (ML 4.9–7.0); PGA varies between 20–260 cm/s2. For each station, mean HVSR curves are calculated for two PGA ranges: <75 cm/s2 and >100 cm/s2 (weak and strong motion). At the soil stations, the “weak” (linear) and “strong” (nonlinear) responses are significantly different. Below 1–1.8 Hz, the nonlinear response exceeds the linear one. Above 2 Hz, the nonlinear response drops below the linear one and above 4–6 Hz below unity (deamplification). From 10 to 16 Hz, the two responses converge. One soil site shows significant negative correlation between resonance frequency and ground acceleration. Such behaviour agrees with other empirical studies and theoretical predictions. Our results imply that the HVSR technique is sensitive to ground-motion intensity and can be used to detect and study nonlinear site response. 相似文献
994.
2017年精河MS6.6地震前地磁异常特征分析 总被引:1,自引:0,他引:1
2017年8月9精河发生MS6.6地震,距震中300km范围内的温泉台、克拉玛依台、乌鲁木齐台捕捉到此次地震前的地磁异常信息,本文对此进行了分析和总结,结论如下:①地磁谐波振幅比反应了观测点深部电阻率变化,2016年呼图壁MS6.2、此次精河MS6.6地震均发生在克拉玛依台地磁谐波振幅比异常下降之后的转折和恢复阶段;②震前83天、21天沿北天山断裂带分布的地磁台站逐日比和加卸载响应比均成组出现超限的高值,且异常高值的空间分布与震中位置有一定关联;③全国大陆地磁台站震前34天出现低点位移突变分界线,且异常集中于震中附近。 相似文献
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998.
农田蒸散量计算方法的比较研究 总被引:8,自引:1,他引:8
根据观测资料,采用波文比法、廓线梯度法和综合阻地等分别计算了农田植被层上的蒸散量,并对比分析了不同方法之间的相对误差和相关程度。结果表明,用波文比法计算的潜热通量存在系统性偏小的现象,而工法和综合阻抗法的计算结果比较一致。文中还讨论了不同方法的优缺点和产生误差的可能原因。 相似文献
999.
基于相关函数理论,提出一种新的计算动三轴试验动模量和阻尼比的方法。假设土体为黏弹性体,采用自相关函数分析应力、应变波形的平均幅值,进而计算动模量;采用互相关函数分析应变滞后于应力时程波形的相位差,进而计算阻尼比。利用饱和珊瑚砂、南京细砂和原状粉质黏土的不排水应变控制分级循环加载试验数据,分析结果表明:动模量和阻尼比计算的相关函数法对不同土样具有普适性;应变幅值小于1×10?4时,相关函数法计算动模量及阻尼比的精度明显优于传统的滞回圈法;应变幅值大于1×10?3时,土体呈现强非线性特性,应力-应变滞回圈不对称,相关函数法计算的阻尼更为可靠。 相似文献
1000.
Formation and development of alternate bars in an engineered mountainous reach of the Arc River, France, is studied using photo analysis, 1D modelling and by applying theoretical and empirical models for alternate bar systems. Alternate bars already existed in the 80s in the form of a stable confined wandering system. In 1994, the river bed was flattened after engineering works. However, aerial photographs and cross-sectional profiles show that bars rapidly recovered within a few years. The alternate bar system evolved rapidly with a reduction of the number of bars and so an increasing bar length. The width-to-depth ratio, the slope change, the bend upstream of the reach, and the sediment supplies are the main controls of bar formation and evolution. The system appears to lead to force bars due to the bend but also due to a bridge in the downstream part of the reach. Nevertheless some free mobile bars are still observed in the middle of the reach. A discussion on the alternate bar formation is provided using empirical and analytical models. Finally, impacts of low flows and vegetation seem to be significant in the stabilization of the system toward a confined wandering system as observed before the engineering works. 相似文献