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91.
Ali Gholami 《Geophysical Prospecting》2014,62(6):1389-1405
Compressed Sensing has recently proved itself as a successful tool to help address the challenges of acquisition and processing seismic data sets. Compressed sensing shows that the information contained in sparse signals can be recovered accurately from a small number of linear measurements using a sparsity‐promoting regularization. This paper investigates two aspects of compressed sensing in seismic exploration: (i) using a general non‐convex regularizer instead of the conventional one‐norm minimization for sparsity promotion and (ii) using a frequency mask to additionally subsample the acquired traces in the frequency‐space () domain. The proposed non‐convex regularizer has better sparse recovery performance compared with one‐norm minimization and the additional frequency mask allows us to incorporate a priori information about the events contained in the wavefields into the reconstruction. For example, (i) seismic data are band‐limited; therefore one can use only a partial set of frequency coefficients in the range of reflections band, where the signal‐to‐noise ratio is high and spatial aliasing is low, to reconstruct the original wavefield, and (ii) low‐frequency characteristics of the coherent ground rolls allow direct elimination of them during reconstruction by disregarding the corresponding frequency coefficients (usually bellow 10 Hz) via a frequency mask. The results of this paper show that some challenges of reconstruction and denoising in seismic exploration can be addressed under a unified formulation. It is illustrated numerically that the compressed sensing performance for seismic data interpolation is improved significantly when an additional coherent subsampling is performed in the domain compared with the domain case. Numerical experiments from both simulated and real field data are included to illustrate the effectiveness of the presented method. 相似文献
92.
Microseismic monitoring has proven invaluable for optimizing hydraulic fracturing stimulations and monitoring reservoir changes. The signal to noise ratio of the recorded microseismic data varies enormously from one dataset to another, and it can often be very low, especially for surface monitoring scenarios. Moreover, the data are often contaminated by correlated noises such as borehole waves in the downhole monitoring case. These issues pose a significant challenge for microseismic event detection. In addition, for downhole monitoring, the location of microseismic events relies on the accurate polarization analysis of the often weak P‐wave to determine the event azimuth. Therefore, enhancing the microseismic signal, especially the low signal to noise ratio P‐wave data, has become an important task. In this study, a statistical approach based on the binary hypothesis test is developed to detect the weak events embedded in high noise. The method constructs a vector space, known as the signal subspace, from previously detected events to represent similar, yet significantly variable microseismic signals from specific source regions. Empirical procedures are presented for building the signal subspace from clusters of events. The distribution of the detection statistics is analysed to determine the parameters of the subspace detector including the signal subspace dimension and detection threshold. The effect of correlated noise is corrected in the statistical analysis. The subspace design and detection approach is illustrated on a dual‐array hydrofracture monitoring dataset. The comparison between the subspace approach, array correlation method, and array short‐time average/long‐time average detector is performed on the data from the far monitoring well. It is shown that, at the same expected false alarm rate, the subspace detector gives fewer false alarms than the array short‐time average/long‐time average detector and more event detections than the array correlation detector. The additionally detected events from the subspace detector are further validated using the data from the nearby monitoring well. The comparison demonstrates the potential benefit of using the subspace approach to improve the microseismic viewing distance. Following event detection, a novel method based on subspace projection is proposed to enhance weak microseismic signals. Examples on field data are presented, indicating the effectiveness of this subspace‐projection‐based signal enhancement procedure. 相似文献
93.
94.
用“震源触发-系统响应”孕震模式，分析讨论了新疆1996-1998年成组强震活动前后地震活动图像的时空演化特征，由此定义了“诱发信号震”，进而分析研究了诱发信号震与后续主震的关系，结果表明，强震发生后触发（或引起）的中等地震活动区及边缘很可能是未来强震发生的地点；强震或其强余震发生当天或稍后1天内发生的信号震具有预报意义，信号震发生后，其周围200km范围内3年发生强震的可能性较大，其震级可能比信号震的震级高0.5-2.0，最后对成组强震活动的机理进行了分析和讨论。 相似文献
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97.
Logistic regression, neural networks and support vector machines are tested for their effectiveness in isolating surface waves in seismic shot records. To distinguish surface waves from other arrivals, we train the algorithms on three distinguishing features of surface-wave dispersion curves in the domain: spectrum coherency of the trace's magnitude spectrum, local dip and the frequency range for a fixed wavenumber k in the spectrum. Numerical tests on synthetic data show that the kernel-based support vector machines algorithm gives the highest accuracy in predicting the surface-wave window in the domain compared to neural networks and logistic regression. This window is also used to automatically pick the fundamental dispersion curve. The other two methods correctly pick the low-frequency part of the dispersion curve but fail at higher frequencies where there is interference with higher-order modes. 相似文献
98.
Earthquake prediction was practiced in Japan to examine the hypothesis that “a pair of earthquakes with similar magnitudes may be a signal of an impending larger earthquake”. In the present study, predictions were announced with expected probabilities of 20–30% (rank A) or 10–20% (rank B). In 2001–2002, excepting the Ogasawara region, 26 and 6 cases among 61 and 30 predictions of ranks A and B, respectively, were successful. Based on a statistical test of time-shift, i.e., one-year shift in this paper, and averaged activity in 1990–1999, the success rate of 43% for rank A was shown to be greater than that expected by chance with a confidence level more than 99%. The success rate of 20% for rank B gave a corresponding confidence level of only about 40%, suggesting that the predictions of rank B were not confident in this period. According to the results, a statistical test of time-shift was found to be useful to evaluate the significance of prediction methods of this type. 相似文献
99.
The characteristics of the strong ground motion accelerograms from the 1999 Kocaeli earthquake are investigated in detail
in this study. The emphasis is on the comparison of the response spectra for the fault normal (FN) and fault parallel (FP)
components of the ground motions. The results show that the near-fault records with directivity effects characterize themselves
with increased base shear demands rather than increased displacement demands for both the FN and FP components and a narrower
velocity sensitive region for the FN component. This study also shows that the effectiveness of base isolation may vary from
site to site and for a given site, from component to component. The site effects in the Marmara region during the 1999 Kocaeli
earthquake are examined. Site amplifications are predicted by the classical spectral ratio (CSR) and the receiver function
(RF) methods. The CSR method gives higher estimates for the site amplifications compared to the RF method and is in better
conformity with the observed damage during the Kocaeli earthquake. The districts of Istanbul that are especially susceptible
to site amplification hazard are determined. It is apparent from the results that the site amplification hazard risk is the
highest for Avcilar and Bakirkoy districts. This study also shows that for sites which have the risk of soil amplification
for long-period structures, liquefaction may not be beneficial as a natural base isolator, and may result in shifting the
eigenperiod of the low- and mid-rise structures to the critical periods with high site amplifications. This may be especially
the case for Avcilar and Bakirkoy districts. In Fatih, Bakirkoy, and Cekmece districts, the predominant period of the ground
motion is calculated to be very close to the eigenperiods of the typical residential buildings. Therefore, these three districts
are expected to experience heavier damages in future earthquakes due to resonance effects. 相似文献
100.
With the increasing number of precise navigation and positioning applications using Global Navigation Satellite Systems (GNSS)
such as the Global Positioning System (GPS), higher order ionospheric effects and their correction become more and more important.
Whereas the first-order error can be completely eliminated by a linear combination of dual- frequency measurements, the second-
and third-order residual effects remain uncorrected in this approach. To quantify the second-order residual effect, a simple
formula has been derived for GNSS users in Germany. Our proposed correction algorithm reduces the second-order effects to
a residual error of fractions of 1 mm up to 2 mm at a vertical total electron content level of 1018 electrons/m2 (100 TECU), depending on satellite azimuth and elevation angles. The correction formula can be implemented in real-time applications
as it does not require the knowledge of the geomagnetic field or the electron density distribution in the ionosphere along
the signal path. It is expected that the correction will enable more accurate positioning using the line-of-sight carrier-phase
measurements. 相似文献