Reservoir time-lapse variations and coda wave interferometry

Coda waves are multiply scattered waves that arrive much later than the major waves. Small seismic velocity variations are observed in reservoirs because of small variations in reservoir properties, which affect the first arrivals. Hence, first arrivals cannot be used to detect small seismic velocity variations. However, small variations can be reliably detected by the coda waves because of the amplification owing to multiple scattering. We investigate the ability of coda wave interferometry to detect seismic velocity variations and monitor time-lapse reservoir characteristics using numerical simulations and experimental data. We use the Marmousi II model and finite-difference methods to build model seismic data and introduce small seismic velocity variations in the target layer. We examine the model seismic data before and after the changes and observe the coda waves. We find that velocity changes can be detected by coda wave interferometry and demonstrate that coda wave interferometry can be used in monitoring time- lapse reservoir characteristics.

Boundary-refl ected waves and ultrasonic coda waves in rock physics experiments

Ultrasonic coda waves are widely usea to stuay hign-trequency scattering, however, ultrasonic coda waves are strongly affected by interference from by boundary-reflected waves. To understand the effect of boundary-reflected waves, we performed ultrasonic experiments using aluminum and shale samples, and the rotating staggered-mesh finite-difference method to simulate the wavefield. We analyzed the wavefield characteristics at the different receiving points and the interference characteristics of the boundary-reflected waves with the ultrasonic coda wave, and the effect of sample geometry on the ultrasonic coda waves. The increase in the aspect ratio of the samples delays the interference effect of the laterally reflected waves and reduces the effect on the ultrasonic coda waves. The main waves interfering with the ultrasonic coda waves are laterally reflected PP-, PS-, PPP-, and PPS-waves. The scattering and attenuation of the high-frequency energy in actual rocks can weaken the interference of laterally reflected waves with the ultrasonic coda waves.

Theoretical researches on three-dimensional coda wave scattering problem

Based on the previous researches on scattering theory, a model of single and multiple scattering in a three-dimensional infinite medium with non-zero hypocentral distance is proposed in this paper. According to the assumptionof three-dimensional medium with numerous, statistically isotropic and uniformly distributed scatterers, we obtain the analytic form of power spectrum of coda waves for single scattering and the integral form of power spectrum of coda waves for multiple scattering.

Discussion on the Measurement Method for the Coda Wave Quality Factor

The Quality factor is the parameter that can be used to describe the energy attenuation on seismic wave. In theory, we can obtain the relationship between the change of the coda wave quality factor with time and the strong earthquake preparation process on the basis of the quality factor of a coda wave in a same ray path. However, in reality the coda wave quality factor measured by different seismic coda waves corresponds to different seismic wave ray paths. The change of the quality factor with time is related to non-elastic characteristics of the medium and the volume of scattering ellipsoid constrained by scattered wave phase fronts, besides the change of regional stress field. This paper discusses the relationship between quality factor, epicenter distance and different lapse time, and then discusses the relationship between quality factor and frequency. Furthermore the determination method of the coda wave quality factor is put forward. The improved determination method of the quality factor, which removes the influence of different earthquakes or propagation depth of scattered waves, may increase measurement precision, thus information pertaining to abnormal changes in quality factor and the relationship between the quality factor and earthquake preparation process can be acquired.

Precise Measurement of Subsurface Seismic Velocity Variation by Coda Wave Interferometry

A filed experiment was conducted continuously for three days,and the velocity variation was measured using coda wave interferometry.The measurement error is estimated to be around 10-4,which coincides well with the theoretical error.The velocity variation during this period is up to 10-3.The relationship between velocity variation and changes in air temperature,barometric pressure and solid earth tide was analyzed with linear least square fitting.The velocity has no dependence on air temperature,but displayed change of the order of 10-6～10-7 when the barometer or earth tide changed one Pa.

Monitoring Media Velocity Variations with Coda Wave Interferometry

Multiply scattered waves are sensitive to media changes owing to the effect of repeated sampling,superposition and amplification. Based on this characteristic,small-medium changes could be detected by using coda wave interferometry. In recent years,coda wave interferometry has been widely used in estimating velocity variation with high precision in areas such as seismology and non-destructive testing. This paper systematically presents the principle and research status of coda wave interferometry,and especial focus is placed on the research of media velocity variations by using repeating earthquakes,artificial sources,and ambient noise. Applications of coda wave interferometry can contribute to the more subtle understanding of dynamic evolution process in the medium.

Spatial variation of coda wave attenuation using aftershocks of the Al Hoceima earthquake of 24 February, 2004, Morocco

On 24th February 2004 a significant earthquake (Md = 6.4) occurred in the north of Moroccocausing great damage in the vicinity of Al Hoceima region. This area is characterized by a complex faulting system as a result of compressional tectonic forces. Three short period seismic stations are set in this area of interest and recordings from these stations were used in this study. In order to complete our knowledge of attenuation, 60 local earthquakes are recorded a few days after the great earthquake with magnitude Ml 2.6 - 5.0 to estimate seismic attenuation. For this purpose, we applied the single backscattering model of Aki & Chouet 1975 inthe frequency range for 1 to 8 Hz. The study of coda waves was limited to a relatively short lapse time (20 Seconds) in order to sample the earth’s crust only. The values of Qc estimated for all the three stations show a strong frequency dependent relationship of the form Qc = Q0fn, where Q0 is Qc at 1 Hz , and n represents the degree of frequency dependence, and reflects the level of crustal heterogeneities to varying degrees. The average frequency dependent attenuation relationship has been obtained which indicates that the attenuation is high in this region. Finally to conclude our work, the values of Q0 suggest that Al Hoceima area is highly heterogeneous and the n parameter indicates a meaning frequency dependence of Qc.

Attenuation of coda waves in the SW of High-Atlas area, Morocco

We investigate attenuation scattering and loss properties in Souss basin(SW of High-Atlas) as a transition zone between the High and Anti Atlas ranges. This district consists in a thinned crustal patch with shallow seismicity and loose sedimentary trenches that perform an important contribution to augment seismic attenuation. So far, no coda waves approach in our knowledge have been used to draw satisfying outputs about the attenuation properties in the region. Therefore, this update suggests to correlate the lateral changes of seismic attenuation to different characteristics and asperities i.e. seismic activity,crustal age and thickness, heat flow, and ground deformation rate. To do so, we analysed coda waves derived from waveform data of more than 23 local earthquakes from seven broadband seismometers recorded during 2010 e2012 period. As a starter, we utilized the backscattering model which defines theseismic attenuation as inversely proportional to quality factor by the equation A ?1=. QQcestimates c were deducted at various central frequency bands 1.5, 3.0, 6.0, 9.0, 12.0 and 18.0 Hz according to different lapses times. The estimated average frequency dependence quality factor gives relation Qc? 120 f1;01,while the average Qcvalues vary from 149 at 1.5 Hz to 1895 at 18 Hz central frequencies. We observed an intimate dependence between quality factor and frequency ranges, which reflects the complexity of geological/geophysical pattern in the Souss basin and the presence of a variety of heterogeneities within the underlying crust.

Moment magnitudes of two large Turkish earthquakes on February 6,2023 from long-period coda

Two large earthquakes(an earthquake doublet)occurred in south-central Turkey on February 6,2023,causing massive damages and casualties.The magnitudes and the relative sizes of the two mainshocks are essential information for scientific research and public awareness.There are obvious discrepancies among the results that have been reported so far,which may be revised and updated later.Here we applied a novel and reliable long-period coda moment magnitude method to the two large earthquakes.The moment magnitudes(with one standard error)are 7.95±0.013 and 7.86±0.012,respectively,which are larger than all the previous reports.The first mainshock,which matches the largest recorded earthquakes in the Turkish history,is slightly larger than the second one by 0.11±0.035 in magnitude or by 0.04 to 0.18 at 95%confidence level.

Attenuation characteristics of coda waves and estimation of Qc values in eastern China

For short-period near-earthquake records in eastern China,from the empirical attenuation formula of coda ground motion amplitude A with timeτ:lgA=G-2.235lgτ,using the single scattering theory modified with epicentral distance,we obtain the curve family of corrected coda amplitude Ac(r,t),and w/2Qc values for each time interval of coda. From this,Qc(f,h) values, which correspond to each observational average frequency and sampling depth,are calculated.The results substantially agree with those observational Qc values in Yunnan,Beijing and central Asia.

Monitoring Contact State of Laboratory Fault with Coda Transmission Waves

We monitored the amplitude changes of coda transmission waves around 500 kHz across the frictional interface of a simulated 1. 5-meter-long fault during normal stress holding test.We find that the amplitude of coda transmission waves increases with the logarithm of stationary contact time. Localized increase amounted to a level ranging from 4% to 16%along the fault is observed during the 1-hour experiment. We discuss that the frictional strength at mesoscopic scale,which is related to the amplitude of coda transmission waves,is responsible for the phenomenon. Combining the reported method with other complementary approaches will enhance the understanding of fault mechanism either at laboratory or on-site applications.

混凝土梁尾波信号环境噪声对照补偿法

为推进超声尾波技术在混凝土结构检测中的应用,研究了环境干扰引起的混凝土梁超声尾波信号特征噪声并提出噪声补偿方法。开展了自然环境下的混凝土梁超声试验,提出平移缩放旋转多特征算法,将尾波信号的波速、频率、幅值特征以时间平移、时轴缩放、向量旋转3种参数形式进行分析。通过两试件超声对照试验,对尾波环境噪声进行补偿,并与尾波干涉法信号降噪结果进行对比,验证本文方法的有效性。结果表明:温度是尾波环境噪声的主要来源,湿度为次要来源;本文方法对尾波信号的降噪效果可达到91%以上,显著优于尾波干涉法;通过对照试件特征参数的噪声补偿可达到90%水平。研究结果可为提高尾波信号稳定性提供研究方法与试验基础。

基于超声尾波法的钢筋混凝土腐蚀监测研究

针对现有超声无损检测方法在钢筋混凝土(reinforced concrete,RC)腐蚀损伤评价方面存在的传播速度偏差问题,提出一种利用尾波干涉(coda wave interference,CWI)表征RC腐蚀情况的监测方法。在混凝土超声传播多重散射基础上,建立CWI腐蚀损伤评价指标。搭建快速电化学试验监测了RC在0,1%,2%腐蚀率下的腐蚀过程,通过逐步CWI分析了相对速度变化Δv/v及去相关系数K_(d)的变化规律,探究了换能器布置方式对尾波腐蚀监测敏感性的影响。研究结果表明:Δv/v随着腐蚀时间而逐渐减小,K_(d)随着腐蚀时间而逐渐增大;CWI方法能够有效地监测RC结构腐蚀裂纹的扩展过程,并且具有较好的敏感性;换能器采用垂直、对侧和同侧三种不同布置方式均能检测出RC结构腐蚀开裂,垂直布置方式对RC腐蚀检测的敏感程度更高。

High frequency S wave envelope synthesis using a multiple non-isotropic scattering model: application to aftershocks from the 2008 Wenchuan earthquake

Based on the formulation of a multiple non-isotropic scattering process, a characteristic source time is introduced to define the initial impulse width of energy density at the source. An analytical expression of the initial intensity spectral density of a seismic wave is incorporated into the integral equation of seismic wave energy density. And, a recursive formula of Green＇s function is derived to obtain the higher order Green＇s function, which is included to describe the stronger non-isotropic scattering process. Then, the effect of the scattering pattern on the energy density envelope is investigated by the modified scattering theory. Significant differences arc found in the decay of the energy density envelopes with distances using different scattering patterns. The envelope synthesized by the forward dominated scattering pattern is larger than the results obtained by the isotropic and backward dominated scattering pattern. Different scattering patterns are also used to fit the observation data from the aftershocks of the 2008 Wenchuan earthquake. It is concluded that the envelopes synthesized by the forward scattering pattern can match the data better than the isotropic and backward dominated scattering cases, and a new interpretation of the coda wave is given. Finally, using the forward dominated scattering pattern, the envelope broadening of the observed data is reproduced.

Research on coda Q distribution in Beijing and its surrounding regions

Based on the single scattering model of seismic coda waves, we have calculated the Q-factor in Beijing and its surrounding regions by means of calculating the power density spectrum in frequency domain with a fixed time window. The digital seismic data of 69 earthquakes from Beijing Telemetered Seismographic Network are used.These earthquakes were recorded from January 1, 1989 to December 31,1990 at 20 stations. This paper shows the variations of the coda Q-factors in the studied region with different sites, frequency and lapse time, and the temporal change of the coda Q-factors in these two years. The results indicate that coda Q-factor depends strongly on the lapse time and frequency. It is assumed that when Qc=Q0f″, for the three time windows of 15 -30 s, 30-60 s and 60-90 s, the average values of Q0 are 48, 115 and 217; and the average values of ηare 0.89, 0. 91 and 0. 74, respectively.

On the generation and decay of the long-period coda energy of large earthquakes

Seismic coda wave is the tail portion of the earthquake record after main arrivals.Studies on the coda usually focus on high-frequency data within several hours after regional events and attribute them to the scattering effect of the heterogeneities inside the earth.Here,we use records of seven large earthquakes at globally distributed seismic stations to examine the decay of long-period(100 s to 300 s)coda in the time window of 10,000 s to 140,000 s after the origin time and fit it with a statistical model.The geometric spreading effect in the estimated initial energy and a locationindependent equivalent attenuation coefficient indicate that the long-period coda energy is less affected by the heterogeneity-induced scattering effect than that of shorterperiod coda.The coda energy can reach the earth’s inner core and can be explained by a 1D earth model,making it more effective for constraining the global attenuation model.It also has the potential to determine the magnitudes of large earthquakes and to explore the interior of planetary bodies.

基于尾波干涉的飞机蒙皮微裂纹检测方法研究

常规超声波无损检测方法的检测极限为超声波波长的1/2,难以检测出机身蒙皮早期微裂纹损伤。文中探究了地震学中的尾波干涉法应用于蒙皮早期微裂纹检伤的可能性。首先,建立尾波的理论模型,以马尔科夫链中的粒子随机游走撞击过程解释了尾波信号的传播特征,并利用有限元工具Abaqus搭建蒙皮模型,明确了蒙皮材料中尾波信号的成因。其次,搭建一系列裂纹损伤程度在μm级的蒙皮模型,将尾波信号与蒙皮的微裂纹损伤特征进行联系。仿真结果表明,随着微裂纹尺寸和微裂纹偏转角度增加,其对应的尾波波速相对变化也线性增加,且微裂纹尺寸的增长对尾波相对波速的变化影响占主导地位。最后,搭建尾波实验台,实验结果验证了尾波方法具有超过常规超声检测方法的检测精度。

基于泰勒展开的混凝土超声测试温度效应研究

为研究环境温度对混凝土超声尾波信号的影响,在室内自然环境条件下,采集环境温度变化下的混凝土工字梁超声尾波信号。利用泰勒展开,分温度区段进行分析处理超声尾波信号,逐步提取信号中各阶温度效应成分并逐一去除,建立各阶信号投影系数与温度效应的拟合关系。结果表明:一阶信号投影系数与温度效应拟合直线的拟合优度高于0.98;去除前二阶温度效应成分后,平均去相关系数可降低97%以上。

Temporal variations of coda Q^（－1） associated with the dahaituoshan， Beijing， earthquake of 1990

emporal variation of coda Q-1 associated with the occurrence of Dahaituoshan earthquake (ML= 5. 4) and its variation with the seismicity in Beijing area were studied by using 90 local earthquakes from the China Digital Seismograph Network (CDSN). The results showed that there was a significant temporal change in coda Q-1 during the period from Jan. 1987 to Aug. 1991 and it was relative to the seismicity of this area. Coda Q-1 was determined for the lapse time window ranging from 10 to 60 s, frequency 1 to 16 Hz. By comparing the average values of Q-1 for different time periods, we found that in region near the mainshock area the coda Q-1 for earthquakes which occurred after the mainshock were higher than those earthquakes that occurred prior to the mainshock, while the coda Q-1 after the mainshock for the surrounding area were lower than before. We also found the temporal changes in η, the exponential in formula Q-1=Q0-1 (f/f0) -η, and Q0, the Q-factor at 1 Hz. Measurements of coda Q-1 from three components showed inconsistency in temporal variations for different frequencies.

基于HVSR法的震源深度引起放大效应的研究——以关东盆地为例

盆地对地震动具有很强的放大作用。文章基于日本关东盆地及其周围20个台站的记录,研究了S波窗口、S波后10秒窗口、尾波窗口之间特征的差异,从理论上解释了这些差异,并研究了三者随震源深度化产生的放大差异。结果表明:尾波窗口、S波后10秒窗口与S波窗口采用HVSR法得到曲线形状相似。浅源地震引起的S波窗口、S波后10秒窗口与尾波窗口放大显著高于深源地震。