A physical-model study of the statistics of seismic waveform fluctuations in random heterogeneous media
A physical-model study of the statistics of seismic waveform fluctuations in random heterogeneous media
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DOI：
10.1046/j.1365-246x.2002.01606.x
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发表时间：
2002-03
影响因子：
2.8
通讯作者：
Chadaram Sivaji;O. Nishizawa;G. Kitagawa;Y. Fukushima
中科院分区：
文献类型：
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作者：
Chadaram Sivaji;O. Nishizawa;G. Kitagawa;Y. Fukushima
摘要利用实验室物理模型，研究了三维非均匀介质中地震波波形的起伏，并评估了短波长随机非均匀性对地震波的影响。我们做了两组实验：（1）用相同的震源信号激发波，通过三种不同尺度非均匀性的花岗岩类岩石传播;（2）用不同的频率激发波，通过同一种岩石传播。将压缩模式压电换能器连接到块的一侧，并且在位于另一侧的具有2 °间隔的10 mm半径圆形阵列处测量波形。在等震源-接收器距离处获得180个波形，其中来自震源的波辐射相等。源信号为单周期或双周期正弦波脉冲，对于不同的非均匀性为0.5MHz，对于相同的非均匀性为0.25、0.5、1和2 MHz。通过使用配备有激光多普勒振动计作为弹性波传感器的系统来记录波形。岩石的非均匀性是由岩石的二维微结构图像获得的一维速度波动来研究的。应用指数自相关函数，三种岩石的非均质性具有相似的波动强度（7.9- 9.3%），但不同的尺度长度范围从0.22至0.92毫米。我们计算了波形的空间相关性，平均波形和观察到的波形之间的相关性，相对于流逝时间的能量分区，以及相关系数、P波走时、P相能量对数等波形参数的统计分布。波形的相关性，能量分配和波形的统计参数作为归一化尺度参数ka的函数进行了研究：波数和非均匀性的特征尺度长度的乘积。我们发现，波形参数改变的趋势，在ka为0.2-0.3。当ka超过0.2-0.3时，散射波变强，波形变得更加复杂。这可能表明地震波在随机非均匀介质中传播时从等效均匀介质向散射介质的过渡。
SUMMARY Using laboratory-scale physical models, we studied seismic waveform fluctuation in 3-D heterogeneous media, and evaluated the effects of short-wavelength random heterogeneity on seismic waves. We made two series of experiments: (1) waves are excited by the same source signal and propagate through three kinds of granitic rocks with different scales of heterogeneity, and (2) waves are excited by different frequencies but propagating through the same rock. A compression-mode piezoelectric transducer was attached to one side of the block, and the waveforms were measured at a 10 mm radius circular array with a 2 ° spacing, located on the other side. 180 waveforms are obtained at equal source–receiver distances with an equal wave radiation from the source. The source signals are one- or two-cycle sine-wave pulse with 0.5 MHz for different heterogeneities, and 0.25, 0.5, 1 and 2 MHz for the same heterogeneity. Waveforms were recorded by using a system equipped with a laser Doppler vibrometer as a sensor of elastic waves. The rock heterogeneities were investigated as the 1-D velocity fluctuations obtained from the 2-D microstructure images of the rock. By applying the exponential autocorrelation function, heterogeneities in the three rocks are characterized by similar fluctuation intensities (7.9–9.3 per cent) but different scale lengths ranging from 0.22 to 0.92 mm. We calculated the spatial correlation of waveforms, the correlation between an averaged waveform and observed waveforms, the energy partition with respect to lapse time, and the statistical distributions of the waveform parameters: correlation coefficients, traveltimes of P wave, and the log values of the P-phase energy. Correlation of waveforms, energy partition and statistical parameters of waveforms are investigated as a function of the normalize scale parameter ka: the product of wave number and the characteristic scale length of heterogeneity. We found that waveform parameters change the trends at ka∼0.2–0.3. When ka exceeds 0.2–0.3, scattered waves become strong and waveforms become more complicated. This may indicate a transition from an equivalent homogeneous medium to a scattering medium for seismic waves propagating through random heterogeneous medium.