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Velocity shift in twodimensional anisotropic random media using the Rytov method
https://niedrepo.bosai.go.jp/records/4754
https://niedrepo.bosai.go.jp/records/47541673b973b2f14526a48d202ad812bf28
Item type  researchmap(1)  

公開日  20230330  
タイトル  
言語  en  
タイトル  Velocity shift in twodimensional anisotropic random media using the Rytov method  
言語  
言語  eng  
著者 
Tatsuhiko Saito
× Tatsuhiko Saito


抄録  
内容記述タイプ  Other  
内容記述  When highfrequency waves propagate through a randomly inhomogeneous medium, the apparent wave velocity is larger than the spatial average of the velocity distribution. The difference between the two velocities is referred to as the velocity shift. The present study formulates the velocity shift in 2D anisotropic random media using the Rytov method. Anisotropic random media in (x, z) coordinates are characterized by autocorrelation functions (ACFs) with a long correlation distance along the xaxis and a short correlation distance along the zaxis, where the velocity structure varies smoothly along the xaxis and rapidly along the zaxis. Note that the spatial average values of the velocity distribution along the xaxis and the zaxis are the same. The formulation gives the velocity shift for general types of ACF. An analytic solution is obtained for the case of a Gaussian ACF. To examine the reliability of the Rytov method, the velocity shift is estimated from numerical simulations of wave propagation using Ricker wavelets with dominant frequencies 80 and 40 Hz. The random media are realized with a spatial average velocity of 2700 m s(1) and an exponential ACF with 5 per cent rms fractional velocity fluctuation, a correlation distance of 80 m along the xaxis and 40 m along the zaxis. Numerical simulations show that waves apparently propagate faster with increasing travel distance, frequency and the angle of incidence measured from the zaxis to the global ray direction. For example, in the case of the 80 Hz Ricker wavelet at a distance of 520 m, the values of the velocity shift are about 0.9 and 0.3 per cent along the xaxis and the zaxis, respectively. The Rytov method quantitatively explains these general tendencies except for short travel distances along the xaxis. The discrepancy at short travel distances could be due to the smallangle scattering approximation and the long travel distance approximation employed in the Rytov method. Observations of Pwave velocity anisotropy have usually been interpreted in terms of preferred orientations of cracks and minerals in past studies. However this study indicates that wave scattering due to anisotropic random media can provide an alternative explanation for those observations.  
言語  en  
書誌情報 
en : GEOPHYSICAL JOURNAL INTERNATIONAL 巻 166, 号 1, p. 293308, 発行日 200607 

出版者  
言語  en  
出版者  WILEYBLACKWELL PUBLISHING, INC  
ISSN  
収録物識別子タイプ  ISSN  
収録物識別子  0956540X  
DOI  
関連識別子  10.1111/j.1365246X.2006.02976.x 