ランダムな不均質による弾性波の散乱
1985 年 38 巻 4 号 p. 513-527
詳細
1985 年 38 巻 4 号 p. 513-527
Elastic wave scattering by random heterogeneities is studied by ultrasonic model experiments; these are carried out using two-dimensional models of media with randomly distributed velocity and density heterogeneities. The P-wave trains composed of several crests and troughs are observed along profiles transversal to the direction of wave propagation. The apparent amplitude attenuation due to the scattering and the amplitude fluctuations along the profiles are examined for the range of ka from 2 to 13.5, where ka is 2π times the ratio of heterogeneity size a to wave length λ. The main experimental results are as follows.
Scattering attenuation Q-1, determined from the ratio of scattering coefficient to wave number, increases remarkably with the increase of ka from 2.5 to 4, and has a peak at ka≅4, and decreases with increasing ka. The scattered energy contributing to the fluctuations of wave field is the order of about 10-1 of the total scattered energy for 2.5<ka<6, but almost all the scattered energy contributes to the fluctuations of wave field for ka>10. The intensity of the fluctuations becomes stronger for P later crests and troughs for 3<ka<6, but the fluctuations are nearly of the same intensity for all P parts for ka<3 and ka>10. The correlation distances of the fluctuations for P initial crests and troughs are seven to eight times the heterogeneity size at ka=2-2.5, and decreases with increasing ka, and are nearly equal to the heterogeneity size for ka>10. The correlation distances of the fluctuations for P later crests rnd troughs are nearly equal to those of the fluctuations for P initial crests and troughs for ka<3 and ka>10, but are considerably shorter than those of the fluctuations for P initial crests and troughs for 3<ka<6.
These results indicate that the scattering effect is different among the following three cases: ka<3; 3<ka<6; ka>10. The scattering effects for ka<3 and 3<ka<6 reflect a very complicated scattering process, respectively; but is well explained by forward scattering due to the randomly distributed heterogeneities for ka>10.
