Springer Online Journal Archives 1860-2000
Abstract To study the effects of strong scattering on elastic waves, spatial fluctuation and scattering attenuation ofP waves were examined by laboratory experiments for 2-D models of random media approximately characterized by a triangular correlation function in the range of 2〈ka〈33, wherek is the wave number anda is the correlation distance of the heterogeneities, i.e., the heterogeneity size. The results obtained are as follows: (1) Forka〉10, both the intensity and the correlation distance of the amplitude fluctuation are approximate for any phase of theP-wave train. The correlation distance nearly agrees with the heterogeneity size. These fluctuation properties are quite consistent with the theoretical prediction by the forward-scattering approximation. (2) For 3〈ka〈6, the fluctuation intensity becomes stronger in later phases of theP-wave train. This shows that scattering is approximately isotropic, and therefore, the scattered energy increases with time within theP-wave train. The correlation distance of the amplitude fluctuation disagrees with the heterogeneity size, and it shows a frequency-dependent property decreasing from 7a to 4a with the increase ofka from 3 to 6. These properties for 3〈ka〈6 have not yet been predicted theoretically. (3) Forka〈3, though the fluctuation is considerably smaller compared with that ofka〉10 and 3〈ka〈6, the fluctuation property is considered similar to that of 3〈ka〈6. (4) The observed scattering attenuation,Q −1, increases withka forka〈3, has a peak aroundka=3∼5, and then decreases withka. (5) When θmin = 15° and σ = 0.075, the theoreticalQ −1 curve, predicted by the approximate theory of Wu, roughly matches the observedQ −1 values, where θmin is the minimum scattering angle measured from the propagation direction of theP waves and σ is the rms of fractional velocity fluctuation. This suggests that the energy scattered in the range of θ〉15° is lost from theP waves, while the energy scattered in the range of θ〈15° is retained; and that the approximate theory overestimates by about three times the σ value of the model media used owing to the neglect of multiple scattering. (6) When the size of velocity heterogeneities responsible for forward scattering at 3〈ka〈6 is estimated from the θmin value of 15° on the basis of Wu's theory, it nearly agrees with the correlation distance for the initial phase of theP-wave train.
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