pure and applied geophysics

, Volume 132, Issue 1–2, pp 197–220 | Cite as

Laboratory measurements of spatial fluctuation and attenuation of elastic waves by scattering due to random heterogeneities

  • Koji Matsunami
Article

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.

Key words

Strong scattering spatial fluctuation scattering attenuation 

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Copyright information

© Birkhäuser Verlag 1990

Authors and Affiliations

  • Koji Matsunami
    • 1
  1. 1.Disaster Prevention Research InstituteKyoto UniversityKyotoJapan

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