Absorption And Scattering Attenuation In Heterogeneous Medium

The energy is attenuated when elastic wave is propagated in the medium. It is more sensitive to study the medium structure change with attenuation than with velocity. The attenuation in the material is mainly caused by the absorption and scattering. These two kinds of attenuation have different mechanism, different change laws and different influencing factors. Distinguishing these two types of attenuation is important for understanding of attenuation mechanism and material properties evaluation. Heterogeneous medium is common in practical geophysical work; therefore, elastic wave propagation in heterogeneous medium has drawn increasing attention. Scattered waves in inhomogeneous media are well developed. According to the relative relationship between wavelength and the non-uniform scale, scattering mode can be divided into four modes. The situation when wavelength is comparable with the non-uniform scale is studied in this thesis. In this case."Mie-scattering" is occurred, and scattering wave is most developed with coda wave phenomenon. Scattering wave energy propagation in heterogeneous media can be seen as a diffuse process, which can be described by a diffusion approximation function. In the study, we intend to use the diffusion approximation to evaluate the material absorption and scattering attenuation. A test system, measurement procedures and analysis method was established. First, the homemade cement samples were tested and their absorption and scattering attenuation coefficient were given. The variation of absorption and scattering attenuation coefficient with frequency, grain size, and stress was analyzed. The results show absorption attenuation coefficient increases with frequency linearly, scattering attenuation coefficient increases with frequency nonlinearly, and the attenuation in cement specimen is mainly caused by scattering. The results also demonstrate that the stress has little influence on attenuation, while heat induced dehydration and cracks has a remarkable effect on both two types of attenuation. Then, the test is carried out for four kinds of rock samples. The results show that diffusion approximation method has some stability. The attenuation change law in rocks is consistent with that in cement specimens. The method can be used to distinguish between absorption and scattering attenuation and to evaluate the scattering characteristics change caused by crack quantitatively, which has potential application in Nondestructive Testing (NDT) and material damage evolution monitoring.