超声实验中谱比法衰减的散射与本征吸收特性

实际地球介质的弹性波衰减，无论是对于地球内部结构反演、含油气储层描述，还是对于地震岩石物理本身的基础研究而言，都十分重要.通过岩石物理实验测量岩石的弹性波衰减是相关研究的基本手段，而谱比法是一种最常用的测量方法.利用谱比法测量的弹性波衰减，简称谱比法衰减，由介质的本征吸收特性和非均质散射特性共同决定.然而，在谱比法理论中，没有将这两种衰减特性的作用区分对待.因此，实验室测量的谱比法衰减中是否含有散射特性，其散射特性是否可以忽略，常不明确.本文基于对岩石物理实验中样品波形特征的基本认识，通过能流理论描述了弹性波的本征吸收和非均质散射过程，以及对应的尾波形成机理.文章还应用能流理论测量岩石的弹性波衰减，即能流法衰减.无论是通过理论推导，还是通过实验验证，谱比法衰减与能流法衰减都显示出良好的一致性.这两种直达波衰减的一致性暗示了谱比法衰减中存在着两种衰减特性.文中还对比分析了谱比法衰减与喷射流本征衰减的实验结果，该结果同样展示了谱比法衰减中可能存在着散射特性.因此，通过能流理论，可以利用样品波形的尾波特征定性判断谱比法衰减中可能存在的散射特性.即异常明显的尾波振幅可能意味着强烈的直达波非均质散射，谱比法衰减中的散射特性因而不可忽略.

Scattering and intrinsic components of attenuation through the spectral ratio method in ultrasonic laboratory experiment

Elastic wave attenuation in the earth media is of great importance whether for the earth structure inversion, hydrocarbon reservoir characterization, or for fundamental studies on seismic rock physics. The laboratory measurement of attenuation is a basic means for rock physical studies while the spectral ratio method is a common approach. The spectral ratio attenuation, measured with the spectral ratio method, is determined jointly by scattering and intrinsic attenuation. Yet, these two effects are not taken into consideration in the theory on the spectral ratio method separately. Therefore, it is neither certain whether there is any scattering effect in the spectral ratio attenuation, nor clear if the underlying scattering effect might be ignorable. Based on the basic understanding of characteristics of laboratory waveform recordings, an energy flux theory is introduced to describe the scattering and intrinsic attenuation of elastic waves, and the corresponding coda excitation mechanism. Through the energy flux theory, the elastic wave attenuation, namely the energy flux attenuation, could also be measured. A good consistency between the energy flux attenuation and the spectral ratio attenuation is shown both in theory and in experiment. The consistency of these two direct wave attenuations implies the coexistence of intrinsic and scattering components in the spectral ratio attenuation. Besides, a comparison of experimental results of the spectral ratio attenuation against the squirt flow intrinsic attenuation is also analyzed, which also suggests the existence of scattering components. Through energy flux theory, the existence of scattering in the spectral ratio attenuation could be identified qualitatively based on the ultrasonic coda. In another word, an extremely significant coda may suggest the existence of intense scattering in the spectral ratio attenuation, also meaning that the scattering component in the spectral ratio attenuation might be unignorable.