强衰减介质中地震波场的频率—空间域特征

Characteristics of seismic wave field in frequency-space domain in strong attenuation media

强衰减模型,是应用Biot理论的基本思想通过运动方程描述介质吸收衰减特征的黏弹性介质模型。相对于传统的黏弹性模型和黏弹孔隙介质模型,可以简便且精准地刻画稠油储层、近地表松散沉积层等的强衰减性。为此,利用优化的25点频率—空间域有限差分算法,模拟了强衰减模型介质中的波场特征,并根据近地表低速层介质的实际情况,研究了孔隙度、流体黏度和介质黏弹性三种物理因素的衰减机理。结果表明:孔隙度、流体黏度和介质黏弹性都是影响地震波强衰减和高频散特征的重要因素,而且对横波衰减的影响都明显强于纵波。其中,介质黏弹性是造成地震波高频衰减的关键,其他因素均能影响地震波有效频率范围内能量的衰减,尤其介质孔隙度的增大引起地震波的衰减最强。通过对比完全弹性、一般黏弹性和强衰减理论下的模拟波场,研究了浅、表层强衰减介质对深层波场信息的影响。结果表明,强衰减模型刻画近地表介质更符合实际,能为涉及不同衰减机制的强衰减介质的研究和强衰减补偿理论的建立奠定基础。

As a viscoelastic medium model,the strong attenuation model applies Biot’s basic idea to modify the viscous dissipation coefficient,and describes the absorption and attenuation characteristics of the medium through the equation of motion.Compared with some conventional viscoeastic medium models and viscoelastic-porous medium models,the strong attenuation model can more easily and accurately describe the strong attenuation properties of media such as heavy oil reservoirs and loose sediments near the surface.In this paper,we implement a method of 25-point frequency-space domain finite difference to simulate the wave field characteristics of a strong attenuation model medium,and study the attenuation mechanism of seismic waves caused by three physical factors including porosity,fluid viscosity and viscoelasticity of medium.From the numerical simulation results,we can find that porosity,fluid viscosity and viscoelasticity are all important factors on inducing strong attenuation and high-velocity dispersion of seismic waves,and their influence on attenuation of S-wave is more obvious than that of P-wave.Of which,viscoelasticity is the key factor on affecting the attenuation of high-frequency component of seismic waves,and both porosity and fluid viscosity can cause the attenuation of energy in the effective frequency range of seismic waves,especially porosity has the strongest attenuation effect.We also study the influence of the shallow medium with strong attenuation on the deep wave field,and find that the strong attenuation model is more practical in describing the lowvelocity medium by comparing the wave field simulation results under the complete elasticity theory and general viscoelasticity.These results provide a reference to the study of strong attenuation media involving different attenuation mechanisms,and also lay a foundation for the establishment of the comprehensive strong attenuation compensation theory.