The effect of viscoelasticity and velocity anisotropy on the reflections of a thin bed

The coal seam is a typical thin bed characterized by much lower elastic impedance than that of the surrounding rock. Coal seam has viscoelasticity and velocity anisotropy and its reflection amplitude changes with frequency. In order to analyze the effect of viscoelasticity and velocity anisotropy on seismic responses of a thin bed, this paper aims at a thin-bed model with strong impedance contrast and simulates seismic responses under three different assumptions, elastic and velocity isotropy, viscoelastic and velocity isotropy, and viscoelastic and velocity anisotropy, through numerical simulation method based on wave equations. Comparison of these seismic reflections shows that the viscoelasticity has non-ignorable influence on the reflection of the thin bed, may reduce the reflected amplitude of elastic waves obviously. While the viscoelastic velocity isotropy and anisotropy have little influence on the reflections of elastic waves of the thin bed. Compared with instantaneous frequencies of the thin bed in elastic and velocity isotropy media, PP-wave instantaneous frequencies in viscoelastic media have a variation with the maximum 8%, while those of PS-wave have only about 3% variation, may be ignored. The above research provides the possibility to predict tectonic coal by using the characteristics of extremely low quality factor of tectonic coal.