Influence of surrounding rock changes on the coal seam reflected wave under thin interbed condition

In order to study the influence of the changes of surrounding rock on the reflected wave of coal seam under the condition of thin interbeds, the coal-bearing strata in the λ/4 thin layer range in seismic exploration is taken as the research object, and three types of models of surrounding rock lithology, thickness and structure change are established. By Brekhovskikh equation in the theory of thin-bed reflection coefficient spectrum, the influence of surrounding rock changes on the AVO curve, attributes and gathers of coal seams are calculated and summarized. The research results show that the changes of surrounding rock lithology in the range of λ/4 have a significant impact on the AVO response of the coal seam. The sandstone roof will significantly increase the AVO intercept and gradient properties of the coal seam. The mudstone roof makes the AVO intercept and gradient properties of the coal seam increase; because of the different roof lithology, the corresponding coal seam AVO gather characteristics will also change. The surrounding rock interbed structure and thickness changes in the range of λ/4 will have a certain impact on the coal seam AVO response, but the impact is small. Among them, the changes of the surrounding rock interbed structure will cause changes in the AVO gather characteristics of the coal seam, and the surrounding rock thickness will cause changes in the AVO intercept attribute of the coal seam. The Zoeppritz equation based on the interface type is not suitable for the normalization of the thin interbedded coal-bearing strata. For simulation, the Brekhovskikh forward equation or other simulation methods that are more suitable for thin interlayers should be selected.