火成岩下伏地层地震反射波场特征研究

Study on seismic reflection wave field characteristics of formations below igneous rock

在火成岩发育区,火成岩对下伏地层的地震成像影响严重,制约了岩下油气藏勘探。研究地震波在火成岩下的传播特征、分析火成岩对地震波场的影响是选择适合火成岩区地震资料采集和处理方法的基础。从Zoeppritz方程出发,分析了火成岩地层中地震波的透射能量与地震波的入射角和界面上下纵波速度比之间的关系;采用一阶弹性波交错网格有限差分对不同火成岩模型以及利用不同主频的子波对非均匀火成岩模型进行了正演模拟,分析了各模型的波场特征;采用反射率法合成了不同主频的火成岩互层模型的τ-p域道集,揭示了火成岩的非均质性对透射能量的衰减作用。模拟分析认为,火成岩发育对下伏地层成像的影响因素除了高速屏蔽还有介质的非均质性导致的散射。研究结果表明,转换波可以更好地对岩下地层成像,低频信号在通过薄互层和非均匀地层时能量损失较小。

In igneous rock development area,igneous rock affects the imaging quality of underlying layers seriously,which restricts the exploration of the underlying reservoir.Studying the propagation characteristics of seismic wave and analyzing the effect of igneous rocks on seismic wave field are the basis of choosing favorable method for seismic data acquisition and processing.Starting from the Zoeppritz equation,we studied the relationship between seismic wave transmission energy,incidence angle and the velocity ratio of compressional wave and shear wave in igneous rock formations.By using first-order elastic wave staggered grid finite difference algorithm, we carried out forward modeling on different igneous rock model and heterogeneitic igneous rock interbedding model by using wavelets with different dominant frequency,and analyzed the wave field characteristics of those models.Reflectivity method was adopted to synthesize gathers of igneous rock interbedding model with different dominant frequency,and analyzed the attenuation effect of heterogeneity of igneous rock on transmission energy.We think that high-velocity shielding effect and scattering caused by heterogeneity of medium are the factors for impacting the imaging quality of underlying formations.The study result shows that converted wave can better image the underlying formations,and the energy lost of low-frequency signal is smaller during passing thin interbedding and heterogeneitic formations.