溶洞地震反射波特征数值模拟研究

Numerical simulation of seismic wave propagation in cave carbonate reservoir

(1.同济大学海洋与地球科学学院,上海200092;2.中国石油化工股份有限公司石油物探技术研究院,江苏南京210014)

School of Ocean and Earth Science,Tongji University,Shanghai 200092,China

采用空间网格大小和时间步长均可任意变化的高阶有限差分地震波传播数值模拟方法,对不同大小和埋深的溶洞系列模型进行了不同主频的地震波数值模拟。结果显示,随着溶洞直径的增大,溶洞反射波最大振幅呈现出先增强、后减弱、再增强、最后趋于稳定的变化规律。低速填充的溶洞对地震波传播存在聚焦作用,致使溶洞反射波的最大振幅不是出现在溶洞顶部的第一次反射,而是出现在经过低速填充的溶洞对地震波聚焦作用后到达溶洞底部时产生的反射波上。当溶洞周长等于地震波波长时,反射波能量最强;当溶洞直径等于地震波波长时,反射波振幅降低到一个极小值。同时,对于相同埋深的溶洞,随着溶洞直径的增大,其产生的反射波的频带逐渐变窄,且向低频端移动。

The research of fracture-cave carbonate reservoir is one of the hottest problems in the world. By use of seismic numerical modeling with variable spatial grids and local time-steps,a series of seismic simulation experiments are conducted for many cave models with different size,different buried depth and different main frequency of wavelet. The results show that the curves of reflection amplitude variation with the cave size display a same tendency. Firstly,the curves increase rapidly to reach a maximum at the cave circumference equal to wavelength. And then they decrease slowly to a minimum value when the cave diameter equal to wavelength. Finally,the curves rise slowly and approach to a stable value when the cave diameter is large enough. The frequency band of the cave reflection becomes narrow and the main frequency decreases when the cave becomes large in the same buried depth.