基于改进气泡振荡理论的有限水体气枪激发模型研究

Air gun sources are more environmentally-friendly, safer and higher repeatability than conventional active sources (such as dynamite). They are suitable for multi-scale subsurface imaging in populated areas and high-resolution monitoring of velocity changes around fault zones. The source model of air guns based on bubble oscillation in infinite water body has boosted the development of air guns in marine seismic surveys. However, it fails when the air guns in land reservoirs are considered, which has a smaller size than ocean and violates the infinite water body assumption. Consequently, studies of land air guns, such as optimizing the excitation strategies and understanding the mechanism of phase generation, lack the support of source models. Therefore, we propose to study the air gun source model in finite water body: (1) improves of the classical theory of bubble oscillation by adding elastic solid-fluid boundary and found the source model of air gun in finite water body; (2) models the wavefield generated by airguns with complex geometry of water bodies considered by hybrid method of analytical source model and spectral-element method; (3) analyzes the effects of the size and geometry of water bodies on excitation of air guns, such as efficiency and azimuthal distribution of energy output, P/S energy ratio. This project could provide theoretical basis for better understanding of the mechanism of air gun excitation in inland water bodies. It could also provide quantitative reference for optimizing excitation strategies, evaluation of airgun experiments, and site selection of fixed airgun transmitting stations, and thus boost the refined studies of airguns.

气枪震源较传统人工震源（如炸药）具有环保、安全、高重复性的优势，适于开展人口密集区的地壳结构多尺度探测和断裂带附近的地下速度高精度变化监测。基于无限水体气泡振荡理论的气枪震源模型对海洋气枪发展发挥了巨大作用，但不适用于尺度较小、不满足无限水体假设的陆地水体，导致陆地气枪研究（如优化激发、震相发育机理等）缺乏震源模型支持。因此，本项目拟开展有限水体气枪激发模型研究：（1）通过加入弹性固液边界，发展考虑气-液-固三相耦合的改进气泡振荡理论，建立有限水体气枪震源激发模型；（2）与谱元法结合，利用混合法计算考虑复杂水体形状的气枪波场；（3）分析陆地水体大小和几何形状对气枪震源激发效果（如能量输出效率和方位分布、P/S波能量比）的影响。本项目可为深入理解气枪在陆地水域的激发机理提供理论基础，对提高气枪激发效率、气枪实验效果预估、固定气枪发射台建设选址提供定量分析支持，进而推动气枪精细化发展。

在陆内水体内激发气枪震源，利用其气泡振荡产生的地震波可以研究地下结构及速度变化。与海洋激发环境不同，陆内有限水体会影响气泡的振荡过程。本项目在经典无限水体气枪激发模型的基础上，增加了球型的固液边界，建立了有限水体气枪激发模型。利用拉格朗日有限元方法，对球对称情况下气泡在有限水体内的振荡过程进行了数值模拟分析，发现气枪震源在陆内水体内的气泡振荡存在临界大小，与气枪激发时产生的激波有关。结果表明，水体较大时（半径大于100 m），可以继续使用无限水体模型；当水体较小时（半径小于50 m），固液边界反射的激波会改变气泡处的压力，进而改变气泡振荡过程，显著地提高气泡振荡的频率，降低振荡的幅度。相关数值模拟结果与实验观测相符，为陆内小水体中高频气枪的源场信号特征提供了合理的解释。基于有限水体的气泡振荡过程分析有助于提升对陆内水体气枪激发过程的理解，为气枪选址、设计、优化提供理论依据。