基于重力异常反演的三维地震波速度结构分析方法研究

An Analysis Method for Three-dimensional Seismic Wave Velocity Structures Based on Gravity Anomaly Inversions

利用重力异常反演测试三维地震波速度结构,存在解不唯一、可靠性不高的问题。将面波反演充分融合到重力异常反演方程中,降低传统反演方法的非唯一性,并提升可靠性。以川滇地区为例,采用融合后的重力异常反演方法分析三维地震波速度结构。通过速度和密度的关系转换,得到对应的重力异常数据。由于面波频射数据主要对地震波横波速度敏感,因此将重力异常数据和初始横波速度相连,依据地震波速度和岩石密度之间的关系,获取重力异常反演方程,用于分析速度结构。选取21.6°~34.2°N、97.1°~105.9°E范围内的川滇地区活动块体作为实验数据,经过实验分析发现:使用该方法迭代反演川滇地区地壳上地幔顶部横波速度,重力异常数据和面波频射数据的残差值分别是6.24mGal和0.027km/s,实际拟合效果较好;分析该地区不同深度切面横波速度发现,在24km深度处,上地壳中含有相对低速层,在44km深度处,中下地壳中存在低速层;且该方法分析川滇地区三维地震波速度结构解的分辨率较高。

Using the inversion of gravity anomaly to test threedimensional seismic wave velocity structures has some drawbacks,e.g.,the solution is not unique and the reliability is not high.The surface wave inversion is fully fused to the gravity anomaly inversion equation,which reduces the nonuniqueness of the traditional inversion method and improves its reliability.Taking the Sichuan-Yunnan area as an example,the 3D seismic velocity structure was analyzed using the fused gravity anomaly inversion method.Through conversion of velocity and density,the corresponding gravity anomaly data was obtained.Because surface wave frequency data were sensitive mainly to seismic shear wave velocity,gravity anomaly data and initial shear wave velocity were connected.According to the relationship between seismic wave velocity and rock density,an inversion equation of the gravity anomaly was obtained to analyze the velocity structure.Active blocks of the Sichuan-Yunnan area,in the range of 21.6°-34.2° N,97.1°-105.9° E,were selected for experimental data.Through experimental analysis,it was found that the residual values of gravity anomaly data and surface wave frequency data were 6.24 mGal and 0.027 km/s,respectively,and the fitting effect was good.Through analysis of shear wave velocities at different depths in this area,it was found that at the 24km depth,there are relatively lowvelocity layers in the upper crust,and lowvelocity layers in the middle and lower crusts at depths of 44 km.Resolution of this method for analyzing the structural solution of threedimensional seismic wave velocity in the SichuanYunnan region was high.