密集线性台阵地震背景噪声速度成像及其在湖南沃溪金钨锑矿勘探中的应用

Seismic Ambient Noise Tomography with Dense Linear Arrays and Its Application in the Exploration of the Woxi Au-Sb-W Deposit in Hunan

多道面波分析技术在近地表勘探领域有着广泛应用,准确的提取频散曲线成为面波勘探成像的关键。文章介绍了一种新的地震背景噪声互相关面波频散成像方法——拓距相移法。该方法在传统相移法的基础上,利用阵内相移对小孔径范围的面波中高频信号进行提取,并利用阵外相移对大孔径范围的面波中低频信号进行提取,然后将两部分频散曲线融合从而得到更宽频带的面波频散曲线用于地下速度结构的反演。该方法在保证对近地表结构进行较高分辨率成像的同时,大大增加对深部结构的有效约束。2019年9月到10月期间,作者在湖南沃溪布设了8条密集测线,进行了1个月的地震背景噪声数据采集,并利用上述拓距相移法提取了0.1~2 s的瑞利面波宽频带相速度频散曲线。通过初步反演其中3条测线的背景噪声数据,获得了该矿区深度2.5 km以浅的地震横波速度结构。经与已知地质资料比对,160测线的地震横波速度反演结果与断层、岩性分界面及矿脉有着较好的对应关系,表明获得的沃溪矿区地震横波速度结构较好地反映了控矿构造、岩性分界以及矿体的分布位置等信息,为该区中—深部找矿提供了重要依据。该研究利用实际数据检验了拓距相移法的有效性,为今后深部找矿提供了一个有效的高精度成像方法。

The multichannel analysis of surface wave(MASW)method has been widely used in near surface explorations.Accurate extraction of dispersion curves is the key to surface wave imaging.In this study,a new seismic surface wave dispersion imaging method,named the extended range phase shift,is introduced.Based on the traditional phase shift method,the newly proposed method uses the internal arrays to extract dispersions in the medium-to-high frequency range,while it uses the external array to extract dispersions in the low-to-medium frequency range.The two dispersion curves in different frequency ranges are then merged to obtain a new dispersion curve with a wider frequency band,which can be used to invert for the subsurface velocity structure.Not only can this method ensure high-resolution imaging of near surface structures,but also greatly increases the depth which can be effectively constrained by the ambient noise data.From September to October in 2019,we deployed 8 dense linear arrays in Woxi,Hunan Province to collect passive seismic ambient noise data for about one month.We extract the Rayleigh surface wave dispersion curves in the frequency band of 0.1-2 s using the proposed method.Through the preliminary inversion using the background noise data from three dense linear arrays,the S-wave velocity structure down to 2.5 km in depth is obtained.Compared with existing geological evidence,the S-wave velocity inversion result for Line 160 shows a good correspondence with known faults,lithological interfaces and the veins.Also,many new geological features can be interpreted based on the tomographic results showing that the new seismic shear-wave structure for part of the Woxi ore field reflects the ore controlling structures,lithological interfaces and ore body distribution.This provides an important foundation for deep ore exploration in this area.In this study,actual data are used to test the effectiveness of the proposed extended range phase shift method,showing that it is an effective and high-precision imaging method for exploring deep ore fields in the future.