| Recently,ambient noise tomography has been widely used in getting near-surface velocity models,and it can be classified as seismic array method and single station method.In this thesis,we studied the near-surface velocity structure of two regions based on the two methods of noise cross-correlation and noise HVSR.Firstly,we applied ambient noise tomography method to the Hutubi gas storage area,which is located at the Hutubi anticline in the southern margin of Junggar basin in Xinjiang.We calculated the cross-correlation functions of the vertical component of continuous ambient noise data of 22 stations in the Hutubi gas storage and its neighboring regions,and measured the fundamental-mode Rayleigh wave group velocity dispersion curves in the period band 0.5?1.5 s.Firstly,using the average dispersion curve in this area,we obtained the one-dimensional average shear-velocity model,and then we used a direct three-dimensional surface-wave tomographic method based on period-dependent raytracing to invert for the 3-D near-surface shear-velocity structure to the maximum depth about 500 meters underground.The results show that the sedimentary layers in this area are thick and have low shear-wave velocity(about 0.4?0.9 km/s)in general.The low velocity structure in the gas storage region is probably caused by the fractured sedimentary rocks due to gas and water extraction and injection.In the southeast and northeast of the gas storage,there are two zones with relatively high velocities,which are probably due to the integrated effects of the local groundwater level and topographic relief.The near-surface velocity model can provide an important basis for understanding the overlying formation characteristics,precise location of local microearthquakes,assessing near-surface site effect,and removing the shallow structural effect in imaging the deep structures.At the same time,the noise HVSR method was applied to 16 stations in the gas storage area,and the peak frequency of the HVSR curve below each station was extracted.Combined with the peak frequency,this thesis introduced and used the empirical formulas provided by four predecessors to obtain the seismic impedance interfaces under the stations.The calculation results show that the depth of the strong seismic impedance interface calculated by different empirical formulas is different,but the trend is the same.The four results show that the interface depth of the gas storage area is the shallowest,the southwest of the gas storage is deeper,and the northeast of the gas storage is the deepest.Moreover,the depth profile through the entire gas storage in the tomographic results shows that the shear wave velocity has a laterally significant increase at a depth of about 140 m,which is similar to the seismic impedance interface depth calculated by different empirical formulas.In summary,we suspected that there is a clear high-low velocity interface at about 140 m below the gas storage area,and according to the lithology well logging data,it may be the interface between conglomerate and mudstone.Then,the noise HVSR method was applied to the dense linear array area of Binchuan,Yunnan Province,and the three-component data recorded by 124 linear stations with a length of about 8 km was used to study the fluctuation of the deposition interface under the stations.The calculation results show that the peak frequencies below all stations have good continuity overall,and the stations in the Chenghai fault area have two significant peak frequencies,and the stations in the left and right areas have only one peak frequency.By comparing the shallower impedance boundary depths calculated by different empirical formulas,it can be considered that there is an interface independent of the fault location at a depth of about 30 m undergrou nd.According to the geological data provided by the predecessors,the depth of 30 m underground may be the interface of the Holocene alluvial,lacustrine deposits and other loose deposits and Pleistocene sandstone and siltstone.It is also not excluded that this interface is an underground aquifer interface.The depth of the deeper interface in the fault zone is between 600 and 800 m,and the variation of the interface depth along the line is in good agreement with the shear wave velocity along the line.The known shear wave velocity profile shows that the depth of the significantly low velocity layer in the fault region is about 800 m,and I guess this interface is an material interface that was contronlled by the Chenghai fault. |
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