Study On Fine Shallow Subsurface Structure Based On Airgun Source And Ambient Noise Methods

It is of great significance for resources exploration and earthquake hazard prevention to study on local-scale(from a few kilometers to tens of kilometers)subsurface structures.A high-resolution velocity structure can provide us a lot of information for our study area and lay an important foundation for relevant research.Tomographic methods based on active source and passive source are the important tools for studying subsurface structures.Active source methods based on airgun shots and passive source methods based on ambient noise are widely used in imaging regional structures.Although numerous efforts had been conducted to image the middle-lower Yangtze River region(MLYR)and the Binchuan basin in Yunnan province,some problems remain unsolved.For example,we lack a reliable shear velocity model in the MLYR and the depth extent of the low-velocity zone in ChengHai Fault(CHF)is still unclear.Through the further development of data processing procedures and the appropriate selection of research technology,we successfully obtained the fine shallow subsurface structure of the two research areas.These models can provide important information for tectonic features,distribution of mineral resources,and evaluation of earthquake hazards.We performed the 3D high-frequency surface-wave tomography to investigate the shallow crustal shear-velocity structure in the MLYR,using airgun signals recorder by dense arrays in a large volume airgun-shot experiment.A new procedure based on the trajectory analysis of particle motion is developed to extract the group-velocity dispersion for the characteristics of high noise level and weak surface-wave energy in this region.We use the direct surface-wave tomographic method to invert all high-frequency Rayleigh-wave group-velocity dispersion data in the 0.1-1.6 s period band simultaneously for 3D variations of shear-velocity structures within 1 km depth.It is the first time that we use the surface wave generated from airgun shots to investigate near-surface structures.It greatly improves the utilization of the data and makes up for the lack of a high-resolution shear-velocity model in this region.In addition,our results specify the distribution of mineral resources and low-velocity deposits.To better know the Binchuan basin,we analyzed the characteristics of seismic wave propagation,which indicate the size and distribution of subsurface velocity anomaly,by using the dense seismic array and large volume airgun shots.12 sub-arrays(67 stations)are chosen in this study.We performed array-based frequency-domain beamforming analysis and single-station-based polarization analysis to investigate the characteristics of seismic wave propagation.The results from these two methods both reveal similar but complex characteristics of seismic wave propagation in the Binchuan basin.The azimuth anomalies off the great-circle path are quite large with values up to 30°,which is caused by strong structural heterogeneity in the very shallow crust.We performed ambient-noise tomography to further analyze the size and distribution of subsurface velocity anomaly quantitatively.We picked group-velocity dispersion curves in the 0.1-5.0 s period band from a 2D seismic array(381 stations in total)in the Binchuan basin.The direct 3D surface-wave inversion method was used to obtain a 3D shallow crustal shear-velocity structure in this area.The path coverage appears to be very good due to a large number of stations and enabled us to obtain a high-resolution model within 3 km depth.Our results show a good correspondence to the geological features,with relatively low shear velocity in Quaternary sediments and high shear velocity in Permian basalts.The densely distributed fault system in the region has an important influence on the velocity structure within 3 km depth.The extension of normal faults plays an important role in the formation of low-velocity anomaly belts.In addition,the dip angle of the southern segment of the CHF gradually decreases from north to south,which reflects the transition of the fault property from strike-slip to extension,indicates the characteristics of the rotation and deformation of the regional block.Research of 3D ambient-noise tomography has shown that the structure of the Binchuan basin is mainly controlled by fault zones.A fine model of the fault zone structure is necessary for further study on the influence of fault zone on the regional structure.Despite the past efforts on imaging high-resolution fault zone structure,the depth extent of the low-velocity fault zone is still a big challenge due to the lack of vertical resolution.We successfully retrieved the P wave reflections below the stations and constrained the depth extent of the low-velocity zone in CHF from ambient noise interferometry and horizontal-to-vertical spectral ratio by using a dense linear array deployed crossing the CHF.An inverted trapezoidal LVZ which extends to?1.0 km depth with a lateral extent of?3 km is constrained in our study.The lateral variations of the interface are well retrieved.Besides,our results from teleseismic P wave coda autocorrelation and ambient noise interferometry also show the potential of deep structure exploration in a fault zone.In this thesis,different methods and data processing procedures are used to study the local-scale shallow subsurface structure,which makes up for the deficiency in previous research in the MLYR and the Binchuan basin,and helps us better understand the distribution of resources and tectonic characteristics in these areas.