Application Of Passive Surface Wave Method In Stratigraphic Classification Of Weihe Basin

The Weihe Basin is located between the Loess Plateau and the Qinling Mountains.Because of its special terrain conditions,a huge thick layer of loose sediment has accumulated in Basin,which has increased the difficulty of geological prospecting in Weihe Basin.Compared with traditional exploration methods such as geological drilling and reflection seismic method,passive surface wave method uses ambient noise to detect underground structures of different scales,which has the advantages in cost,proficiency,and data processing.In this thesis,I adopt the spatial autocorrelation(SPAC)and Aki’s formulation to detect the shear-wave velocity structure of Weihe Basin.Before processing the actual data,staggered-grid finite difference method is applied to synthesize the ambient noise data.And then using SPAC method and Aki’s formulation to measure the dispersion curve.At last,we find the curve fitting is good through the comparison between calculation result and theoretical dispersion,which proves the validity and accuracy of the passive surface wave method.Then the passive surface wave method is applied to the detection of underground structures in the Weihe Basin,including imaging of small-scale and large-scale stratigraphic structures.Firstly,the Aki’s formulation is used for surface-wave tomography of the western region of the Weihe Basin(Fengxiang County,Baoji City).And the result showed combined use of active and passive surface wave method improved the imaging.The obtained shear-wave velocity profile is basically consistent with the actual logging data in the obvious interfaces,which also proves that the passive surface wave method is feasible in practical exploration.After that,the study area was extended to the entire Weihe Basin.We selected six sites to record ambient noise data in the basin,and then processed the ambient noise data using the Aki’s formulation and the SPAC method,respectively.Firstly,three representative profiles were selected in the study area,and the ambient noise data on each profile was processed using Aki’s formulation to obtain shear wave velocity profile which is consistent with the actual geological results.Secondly,the SPAC method was used to obtain the average dispersion information within the study area.And then through the inversion,I retrieved the average shear wave velocity profile of the study area,and divided the crustal structure.The crustal structure in this thesis and the result obtained from a seismic refraction experiment section in Menyuan-Pingliang-Weinan are basically consistent.Measured crustal thickness in the study area at the end is basically the same as the result in Xi’an(42 ± 3km)obtained by Ding Yunyu in 1965.