A Study On The Shallow Structure And Buried Faults In The Yinchuan Basin By The Seismic Refraction And Reflection Data

Seismological method has its unique advantages inobtaining the aspect of deep-shallow structure and tectonic,it has a wide range of application on the research of engineering survey,mineral resource exploration,deep structure,seismic dynamics,and so on.In addition to the natural seismic method,the artificial seismic exploration method is the main technical method todetect and study the fine structure and tectonic of the crust.The Yinchuan basin is a Cenozoic rifted basin in the western of the North China Craton with relatively stronger tectonic activity,in which the geological structure is complex and the active faults are developed.In order to study the shallow structure and active faults characteristics of the Yinchuan basin,we applied a first-break wave imaging method to obtain the high-precision basement P-wave velocity structure of the Yinchuan basin,Which using a large amount of first-break wave data from deep seismic reflection profile of 135 km was acquired in 2014.Since it is difficult to determine the accurate position,the upper-breakpoint depth of the fault and analyze the near-surface tectonic patterns and activities of the fault only according to the velocity structure profile.The high-resolution shallow seismic reflection exploration method also used to imaging the buried faults of the Yinchuan basin and the shallow structure of the surface rupture zone in the 1739 Pingluo M8.0 earthquake,obtaining the stacked or migration section of shallow seismic reflection.A further analysis in basic morphology of stratum and fracture characteristics of Yinchuan basin is developed on the basis of P-wave velocity structure of the basement.The research has shown that the combination of seismic refracted wave and reflected wave data can be used to reflect the different information of structure and tectonic underground,which can help us to study the changes of structure and tectonic underground from different aspects.The results of the first-break tomography reveals that the Yinchuan basin shows a obvious low-velocity structure and the basement surface exhibits large fluctuations,the deepest part of the basement is below the Yinchuan city between the Luhuatai fault and the Yinchuan fault,and its depth is about 7000 ~ 7200 m.Faults in Yinchuan basin play an important role in controlling the formation of the Yinchuan basin and the thickness of the Cenozoic strata in the basin and the Quaternary subsidence center.The Yinchuan basin is divided into three structural units in the western fault zone,the central depression area and the eastern western slope area.Shallow seismic reflection profile shows that the stratigraphic reflection interface morphology and the shallow structure of the fault are clear.The Yellow River fault,the Eastern Piedmont of the Helan Mountains,the Yinchuan fault and the Luhuatai fault are not only faults cutting the basement of the basin,but also are active faults since the Quaternary.In the near surface,these faults present as the "Y-shape" tectonic form being composed of 2 ~ 3 faults,and the latest activities of main faults can be traced to the late Pleistocene or the Holocene,which is the result of tectonic inheritance activities.During the seismic data acquisition,processing and interpretation,the forward modeling method,which has been widely used in oil,coal and engineering seismic exploration,is an effective way to verify the parameters of seismic exploration and data processing in complicated tectonic region.Considering the complex tectonic phenomena in seismic reflection profile,a simplified geological model was built based on the actual seismic data processing parameters and seismic reflection profile,in which we take the Yellow River fault as an example.We obtaining the seismic response by using the forward modeling method.The simulation results show that the characteristics of the main reflection wave groups in the section are in good accord with the seismic reflection profile of the actual seismic profile,which demonstratesthe velocity parameters in actual data processing and interpretation are reliable,and that the forward modeling has important guiding significance and reference valuein the application of data processing.