Research On Key Technology Of Reflection Seismic Exploration In Ultra Mafic Rock Region

Deep Earth exploration have great significances for scientific research and guidance for prospecting mineral.In this paper,we research on seismic reflection exploration techniques in ultramafic rock area to meet the deep seismic exploration needs of the Luobusha ultramafic rock area in Tibet based on the project " Pre-study of the scientific drilling site selection in Tibet Luobusha chromite deposit area".We designed two deep reflection seismic long survey lines that is north-south direction at the eastern part of the ophiolite belt on the south bank of the Yarlung Zangbo River in the ultramafic rock area,and study the acquisition of seismic data from ultramafic rock areas.For the deep reflection seismic data,such problems as strong background noise,low signal-to-noise ratio,discontinuity of reflection events,multiple refraction noise,week signal in the deep,etc.we analyze the seismic wave propagation regularity and response characteristics in ultrabasic rock areas,and study the corresponding data processing methods and techniques of seismic exploration and interpretation.The main results are as follows:(1)We choose the appropriate seismic data acquisition parameters through theoretical analysis,seismic forward modeling and field experiments research,which can provide the basis for the later seismic ultra-basic rock reflection acquisition and designation.(2)Combined with the characteristics of forward modeling wave field,real seismic single shot records and attribute analysis methods,we used attribute analysis method to analyze the post-stack section and migration section,and study the characteristics of the seismic reflection wave in the ultrabasic rock area:they have mixture for the reflection wave and first-arrival wave at the near rock mass outcrop.The reflection wave of upper interface is clear,and the reflection wave of lower interface is weak due to the function of shielding.It have reversed interface between the upper and lower interfaces of the rock body.The chromite will reduce the local frequency and energy of the bottom interface.There are discontinuous and weakly-like events on the inside.The result of the above research can provide some reference for later seismic reflection exploration in ultramafic rock areas..(3)We combined the tomography static correction and refraction static correction method to solve the problems of static correction in ultrabasic rock area.The multiple refraction wave cover up the reflection wave,and they have same frequencies and velocities.We studied and tested the different parameters on both sides of the shot in the de-noising process,which can effectively remove the multiple refractions and stand out the weak signal.We use the different parameters to remove the surface waves for the shallow and deep part,which can eliminate the deep surface wave residuals and increase the weak reflection signals.(4)The post-stack reverse time depth migration method is applied to deep reflection seismic post-stack data.It can be obtained the effective reflection information from the 20 km in depth.We investigate the distribution characteristics of the ultrabasic rock mass,analyze the chromite origin and material sources,and predicted the information of underground deposits.In this project,we combine the geological data of the ultrabasic rock area,the deep borehole data and the deep reflection seismic profile,and the aims to provide a structural basis for the deep ore prospecting of the Luobusha chromite deposit.Our work can not only provide scientific advice for the scientific drilling site selection,but also provides more information for the study of the evolution mechanism of the Brahmaputra suture zone.