Geophysical Survey Research To Site Screening Of Clays As The Host Rocks For High Level Radioactivity

Sitting of geological disposal for nuclear waste is a key problem for the long-term safety of high level radioactive waste disposal and type of surrounding rock is must be considered for the setting.At present,surrounding rock of granite and clay are focused on the research of the disposal sites in the world.Compared with the previous research on the surrounding rock of granite,material of surrounding rock of clay has been carried out in recent years.Based on comprehensive analysis of geological data,petrophysical parameters and regional gravity and magnetic field characteristics,the comprehensive geophysical survey for the sitting of geological disposal of clay rock is carried out,which relies on the implementation of “the study on the screening and evaluation of clay rock area in the northwest area of high level radioactive waste disposal repository”.The aim of this article is to provide scientific support for the sitting of geological disposal.Based on regional gravity and magnetic data,seven faults are defined as the main faults(lithological contact zone)in the north east,northwest and near east and west,using new techniques for potential-field data processing,such as the method of theta map,the improved normalized standard deviation,the Tilt gradient and the analytic signal method.Inversion results of gravity and magnetic anomaly reveal spatial distribution of density and magnetic in the study area.In addition,three suitable areas for further work are screened.Based on measured magnetotelluric sounding data,correlated interferences are suppressed effectively using the far-reference technique,apparent resistivity and impedance phase curves and quasi-sectional views are plotted and analyzed,and distribution of resistivity for the study area is revealed preliminarily.Results show that the resistivity changes as the depth increases.The L4 line is selected for setting some inversion parameters for 2D inversion using nonlinear conjugate gradient,and results show that the best inversion parameters for this region are as follows: polarization mode is TE(TM),the lagrange multiplier is 3,the background resistivity is 100 ohms,the horizontal mesh is 500 m and the vertical mesh sets 1.1 times the minimum depth of exploration.The results of 2D profile inversion and 3D slice reveal spatial distribution of sedimentary strata and fluctuation of basement.According to geological data,it is presumed that the thickness of the mudstone in the upper section of the Bayingebi formation is more than 500 m and the continuous thickness is more than 100 m.Finally,the favorable area of about 200 km2 is delineated in the screening area,combined with geological and geophysical survey results and the selection criteria of high-level radioactive waste disposal site clay rock site of China.