Application of 2D microtremor section survey method in covered karst area, taking Yongan Dahu Basin,Fujian Province as example

Based on the importance of karst development location and development degree, and the construction of modern cities,this paper uses the two-dimensional microtremor section detection method to detect the formation struc- ture of limestone karst area,and obtains a new understanding of the development law of hidden karst caves. Two-di- mensional microtremor section method makes use of the advantages of natural source surface wave and is not limited by the interference of human living environment,which highlights its advantages in shallow geological structure detection. In the Dahu basin of Fujian Yongan China,the coverage area of karst development law is studied by using this meth- od. There are two karst groundwater monitoring wells in the area,where the analysis of well drilling cores S wave veloc- ity test results summarizes the formation velocity law. At the same time,the two-dimensional microtremor profile detec- tion results compared with drilling results reveal that the S wave velocities at the sections with limestone karst develop- ment and no development are 1 000-1 600 m / s and more than 1 700 m / s respectively. The S wave velocity at lime- stone cave is 300-850 m / s. There are significant differences in those sections. Based on the velocity profile of S-wave, it is further revealed that although the overall development pattern of underground karst caves in the Kengbian and Xi- ahe area is stratified,the development pattern is mainly karst pipeline. The karst depth in the area is 160 m,and the development depth is correlated with topographic relief. Based on the summative law of the two test areas,the karst de- velopment law of another microtremor survey line in the Dahu basin is analyzed and verified. It can be seen that by comparing and analyzing the law of microtremor profile results with that of borehole,the karst development in other non-borehole areas in this region can be inferred based on the law of microtremor profile results. The results show that the microtremor detection can directly detect the development and spatial distribution of underground limestone karst caves in the overlying karst areas,and can track and analyze the lateral variation rules of karst.