Estimating the shear wave velocity structure above the fresh bedrock based on small scale microtremor observation array

Abstract

S-wave velocity and thickness of weathered strata are the important parameters to engineering construction. In this study, array measurements of microtremors were carried out at 50 sites in Jimo, China. It revised the observation array that a small scale array was placed in the center of traditional regular triangle observation array. The field data of Jimo verified that it is effective to identify the different degrees of weathered strata above fresh bedrock. Based on the spatial autocorrelation (SPAC) method, the dispersion curves were achieved. While the S-wave velocity structure was estimated using the fork genetic algorithm. By using small scale observation array, it identifies different strata including weathered layer in detail. In order to test the accuracy of inversion results, 5 sites were chosen to drill. Comparison between inversion results and boreholes, it shows agreement with logs data. At last, the S-wave velocity structures in different depths above the fresh bedrock were plotted. We delineated the S-wave velocity structure for the first time in this research region using revised measurement of microtremors signal. The depth of fresh bedrock fluctuates strongly which is affected by weathered. This method provides another means to distinguish the degree of weathered, depth of fresh bedrock, and S-wave velocity structure of shallow strata.