The Passive Surface Wave Methods For Shallow Engineering Exploration Based On The SPAC And NCF Technology

The shear wave velocity is an important parameter for site classification and seismic response analysis. The average shear wave velocity from the surface to the depth of 30 meters is an international standard for site classification. In domestic, the equivalent shear wave velocity in the depth ranged form the surface to 20 meters and the thickness of covering soil are thought as the standard for site classification. The shear wave velocity is also an important parameter in seismic response analysis calculation. Therefore, it is very important to test the shear wave velocity structure in engineering investigation. At present, drilling test and surface wave method are the main techniques to investigate the shear wave structure of the site. The result from drilling test is intuitive, reliable, but the cost is high. Also, it is a destructive method and not suit to carry out in a large area. Surface wave method has now found its wide application in shallow engineering exploration due to its nondestructive and costless.The theory of the surface waves propagated in the layered media, active and passive (mainly the SPAC technique of microtremor surface wave method and seismic tomography using the ambient noise correlation function) methods are reviewed in this paper. Based on the consistency of the physical basis of SPAC and NCF, a scheme is proposed in this paper, which combined the active surface wave method, traditional passive SPAC technique and NCF method developed recently in global seismic tomography. The joint method and imaging method used in global scale are proposed to be applied in the shallow geophysical prospecting.To realize the idea and research purpose, a circle array with 16m diameter and 23 sensors was setup in YuXi, Yunnan province. The field experiment of active and passive measurements was conducted. For SPAC method, a method to extract the dispersion curve by processing the SPAC coefficients properly was proposed. The reliable dispersion curves ranged frequency of 6.7-23Hz are extracted. The S-wave velocity structure is inversed by fitting the observed and predicted dispersion curves. The inversed structure agreed with that obtained by drilling test under the array. For NCF method, the correlation functions for different paths are obtained by cross-correlating the ambient noise records. The group velocity dispersion curves at the paths with high signal-noise ratio NCF are extracted. For active surface wave method, the dispersion curves for different paths are obtained by SASW method. The one-dimensional S-wave velocity profiles are inverted for different paths based on the dispersion curves from NCF and SASW methods. The three-dimensional S-wave velocity structure is achieved preliminary by combing the inversed results from active and passive surface methods.