Development of Surface-wave Methods and Its Application to Site Investigations

Abstract

We have studied surface-wave propagation in two-dimensional space and applied surface-wave methods to near-surface S-wave velocity delineation for civil engineering applications. This dissertation describes fundamental theory of surface-wave propagation, numerical and physical modeling, surface-wave data acquisition and analysis methods that we have developed and application examples of the methods as well. We have proposed a new analysis method “CMP cross-correlation” that can greatly improve horizontal resolution of the surface-wave method. The CMP cross-correlation gathers of the multi-channel and multi-shot surface waves give accurate phase-velocity curves and enable us to reconstruct two-dimensional velocity structures with high resolutions. Data acquisition for the CMP cross-correlation analysis is similar to a 2D seismic reflection survey. Data processing seems similar to the CDP analysis of the 2D seismic reflection survey but it differs in the point that the cross-correlation of original waveform is calculated before making CMP gathers. Data processing of the CMP cross-correlation analysis consists of following four steps: First, cross-correlations are calculated for every pairs of two traces in each shot gather. Second, correlation traces having common mid-point are gathered and the traces that have equal spacing are stacked in a time domain. Resultant cross-correlation gathers resembles to shot gathers and named as CMP cross-correlation gathers. Third, a multi-channel analysis of surface waves is applied to the CMP cross-correlation gathers for calculating phase-velocities. Finally, 2D S-wave velocity profile is reconstructed through non-linear least square inversion. Analyses of waveform data from numerical modeling and field observations indicated that the new method could greatly improve the accuracy and resolution of underground S-velocity structure, compared to the conventional surface wave methods. We have performed numerical and physical modeling of surface waves in order to evaluate the applicability of the method. A finite-difference method is used in the numerical modeling and a Laser Doppler Vibrometer is used in the physical modeling. Both numerical and physical modeling has revealed that the surface-waves can be used for imaging two-dimensional velocity models. The modeling also clearly shows the applicability of the new method. The new method was applied to the real seismic data too. The data acquisition was similar to the shallow P-wave seismic reflection methods. The CMP cross-correlation analysis calculates dispersion curves from shot gathers. A non-linear least square inversion was applied to each dispersion curve in order to obtain one-dimensional S-wave velocity models. The velocity models down to depth of ten meters obtained through the CMP cross-correlation analysis agreed with known geological information very well. We have modified a passive surface-wave method, so called a micro-tremors array measurement, and applied it to near-surface investigation in civil engineering purposes. We have developed irregular arrays methods, such as L-shape array or linear array, for the micro-tremors array measurement and evaluated the applicability of them in comparison with isotropic array. These results lead to the conclusion that irregular arrays can be used for small-scale passive surface-wave method in which relatively high-frequency (1 to 10Hz) micro-tremors are used. Our new surface-wave methods have been applied to several different purposes in civil engineering, such as housing site investigations, earthquake disaster mitigation, levee inspections, and environmental issues. All these application examples prove that the surface-wave methods are very effective tool for estimating subsurface S-wave velocity model. The most important character of the surface-wave methods is that the method can estimate subsurface rigidity non-destructively from ground surface in soil engineering applications. Traditional geophysical methods in engineering field, such as a seismic refraction survey and a resistivity survey, are mainly used in rock mechanics field. No geophysical method has been widely used in soil engineering except loggings. The surface-wave methods can be first non-destructive investigation in soil engineering and it implies that the method will be used very widely. We believe that the surface-wave methods must become one of the standard methods in civil engineering investigations.