Analysis of Surface-Wave Data Including Higher Modes Using the Genetic Algorithm

Abstract

Analysis of surface wave data generally assumes that a dispersion curve mainly consists of a fundamental mode. Higher modes may dominate in several types of velocity structures, such as a model in which a high-velocity layer overlays on a low-velocity layer or a model in which a high-velocity layer is embedded in lowvelocity layers. In these types of complex velocity structures, higher modes may dominate in particular frequency range and observed dispersion curves look very complex. It is generally difficult to separate the fundamental mode and the higher modes correctly and traditional inversion methods based on the Jacobiam matrix cannot be applied. In order to overcome these difficulties, we have developed a new inversion method using a genetic algorithm (GA). In this new method, phase velocities and relative amplitude for the fundamental and higher modes are calculated. For each frequency, residual between observed and theoretical phase velocities is defined as the difference of an observed phase velocity and a synthetic phase velocity that has maximum relative amplitude in all modes. The GA is applied to obtain a velocity model that provides minimum residual. In this paper, we describe typical examples of dispersion curves in which higher modes dominate. Secondly, the theory of the new inversion method and numerical examples are shown. Finally, application of the new method to an engineering site investigation is demonstrated.