Inversion Of Surface Wave Phase Velocity Using New Genetic Algorithm Technique For Geotechnical Site Investigation

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Date
2011-05
Authors
La, Hamimu
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Publisher
Universiti Sains Malaysia
Abstract
The inverse procedure of surface wave phase velocity is a complicated problem for linear inversion approach because of its multi-model and highly nonlinear nature. Therefore the use of genetic algorithm (GA) optimization technique which is one of nonlinear optimization methods is an appropriate choice to solve surface wave inversion problem having high nonlinearity and multimodality. A novel inversion procedure of GA (NIPGA) suggested in this study was presented an effective technique for surface wave phase velocity inversion. This procedure adopted standard inversion procedure of GA (SIPGA) inserting a new loop scheme for updating the search limits of model parameters i.e., shear wave velocity (Vs) and thickness (H). Inversion codes of NIPGA developed under Visual Compact FORTRAN Version 6.6 were successfully applied to the synthetic and field phase velocity inversions. Two different synthetic profiles representing the high velocity layer (HVL) and low velocity layer (LVL) cases were used. These profiles are considered to simulate situation commonly encountered in geotechnical site investigation. Effective Rayleigh wave and multimode Love wave phase velocities are synthesized from these profiles using full P-SV waveform reflectivity and full SH waveform reflectivity, respectively. The phase velocities are then inverted using codes of NIPGA with three different approaches, namely single inversion of the effective Rayleigh wave phase velocity, single inversion of multimode Love wave phase velocities and joint inversion of them. To assess the accuracy of each inversion approach, differences between the true and inverted shear wave velocity profile are quantified in terms of shear wave velocity error, ES. Our numerical modeling showed that the shear wave velocity errors of the joint inversion approach based on NIPGA codes are relatively smaller than the single inversion approach. These errors indicate that the accuracy of shear wave velocity reversal can be improved by jointly inverting of Rayleigh wave and Love wave phase velocities.
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Keywords
Surface wave phase velocity , use of genetic algorithm
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