Study On Rayleigh Waveform Inversion Of Shallow Structure

At present,although the conventional Multi-channel Analysis of Surface Waves(MASW)technology has been widely used.But it is based on the assumption of horizontally layered-elastic media,so MASW techniques are only applicable to the horizontal layer models and those earth models with slow horizontal variation in S-Wave velocity.If there is a dramatic change in lateral velocity,the MASW technique may erroneously identify this lateral change as a vertical change in S-Wave velocity,which may reduce the lateral resolution of the MASW method.To overcome this shortcoming of MASW technology,and avoid to calculate the theoretical dispersion curves of the being updated model by using the dispersion equation for horizontally layered-elastic media,we propose a Rayleigh waveform inversion method to obtain S wave velocity of the shallow structure.Based on the stress-velocity wave equation,the finite difference method is used to simulate the forward value of Rayleigh waves.And its accuracy is also verified in this thesis.In the inversion,a normalized L2 objective function is used.In order to obtain a reliable initial model,a 2-D velocity distribution of the underground structure is obtained firstly by using the conventional MASW method.Secondly,this 2-D velocity distribution can be used as the initial model in waveform inversion.In order to prevent the inversion from getting to a local minimum,a time domain multi-scale inversion method is adopted this thesis.That is the observed data and seismic wavelet are decomposed into different frequency bands firstly.And then,the inversion is done from the low frequency band to the high frequency band,where,the low frequency band inversion result is used as the initial model in the high frequency band inversion.This strategy ensures the inversion process to be stable.The inversion results of the theoretical shallow complex media models(void,dip fault,etc)show that the high-resolution S-wave velocity structure can be obtained by Rayleigh waveform inversion.The effect of quality factor Q on the Rayleigh waveform inversion in viscoelastic media is also discussed in detail.It is found that to select a suitable quality factorQ is very important in order to get a accura inversion result.Finally,a field test experiment is made over an underground manmade tunnel.The velocity structure reconstruction of the tunnel is realized by Rayleigh waveform inversion.The inversion results show that the imaging accuracy is high and the quantitative interpretation can be achieved.It shows that Rayleigh waveform inversion has great potential in shallow structure imaging.