Recent Advances in High-Frequency Surface-wave Methods

摘要

Multi-channel Analysis of Surface Waves (MASW) analyzes high-frequency Rayleigh waves to determine near-surface shear (S)-wave velocities.The method is getting increasingly attention in the near-surface geophysics and geotechnique community in the past 20 years because of its non-invasive,non-destructive,efficient,and low-cost advantages.They are viewed by near-surface geophysics community as one of most promise techniques in the future.However,they face unique problems related to extremely irregular velocity variations in near-surface geology or man-made constructions,for example,highway,foundation,dam,levee,jetty,etc.,which are not solvable by techniques or algorithms widely used in earthquake seismology or oil/gas seismic exploration.Calculation of dispersion curves by existing algorithms may fail for some special velocity models due to velocity inverse (a high-velocity layer on the top of a low-velocity layer).Two velocity models are most common in near-surface applications.One is a low-velocity half space model and the other a high-velocity surface layer.The former model results in a complex matrix that no roots can be found in the real number domain,which implies that no phase velocities can be calculated in certain frequency ranges based on current existing algorithms.A solution is to use the real part of the root of the complex number.It is well-known that phase velocities approach about91％ of the S-wave velocity of the surface layer when wavelengths are much shorter than the thickness of the surface layer.The later model,however,results in that phase velocities in a high-frequency range,calculated using the current algorithms,approach a velocity associated with the S-wave velocity of the low-velocity layer NOT the surface layer.A solution to this problem is to use an equivalent layer model to calculate phase velocities when wavelengths are shorter than certain threshold (around 2h,h is the total thickness of the layers on the top of the low-velocity layer).The algorithm that we newly developed can handle any arbitrary velocity models,which is the foundation of high-frequency Rayleigh-wave methods and is critical to near-surface applications.Non-geometric wave exists uniquely in near-surface materials,especially in unconsolidated sediments.It occurs in near-offsets in high-frequency Rayleigh data.It is valuable for a quick and accurate estimation of S-wave velocity of the surface layer.Our study shows that non-geometric waves are leaky waves and dispersive.Leaky surface wave could cause misidentification when treating the leaky-wave energy as fundamental or higher modes.Such misidentification will result in wrong inversion results.Recently developed Multi-channel Analysis of Love Waves (MALW) is another powerful tool in determining near-surface S-wave velocities.Unlike Rayleigh waves,the dispersive nature of Love waves is independent of P-wave velocity.Approximating SH-wave velocities using MALW in near-surface applications may become more appealing than MASW because it possesses three advantages: 1) Love-wave dispersion curves simpler than Rayleigh waves; 2) Dispersion images of Love-wave energy have a higher signal to noise ratio and more focus than those generated from Rayleigh waves; and 3) Inversion of Love-wave dispersion curves is less dependent on initial models and more stable than Rayleigh waves.