Use of Surface Waves for Near-Surface Characterization and Drilling Hazard Reduction

Abstract

To overcome the lack of information on the most superficial part of the near surface obtained by the use of the First Arrivals (refracted waves), we have implemented an innovative combined workflow that use the information from the Surface Waves (Rayleigh waves) to complement the first-break measurements. As the Rayleigh waves propagates along the free surface interface, they carry significantly more detailed information of the near-surface characteristics which can be used to better constrain the first-break inversion. The first step of the workflow starts with the surface wave dispersion curve picking. As reliability of the results directly depends on the quality of that picking, data regularization are used to improve picks accuracy on both low and high frequency of the phase velocity/frequency spectra. A surface wave tomography process is then applied to convert the spatially irregular frequency-dependent picks into a regularized (x, y, frequency) Rayleigh wave’s velocity volume. Lastly a laterally constrained depth inversion is performed, delivering a 3D shear wave’s near-surface depth velocity model. The second step of this workflow uses this S-wave velocity model, which contains the near-surface details captured by the Surface-Waves, to constrain the refracted P-wave first-break tomography. A regional scaling, here a 1D VP/VS ratio estimated from knowledge over the area or from fast-track refraction first-break analysis, is required to convert the S-wave velocity into a P-wave model. The constrained tomography aims to scale the trend from the high-resolution S-waves velocity model by fitting it with the trend of the P-wave field derived from the first arrivals. This corresponds in a sense in inverting the VP/VS ratio in such a way to preserve the high-resolution feature scaptured by the surface waves while remaining consistent with P-wave information. The resulting near-surface velocity model looks more geologic, better respects P-wave travel times and can be used with more confidence to compute the primary statics solution than the conventional P-wave field only obtained from the first-arrivals tomography… Furthermore, this accurate update of VP/VS ratio can be used to estimate the Poison’s ratio. It can be used to better plan geotechnical survey in order to reduce shallow drilling hazards.