Full-waveform P-SV reflectivity inversion of surface waves for shallow engineering applications

Existing surface wave modelling methods fail to correctly interpret some velocity structures that are critical from a geotechnical perspective, for example those with large velocity contrasts and reversals. An inversion scheme based on the observation of an 'effective' phase velocity has proven more successful than conventional methods in such situations. Our new approach incorporates dominant higher modes into the dispersion curve inversion by using a forward calculation with full-waveform P-SV reflectivity synthetic shot gathers to reproduce all wavefields. We then extract the theoretical surface wave dispersion from the plane-wave transformed synthetic gathers, removing the need for mode identification. The measured field dispersion is then inverted, using the forward modelling scheme and a linearised optimisation, to a flat-layered, shear velocity model. With the new method, velocity reversals are better modelled than with conventional inversion methods. In general, low velocity layers directly under a surface caprock are inverted with more accuracy than those masked by buried high velocity layers. Limitations of surface wave inversion which remain include the need to assume layer thicknesses, and the rapid loss of resolution with depth.