SeisMike measures surface waves in either leaky (e.g., Lamb) or evanescent (e.g., Rayleigh) mode. It can also record acoustic waves emerged from subsurface such as reflected, refracted, and diffracted pressure (P) waves. The comparison of velocity (Vs) and common-offset sections below indicates there are acoustic wavefields in the raw SeisMike data as noticed by many hyperbolic features observed in the common-offset sections. On the other hand, the overall resemblance between the two sections must originate from the surface wave characteristics that change laterally.
It seems that the optimum field logistics for acoustic imaging of subsurface below pavement is different than those for the surface waves in the top pavement layer. A longer offset and a stronger impact source may be needed to capture such acoustic waves more effectively. This is currently under active investigation. When the SeisMike rolling system is available, this will be tested again over various subsurface settings with different receiver offsets and impact sources.
