Currently, SeisMike is mainly used to investigate pavement shear-wave velocity (Vs) and thickness (H) in a 2D cross section. The Vs is known to be the best indicator of material’s stiffness. Analysis of both parameters (Vs and H) follows the ordinary procedures with MASW analysis. In general, the accuracy in Vs evaluation (e.g., ≥ 95%) is higher than that in H evaluation (e.g., ≥ 85%).
Seismic imaging of ultra-shallow depths (e.g., ≤ 5m) utilizing the reflected acoustic pressure (P) waves can also be attempted using the SeisMike approach. Conventional geophone approach has not been effective for this depth because of the limitations in recording high frequencies (e.g., ≥ 1 kHz) and inherent issues with receiver statics and coupling with ground. All of them do not exist with SeisMike. Considering continuous rolling capability with SeisMike, it can produce a subsurface image of reflection cross section in real time, in a similar manner to GPR (for much deeper depths). This approach, however, requires more studies in the optimum survey logistics and also in the impact source. Nonetheless, this is an excellent R&D topic highly promising for diverse civil-engineering applications.
Evanescent Waves for Geotechnical Survey
Although it has been extensively studied in the fiber-optic area, there are little studies about the evanescent characteristics of Rayleigh waves used in the common geotechnical applications with MASW for relatively deep depths (e.g., 30 m). The seismic evanescent waves have been known to be extremely “thin” with an intensity rapidly decaying from the interface (Claerbout, 1985). Ryden et al. (2019) reported a successful measurement of evanescent Rayleigh waves propagating at a relatively low velocity (e.g., 100 m/s) using a microphone hanging 15 cm above the ground surface. This opens up the possibility of SeisMike even for the normal geotechnical MASW applications by replacing current heavy and bulky land streamers. It will also provide a one-step-further possibility of rolling geotechnical MASW surveys if a proper rolling impact source is invented. This topic needs extensive investigations.