Seismo-acoustic ground coupling: Wave types, transfer efficiency, and near-surface structure

Pressure perturbations such as e.g. impulsive acoustic waves can couple into solid earth through the long-known phenomenom of seismo-acoustic coupling. Yet, the associated mechanisms are not always clear. Most studies investigate seismo-acoustic through low-frequency and high amplitude signals generated by e.g. natural or man-made explosions.

We conducted a small-scale field experiment with firecrackers as acoustic sources and hundred 3-component nodal geophones as receivers in a 20m diameter ring layout, some of them co-located with seismically decoupled Hyperion IFS-5111 infrasound sensors. This allowed us to investigate seismo-acousting coupling for higher frequencies and very small (meter scale) offsets.

The large receiver density enabled us to observe and distinguish different wave types induced by acoustic sources, including direct air waves, air-coupled Rayleigh waves, and possibly slow Biot waves. Having co-located seismic and pressure sensors additionally allowed us to investigate the coupling efficiency, which is in the order of 10^-7 and thus similar to many of the low-frequency and large-offset studies. Furthermore, we can deduct soil properties such as rigidity, bulk modulus, and density from the co-located sensors, and efficiently infer near-surface (soil) properties using cheap acoustic sources.