Propagation characteristic changes of elastic waves transmitted through a sand soil in water injection and drainage processes

It is still veiled how elastic waves propagate in partially water-saturated unconsolidated media. Barrière et al. (2012 GJI) found the decrease and the increase of phase velocity of P-wave between 1.6-1.8 kHz in a sand soil during saturation and dehydration, respectively. Nakayama et al. (2018 AGU) found that the phase velocity of the broader frequency band between 10-50 kHz had increased gradually during the air-drying process of the completely saturated sand soil, whereas the spectral amplitude had varied among sensors, so that they speculated that the amplitude change was caused by not only the change in the propagation characteristic on the ray path but also the change in environment in the vicinity of the shaker. Therefore, in the sand soil used in this study, we prepare a water-repellent area for embedding the shaker. Also, we focused on the change of propagation characteristics in quasi-static situations like water injection and drainage processes.

We made sand soil and installed some uniaxial accelerometers and soil moisture meters (Fig. 1). We conducted water injection and drainage tests. We designed a linear sweep signal (1-50 kHz) and applied it to the shaker repeatedly and recorded the transmitted waves at the sampling rate of 204.8 ksps.

From the results of the moisture meters installed in the water-repellent area, we succeeded in preventing water infiltration into it.

For the injection test, spectral amplitude every 5 kHz between 15-45 kHz decreased by 20-50 % of the initial value, whereas time lag increased by ~ 2 μs. These tendencies were an agreement with the results of Barrière et al. (2012). We also found that the frequency dispersion of the waves between 15-45 kHz was not observed.

For the drainage test, the amplitude and time lag had changed within ±10 % and ±1 μs, respectively, which were almost the same fluctuations obtained even in the statically saturated sand soil. Therefore, we concluded no significant change observed.