Fig. 1

Parameter space between wavelength \(\lambda\) and sensor intervals \(x\). We used a velocity at a condition of dry or lower degree of water saturation or velocity profiles in situ (Mainsant et al. 2012) together with frequency of input signals or analyzed frequency, when we calculated \(\lambda\). The velocities by George et al. (2009), Barrière et al. (2012), Mainsant et al. (2012), Lorenzo et al. (2013), Taylor et al. (2019), and this study were 250 m/s, 160 m/s, 475 m/s, 170 m/s, 264 m/s, and 65 m/s (refer to “Monitoring capability for water content change in our system” section), respectively. The light red solid rectangle represents our experimental conditions, and the red solid rectangle represents the range that we analyzed in detail