Experimental observation of compression wave dispersion in quasi-saturated soils

Due to the sensitivity of compression waves to the presence of water in porous media, P-wave velocity has regularly been proposed as a method for estimating the water content of soils. However, several studies have shown that compression wave velocity depends not only on the degree of water saturation of the soil, but also on pore fluid distribution. Meanwhile, in the field of rock physics, several physical phenomena have been shown to induce dispersion of compression waves in partially saturated rocks, meaning that wave velocity is also a function of its frequency. Through an experimental study on Fontainebleau sand, we show that similar observations can be made in quasi-saturated sands: compression wave velocity is highly dependent upon probing frequency. These findings are then analysed through the lens of several existing compression wave dispersion mechanisms. Finally, in situ cross-hole tests recorded in quasi-saturated sands and previously published in the literature are reanalysed.The conclusion of the study is that no reliable estimates of the degree of water saturation can be made in this way unless: i) great care is paid to characterising the frequency of the emitted wave and ii) potential wave dispersion mechanisms are investigated.