A field trial was conducted to test the existence of subsonic (Vp <331 m>/s) P-waves previously reported in the literature. A 1-m-long reverse profile was acquired with three-component (3C) geophones on a sandy silt (unified classification ML). The silt had a porosity of 54%, a degree of water saturation of 63%, and a plasticity index of 10. No subsonic P-waves were observed, although high frequency (up to 1200 Hz) Rayleigh waves were identified by hodogram analysis. These surface waves were observed with horizontal velocities that varied from 40 to 200 m/s. Hodogram observations and theory suggest that a portion of the data were also in the near-field.
Michaels, Paul. (2002). "Identification of Subsonic P-Waves". Geophysics, 67(3), 909. http://dx.doi.org/10.1190/1.1484533