HIGH-RESOLUTION SHALLOW-SEISMIC EXPERIMENTS IN SAND, PART II - VELOCITIES IN SHALLOW UNCONSOLIDATED SAND

We conducted a shallow high-resolution seismic reflection and refracti on experiment on a sandy beach. The depth of investigation was about 2 m. We interpret the data using the Hertz-Mindlin contact theory combi ned with Gassmann's equation. These were used to obtain the vertical v elocity profile. Then the profile was computed from seismic data using the turning-rays approximation. The normal moveout (NMO) velocity at the depth of 2 m matches the velocity profile. As a result, we develop ed a method to invert measured velocity from first arrivals, i.e., vel ocity versus distance into velocity versus depth using only one adjust able parameter. This parameter contains all the information about the internal structure and elasticity of the sand. The lowest velocity obs erved was about 40 m/s. It is noteworthy that the theoretical lower bo und for velocity in dry sand with air is as low as 13 m/s. We find tha t modeling sand as a quartz sphere pack does not quantitatively agree with the measured data. However, the theoretical functional form prove s to be useful for the inversion.