P-Wave Velocity at Shallow Depths in Sand Dunes and Its Effect on Static Correction

Summary We address the problem of measuring the P-wave velocity at a very shallow depth in unconsolidated dune sand. Because the overburden stress is very small at shallow depth, the respective velocity is small and the signal is weak. Hence, such data are scarce, both in the lab and in the field. We used in our approach a high-resolution seismic experiment with geophone intervals ranging between 10 and 25 cm. The outcome is a velocity-depth relation in the upper 1 m interval. These results were combined with a conventional survey where the geophone spacing was 2 m. The latter results gave us the velocity profile in the deeper interval between 1 and 7 m, down to the bottom of the dune. The calculated velocities ranging between 87 m/s at a depth of few centimetres to 390 m/s at 7 m depth. This is the first study where such low velocity was recorded at extremely shallow depths in the sand dune environment. The velocity profile thus generated is statistically fitted with a simple analytical equation. We show that using a replacement or tomogram velocities instead of the accurately measured velocity profile may result in 23% to 44% error in static correction.