P-wave velocity profile at very shallow depths in sand dunesP-wave velocities at very shallow depths

摘要

We have addressed the problem of measuring the compressional wave velocity at a very shallow depth in unconsolidated dune sand. Because the overburden stress is very small at shallow depths, the respective velocity is small and the seismic signal is weak. This is why such data are scarce, in the lab and in the field. Our approach is to stage a high-resolution seismic experiment with a dense geophone line with spacing varying between 10 and 25?cm, allowing us to produce a velocity-depth relation in the upper 1?m interval. These results are combined with another survey in which the geophone spacing is 2?m and the dominant frequency is an order of magnitude lower than in the first survey. The latter results give us the velocity profile in the deeper interval between 1 and 7?m, down to the base of the dune. The velocity rapidly increases from about 48?m/s in the first few centimeters to 231?m/s at 1?m depth and then gradually increases to 425?m/s at 7?m depth. This is the first time when such a low velocity has been recorded at extremely shallow depths in sand in situ. The velocity profile thus generated is statistically fitted with a simple analytical equation. Our velocity values are higher than those published previously for beach sand. We find that using replacement or tomogram velocities instead of an accurately measured velocity profile may result in 23%–44% error in the static correction.