CONCLUSION

Diffraction theory teaches us that there is not a single P-wave traveling in a porous medium. The arrival time picked in the lab measurements is generally the most ``beautiful'' amplitude: the highest and earliest. However, near the limit between suspension and load-bearing rock, picking the highest and earliest arrival is a very hard task, and, instead of picking a single peak, it is probably more interesting to pick a range of arrival times with equivalently high amplitudes that would yield a range of possible velocities.

I have derived a very simple expression relating the diameter of the grains to the frequency of the measurement. The validity of this expression is limited to very small grain sizes since we assume a perfect scattering behavior. However, the expression has not yet been experimentally tested.