HR: 0803h
AN: NS11D-0793    [Abstracts]
TI: A Mystery Unraveled: Booming Sand Dunes
AU: * Vriend, N M
EM: nmvriend@caltech.edu
AF: Division of Engineering and Applied Science, California Institute of Technology, 1200 East California Boulevard M/C 104-44, Pasadena, CA 91125, United States
AU: Hunt, M L
EM: hunt@caltech.edu
AF: Division of Engineering and Applied Science, California Institute of Technology, 1200 East California Boulevard M/C 104-44, Pasadena, CA 91125, United States
AU: Clayton, R W
EM: clay@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard M/C 252-21, Pasadena, CA 91125, United States
AB: "Booming" sand dunes have intrigued travelers and scientist for centuries. These dunes emit a persistent, low-frequency sound during a slumping event or a natural avalanche on the leeward face of the dune. The sound can last for several minutes and be audible from miles away. The resulting acoustic emission is characterized by a dominant audible frequency (70 - 105 Hz) and several higher harmonics. \par In the work of Vriend et al. (2007), seismic refraction experiments proved the existence of a multi-layer internal structure in the dune that acts as a waveguide for the acoustic energy. Constructive interference between the reflecting waves enables the amplification and sets the frequency of each boom. A relationship was established that correctly predicts the measured frequency in terms of the thickness (~ 2.0 m) and the seismic body wave velocity of the loose, dry surficial layer (~ 240 m/s) and the substrate half-space (~ 350 m/s). \par The current work highlights additional measurements and simulations supporting the waveguide model for booming sand dunes. Experiments with ground penetrating radar continuously display the subsurface features which confirm the layered subsurface structure within the dune. Cross-correlation analysis shows that the booming sound propagates at speeds close to the measured body wave velocity. Squeaking sounds, which are generated during the onset of the slide and precede the sustained booming emission, have been found to have distinctly different characteristics. These short bursts of sound are emitted at a lower frequency (50 - 65 Hz) and propagate at a lower propagation speed (125 m/s) than the booming emission. \par The acoustic and elastic wave propagation in the dune has been simulated with a finite difference code. The interaction between the air and the ground produces a coupling wave along the surface. The reflections in the surficial layer propagate in a dispersive band at a group velocity that is slower than the phase velocity of the layer. Different source mechanisms are used in order to simulate the squeaking and booming emission within the dune. \vspace{.1in} \noindent{\footnotesize Vriend, N. M., M. L. Hunt, R. W. Clayton, C E. Brennen, K. S. Brantley, and A. Ruiz-Angulo (2007), Solving the mystery of booming sand dunes, Geophys. Res. Lett., 34, L16306, doi:10.1029/2007GL030276.}
UR: http://www.its.caltech.edu/~nmvriend/research/
DE: 0902 Computational methods: seismic
DE: 0935 Seismic methods (3025, 7294)
DE: 3285 Wave propagation (0689, 2487, 4275, 4455, 6934)
DE: 7203 Body waves
DE: 7255 Surface waves and free oscillations
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting