Geoscience Reference
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Fig. 10.2
A summary of the observed booming frequency of several
dunes as an exercise in chartsmanship. These two plots, discussed with
some retrospection by Andreotti (2012), actually show the same data.
A variant of the plot on the right was used by Vriend et al. (2007) to
argue grain size had no effect on the frequency, while the plot on
the left emphasizes a subset of points by choice of symbols, and
transforms the grainsize into a predicted frequency, as shown in
Andreotti's (2007) comment. When all data points are considered, and
especially when error bars in both dimensions are shown, it is
difficult to conclude anything, except perhaps about the subjectivity
of
the
scientific
process
and
how
chartmanship
can
be
used
to
advantage
('Sturtzstroms', Collins and Melosh 2003) to run out far
further horizontally than you would expect, 20-30 times the
vertical drop. Similarly, this acoustic fluidization process is
important in allowing a seismically-shocked planetary crust
to behave locally as a liquid long enough in a large impact
event to allow the crater to collapse and form a central peak
or (in more energetic events where the rattling takes longer
to decay) a peak ring (Melosh and Gaffney 1983).
Such a granular 'gas' has a very low sound speed (per-
haps just a few m/s—Andreotti's chapter expresses surprise
that their measurements indicate a propagation speed an
order of magnitude lower than they'd expect), making a
strongly-reflecting boundary between the avalanche and air
(340 m/s) and between avalanche and the packed sand of
the dune (400 m/s). Where there is a difference in sound
speed and density, sound is reflected, so seismoacoustic
energy will bounce back within the avalanching area. Since
the 'slab' of sliding sand probably has a somewhat uniform
thickness, the time for a sound wave to cross the slab and
back (a 10 cm thick layer and sound speed of 10 m/s would
give 50 Hz) could allow a fairly narrow band resonance.
Thus while Vriend et al. are right to point out the
importance of layering in a dune, we suggest that its seismic
properties are largely irrelevant in the sense of controlling
the sound directly. The layering of the dune may be critical,
however, in controlling how deep the avalanching layer is,
and that in turn may influence the frequency of emission. It
is not enough to build a dune from sand that sings in the
laboratory to make a booming dune. Similarly, the grain
microscale properties are likely crucial in influencing how
sliding generates the acoustic excitation: even a dune with
the fancy layering beloved by the Caltech group will not
boom if its sand is silent.
It is probably bad form for writers to advance their half-
formed hypotheses in a book without the cut and thrust of
peer review, but science is a journey, not a destination. We
suggest some fruitful avenues to pursue may be field
experiments wherein the depth of the avalanche is con-
trolled, or perhaps experiments with very high- or low-
density grains. Similarly, if methods can be devised to
measure in the field the depth of and sound speed in a
flowing avalanche, such information will be of value for any
theory.
Lest it be thought that the study of booming dunes is a
frivolous pastime, it should be noted that the flow of
granular materials is of vital import to industry, whether
plastic pellets, construction material, cornflour or any of a
vast array of materials. These are usually stored in tall
vertical cylinders or silos, such that they can be poured from
the bottom. When this flow occurs—essentially a contained
avalanche—oscillations analogous to booming dunes can be
excited that may influence the flow or even damage the
structure; the audible manifestation of these go by the
delightful name of 'silo honking' (e.g., Buick et al. 2005).
Vibrations in sand are of interest in civil engineering (e.g.,
piledriving, or the liquefaction of sand during an earth-
quake), and are a matter of life and death to the desert
scorpion, which uses them to find prey (Brownell 1977).
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