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One of the common assertions in the literature is that booming dunes are rare. The standard test of whether a dune booms or not is to climb up to the top of a nearby slipface and forcefully dislodge sand with a boot. The Kelso Dunes in the Mojave Desert of California feature well-known booming sands, and the boot test often gives spectacular and memorable results (10). But not always. Booming dunes are temperamental, sometimes booming, sometimes mute. Some days will find the Kelso sands uncooperatively silent. Moreover, some slipfaces on a nominally booming dune such as at Kelso can never be made to boom.
A laboratory test for booming would clearly be of great value. Lewis showed that this was possible (7). He noted that bags of sand brought back to Pretoria would "roar" when disturbed. In this spirit, we collected one gallon plastic milk jugs of sand from a number of desert dune sites and took them back to the laboratory. On the dunes that boomed when booted, it was discovered that the loudest sounds are invariably produced near the dune crests and that basal regions of the dune bark only weakly if at all. On the basis of this observation, samples were collected from these two regions.
Another immediate and satisfying observation was that when booming sand is introduced into a milk jug and sloshed back and forth, a definite rhythm, like fingers on a washboard, is detectable in the vessel wall, and a pleasing thrum issues from the bottle. Thus a test of laboratory-sized samples became available which gives a definite signal, thrumming, when applied to samples of known booming sand but which produces only an uninspiring hiss from silent sand. The bottle test was subsequently refined to the beaker test, as described below.
As Criswell (5), Lewis (7), and earlier authors had observed, the booming property of a known booming dune is strongly affected by the moisture content of the sand. When damp, the booming dunes fall silent. Even when to the eye and touch the dune surface appears to have dried, it will remain silent until such time has passed at a sufficiently low humidity that most adsorbed water has evaporated. Although not explaining why some dunes never boom, the moisture effect is an obvious explanation of why booming dunes appear to be temperamental; adsorbed moisture on grain surfaces is often undetectable in the field, and yet it can choke off booming entirely. We note, however, the existence of booming beach dunes, such as those south of the Napali Cliffs on the island of Kauai, Hawaii, which are certainly never as dry as desert sands (6, 11); these dunes are calcareous, not quartzitic, and evidently the environmental conditions for booming are dependent on the nature of the constituent grains. This observation suggests that the booming property is determined by a particular grain grain contact force which is attained without water for clean quartz grains, but requires some water adsorption for the calcareous sand.
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发表于 2019-10-15 20:47:37
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