George Rossman, I want to thank you for your insight into the geochemistry of sand and your offer to use your microscopes to examine the details of the grains. The frequency of the rumbling is a direct function of the dimensions and velocities in the waveguide.
Characteristics of Desert, Dunes and Sand
The divergence depends on the specific gravity of sand, the distance of migration of the dune and the local slope angle θ. The deviations at the extreme ends of the distribution are due to the absence of the finest and coarsest sediments in the sample.
Acoustic Emissions from a Granular Material
The width of the shear band depends on the diameter of the pole and on the static and dynamic friction angles. The accelerations in the granular material are synchronized with the silo wall accelerations but are much larger in magnitude.
B, K, internet
Previous Scientific Work
In a later paper Bagnold (1966) developed a theory of sound generation based on grain shearing and expansion in a sliding plane and derived a relation between the boom frequency f and the particle diameter D. Patitsas (2003) contributed a quan new - titular explanation of bloom frequency in terms of granular fluidized beds, similar.
Thesis Outline
Andreotti presented a new theoretical model based on elastic waves generated by the avalanche synchronizing the avalanche sand grains and proposed a wave-particle mode locking mechanism. proposed an alternative theory for the generation of the booming sound, in which grains synchronize their motion through a slowly propagating coupling wave in the sheared layer. The correct mechanism for generating the booming sound should explain all the characteristics and be consistent with the field observations.
Existing Theories
The frequency of acoustic emission vibrations directly depends on the width of the slide channel. Douady and colleagues proposed that sound frequency is related to the relative movement of sand grains (Douady et al., 2006).
Constructive Interference in a Waveguide
The propagating waves move inward in phase. the waveguide in case of constructive interference. As the subsurface structure of the dune changes uphill or downhill (illustrated in Figure 2.1), the phase velocity also changes independently of the frequency of the source.
The Interaction between the Waveguide and Booming
The cutoff frequency determined by the waveguide dimensions must overlap with part of the source spectrum of the burping emission. A similar mode spectrum occurs if the waveguide depth is very shallow (figure 2.6d) and the cutoff frequency is beyond the maximum frequency of the source.
Introduction
The rumbling frequency is determined by the depth of the surface layer of dry, loose sand compressed between two areas of higher compressional body wave velocity. The present work presents new experimental evidence supporting an alternative resonant waveguide-based explanation of humming.
Method
At each location, the dune had a clear sliding surface beneath the ridge at the angle of repose of the sand. Spectrograms from microphone and geophone recordings of a booming avalanche - the change in amplitude and sustained dominant frequency along the dune are illustrated from Figure 2b top to Figure 2e bottom.
Results
The boom sound cannot be generated where the speed of the dune surface layer approaches or exceeds the air speed. The sustained tone and its harmonics are not affected by the speed of the avalanche.
Conclusion
However, the slower slide included a larger surface area involved in the avalanche and the amplitude of the acoustic emission was a factor of two higher. Consequently, the amplitude of the bloom increases with the amount of sand falling, as shown for the large slide in auxiliary material Animation S2.
Acknowledgments
Auxiliary Material
This high-quality sound recording captured the release of a slow slide with a sliding speed of approximately V = 1 m/s. This high-quality sound recording captured the radiation of a fast slide with a sliding speed of about V = 2 m/s.
Curved Ray Paths and the Existence of a Resonance Con- dition
The summations of the first arrivals are indicated in red points and show a constant velocity in the surface layer. The method determines the velocity from the travel time of the first arrival wave, but does not relate to the resonant frequency of spontaneous surge.
Relation between the Resonance Frequency and the Method of Initiationof Initiation
Images of these natural avalanches showed a 20 Hertz difference in the sustained rumble frequency, while the subsurface structure showed a quantitative difference for these two cases from ground-penetrating radar images (Vriend et al., 2010a). This is in direct contrast to the finding of Andreotti et al. (2008) that the frequency is constant for one location.
Variation of Resonance Frequency with Grain Size
This is in direct contrast to the observation of Andreotti et al. (2008) that the frequency is constant for a site. a) Boom frequency as a function of 0.4p g/D. Ground-penetrating radar surveys conducted in the summer of 2007 provide a better estimate of the waveguide depth, which determines the boundary of the resonant frequencies (Vriend et al., 2010a).
Auxiliary Material
Geophone measurements of the wave propagation through the sand in the dune contain both surface and body waves. This paper demonstrates that upwelling is due to the trapping of the body waves in the surface layer;.
Introduction
Douady et al. (2006) observed that burr frequency depends on the shear rate and particle diameter of the sample. The analysis presented in Vriend et al. (2007) explained sound amplification through the constructive interference of a compressional P-wave within a natural waveguide within the upper 2 m of the dune.
Background
Booming does not occur in winter because the moisture in the upper layer increases the speed of sound in the surface layer c1 such that the cutoff frequency is above the excitation frequency and effectively eliminates the sandwich structure that holds the energy inside the waveguide.
Source Mechanism
A hammer impact on an aluminum plate placed on the surface of the sliding surface produces a repeatable pressure pulse. This increase in frequency is likely due to the propagation of waves from the source into the region of changing subsurface structure.
Wave Propagation
The amplitude trend is inversely related to the square root of the distance from the source 1/√. The discrepancy between the phase velocity of the wave in the burp experiment (Vpb=110-148 m/s) and the Rayleigh wave in the refraction experiment (Vp = 87 m/s) is due to the nonlinear behavior of the wave propagation.
Conclusion
The exact interplay between nonlinearity and dispersive behavior for burr emission remains an open question. Further work should include more extensive work on quantifying the exact nature of the nonlinear and dispersive effects.
Acknowledgments
Subsurface features of the sand dune fields of the Mojave Desert show evidence of dune construction, wind regime, and precipitation history. The dune stratigraphy shows a strong internal layering with a cemented structure that can immobilize and influence the migration of dune spaces.
Introduction
Long-term climatic changes affect the sediment supply in a region, including particle size distribution and chemical content of the sand. The current paper presents geophysical observations and field measurements of the stratigraphy of two large desert dunes in the Dumont and Eureka dune fields of the Mojave Desert and relates the observations to the short- and long-term climatic history.
Regional Geologic and Climatic Setting
The highest dune in the center of the dune field rises 120 meters (Nielson and Kocurek, 1987) above the desert floor. Seasonal changes in wind direction also resulted in changes in the surface features of the dune.
Layering Structure of a Dune
The reflection hyperbola originates from discrete layers in the subsurface - the curvature determines the radar speed, and the intersection with the origin determines the depth of the layer. The structure on the leeward side is dominated by parallel layers at the angle of repose in the upper areas of the dune.
Discussion on the Stratigraphy
The cementation of the sand grains leads to a decrease in porosity and a strong increase in velocity. The regular subsurface pattern on the windward face of the Dumont dune shows descending layers close to the angle of repose.
Near-surface Structure of a Dune
The topography was not directly measured and therefore a local interpretation of the topography is used as a framework for the radar results. The topography was not measured directly and a local interpretation of the topography is used as a framework for the radar results.
Velocity Structure of a Dune
The combination of the seismic velocity structure of the dunes with the ground penetrating radar profiles provides a comprehensive picture of the underground structure. The seismic velocity is not measured but estimated beyond 24 m from the summit based on continuation of the profile.
Conclusion
Summary
The frequency of the booming increased for both recordings as the natural avalanche progressed down the slope. The phase velocity of the boom shows large variations between different field dates – the phase velocity at the top varies between 180 and 260 m/s and is strongly correlated with the top layer seismic velocity.
Future Perspective
A two-dimensional array of geophones positioned in a grid will provide information on the radial distribution of the acoustic emission. A continuum model of the acoustic propagation in the dune will provide insight into the elastic wave propagation in the layered structure found in a desert dune.
Booming Measurements
The second column explains whether surge can be generated and the range of peak frequencies measured with acoustic and seismic measurements. The third, fourth, fifth and sixth columns indicate whether measurements were made covering 1 wave propagation characteristics of the acoustic emission, 2 estimated or measured topography of the slip plane, 3 ground penetrating radar surveys and/or 4 seismic refraction surveys.
Statistical Methods on Sand
The velocity shows a diffusive increase in velocity (from 200 m/s to 350 m/s) without refraction horizons and clear internal layering. Superimposed on the seismic profile are the indicated areas where booms could be generated, the frequency and phase velocity measured at the local geophones.
Shape of Sand
Theoretical Background: Wave Equation
An exact soliton solution of this equation describes the displacement as: with amplitudeA and solitonic phase speed:. The important nonlinear feature is that the amplitude linearly affects the velocity of the phase V in the dispersion relation.
Reflection and Transmission Coefficients
Methods and Materials
The margin of error on the velocity measurement depends on the magnitude of the velocities and the amount of geophones captured in the refraction. A review of the effects of surface moisture content on aeolian sand transport in desert aeolian processes.
