1,547 research outputs found
Propagation of an Acoustic Pulse of Finite Amplitude in a Granular Medium
A study of propagation of a wide-band acoustic signal in a granular medium is reported. Experimental data on the propagation of pulses with an amplitude up to 3 MPa and characteristic length about 1 µs through a sample of cobalt-manganese nodules are compared with a computer model of the process. An anomalous sig'rfal absorption in the high-frequency range observed with relatively weak sounding pulses is explained under the assumption of a fractal sample structure on a certain scale. When the signal amplitude increases, the ahsorption assumes a normal power form which is evidence of substance structural changes
A model for the simulation of sidescan sonar
This thesis presents the development of a computer model for the simulation of the
sidescan sonar process. The motivation for the development of this model is the creation
of a unique and powerful visualisation tool to improve understanding and interpretation
of the sidescan sonar process and the images created by it. Existing models tend to generate
graphical or numerical results, but this model produces synthetic sidescan sonar
images as the output. This permits the direct visualisation of the influence of individual
parameters and features of the sonar process on the sidescan images.
The model considers the main deterministic aspects of the underlying physical
processes which result in the generation of sidescan sonar images. These include the
propagation of the transmitted pulse of acoustic energy through the water column to its
subsequent interaction and scattering from the rough seafloor. The directivity and motion
characteristics of the sonar transducer are also incorporated. The thesis documents the
development of the model to include each of these phenomena and their subsequent effect
on the sidescan sonar images. Finally, techniques are presented for the investigation and
verification of the synthetic sidescan images produced by the model.Defence Research Agenc
A general framework for waves in random media with long-range correlations
We consider waves propagating in a randomly layered medium with long-range
correlations. An example of such a medium is studied in \citeMS and leads, in
particular, to an asymptotic travel time described in terms of a fractional
Brownian motion. Here we study the asymptotic transmitted pulse under very
general assumptions on the long-range correlations. In the framework that we
introduce in this paper, we prove in particular that the asymptotic time-shift
can be described in terms of non-Gaussian and/or multifractal processes.Comment: Published in at http://dx.doi.org/10.1214/10-AAP689 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Intermittency and roughening in the failure of brittle heterogeneous materials
Stress enhancement in the vicinity of brittle cracks makes the macro-scale
failure properties extremely sensitive to the micro-scale material disorder.
Therefore: (i) Fracturing systems often display a jerky dynamics, so-called
crackling noise, with seemingly random sudden energy release spanning over a
broad range of scales, reminiscent of earthquakes; (ii) Fracture surfaces
exhibit roughness at scales much larger than that of material micro-structure.
Here, I provide a critical review of experiments and simulations performed in
this context, highlighting the existence of universal scaling features,
independent of both the material and the loading conditions, reminiscent of
critical phenomena. I finally discuss recent stochastic descriptions of crack
growth in brittle disordered media that seem to capture qualitatively - and
sometimes quantitatively - these scaling features.Comment: 38 pages, invited review for J. Phys. D cluster issue on "Fracture:
from the Atomic to the Geophysics Scale
Finite Difference Simulations of Seismic Scattering: Implications for the Propagation of Short-Period Seismic Waves in the Crust and Models of Crustal Heterogeneity
Synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two-dimensional media with random spatial variations in seismic velocity. The results of this study provide important insights about the propagation of short-period ( 5), the self-similar medium is characterized by a scattering Q that is constant with frequency, whereas theory predicts that the apparent Q in an exponential medium is proportional to frequency. These alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal Q at frequencies greater than about 1 Hz, assuming that scattering is responsible for most of the attenuation at these frequencies. Measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation (scattering Q ≤ 200). In these cases, Q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission Q of the medium. The cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium. The self-similar random medium proposed here for the crust produces waveform variations at high frequencies (15–30 Hz) similar to those reported for actual small-scale seismic arrays with apertures of hundreds of meters
Pulse propagation in time dependent randomly layered media
We study cumulative scattering effects on wave front propagation in time
dependent randomly layered media. It is well known that the wave front has a
deterministic characterization in time independent media, aside from a small
random shift in the travel time. That is, the pulse shape is predictable, but
faded and smeared as described mathematically by a convolution kernel
determined by the autocorrelation of the random fluctuations of the wave speed.
The main result of this paper is the extension of the pulse stabilization
results to time dependent randomly layered media. When the media change slowly,
on time scales that are longer than the pulse width and the time it takes the
waves to traverse a correlation length, the pulse is not affected by the time
fluctuations. In rapidly changing media, where these time scales are similar,
both the pulse shape and the random component of the arrival time are affected
by the statistics of the time fluctuations of the wave speed. We obtain an
integral equation for the wave front, that is more complicated than in time
independent media, and cannot be solved analytically, in general. We also give
examples of media where the equation simplifies, and the wave front can be
analyzed explicitly. We illustrate with these examples how the time
fluctuations feed energy into the pulse
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