Characterization of Suspended Sediment by Acoustic Techniques. Part I: Theoretical and Experimental Validation of Attenuation Spectroscopy
A combined experimental and theoretical study of the attenuation cross section of irregular shaped sediment particles is presented. Numerical calculations, using finite and boundary element methods with a Variational Helmholtz Gradient Formulation (VHGF) approach, were carried out for rigid immobile particles for four different non-spherical (polyhedral) shapes for values of the normalized frequency ka < 10. Attenuation spectra were measured in frequency range of 1–100 MHz for some suspensions of sand and quartz particles in the size range of 30 to 200 μm. It was observed that the attenuation for ka > 1 is enhanced with respect to the spherical case. The enhancement of the attenuation is larger the more irregular the particle shape is, up to a factor of 1.8 found experimentally. The numerical calculations also show an enhancement of the attenuation cross section at high values of ka which is proportional to the orientation average projected area of the particle. This provides an explanation of the trend found experimentally, in terms of the particle shape. Quantitatively, the agreement is reasonable. Possible reasons for a discrepancy of about 20% are discussed. Also, the implications of these results for the use and further development of both attenuation spectroscopy and acoustic backscattering (profiling) methods for in situ measurements of suspended sediments are discussed.
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Document Type: Research Article
Publication date: March 1, 1998