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. Numerica l calculations, using finite and boundary element methods with a Varia tional Helmholtz Gradient Formulation (VHGF) approach, were carried ou t for rigid immobile particles for four different non-spherical (polyh edral) shapes for values of the normalized frequency ka < 10. Attenuat ion spectra were measured in frequency range of 1-100 MHz for some sus pensions of sand and quartz particles in the size range of 30 to 200 m u m. It was observed that the attenuation for ka > 1 is enhanced with respect to the spherical case. The enhancement of the attenuation is l arger the more irregular the particle shape is, up to a factor of 1.8 found experimentally. The numerical calculations also show an enhancem ent of the attenuation cross section at high values of ka which is pro portional to the orientation average projected area of the particle. T his provides an explanation of the trend found experimentally, in term s of the particle shape. Quantitatively, the agreement is reasonable. Possible reasons for a discrepancy of about 20% are discussed. Also, t he implications of these results for the use and further development o f both attenuation spectroscopy and acoustic backscattering (profiling ) methods for in situ measurements of suspended sediments are discusse d.