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The vertical structure of downslope velocity within sub-aqueous gravity-driven flows of (smoother) glass beads and (rougher) natural sand is investigated for both fixed roughness and erodible beds using high-resolution, MHz-frequency acoustics. The observed velocity profiles within the O(1) cm thick, O(10) cm/s flows exhibit a negative shear layer extending downward from the sediment–water interface to a velocity maximum at ∼ 9 grain diameters depth within the layer, below which the velocities decrease to near-zero values at the pre-flow bed location for fixed roughness beds and to non-zero values for mobile beds. The attenuation of sound transmitted through the moving layer is used to constrain the depth-averaged solids concentration to a value of ∼ 0.52. The observed negative shear at the interface indicates that, unlike the sub-aerial case, interfacial friction is dynamically important in gravity-driven sub-aqueous granular flows. It is shown that the observed vertical structure of velocity within the layer can be well represented by continuum viscous flow models. Solids concentration and effective viscosity are estimated from the best-fit model parameters using the Zarraga–Hill–Leighton (2000) empirical relation for suspensions of negatively buoyant particles, yielding vertically averaged values ∼ 0.57. While the sub-millimeter vertical resolution of the measurements is too coarse to provide precise estimates of the friction velocity at the interface, the model-data comparisons nevertheless indicate that the vertical structure of the downslope flow consists of a weakly stratified dense layer and a thin, dilute transition layer between the dense flow and the overlying water. [ABSTRACT FROM AUTHOR]

Acoustic reflection coefficients are reported for water-saturated granular media at frequencies from 1.2 to 2.0MHz using a narrow-beam broadband transducer in a monostatic geometry at near-normal incidence. Natural sand and glass beads with median grain diameters ranging from 0.22 to 0.40mm were used. For each granular medium, bed elevation and root-mean-square roughness were measured using side-on photographs of the sediment-water interface. The probability density distributions of the bed elevations are Gaussian. The roughness parameter is close to 1, indicating that the reflected pressure field is mainly due to coherent scattering. The probability distribution of the observed reflection coefficients is nearly Gaussian, consistent with the predictions from a coherent single-scattering model. The horizontal decorrelation length of the observed reflection coefficients is˜4 mm, with no consistent dependence on either frequency or grain size, and approximately equal to 20% of the transducer diameter. This behaviour, which is reproduced by the single-scattering model, is due to speckle. The size/frequency-dependence of the reflection coefficients are well described by Eckart’s [(1953). J. Acoust. Soc. Am. 25(3), 566–570] prediction for a rough surface with Gaussian-distributed surface elevations. Comparisons are made to previously reported reflection coefficient measurements. [ABSTRACT FROM AUTHOR]

Measurements of the phase velocity of compressional sound waves in water-saturated granular materials are reported for the 1.0–2.0 MHz frequency range. The sound speed estimates are based on travel times through granular layer thicknesses ranging from 8 to 17 mm. Three types of granular media were used: 336 μm median diameter glass beads and two natural sands with median diameters of 219 and 406 μm. These grain sizes and frequency range correspond to 0.5 < k a < 1.2 , where k is the wavenumber and a the grain radius. To remove trapped air, the samples were boiled under pressure before transfer to the measurement tank. The results are compared to previously reported experimental results and to the Schwartz and Plona [J. Appl. Phys. 55(11), 3971–3977 (1984)] multiple scattering prediction, confirming negative dispersion for ka > 0.5. Scaling the data by a factor depending on porosity and grain density reduces the spread among the available phase speed estimates by nearly a factor of 2, from 12.5% to 6.9%. [ABSTRACT FROM AUTHOR]

Attenuation and group speed measurements are reported for water-saturated granular materials (natural sand and glass beads) at frequencies of 1.0 to 1.8 MHz. Median grain diameters were 219 to 497 μm, corresponding to k d ≳ 1 , i.e., the scattering regime. The measurements were made for different thicknesses of sediment resting on a reflective surface using a monostatic geometry. The attenuation estimates compare well with previously reported experimental results and to the predictions of multiple scattering theory, confirming in particular the tendency toward f4 dependence for k d ≳ 1. Group speed estimates exhibit the negative dispersion predicted by theory and are comparable in magnitude to previously reported measurements made using transmission geometries. It is found that the available data exhibit a O(10)% spread among the sound speed measurements at a given kd value, and that this spread is reduced to 2.2% when the data are scaled by a factor dependent on porosity and grain density, and that essentially all of the reduction can be attributed to differences in porosity. [ABSTRACT FROM AUTHOR]

The short-pulse acoustic backscatter amplitude calibration technique of Dragonette, Numrich, and Frank [(1981), J. Acoust. Soc. Am. 69, 1186–1189] is evaluated for MHz frequency systems, as are used to measure environmental suspended sediment concentration. The technique involves measuring backscatter from a short transmit pulse reflected by a solid spherical target, and treating the first received reflection as equivalent to that from a perfectly rigid sphere. In this implementation, the target sphere is 16mm diameter tungsten carbide. The target is mounted on a stiff support rod rather than being suspended on a filament, enabling precise positioning of the sphere relative to the transducer. That the support rod does not significantly influence the calibration is demonstrated by reconstructing the backscatter time series, including the sequence of pulses from surface (Rayleigh) waves, using elastic scattering theory. Calibration results are verified in a separate experiment in which the backscatter from dilute aqueous suspensions of 400–600 lm diameter polystyrene beads in water is measured within the 1–2MHz frequency band. After correcting for the additional attenuation within the scatterers, the resulting values of the backscatter form factor are within 20% of the theoretical predictions. [ABSTRACT FROM AUTHOR]

A methodology is developed and tested for simultaneously extracting time-resolved one-dimensional profiles of the mass-concentration and velocity of two different particle types in a mixed suspension, using a multi-frequency pulse-to-pulse coherent Doppler instrument. The technique involves inversion of a model for frequency-dependent acoustic backscatter amplitude and phase. Results are presented from a laboratory settling column experiment, measuring a mixture of polystyrene beads (slowly-settling, strongly-scattering) and glass beads (quickly-settling, weakly-scattering) in a vertical pipe section. [ABSTRACT FROM AUTHOR]

Sound scattering by solid particles suspended in a turbulent jet is investigated. Measurements of the scattered amplitude were made in a bistatic geometry at frequencies between 1.5 and 4.0 MHz, and at scattering angles from 95° to 165° relative to the forward direction. Two types of particle were used: nearly spherical lead-glass beads and aspherical natural sand grains. For each particle type, experiments were carried out using ∼200 and ∼500 μm median diameter grain sizes, corresponding to 0.7≤=ka≤=4. The sphericity of the sand grains, defined as the ratio of projected perimeter size to projected area size, was 1.08. The lead-glass bead results are consistent with an elastic sphere model. A rigid movable sphere model provides the best fit to the sand data, and the best-fit diameter is within 4% of the equivalent volume size. However, the scattering pattern for sand is systematically smoother than predicted: that is, the undulations in the angular scattering pattern predicted by spherical scatterer theory are present, but muted. This observed departure from spherical scatterer theory is attributed to disruption of the interference among creeping waves by the irregular surfaces of natural sand grains. [ABSTRACT FROM AUTHOR]

Multifrequency acoustic scattering experiments were carried out in the laboratory using a free, turbulent water jet carrying solid particles in suspension. A crossed-beam geometry was used, in which the axis of the jet was perpendicular, or nearly so, to the direction of the incident sound. The suspended particles were either natural sand grains or lead-glass beads, 100-500 μm in diameter. The results, based on measurements made simultaneously at 1, 2.25, and 5 MHz, include backscatter and attenuation as a function of suspended sediment concentration, measurements of the total scattering and backscattering cross sections of natural sand grains and glass beads as a function of frequency and particle size, and two-point correlation measurements of the fluctuations in apparent suspended sediment concentration. The lead-glass bead cross sections exhibit both the resonance and diffraction extrema expected of solid spherical scatterers; the sand cross sections do not and, for acoustic wavelengths comparable to or less than the particle circumference, are larger than spherical scatterer theory would predict. The two-point correlation estimates of jet velocity and measurements of the time-averaged jet width as a function of particle size are related to the mean and turbulent structure of two-phase jets and illustrate the potential of acoustic methods for noninvasive investigations of two-phase turbulent flow. [ABSTRACT FROM AUTHOR]

Measured and theoretical total cross sections for aqueous suspensions of lead glass and polystyrene beads are presented. Attenuation is measured from 1 to 100 MHz in a recirculating suspension circuit using a swept-frequency system and broadband transducer pairs. The theoretical calculations employ the phase shift formalism and the coherent removal of a rigid scatterer background. Good agreement between theory and experiment is obtained for both types of particle, and departures from the rigid sphere cross section are due to resonance scattering. The resonance features in the glass bead cross sections are dominated by broad Rayleigh wave resonances, with frequencies close to those given by the in vacuo free-body resonance condition, and represent departures from the rigid sphere cross section of, at most, 25%. In contrast, the polystyrene bead cross sections are dominated by narrow resonance features with maximum amplitudes up to five times greater than the geometric scattering cross section. It is shown that these resonances, which we call giant resonances, occur at frequencies given by an immersed free-body resonance condition that is derived by assuming the existence of wave trapping in the fluid at the surface of the sphere. This condition yields frequencies that are wholly real, consistent with the observed narrow resonance widths, and leads to an approximate expression for the resonance amplitudes that is in good agreement with the exact calculations. The meridional wave phase speeds are consistent in the appropriate limit with Stonely waves. The giant resonances cease to exist if the meridional wave phase speed approaches the speed of sound in the ambient fluid. [ABSTRACT FROM AUTHOR]

The available data for scattered acoustic intensity and attenuation in dilute aqueous suspensions of sand are compared with theory. In theoretical calculations, the scatterer is assumed to be spherical and elastic, or rigid and movable, or rigid and immovable. The rigid movable model provides the best fit to the data. The failure of the elastic model in comparison to the rigid sphere models indicates that resonance excitation does not occur in natural sand grains, probably because of irregularities in shape. The fact that better agreement with experiment is obtained with the rigid movable model than with the rigid immovable model indicates that the inertia of the particles is important. Additional approximate expressions for the form factor and attenuation coefficient have been constructed based on a modified form of the so-called high-pass model introduced by Johnson [J. Acoust. Soc. Am. 61, 275-277 (1977)]. The modified high-pass model provides a fit to the data that is as good as, or better than, the rigid movable case. [ABSTRACT FROM AUTHOR]

An explicit expression is derived for the phase shifts of the partial waves scattered by a solid elastic sphere, incorporating the effects of the viscosity of the ambient fluid. The solution is applicable to frequencies and scatterer sizes for which the viscous boundary layer thickness is much less than the radius of the sphere. For particle sizes in the sand-size range (30 μm

The theory of sound scattering and attenuation by a mobile, elastic sphere in a viscous, heat-conducting fluid is formulated in terms of phase angles for the partial scattered waves. The solution reduces in the appropriate limit to that for the inviscid, nonconducting case. Specializing to the long-wavelength limit, an important case in marine acoustic remote sensing applications, expressions are obtained for the sound pressure amplitude of the scattered wave including the effects of viscous and thermal absorption at the fluid-particle interface. Furthermore, an expression is obtained for the ratio of thermal to viscous absorption and it is shown that this ratio is small for particles with bulk densities greater than 1.3-1.7 g cm-3 suspended in water, depending upon the thermal and elastic properties of the solid, and provided the acoustic frequency is such that the skin depths of the thermal and viscous waves are less than the particle radius. [ABSTRACT FROM AUTHOR]

Results are presented from an acoustic attenuation spectroscopy study of suspensions of irregularly shaped sediment particles and of nominally spherical lead-glass beads. The measured spectra cover a wide frequency band of 1-100 MHz. It is shown that the spectra can be brought into close agreement with the rigid movable sphere model using two equivalent sphere size parameters. One parameter is the diameter dG of an equal cross-sectional area sphere, and is used to scale the frequency to obtain the nondimensional size/frequency parameter ka. The other parameter is the diameter dp of an equal volume sphere, and is used to scale the particle volume to obtain the particle number density N. It was observed that for the irregular particles the attenuation for ka>1 is enhanced with respect to the sphere case. The observed enhancement factors are considerable: up to 1.6 for natural sand particles and 1.8 for the most irregular particles studied (ground quartz). In terms of the two-parameter model the enhancement factor is equal to a shape parameter b0=(dG/dp)3 and can indeed be explained as a shape effect. The implications of this result for the acoustic detection of suspended sediments in aqueous environments are discussed. © 1997 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

An inversion algorithm for extracting suspended sand size and concentration from simultaneous backscattered acoustic pressure amplitude at three operating frequencies is presented. The algorithm is based on the differences in signal amplitude between different frequency pairs, and is tested using laboratory measurements of multifrequency backscatter from a turbulent sediment-carrying jet. Concentration and size profiles inverted from field and laboratory data are compared with results from a previously developed algorithm based on signal ratios. The difference inversion scheme is less sensitive to errors arising from low signal levels, allowing the size/concentration measurement range to be extended to regions of lower concentration. The concentrations from the field data agree well with independent optically determined estimates. The results demonstrate sensitivity to the backscatter form factor. [ABSTRACT FROM AUTHOR]

Analytical results are obtained for the amplitude of a spherical wave backscattered from a solid cylinder immersed in water. The effects of transducer directivity and transmitted pulse duration are included. Backscatter measurements for comparison with theory are made using straight stainless steel wires ranging from 0.13 to 0.26 mm diameter, and acoustic transceivers operating at 1, 2.25, and 5 MHz. The results are used to evaluate the use of thin wires as standard targets. [ABSTRACT FROM AUTHOR]

A method using a broadband transducer to measure the acoustic backscatter cross section of suspended sand particles is investigated. The frequencies used range from 1.3 to 2.8 MHz, and the sand sizes from 100- to 350-μm diameter. The measurements are made in the transducer near field. The measured form factor is compared with the theoretical result for the movable rigid sphere model, and with previous narrow-band measurements. [ABSTRACT FROM AUTHOR]

A closed-form expression for the pressure field produced by a circular piston radiator with simply supported boundary condition and continuous-wave excitation is derived. The method of Hasegawa et al. [J. Acoust. Soc. Am. 74, 1044-1047 (1983)], developed for a uniformly vibrating piston, is extended to include the nonuniform case. The resulting expression is valid for all field points and for large ratio of source diameter to wavelength. The average pressure on the surface of a circular receiver as a function of receiver size and distance from the transducer is also calculated. [ABSTRACT FROM AUTHOR]

An expression for the viscous attenuation coefficient in a suspension of solid particles is derived using the viscosity-modified phase shifts for a solid sphere presented in a previous article [Hay and Mercer, J. Acoust. Soc. Am. 78, 1761-1771 (1985)]. This expression reduces in the appropriate limits to the well-known result for viscous attenuation by small rigid mobile particles. [ABSTRACT FROM AUTHOR]

Sediment transport in the oceanic bottom boundary layer involves threefold coupling among: (1) the motion of sediment grains; (2) the response of the bed; and (3) the fluid forcing. The appeal of acoustic remote sensing methods is that in principle all three components of the problem can be addressed with minimal disturbance to the system. This paper reviews the progress that has been made to date toward developing acoustic instrumentation systems for sediment transport measurements in the nearshore zone, where sand is the principal component of the bottom sediments. Instrument developments are discussed in relation to several types of measurement requiring multiple acoustic beams: (1) range-gated multifrequency backscatter to obtain profiles of suspended sediment concentration and size, and the associated inverse problem; (2) bedform imaging with fan-beam sonars; (3) suspended sediment fluctuations under waves. The results provide revealing insights into the sediment transport process, and thus demonstrate the effectiveness of the multibeam approach. The results also raise a number of fundamental questions. Some of these questions can be addressed with the instrumentation currently at hand. Others will require a new generation of instruments. [ABSTRACT FROM AUTHOR]

Results are presented from the acoustic remote sensing component of Dalhousie University’s interdisciplinary Coastal Ocean Observatory in Lunenburg Bay, Nova Scotia, operational since June 2002 (www.cmep.ca/bay). Remote acoustic measurements of water column velocity and backscatter are made from bottom-mounted instruments hard-wired to surface buoys. The data are telemetered in near real-time to a shore station via wireless Ethernet, and to Dalhousie over the Internet. The fixed-point measurements from the bottom pods are augmented by a vessel-mounted profiling system, and by opportunistic deployments of internally-recording instruments. Highlights from the first three years of the observing system operation will be presented, including sediment suspension from the seabed and bubble penetration from the surface during Hurricane Juan. [ABSTRACT FROM AUTHOR]

The field of sediment transport measurement using acoustic remote sensing technique is reviewed, summarizing recent advances and possible directions for the future. The focus is on elucidating sediment transport mechanisms in the bottom boundary layer, and particularly on estimating terms in the sediment mass transport equation. A key problem is the inversion of acoustic backscatter profiles to obtain suspended sediment size and concentration. Several fundamental issues are involved: the scattering cross sections of irregular particles; estimating scatterer concentration fluctuations from fluctuations in signal amplitude; the related problem of inversion stability in the presence of fluctuations; the spatial coherence scales of the concentration field; and sediment mass flux estimation. These issues are illustrated with results from measurements made both in the laboratory and in the field. While several of the above issues remain unsolved, the results are nevertheless quite promising, and indicate a bright future for the application of acoustics to the sediment transport measurement problem. [ABSTRACT FROM AUTHOR]