Brian T. Hefner

Sound speed and attenuation measurements in unconsolidated glass-bead sediments saturated with viscous pore fluids

As part of a recent ocean sediment acoustics experiment, a number of independent sound speed and attenuation measurements were made in a well-characterized sandy sediment. These measurements covered a broad frequency range and were used to test both Biot-Stoll theory and Buckingham’s more recent grain-to-grain shearing model. While Biot theory was able to model the sound speed well, it was unable to predict the attenuation measured above 50kHz. This paper presents a series of measurements made in the laboratory on a simple glass-bead sediment. One goal of these measurements was to test the hypothesis that the attenuation measured at-sea was a result of scattering from shells within the sediment. The laboratory sediments used were saturated with fluids with different viscosities in order (assuming that Biot-Stoll theory is correct) to shift the dispersion into the frequency range of the measurement system. The measured attenuation in the glass-bead sediments exhibited the same frequency dependence as obser...

Mid- to High-Frequency Acoustic Penetration and Propagation Measurements in a Sandy Sediment

During the recent 2004 sediment acoustics experiment (SAX04), a buried hydrophone array was deployed in a sandy sediment near Fort Walton Beach, FL. The array was used to measure both the acoustic penetration into the sediment and sound speed and attenuation within the sediment while a smaller, diver-deployed array was also used to measure sound speed and attenuation. Both of these systems had been deployed previously during the 1999 Sediment Acoustics Experiment (SAX99). In that experiment, the buried array was used to make measurements in the 11-50-kHz range while the diver-deployed array made measurements in the 80-260-kHz range. For the SAX04 deployment, the frequency range for the measurements using the buried array was lowered to 2 kHz. The diver-deployed array was also modified to cover the 40-260-kHz range. Unlike the SAX99 deployment, there were no obvious sand ripples at the SAX04 buried array site at the time of the measurements. To examine the role of sand ripples in acoustic penetration over this new frequency range, artificial ripple fields were created. For the high frequencies, the penetration was consistent with the model predictions using small-roughness perturbation theory as in SAX99. As the frequency of the incident acoustic field decreased, the evanescent field became the dominant penetration mechanism. The sound speed measured using the buried array exhibits dispersion consistent with the Biot theory while the measured attenuation exceeds the theory predictions at frequencies above 200 kHz.

Backscattering enhancements associated with the excitation of symmetric Lamb waves on a circular plate: direct and holographic observations

Significant backscattering enhancements occur for thin circular plates tilted close to the extensional wave coupling angle. Two mechanisms responsible for these enhancements are explored. The response that is initially the largest is from a leaky extensional wave that travels along the diameter and reflects from the plate edge. The second response is due to extensional waves traveling along off-diameter paths. When these waves strike the plate edge, they mode convert into shear waves. These shear waves strike the opposite edge and mode convert into extensional waves that can reradiate back towards the source. These mechanisms are confirmed with backscattering measurements and images of the plate motion from acoustic holography.

Acoustical helicoidal waves and Laguerre–Gaussian beams: Applications to scattering and to angular momentum transport

Traveling waves having helicoidal wavefronts are cylindrically symmetric except for an azimuthal angular dependence of exp(imφ−iωt), where m is an integer. The amplitude vanishes at the screw‐phase dislocation [J. F. Nye and M. V. Berry, Proc. R. Soc. London Ser. A 336, 165–190 (1974)] on the z propagation axis. Examples of paraxial helicoidal waves are the Laguerre–Gaussian beam solutions of the parabolic wave equation. Some potentially useful properties of acoustical helicoidal waves are analyzed. Just as for linearly polarized optical Laguerre–Gaussian beams [L. Allen et al., Phys. Rev. A 45, 8185–8189 (1992)], the ratio of the axial angular momentum flux to the acoustical beam power is found to be m/ω so that axial radiation torques are generated when the acoustical energy is absorbed. Helicoidal waves are predicted to backscatter in a way which reveals the axisymmetry of the scatterer when combined with a helicoidal mode‐selective detector. Some strategies for acoustical helicoidal wave generation an...

Evaluation of Laser Scanning and Stereo Photography Roughness Measurement Systems Using a Realistic Model Seabed Surface

The topography of the seabed is influenced by sediment transport due to wave motion, current disturbance, and biological activities. The bottom roughness generated by these processes can substantially alter acoustic wave penetration into and scattering from the bottom, and therefore, it is essential to make accurate measurements of the bottom roughness for such acoustic applications. Methods to make direct measurements of bottom roughness include stereo photography, laser line scanning, and sediment conductivity. Roughness can also be measured indirectly using high-frequency sound backscatter. For optically-based methods, the accuracy of these measurements is typically evaluated using the elevations, lengths, or diameters of simple surface features of known dimensions. However, for acoustic applications, the statistical characteristics of the surface, e.g., the roughness spectrum, are more meaningful. In this paper, we present a fabricated rough surface milled into a 40 times 60 cm2 plastic block for use as a benchmark in the assessment of two in situ roughness measurement systems: a laser scanning system and a digital stereo photography system. The surface has a realistic roughness power spectrum that is derived from the bottom roughness measured during the 1999 Sediment Acoustics Experiment (SAX99) and was fabricated by a computer numerical controlled milling machine. By comparing the fabricated surface spectrum to the measured spectrum, a determination of the accuracy of the roughness measurement is evaluated, which is of direct relevance to acoustic applications.

Dispersion and attenuation due to scattering from heterogeneities of the frame bulk modulus of a poroelastic medium

While Biot theory can successfully account for the dispersion observed in sand sediments, the attenuation at high frequencies has been observed to increase more rapidly than Biot theory would predict. In an effort to account for this additional loss, perturbation theory is applied to Biots poroelastic equations to model the loss due to the scattering of energy from heterogeneities in the sediment. A general theory for propagation loss is developed and applied to a medium with a randomly varying frame bulk modulus. The theory predicts that these heterogeneities produce an overall softening of the medium as well as scattering of energy from the mean fast compressional wave into incoherent fast and slow compressional waves. This theory is applied to two poroelastic media: a weakly consolidated sand sediment and a consolidated sintered glass bead pack. The random variations in the frame modulus do not have significant effects on the propagation through the sand sediment but do play an important role in the propagation through the consolidated medium.

Underwater Acoustic Navigation Using a Beacon With a Spiral Wave Front

In this paper, a method for performing underwater acoustic navigation using a spiral wave-front beacon is examined. A transducer designed to emit a signal whose phase changes by 360° in one revolution can be used in conjunction with a reference signal to determine the aspect of a remote receiver relative to the beacon. Experiments are conducted comparing spiral wave-front beacon navigation to Global Positioning System (GPS) onboard an unmanned surface vehicle. The advantages and disadvantages of several outgoing signals and processing techniques are compared. The most successful technique involves the use of a phased array projector utilizing a broadband signal. Aspect is determined by using a weighted mean over frequencies. Sources of error for each of the techniques are also examined.

Transport Theory for Shallow Water Propagation with Rough Boundaries

At frequencies of about 1 kHz and higher, forward scattering from a rough sea surface (and/or a rough bottom) can strongly affect shallow water propagation and reverberation. The need exists for a fast, yet accurate method for modeling such propagation where multiple forward scattering occurs. A transport theory method based on mode coupling is described that yields the first and second moments of the field. This approach shows promise for accurately treating multiple forward scattering in one‐way propagation. The method is presently formulated in two space dimensions, and Monte‐Carlo rough surface PE simulations are used for assessing the accuracy of transport theory results.

Magnetic excitation and acoustical detection of torsional and quasi-flexural modes of spherical shells in water

A simple electromagnetic acoustic wave transducer (EMAT) based on a wire coil was used to excite the resonant modes of a stainless steel spherical shell without direct mechanical contact. The coupling produced by this EMAT was examined first for shells in air and then for shells immersed in water to examine the effects of fluid loading on the shell’s spectrum. It was found that the torsional modes were excited using this method and these modes radiated sound into the surrounding water contrary to expectations. This excitation is shown to depend on the presence of a permanent magnetization in the shell itself or on the presence of a static external field applied at right angles to the axis of the coil. Possible mechanisms for the excitation and the acoustic radiation of the torsional modes are considered. The excitation of quasi-flexural shell modes is also discussed for shells in air and in water. The shell responds at the oscillation frequency of the applied field and at twice the frequency. Some potenti...

Backscattering enhancements associated with antisymmetric Lamb waves confined to the edge of a circular plate: Direct and holographic observations

Significant backscattering enhancements occur for thin circular plates tilted close to the coupling angle for the second antisymmetric leaky wave, the a 1 wave. Imaging the surface of the plate using acoustic holography shows that the mechanism responsible for this enhancement is associated with the excitation of a leaky wave that is confined to the edge of the plate. This “circumferential” wave couples to the incident field at the top and bottom of the plate and travels around the plate with a group velocity that is much slower than that of the a 1 wave. This circumferential wave may be a whispering gallery mode associated with the a 1 wave or a higher order flexural edge wave.