Theory for acoustic propagation in materials which can support stress and which contain gas bubbles, with applications to acoustic effects in marine sediment and tissue

Whilst there is a considerable body of work in the literature on the theory of acousticpropagation in marine sediment, the incorporation of gas bubbles into such theories isdone with the inclusion of assumptions which severely limit the applicability of thosemodels to practical gas-laden marine sediments.Section 2 develops a theory appropriate for predicting the acoustically-drivendynamics of a single spherical gas bubble embedded in an incompressible lossyelastic solid. Use of this theory to calculate propagation parameters requirescalculation of the gas pressure component of section 2, and the options are outlined insection 3. The incorporation of radiation losses is discussed in section 4. Section 5discusses how the entire scheme can be incorporated into a nonlinear, time-dependentpropagation model

A method for estimating sound speed and the void fraction of bubbles from sub-bottom sonar images of gassy seabeds

There is increasing interest in the effect of bubbles in gassy sediment. This is, first, because ofthe impact those bubbles have on the structural integrity and load-bearing capabilities of thesediment; second, because the presence of bubbles can be indicative of a range of biological,chemical or geophysical processes (such as the climatologically-important flux of methanefrom the seabed to the atmosphere); and third, because of the effect which the bubbles haveon any acoustic systems used to characterise the sediment. For this reason, a range of methodshave been investigated for their ability to estimate the bubble population in the seabed. Withinsuch a range, there will a mix of advantages and limitation to given techniques. This reportoutlines a very basic method by which an observations which have already been taken forother purposes (sub-bottom profiles) may be subjected to a rapid analysis to obtain anestimate of the effect of bubbles on the sound speed in the sediment, and from there toprovide a rapid preliminary estimate of the void fraction of bubbles present (assuming quasistaticbubble dynamics). This approach is not meant to compete with large-scale field trialswhich deploy specialist equipment to monitor gas bubbles in sediment, but rather to provide amethod to exploit archived sub-bottom profiles, or to survey a large area rapidly withcommercial equipment from a small vessel, in order to obtain an estimation of the local voidfractions present, and their location and extent in three dimensions

The use of acoustics in space exploration

In recent years increased attention has been paid to the potential uses of acoustics forextraterrestrial exploration. The extent to which acoustics per se is used in these studiesvaries greatly. First, there are the cases in which acoustics is simply the medium throughwhich some other time-varying non-acoustic signal (such as the output of a cosmic raydetector) is communicated to humans. Second, perturbations in a non-acoustic signal (e.g.EM) are interpreted through mechanisms relating to acoustic perturbations in the sourcematerial itself. Third, some probes have made direct measurements of acoustic signalswhich have been generated by the probe itself, as is done for example to infer the localatmospheric sound speed from the time-of-flight of an acoustic pulses over a shortdistance (O(10 cm)). Fourth, some studies have discussed ways of interpreting thenatural acoustic signals generated by the extraterrestrial environment itself. The reportdiscusses these cases and the limitations, implications and opportunities forextraterrestrial exploration using acoustics

Derivation of the Rayleigh-Plesset equation in terms of volume

The most common nonlinear equations of motion for the pulsation of a spherical gas bubble in an infinite body of liquid arise in the various forms of the Rayleigh-Plesset equation, expressed in terms of the dependency of the bubble radius on the conditions pertaining in the gas and liquid. However over the past few decades several important analyses have begun with a heuristically-derived form of the Rayleigh-Plesset equation which considers the bubble volume, instead of the radius, as the parameter of interest, and for which the dissipation term is not derived from first principles. The predictions of these two sets of equations can differ in important ways, largely through differences between the methods chosen to incorporate damping. As a result this report derives the Rayleigh-Plesset equation in terms of the bubble volume from first principles in such a way that it has the same physics for dissipation (viscous shear) as is used in the radius fram

Theory for acoustic propagation in solid containing gas bubbles, with applications to marine sediment and tissue

Whilst there is a considerable body of work in the literature on the theory of acoustic propagation in marine sediment, the incorporation of gas bubbles into such theories is done with the inclusion of assumptions which severely limit the applicability of those models to practical gas-laden marine sediments.Following an Introduction (section 1), section 2 develops a theory appropriate for predicting the acoustically-driven dynamics of a single spherical gas bubble embedded in an incompressible lossy elastic solid. Use of this theory to calculate propagation parameters requires calculation of the gas pressure component of section 2, and the options are outlined in section 3, with the implications for the description of dissipation. This leads to a discussion in section 4 into further of how dissipation enters the description, and in section 5 how the entire scheme can be incorporated into a propagation model

Dolphin-inspired target detection for sonar and radar

Gas bubbles in the ocean are produced by breaking waves, rainfall, methane seeps, exsolution, and a range of biological processes including decomposition, photosynthesis, respiration and digestion. However one biological process that produces particularly dense clouds of large bubbles, is bubble netting. This is practiced by several species of cetacean. Given their propensity to use acoustics, and the powerful acoustical attenuation and scattering that bubbles can cause, the relationship between sound and bubble nets is intriguing. It has been postulated that humpback whales produce ‘walls of sound’ at audio frequencies in their bubble nets, trapping prey. Dolphins, on the other hand, use high frequency acoustics for echolocation. This begs the question of whether, in producing bubble nets, they are generating echolocation clutter that potentially helps prey avoid detection (as their bubble nets would do with man-made sonar), or whether they have developed sonar techniques to detect prey within such bubble nets and distinguish it from clutter. Possible sonar schemes that could detect targets in bubble clouds are proposed, and shown to work both in the laboratory and at sea. Following this, similar radar schemes are proposed for the detection of buried explosives and catastrophe victims, and successful laboratory tests are undertaken

The use of capillary as a sensor of cavitation

The dramatic rise in height of the meniscus within a capillary, the other end of which is immersed in a liquid undergoing ultrasonic cavitation, represents a novel, robust and inexpensive method for monitoring cavitational activity. Here this effect is compared quantitatively with the multibubble sonoluminescence, which is simultaneously being emitted by the sample. A comparison is made of both the thresholds of the two measures, and of their magnitudes in the super-threshold condition

Studies into the detection of buried objects (particularly optical fibres) in saturated sediment. Part 2: design and commissioning of test tank

This report is the second in a series of five, designed to investigate the detection oftargets buried in saturated sediment, primarily through acoustical or acoustics-relatedmethods. Although steel targets are included for comparison, the major interest is intargets (polyethylene cylinders and optical fibres) which have a poor acousticimpedance mismatch with the host sediment. This particular report details theconstruction of a laboratory-scale test facility. This consisted of three maincomponents. Budget constraints were an over-riding consideration in the design.First, there is the design and production of a tank containing saturated sediment. Itwas the intention that the physical and acoustical properties of the laboratory systemshould be similar to those found in a real seafloor environment. Particularconsideration is given to those features of the test system which might affect theacoustic performance, such as reverberation, the presence of gas bubbles in thesediment, or a suspension of particles above it. Sound speed and attenuation wereidentified as being critical parameters, requiring particular attention. Hence, thesewere investigated separately for each component of the acoustic path.Second, there is the design and production of a transducer system. It was the intentionthat this would be suitable for an investigation into the non-invasive acousticdetection of buried objects. A focused reflector is considered to be the most costeffectiveway of achieving a high acoustic power and narrow beamwidth. Acomparison of different reflector sizes suggested that a larger aperture would result inless spherical aberration, thus producing a more uniform sound field. Diffractioneffects are reduced by specifying a tolerance of much less than an acousticwavelength over the reflector surface. The free-field performance of the transducerswas found to be in agreement with the model prediction. Several parameters havebeen determined in this report that pertain to the acoustical characteristics of the waterand sediment in the laboratory tank in the 10 – 100 kHz frequency range.Third, there is the design and production of an automated control system wasdeveloped to simplify the data acquisition process. This was, primarily, a motordrivenposition control system which allowed the transducers to be accuratelypositioned in the two-dimensional plane above the sediment. Thus, it was possible forthe combined signal generation, data acquisition and position control process to be coordinatedfrom a central computer.This series of reports is written in support of the article “The detection by sonar ofxdifficult targets (including centimetre-scale plastic objects and optical fibres) buriedin saturated sediment” by T G Leighton and R C P Evans, written for a Special Issueof Applied Acoustics which contains articles on the topic of the detection of objectsburied in marine sediment. Further support material can be found athttp://www.isvr.soton.ac.uk/FDAG/uaua/target_in_sand.HTM

Acoustic bubble detection - I. The detection of stable gas bodies

The ability to detect and measure bubbles within liquids is of importance to a wide range of applications. In this paper the optical and especially the acoustic techniques appropriate to populations of stable bubbles, or gas bodies, will be examined. In a second paper, techniques appropriate to transient cavitation will be discussed

Acoustic bubble detection - II. The detection of transient cavitation

This paper follows Part I, which described primarily acoustic techniques for the detection of stable bubbles (gas bodies). Other techniques, based on optical, chemical, and erosive effects, were discussed. This paper describes techniques in the same four categories, though again concentrating primarily on acoustic methods, for the detection of transient cavitation, which is characterised by the sudden growth of a bubble from a small seed nucleus, followed by an energetic collapse and reboun