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

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

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

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

Stress affects reproduction

A short account of how stress interferes with reproduction in farmed fish

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

Volunteers and mega sporting events : developing a research framework

Interest in all aspects of the politics, financing, planning, management and operation of mega sporting events has been highlighted both by success stories and ongoing problems associated with Olympic Games, Football World Cups and other similar events. There is a growing literature that addresses these and related matters through both case history and comparative analyses. Within the context of mega sporting events, the issue of employment creation is an important motivator for host cities and features high on the political justification agenda for bids to host events. At the same time, the most significant working contribution to major mega events in sports, as in other areas, is provided by the very large numbers of volunteers who undertake tasks across the range of opportunities afforded by such events. Numbers of volunteers between 40,000 and 60,000 have been noted for some recent major events. Relatively little is known about these volunteers at mega sporting events and yet their contribution and wider impact is very significant, both to the events themselves and within the host community. This paper seeks to identify the evident gaps that exist in understanding areas such as what volunteers do at mega sporting events; who they are; what motivates them; how volunteering impacts upon their lives; what associated activities they do surrounding the event in the host city; and the extent to which volunteering is recidivistic. The paper concludes with the presentation of a tentative research framework agenda in order to guide future study of this important area

An investigation into the tourism and hospitality scholarly activity currently being undertaken in Irish higher education institutions

The purpose of this article is to describe the tourism and hospitality scholarly activity currently taking place in Irish higher education (HE) institutions. A number of approaches to tourism and hospitality research are identified in the literature. In order to understand the policy and practice within the Irish HE sector, qualitative research was conducted through a survey of tourism and hospitality academics and researchers within Irish HE institutions. The findings suggest that tourism and hospitality scholarly activity in Ireland is being carried out in many ways, including through joint research projects between Irish HE institutions and industry. This is a positive step towards achieving research and industry harmony. The development of stand-alone research centres within these institutions can also promote and attract funding for tourism and hospitality research activities