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

Comparison of theories for acoustic wave propagation in gassy marine sediments

More than three decades ago, Anderson and Hampton [1, 2] (A&H) presented theories for wave propagation in gassy water, saturated sediments and gassy sediments in their two part review, which has been cited by many researchers in the geoacoustics and underwater acoustics areas. They gave an empirical formulation based on the theory of Spitzer [3] for the wave propagation in gassy water by adapting that for a viscoelastic, lossy medium. Following Leighton [4], this paper presents a theory based on non-stationary nonlinear dynamics of spherical gas bubbles and extends that 2007 paper to include liquid compressibility and thermal damping effects. The paper then shows how that nonlinear formulation can be reduced to the linear limit, and derives the expressions for the damping coefficients, the scattering cross section, the speed of sound and the attenuation, and compares these with the A&H theory. The current formulation has certain advantages over A&H theory such as implementing an energy conservation based nonlinear model for the gas pressure inside the bubble, having no sign ambiguity for the speed of sound formula (which is important when estimating the bubble void fraction) and correcting the ambiguity on the expression for scattering cross section, as identified in the recent work of Ainslie and Leighton [5]. Moreover, the theory presented here forms a basis for a nonlinear, time-dependent acoustic estimation model for gas bubble distributions in viscoelastic mediums since it avoids the commonly encountered assumptions on the bubble dynamics such as linearity, steady-state behaviour and monochromaticity

Passive acoustic quantification of gas releases

The assessment of undersea gas leakages from anthropogenic and natural sources is becoming increasingly important. This includes the detection of gas leaks and the quantification of gas flux. This has applications within oceanography (e.g. natural methane seeps) and the oil and gas industry (e.g. leaks from undersea gas pipelines, carbon capture and storage facilities). Gas escaping underwater can result in the formation of gas bubbles, and this leads to specific acoustic pressure fluctuations (sound) which can be analysed using passive acoustic systems. Such a technique offers the advantage of lower electrical power requirements for long term monitoring. It is common practice for researchers to identify single bubble injection events from time histories or time frequency representations of hydrophone data, and infer bubble sizes from the centre frequency of the emission. Such a technique is well suited for gas releases that represent low flow rates, and involving solitary bubble release. However, for larger events, with the overlapping of bubble acoustic emissions, the inability to discriminate each individual bubble injection event makes this approach inappropriate. In this study, an inverse method to quantify such release is used. The model is first outlined and following this its accuracy at different flow rate regimes is tested against experimental data collected from tests which took place in a large water tank. The direct measurements are compared to estimates inferred from acoustics.<br/

Development of a new diagnostic device for extracorporeal shock-wave lithotripsy

Extracorporeal Shock-Wave Lithotripsy (ESWL) is the leading technique for the non-invasive treatment of urinary stones. Thousands of ultrasound shocks are focused on the stones in order to break them into fragments small enough to be passed naturally by the body. The procedure is well established, though the re-treatment rate is around 50%. One of the limits of the procedure is that there is no capability for on-line monitoring of the degree of fragmentation of the stone. The output of the treatments could probably be improved if this facility was made available. The underlying physical mechanisms responsible for the break-up of the stone are still subject to investigation. However both direct stress damage and indirect cavitation erosion seem to be necessary to obtain eliminable fragments. In previous studies, Coleman et al. monitored cavitation in-vivo through the associated acoustic emissions. The objective of this research was to design a new diagnostic device for lithotripsy, exploiting the information carried by these acoustic emissions. After preliminary laboratory experiments some clinical prototypes were developed in collaboration with Precision Acoustic Ltd., UK. The prototypes are currently been tested in the clinic

The estimation of geoacoustic properties from broadband acoustic data, focusing on instantaneous frequency techniques

The compressional wave velocity and attenuation of marine sediments are fundamental to marine science. In order to obtain reliable estimates of these parameters it is necessary to examine in situ acoustic data, which is generally broadband. A variety of techniques for estimating the compressional wave velocity and attenuation from broadband acoustic data are reviewed. The application of Instantaneous Frequency (IF) techniques to data collected from a normal-incidence chirp profiler is examined. For the datasets examined the best estimates of IF are obtained by dividing the chirp profile into a series of sections, estimating the IF of each trace in the section using the first moments of the Wigner Ville distribution, and stacking the resulting IF to obtain a composite IF for the section. As the datasets examined cover both gassy and saturated sediments, this is likely to be the optimum technique for chirp datasets collected from all sediment environments

Frequency dependence of acoustic waves in marine sediments

In situ techniques provide the most reliable method of examining the geoacoustical properties of marine sediments. In the past, individual in situ surveys have only been able to examine compressional waves over a maximum frequency range of 100 Hz to 50 kHz. A new in situ acoustic device, the Sediment Probing Acoustic Detection Equipment, or SPADE, has been developed, which can emit a variety of pulses, e.g. tonal and swept-frequency, over a continuous frequency range of 10 - 100 kHz. Data from a recent field trial are analysed to obtain the in situ velocity and attenuation over frequency increments of 5 kHz between 10 - 75 kHz. Results imply that scattering is a dominant attenuation mechanism from 10-75 kHz and the media is dispersive for frequencies between 60 and 70 kHz and below 20 kHz. Biot theory cannot accurately model the observed velocity and attenuation

Soil organic matter and the extracellular microbial matrix show contrasting responses to C and N availability

AbstractAn emerging paradigm in soil science suggests microbes can perform ‘N mining’ from recalcitrant soil organic matter (SOM) in conditions of low N availability. However, this requires the production of extracellular structures rich in N (including enzymes and structural components) and thus defies stoichiometric expectation. We set out to extract newly synthesised peptides from the extracellular matrix in soil and compare the amino acid (AA) profiles, N incorporation and AA dynamics in response to labile inputs of contrasting C/N ratio. Glycerol was added both with and without an inorganic source of N (10% 15N labelled NH4NO3) to a soil already containing a large pool of refractory SOM and incubated for 10 days. The resulting total soil peptide (TSP) and extracellular pools were compared using colorimetric methods, gas chromatography, and isotope ratio mass spectrometry. N isotope compositions showed that the extracellular polymeric substance (EPS) contained a greater proportion of products formed de novo than did TSP, with hydrophobic EPS-AAs (leucine, isoleucine, phenylalanine, hydroxyproline and tyrosine) deriving substantially more N from the inorganic source provided. Quantitative comparison between extracts showed that the EPS contained greater relative proportions of alanine, glycine, proline, phenylalanine and tyrosine. The greatest increases in EPS-peptide and EPS-polysaccharide concentrations occurred at the highest C/N ratios. All EPS-AAs responded similarly to treatment whereas the responses of TSP were more complex. The results suggest that extracellular investment of N (as EPS peptides) is a microbial survival mechanism in conditions of low N/high C which, from an evolutionary perspective, must ultimately lead to the tendency for increased N returns to the microbial biomass. A conceptual model is proposed that describes the dynamics of the extracellular matrix in response to the C/N ratio of labile inputs