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

Multiwavelength studies of MHD waves in the solar chromosphere: An overview of recent results

The chromosphere is a thin layer of the solar atmosphere that bridges the relatively cool photosphere and the intensely heated transition region and corona. Compressible and incompressible waves propagating through the chromosphere can supply significant amounts of energy to the interface region and corona. In recent years an abundance of high-resolution observations from state-of-the-art facilities have provided new and exciting ways of disentangling the characteristics of oscillatory phenomena propagating through the dynamic chromosphere. Coupled with rapid advancements in magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly investigate the role waves play in supplying energy to sustain chromospheric and coronal heating. Here, we review the recent progress made in characterising, categorising and interpreting oscillations manifesting in the solar chromosphere, with an impetus placed on their intrinsic energetics.Comment: 48 pages, 25 figures, accepted into Space Science Review

Factors Associated with Revision Surgery after Internal Fixation of Hip Fractures

Background: Femoral neck fractures are associated with high rates of revision surgery after management with internal fixation. Using data from the Fixation using Alternative Implants for the Treatment of Hip fractures (FAITH) trial evaluating methods of internal fixation in patients with femoral neck fractures, we investigated associations between baseline and surgical factors and the need for revision surgery to promote healing, relieve pain, treat infection or improve function over 24 months postsurgery. Additionally, we investigated factors associated with (1) hardware removal and (2) implant exchange from cancellous screws (CS) or sliding hip screw (SHS) to total hip arthroplasty, hemiarthroplasty, or another internal fixation device. Methods: We identified 15 potential factors a priori that may be associated with revision surgery, 7 with hardware removal, and 14 with implant exchange. We used multivariable Cox proportional hazards analyses in our investigation. Results: Factors associated with increased risk of revision surgery included: female sex, [hazard ratio (HR) 1.79, 95% confidence interval (CI) 1.25-2.50; P = 0.001], higher body mass index (fo

The measurement of the in situ compressional wave properties of marine sediments

Geoacoustic inversion requires a generic knowledge of the frequency-dependence of compressional wave properties in marine sediments, the nature of which is still under debate. The use of in situ probes to measure sediment acoustic properties introduces a number of experimental difficulties that must be overcome. To this end, a series of well-constrained in situ acoustic transmission experiments were undertaken on inter-tidal sediments using a purpose-built in situ device, the Sediment Probing Acoustic Detection Equipment. Compressional wave velocity and attenuation coefficient were measured from 16 to 100 kHz in medium to fine sands and coarse to medium silts. Spreading losses, which were adjusted for sediment type, were incorporated into the data processing, as were a thorough error analysis and an examination of the repeatability of both the acoustic wave emitted by the source and the coupling between probes and sediment. Over the experimental frequency range and source-to-receiver separations of 0.99 – 8.1 m, resulting velocities are accurate to between + 1.1 to + 4.5 % in sands and less than + 1.9 % in silts, while attenuation coefficients are accurate to between + 1 to + 7 dB•m-1 in both sands and silts. Preliminary results indicate no velocity dispersion and an attenuation coefficient which is proportional to frequency

Inferring bubble populations in intertidal sediments from attenuation and scattering measurements

The presence of gas and gas bubbles, and their effects on the geoacoustic properties of sediments, have being the topics of many investigations. In the case of cohesive gassy sediments, the shape of the gas in the host medium has also been seen as important. In many cases the gas in muds appears to form cracks or take the form of oblate spheroids. However to make theory of the acoustic interactions tractable, they are often assumed to be spherical. In this paper, acoustic propagation and backscattering measurements are interpreted in order to infer shape information of gas inclusions (e.g. slabs, cracks or spheroids) The measurements were carried out at intertidal sites on the South coast of England. The bubble population was estimated from backscattering measurements using a nonlinear bubble dynamics inversion method. This method assumes the presence of spherical bubbles only and uses a 'mixed-frequency' insonification which limits ambiguities in interpretation of the measurements. Then the computed bubble population is converted to attenuation values. These attenuation values are compared to attenuation values obtained from the propagation measurements in order to compare the two methods and speculate the sphericity of the bubbles present

Acoustic backscatter at very high frequencies from rough seabeds

Acoustic backscatter data were gathered from a variety of seabed types, both in the laboratory tank and in coastal waters. Data were gathered from sandy sediments in the tank, with a variety of characterized rough surfaces, using narrow-band pulses at frequencies between 100 and 950 kHz. Wideband data gathered at sea were obtained at frequencies between 80 and 650 kHz. Data gathered at sea from the Acoustic Range at QinetiQ Bincleaves included backscatter from the natural sandy seabed, but also scatter from several artificial sediments (sand, gravel, and pebbles) placed in a rotator. The latter equipment allowed acoustic interrogation of the same patch of seabed from multiple angles. Experimental data are compared with fluid, poroelastic, and discrete scatterer models, with a view to recommendations for the modeling of seabed backscatter in the frequency band 100 kHz to 1 MHz

Clutter suppression and classification using twin inverted pulse sonar in ship wakes

Twin inverted pulse sonar (TWIPS) is here deployed in the wake of a moored rigid inflatable boat (RIB) with propeller turning, and then in the wake of a moving tanker of 4580 dry weight tonnage (the Whitchallenger). This is done first to test its ability to distinguish between scatter from the wake and scatter from the seabed, and second to test its ability to improve detectability of the seabed through the wake, compared to conventional sonar processing techniques. TWIPS does this by distinguishing between linear and nonlinear scatterers and has the further property of distinguishing those nonlinear targets which scatter energy at the even-powered harmonics from those which scatter in the odd-powered harmonics. TWIPS can also, in some manifestations, require no range correction (and therefore does not require the a priori environment knowledge necessary for most remote detection technologies)