Variability of the low-frequency acoustic response along leaf blades and between species of seagrass (Posidonia oceanica and Cymodocea nodosa).

An acoustic resonator was used to measure the low-frequency (1-5 kHz) effective acoustic properties of the leaf blades of two Mediterranean seagrass species (Posidonia oceanica and Cymodocea nodosa). Variability along blades was assessed by measuring the effective change in sound speed per gram blade biomass of the basal and apical halves of P. oceanica leaves separately (-11 and -1.5 m s-1 g-1, respectively). Large differences in the effective sound speed per unit biomass between P. oceanica and C. nodosa (43-52 m s-1 g-1 larger for C. nodosa) are discussed using microscopic imagery of blade cross-sections.

Low frequency acoustic properties of Posidonia oceanica seagrass leaf blades.

The acoustics of seagrass meadows impacts naval and oceanographic sonar applications. To study this environment, a one-dimensional resonator was used to assess the low-frequency (1-5 kHz) acoustic response of the leaf blades of the Mediterranean seagrass Posidonia oceanica in water. Three separate collections of plants from Crete, Greece, and Sicily, Italy were investigated. A high consistency in effective sound speed was observed within each collection while a strong variability was observed between different collections. Average size, mass, and epiphytic coverage within each collection were quantified, and discoloration and stiffness are discussed qualitatively with respect to the observed acoustic variability.

Measurement of low-frequency tissue response of the seagrass Posidonia oceanica.

A one-dimensional acoustic resonator technique was used to study leaves of the Mediterranean seagrass species Posidonia oceanica collected from Crete and Sicily. The leaf blades were finely divided, mixed with artificial seawater, and degassed to create a suspension of tissue independent of leaf structure and free bubbles or internal voids. The low-frequency (1 to 8 kHz) bulk modulus of the leaf tissue was inferred from the acoustic measurements and independent density measurements. The measured density of the seagrass tissue was 960 ± 20 kg/m3 which agrees with previously published values. The inferred bulk modulus was 2.1 GPa with 90% confidence limits 1.0-5.0 GPa.

In vivo tear film thickness measurement and tear film dynamics visualization using spectral domain optical coherence tomography.

Dry eye syndrome is a highly prevalent disease of the ocular surface characterized by an instability of the tear film. Traditional methods used for the evaluation of tear film stability are invasive or show limited repeatability. Here we propose a new non-invasive fully automated approach to measure tear film thickness based on spectral domain optical coherence tomography and on an efficient delay estimator. Silicon wafer phantom were used to validate the thickness measurement. The technique was applied in vivo in healthy subjects. Series of tear film thickness maps were generated, allowing for the visualization of tear film dynamics. Our results show that the in vivo central tear film thickness measurements are precise and repeatable with a coefficient of variation of about 0.65% and that repeatable tear film dynamics can be observed. The presented approach could be used in clinical setting to study patients with dry eye disease and monitor their treatments.

A Monte Carlo experiment for measuring acoustic properties of macroalgae living tissue.

A methodology is developed to measure ex situ ultrasonic velocity of submerged aquatic vegetation tissue, in particular, macroalgae, in a nondestructive and efficient manner. An entire thallus is submerged in artificial seawater-filled tank through which many ultrasonic pulse-echo measurements are recorded while thallus parts are randomly displaced. Average sound speed of tissue is estimated from normal fit to extracted travel times given measured total volume fraction of tissue and travel time in water alone. For species Ecklonia radiata the resulting values for sound speed 1573.4 ± 4.8 m s(-1) and adiabatic compressibility 3.134 ×10(-10) ± 1.34 ×10(-11) Pa(-1) at 18 °C agree with more laborious and destructive methods.

Acoustic interferometry for geoacoustic characterization in a soft-layered sediment environment.

The broadband spectrogram of a moving surface ship usually exhibits striations. Their structure is determined by bottom conditions of the shallow water waveguide and can therefore be used for environmental characterization. A two-step acoustic interferometry technique is proposed to estimate main geoacoustic properties of unconsolidated sediment by exploiting local features of the striations. Their positions at low frequencies are first used to detect the changes in sediment properties with respect to a reference sediment and provide a reliable estimation of the changes through the determination of a frequency shift. Then toward higher frequencies, local frequency-range areas with salient striations are selected to refine the solution with their structure features. The technique is tested with passive acoustic ship run data collected southeast of the island of Elba in the Mediterranean Sea in 2007. Data from the four receivers of a shallow sparse vertical array are processed to estimate the thickness and compression wave speed of a soft clay layer overlying a harder bottom. The results from individual receivers are close and agree well with active inversion results and seismic profiles in the same area. Moreover, a better resolution is obtained by combining these results. This method is demonstrated to be robust to source range uncertainties due to the striation stability to its small variation. The good experimental results suggest the technique is an effective tool for mapping the geoacoustic properties of wide coastal areas with easily deployed receiver systems or even one single receiver.

Sequential Bayesian geoacoustic inversion for mobile and compact source-receiver configuration.

Geoacoustic characterization of wide areas through inversion requires easily deployable configurations including free-drifting platforms, underwater gliders and autonomous vehicles, typically performing repeated transmissions during their course. In this paper, the inverse problem is formulated as sequential Bayesian filtering to take advantage of repeated transmission measurements. Nonlinear Kalman filters implement a random-walk model for geometry and environment and an acoustic propagation code in the measurement model. Data from MREA/BP07 sea trials are tested consisting of multitone and frequency-modulated signals (bands: 0.25-0.8 and 0.8-1.6 kHz) received on a shallow vertical array of four hydrophones 5-m spaced drifting over 0.7-1.6 km range. Space- and time-coherent processing are applied to the respective signal types. Kalman filter outputs are compared to a sequence of global optimizations performed independently on each received signal. For both signal types, the sequential approach is more accurate but also more efficient. Due to frequency diversity, the processing of modulated signals produces a more stable tracking. Although an extended Kalman filter provides comparable estimates of the tracked parameters, the ensemble Kalman filter is necessary to properly assess uncertainty. In spite of mild range dependence and simplified bottom model, all tracked geoacoustic parameters are consistent with high-resolution seismic profiling, core logging P-wave velocity, and previous inversion results with fixed geometries.

Optimal nonlocal boundary control of the wide-angle parabolic equation for inversion of a waveguide acoustic field.

This paper applies the concept of optimal boundary control for solving inverse problems in shallow water acoustics. To treat the controllability problem, a continuous analytic adjoint model is derived for the Claerbout wide-angle parabolic equation (PE) using a generalized nonlocal impedance boundary condition at the water-bottom interface. While the potential of adjoint methodology has been recently demonstrated for ocean acoustic tomography, this approach combines the advantages of exact transparent boundary conditions for the wide-angle PE with the concept of adjoint-based optimal control. In contrast to meta-heuristic approaches the inversion procedure itself is directly controlled by the waveguide physics and, in a numerical implementation based on conjugate gradient optimization, many fewer iterations are required for assessment of an environment that is supported by the underlying subbottom model. Furthermore, since regularization schemes are particularly important to enhance the performance of full-field acoustic inversion, special attention is devoted to the application of penalization methods to the adjoint optimization formalism. Regularization incorporates additional information about the desired solution in order to stabilize ill-posed inverse problems and identify useful solutions, a feature that is of particular importance for inversion of field data sampled on a vertical receiver array in the presence of measurement noise and modeling uncertainty. Results with test data show that the acoustic field and the bottom properties embedded in the control parameters can be efficiently retrieved.