Detect Surface Waves Research Articles

Sun glitter imagery of ocean surface waves. Part 1: Directional spectrum retrieval and validation

A practical method is suggested to quantitatively retrieve directional spectra of ocean surface waves from high-resolution satellite sun glitter imagery (SSGI). The method builds on direct determination of the imaging transfer function from the large-scale smoothed shape of sun glitter. Observed brightness modulations are then converted into sea surface elevations to perform directional spectral analysis. The method is applied to the Copernicus Sentinel-2 Multi-Spectral Instrument (MSI) measurements. Owing to the specific instrumental configuration of MSI (which has a primary mission dedicated to mapping land surfaces), a physical angular difference between channel detectors on the instrument focal plane array can be used to efficiently determine the surface brightness gradients in two directions, i.e. in sensor zenith and azimuthal directions. In addition, the detector configuration of MSI means that a small temporal lag between channel acquisitions exists. This feature can be exploited to detect surface waves and infer their space-time characteristics using cross-channel correlation. We demonstrate how this can be used to remove directional ambiguity in 2D detected wave spectra, and to obtain information describing local dispersion of surface waves. Directional spectra derived from Sentinel-2 MSI SSGI are compared with in situ buoy measurements. We report an encouraging agreement between SSGI-derived wave spectra and in situ measurements. This article is protected by copyright. All rights reserved.

A noninvasive ultrasound elastography technique for measuring surface waves on the lung

The purpose of this work was to demonstrate an ultrasound based surface wave elastography (SWE) technique for generating and detecting surface waves on the lung. The motivation was to develop a noninvasive technique for assessing superficial lung tissue disease including interstitial lung disease (ILD). ILD comprises a number of lung disorders in which the lung tissue is stiffened and damaged due to fibrosis of the lung tissue. Currently, chest radiographs and computed tomography (CT) are the most common clinical methods for evaluating lung disease, but they are associated with radiation and cannot measure lung mechanical properties. The novelty of SWE is to develop a noninvasive and nonionizing technique to measure the elastic properties of superficial lung tissue. We propose to generate waves on the lung surface through wave propagation from a local harmonic vibration excitation on the chest through an intercostal space. The resulting surface wave propagation on the lung is detected using an ultrasound probe through the intercostal space. To demonstrate that surface waves can be generated on the lung, an ex vivo muscle-lung model was developed to evaluate lung surface wave generation and detection. In this model, swine muscle was laid atop a swine lung. A vibration excitation of 0.1s 100Hz wave was generated on the muscle surface and the surface waves on the lung were detected using a linear array ultrasound probe at 5MHz. To test its feasibility for patient use, SWE was used to measure both lungs of an ILD patient through eight intercostal spaces. The mean wave speed was 1.71±0.20m/s (±SD) at the functional residual capacity, while the mean wave speed was 2.36±0.33m/s at the total lung capacity. These studies support the feasibility of SWE for noninvasive measurement of elastic properties of lung and demonstrate potential for assessment of ILD.

Excitation and Detection of Surface Waves on Normal and Superfluid 3He

We describe the measurement principles to excite and detect surface waves on liquid helium at very low temperatures. We excited the waves mechanically by rocking the whole cryostat with frequencies of few hertz. The waves were detected capacitively with an interdigital capacitor on a vertical wall of the cuboid experimental volume. In superfluid 3He at around 0.2 mK at least eleven surface resonances were observed below 11 Hz whereas in normal fluid only a few resonances were observable above 50 mK.

Simulations of surface acoustic wave devices built on stratified media using a mixed finite element/boundary integral formulation

The demand for high frequency surface acoustic wave devices for modern telecommunication applications imposes the development of devices able to answer the manufacturer requirements. The use of high velocity substrates for which a piezoelectric layer is required to excite and detect surface waves has been widely investigated and requires the implementation of accurate theoretical tools to identify the best combinations of material. The present paper proposes a mixed formulation combining finite element analysis with a boundary integral method to accurately simulate the capability of massive periodic interdigital transducers to excite and detect guided acoustic waves in layered media. The proposed model is exploited for different typical configurations.

Depth Measurement of Surface-Breaking Cracks Using Point-Source/Point-Receiver Acoustic Transducer

ABSTRACTA newly developed point-source/point-receiver (PS/PR) acoustic transducer is used for measuring the depth of surface-breaking cracks. The transducer consists of two miniature conical PZT elements which form a transmitter/receiver pair for generating and detecting surface waves. A tone-burst measurement system operates the PS/PR transducer at a fixed frequency. When a surface-breaking crack is present in between the transmitter and receiver, the change in time-of-flight of surface wave propagation caused by the crack is measured and used to estimate the crack depth. To verify the feasibility of this method, machined slots on a carbon steel sample are tested by the PS/PR transducer for crack depth determination. Good experimental results are obtained. Future applications and improvements on the PS/PR transducer system are addressed.

Vibration Sensitivity and a Computational Theory for Prey-Localizing Behavior in Sand Scorpions1

As burrowing, nocturnal predators of small arthropods, sand scorpions have evolved exquisite sensitivity to vibrational information that comes to them through the substrate they live on, dry sand. Over distances of a few decimeters, sand conducts low velocity (∼50 m/sec) surface (Rayleigh) waves of sufficient amplitude and bandwidth (200<f<500 Hz) to be biologically detectable. Eight acceleration-sensitive receptors (slit sensilla) at the tips of the scorpion's circularly arranged legs detect surface waves generated by prey movements or “juddering” signals from other scorpions. From this input alone, direction of the disturbance source is calculated up to 20 cm distance. By ablating slit sensilla in various combinations on the eight legs, the contribution each makes in computing target location can be assessed. Other behavioral experiments show that differential timing of surface wave arrival at each sensor is most likely the cue that determines target location. Given the simplicity of this sensory system, a computational theory to account for wave source localization has been developed using a population of second-order neurons, each receiving excitatory input from one vibration receptor and inhibition from the triad of receptors opposite to it in the eight-element array. Input from a passing surface wave opens and closes a time widow, the width of which determines the firing probability of second-order neurons. Target direction is encoded as the relative excitation of these neurons, and stochastic optimization tunes the relative strengths of excitatory and inhibitory inputs for accuracy of response. The excellent agreement between predictions of the model and observed behavior of sand scorpions confirms a simple theory for computational mapping of surface vibration space.

In vitro experimental evaluation of contrast agents

In this presentation, techniques for in vitro characterization of contrast agents will be reviewed. Contrast agents contain air or a lower diffusivity gas, with a shell that can be composed of albumin, polymer, a lipid monolayer, a surfactant or oil. The range of possible shell properties creates a wide range of physical properties, and resulting variations in behavior upon insonation. In order to characterize the agents, several methods have been employed, including acoustical and optical interrogation of single bubbles and ensembles. High-speed optical techniques can be employed to measure the radius-time curves, to determine the mechanisms of destruction, and to detect surface waves. The maximum radius and wall velocity, and mechanisms of destruction, are significantly affected by both the gas and shell properties. Observed mechanisms of destruction include static diffusion, acoustically driven diffusion and fragmentation. These destruction mechanisms are characterized by unique time scales which have been determined through both optical and acoustical methods. The results of acoustical techniques that have been applied to determine the linear resonance frequency and to quantify the scattering and attenuation cross sections will also be presented.

Surface defect inspection of spherical objects by the resonant sphere technique

Ceramic materials, such as silicon nitride and silicon carbide, are important because of their high mechanical strength at high temperature and in nonlubricative environments. The weakness of these materials lies in their brittleness. Stress concentration caused by the existence of surface or subsurface cracks can lead to the total mechanical failure of parts. In this letter, we describe the application of the resonant-sphere technique to perform nondestructive evaluation of ceramic bearing balls. The technique had been shown to be capable of measuring various material properties, such as Vs (shear wave velocity) and ν (Poisson’s ratio). We use phase and amplitude measurements to simplify the evaluation of various resonance quality factors (Q), and we present, for the first time, the use of a resonance technique to propagate and detect surface waves on a sphere. We also show a decrease in Q for surface-wave modes due to the existence of surface cracks.

Viscosity and surface-tension measurements on cyanobenzylidene octyloxyaniline using propagating capillary waves: Critical behavior

The shear viscosities and surface tension of a liquid crystal CBOOA (cyanobenzylidene octyloxyaniline) were measured by the use of the technique of generating and detecting surface waves (capillary waves) developed by the authors. For the wave propagation perpendicular to the direction, the surface tension $\ensuremath{\sigma}$ decreased sharply upon entering into the smectic-$A$ phase. When the waves propagate parallel to the director, the attenuation due to the diverging viscosity ${\ensuremath{\eta}}_{3}$ near the nematic to smectic-$A$ transition became so large that measurements in the smectic phase were not possible. The viscosity ${\ensuremath{\eta}}_{3}$ was found to diverge as ${(T\ensuremath{-}{T}_{c})}^{\ensuremath{-}\ensuremath{\nu}}$, where $\ensuremath{\nu}=1.0\ifmmode\pm\else\textpm\fi{}0.1$ at 200 Hz and $\ensuremath{\nu}=0.83\ifmmode\pm\else\textpm\fi{}0.15$ at 400 Hz.

Novel technique for dynamic surface tension and viscosity measurements at liquid–gas interfaces

A novel means of generating and detecting surface waves at liquid-gas interfaces has been successfully developed. Electrocapillarity is used to generate the waves which are detected via specular reflection of a laser beam from the fluid surface to a position sensitive photodiode. Such a scheme is compact, sensitive, and does not mechanically touch the fluid surface. A preliminary study of highly damped waves on the magnetically oriented liquid crystal MBBA is reported.

Initial Discrimination Results from the Norwegian Seismic Array

Summary A study has been made of the capability of the Norwegian Seismic Array to discriminate between earthquakes and underground explosions occurring in central Asia and western Russia. The ratio of surface to body wave magnitudes (Ms,: mb) has been used exclusively, the chief application of the array being in the detection and measurement of low amplitude surface waves. Beamforming and matched filtering were the signal enhancement techniques applied. Of 34 events in central Asia studied, 10 were identified as explosions, 22 earthquakes, and two were unidentifiable because of high, long period background noise. Five events in aseismic western Russia, all presumed to be explosions, showed wide variation in Ms,: mb; two of these measurements being close to the earth-quake population of central Asia. The Ms: mb measurements of central Asia explosions made in this and another study are compared with measurements of Nevada explosions. For a given mb the latter consistently have higher Ms values. The possible causes of this are discussed and, depending on the cause, the array seems capable of detecting surface waves from an explosion of 4–16 kt yield at 40° distance in central Asia. Strong seismic noise variations in Norway make this long period capability time dependent.

Surface Wave Detection with a Broad Band Accelerometer

WE report preliminary results of a study to determine the capability of a broad band vertical accelerometer to detect surface waves from small earthquakes. The instrument uses a quartz fibre in torsion and capacitive position sensing1. It was designed as a low noise, low drift broad band accelerometer and has produced data on Earth normal mode excitation after earthquakes (5 to 40 cycle h−1, ref. 2). For this study, the output was modified by an active filter to copy the response of Pomeroy et al.3 and Molnar et al.4. Such a response does not use the wide band aspects of this instrument and the shape was dictated primarily by the shape of the ground noise spectrum, which has a minimum in amplitude between periods of 20 and 30 s. This minimum may be inferred from the known decrease in noise amplitude with increasing period at shorter periods and seismic background noise data5. It is also shown in the data of Savino et al.6.