Surface Wave Components Research Articles

Intensity statistics of very high frequency sound scattered from wind-driven waves.

The interaction of vhf 100-1000 kHz underwater sound with the ocean surface is explored. The bistatic forward scatter of 300 kHz sound is measured in a wind driven wave channel. Fluctuations in arrival amplitude are described by the scintillation index (SI) which is a measure of arrival intensity variance. SI initially increases with wind speed but eventually saturates to a value of 0.5 when the root-mean-square (rms) roughness is 0.5 mm. An adjusted scintillation index (SI*) is suggested that accounts for the multiple arrivals and properly saturates to a value of 1. Fluctuations in arrival time do not saturate and increase proportionately to the dominant surface wave component. Forward scattering is modeled at frequencies ranging from 50 to 2000 kHz using the Helmholtz-Kirchhoff integral with surface wave realizations derived from wave gauge data. The amplitude and temporal statistics of the simulated scattering agree well with measured data. Intensity saturation occurs at lower wind speeds for higher frequency sound. Both measured and modeled vhf sound is characterized by many surface arrivals at saturation. Doppler shifts associated with wave motion are expected to vary rapidly for vhf sound however further analysis is required.

Statistics of very high frequency sound scattered from wind-driven waves

The amplitude, Doppler spread, and temporal coherence of VHF scattering is important for the performance of high frequency sonars and underwater communications systems in operating scenarios where energy from the sea surface cannot be screened. For this talk, the amplitude and arrival time statistics of very high frequency (50 kHz–2000 kHz) sound scattered from a rough wind-driven wave surface are modeled. Saturation occurs in the normalized second moment of acoustic intensity when the surface correlation length exceeds a Fresnel zone length. Fluctuations in arrival time do not saturate and increase proportionally to the dominant surface wave component. Data collected at 300 kHz agree with the model results. Energy is first detected in the wind-driven surface wave field at wavelengths of 1.7 cm and 3.4 cm. The former wave corresponds to a gravity-capillary wave whose phase speed is at a minimum. The principal appearance of this type of wave has threshold implications for the range of acoustic transmission frequencies susceptible to saturation in amplitude statistics.

A numerically stable formulation of the Green's function parabolic equation: Subtracting the surface-wave pole

The original formulation of the Green's function parabolic equation (GFPE) can have numerical accuracy problems for large normalized surface impedances. To solve the accuracy problem, an improved form of the GFPE has been developed. The improved GFPE formulation is similar to the original formulation, but it has the surface-wave pole "subtracted." The improved GFPE is shown to be accurate for surface impedances varying over 2 orders of magnitude, with the largest having a magnitude exceeding 1000. Also, the improved formulation is slightly faster than the original formulation because the surface-wave component does not have to be computed separately.

Application of passive source surface-wave method in site engineering seismic survey

Site engineering seismic survey provides basic data for seismic effect analysis. As an important parameter of soil, shear-wave velocity is usually obtained through wave velocity testing in borehole. In this paper, the passive source surface-wave method is introduced into the site engineering seismic survey and practically applied in an engineering site of Shijingshan District. By recording the ubiquitous weak vibration on the earth surface, extract the dispersion curve from the surface-wave components using the SPAC method and obtain the shear-wave velocity structure from inversion. Over the depth of 42 m underground, it totally consists of five layers with interface depth of 3.31, 4.50, 7.23, 17.41, and 42.00 m; and shear-wave velocity of 144.0, 198.3, 339.4, 744.2, and 903.7 m/s, respectively. The inversion result is used to evaluate site classification, determine the maximum shear modulus of soil, provide basis for further seismic hazard analysis and site assessment or site zoning, etc. The result shows that the passive source surface-wave method is feasible in the site engineering seismic survey and can replace boreholes, shorten survey period, and reduce engineering cost to some extent.

Short surface waves in the Canadian Arctic in 2007 and 2008

[1] We have collected time series data of short oceanic waves as a part of the International Polar Year (IPY) 2007–2008. Using a shipboard laser wave slope (LAWAS) system operating at 900 nm, we have obtained wave slopes measurements up to 60 rad m−1 wave number. We have compared our in situ wave slopes with collocated and concurrent high-resolution upwind Normalized Radar Cross Sections (NRCS) collected by QuikSCAT. The LAWAS measured wave slope spectra were consistent with local wind speeds and QuikSCAT measured NRCS. Our measured short wave mean slopes indicate their enhancement by long waves (0–1 rad m−1) at small values of long-wave slope. Concurrent with wave slope measurements, the strength of the reflected LAWAS light beam was analyzed in terms of the light attenuation coefficient at 900 nm. We have observed a correlation between surface elevation and light attenuation. The mechanism of wave modulated beam attenuation was found to be related to the instantaneous long wave skewness.

Seismic property characterization of lithotypes cropping out in the Siracusa urban area, Italy

The town of Siracusa is located in the south-eastern coast of Sicily (southern Italy) in a wide inlet engraved into a limestone formation. This lithotype, which is overlaid by soft sediments such as sandy clays and detritus, can be considered as the local bedrock. Present study aims to investigate on the dynamic properties of main lithotypes outcropping in the study area and their relationships with the local seismic response. Non-invasive seismic prospecting techniques using the vertical component of surface waves (MASW and ReMi) were adopted, as well as ambient noise measurements, processed through the Nakamura technique. The best solution for the experimental dispersion curves was selected using the neighbourhood algorithm and information about the deeper part of the estimated shear wave profiles was obtained by considering the ellipticity of the fundamental peak. Moreover, a cluster analysis was performed to subdivide into homogeneous groups the experimental noise spectral ratios. Results obtained pointed out that the use of combined different methods provides a robust way to characterize the investigated soils and to reduce the problems linked to the non-uniqueness of solutions during the interpretation of geophysical data.

Comparison of polarimetric techniques for the identification of biological and chemical materials using Mueller matrices, lateral waves, and surface waves

Optical polarimetric techniques to identify and characterize biological and chemical materials have received much attention recently for their broad applications in biophotonics, biochemistry, biomedicine, and pharmacology. We present here several options for the measurement of optical rotation, diattenuation, and the index of depolarization. These include polar decomposition, identification of specific pairs of Mueller matrix elements that are proportional to optical activity, and the cross-polarized components of lateral waves and surface waves at the interface between free space and the optically active material.

Body wave observations from cross-correlations of ambient seismic noise: A case study from the Karoo, RSA

[1] In the past decade the analysis of seismic noise has become an efficient tool to recover the Green's function between pairs of receivers by cross-correlation of seismic traces. Most studies focus on the investigation of the surface wave component of the ambient noise. Several attempts to recover the body wave part of the Green's function have been documented. In this paper I present the results of cross-correlation of seismic noise and the retrieval of refracted and reflected P-waves along a seismic line in the Karoo region (Republic of South Africa). Body wave refractions (direct phases) and reflections have been observed in the Green's functions derived from ambient noise records of up to 60 hours. The results are compared with shot gathers from a controlled source experiment (borehole explosions), carried out along the same line. The significant potential of ambient noise analysis, especially with respect to P-wave reflections will be shown and discussed.

Modeling Resuspension in a Dynamic Water Supply Reservoir

Enhancements to the two-dimensional lake and reservoir water quality model W2Tn to simulate the effects of currents and waves on sediment resuspension and turbidity are described. Bed stress attributable to currents was computed by the hydrothermal component of W2Tn, whereas a surface wave component was added to W2Tn to determine bed stress owing to waves. Resuspension flux is computed from bed stress and is included as a source of turbidity to the water column. The model is tested through application to Schoharie Reservoir, a drinking water supply that experiences episodes of elevated turbidity caused by runoff events and exacerbated by drawdown. Model predictions of bed stress attributed to currents are validated by using measurements obtained from acoustic Doppler instrumentation. The surface wave component of the model is established on a framework that has been previously validated for Schoharie Reservoir. Testing of the enhanced turbidity component of W2Tn was completed for a 3.5-year period of historical observations, which included a number of runoff events covering a range of severity and variations in reservoir drawdown. The enhanced model performed well in simulating observed conditions in the water column. The resuspension mechanism made a significant contribution to the predicted turbidity during periods of reservoir drawdown and during a severe runoff event. The model also performed well in simulating the observed turbidity of the drinking water withdrawal. Resuspension of particles contributing to turbidity was largely attributable to reservoir currents with surface wave-induced resuspension playing a smaller role. The potential application of this model to other water bodies and water quality issues is discussed.

Pixel level optical-transfer-function design based on the surface-wave-interferometry aperture.

The design of optical transfer function (OTF) is of significant importance for optical information processing in various imaging and vision systems. Typically, OTF design relies on sophisticated bulk optical arrangement in the light path of the optical systems. In this letter, we demonstrate a surface-wave-interferometry aperture (SWIA) that can be directly incorporated onto optical sensors to accomplish OTF design on the pixel level. The whole aperture design is based on the bull’s eye structure. It composes of a central hole (diameter of 300 nm) and periodic groove (period of 560 nm) on a 340 nm thick gold layer. We show, with both simulation and experiment, that different types of optical transfer functions (notch, highpass and lowpass filter) can be achieved by manipulating the interference between the direct transmission of the central hole and the surface wave (SW) component induced from the periodic groove. Pixel level OTF design provides a low-cost, ultra robust, highly compact method for numerous applications such as optofluidic microscopy, wavefront detection, darkfield imaging, and computational photography.

Surface-wave-enabled darkfield aperture for background suppression during weak signal detection

Sensitive optical signal detection can often be confounded by the presence of a significant background, and, as such, predetection background suppression is substantively important for weak signal detection. In this paper, we present a novel optical structure design, termed surface-wave-enabled darkfield aperture (SWEDA), which can be directly incorporated onto optical sensors to accomplish predetection background suppression. This SWEDA structure consists of a central hole and a set of groove pattern that channels incident light to the central hole via surface plasmon wave and surface-scattered wave coupling. We show that the surface wave component can mutually cancel the direct transmission component, resulting in near-zero net transmission under uniform normal incidence illumination. Here, we report the implementation of two SWEDA structures. The first structure, circular-groove-based SWEDA, is able to provide polarization-independent suppression of uniform illumination with a suppression factor of 1230. The second structure, linear-groove-based SWEDA, is able to provide a suppression factor of 5080 for transverse-magnetic wave and can serve as a highly compact (5.5 micrometer length) polarization sensor (the measured transmission ratio of two orthogonal polarizations is 6100). Because the exact destructive interference balance is highly delicate and can be easily disrupted by the nonuniformity of the localized light field or light field deviation from normal incidence, the SWEDA can therefore be used to suppress a bright background and allow for sensitive darkfield sensing and imaging (observed image contrast enhancement of 27 dB for the first SWEDA).

Asymptotics of surface waves over random bathymetry

This paper addresses the propagation of free surface water waves over a variable seabed in the long wavelength scaling regime. We consider the situation in which the bathymetry is given by a stationary random process which has a correlation length substantially shorter than the wavelength of the principal surface wave components. An asymptotic description shows that the water waves problem is modeled by an effective system of equations that is related to the KdV, however in a reference frame given in terms of random characteristic coordinates, and in addition with a random amplitude modulation and random scattered component. The resulting random processes are strongly correlated and have canonical limits due to the Donsker invariance principle. Our analysis is based on the Hamiltonian description of water waves and long wave perturbation theory and a new criterion for asymptotic expansions of partial differential equations with rapidly varying coefficients. In this paper we give a detailed analysis of the transformation to random characteristic coordinates and the asymptotic form of the resulting transformed partial differential equations. A companion paper (de Bouard, A., Craig, W., Diaz-Espinosa, O., Guyenne, P., Sulem, C., Long wave expansions for water waves over random topography, Nonlinearity 21 (2008), 2143-2178) analyses in detail the asymptotic behavior of the resulting expression for solutions, and their consistency with the derivation of the effective model equations.

The physics of pulse propagation in the nocturnal atmospheric boundary layer: measurement and theory

A series of experiments designed to probe the effect of the nocturnal atmosphere on low‐frequency (10‐ 500 Hz) sound propagation will be discussed, and their ramifications explored. These experiments involve detecting arrivals from a propane cannon on a vertical array of microphones 1‐3 km from the source, which were collected simultaneously with meteorological experiments designed to measure the vertical temperature and horizontal wind velocity profiles. Chief among the results of these experiments is the observation of a nocturnal model structure that has a significant surface wave component at frequencies below 150 Hz. At higher frequencies and longer propagation distances (>1.5 km), the surface wave is not observed due to attenuation from its interaction with the ground. At higher frequencies, the model structure displays a characteristic “quiet height” first described in Waxler et al. (2006). The potential application of these results for remote sensing the atmospheric boundary layer are discussed, and contrasted with other methods of measurement of the atmospheric profile.

Horizontal wave number spectra of atmospheric acoustic fields in range-dependent environments

The parabolic equation (PE) is a powerful method for computing sound propagation in range-dependent environments. However, the PE gives only the total field, with no information on the horizontal wave number spectrum of the acoustic field. In many situations, e.g., for propagation over irregular terrain, the horizontal wave number spectrum is valuable for analyzing the modal content of the acoustic field. For example, one might want to know the strength of the surface wave component at different ranges. A spectral decomposition method developed by Gilbert and Evans for ocean acoustics is applied here to atmospheric propagation over irregular terrain. This paper shows that for a PE field p(z) at range r, the horizontal wave number spectrum φ(κ,z) can be obtained directly and efficiently by solving a one-dimensional elliptic equation that uses the field p(z) as a source term. The derivation of the spectral decomposition equation is given along with a numerical method for solving it. Examples are presented and discussed for propagation over irregular terrain. [Research supported by the U. S. Army TACOM-ARDEC at Picatinny Arsenal, New Jersey]

Depth evaluation of surface-breaking cracks using laser-generated transmitted Rayleigh waves

A theoretical and experimental study of nondestructive evaluation of surface-breaking cracks with linear surface acoustic wave (SAW) pulses is presented. Schwarz–Christoffel conformal mapping was used to introduce a special orthogonal coordinate system that conserves the profile of the cracked surface. The inverse problem for two dimensions has been solved by means of conformal mapping. Thermoelastically generated broadband SAW pulses were employed to study the scattering of linear SAW pulses by a single crack. The surface wave component transmitted through the isolated microcrack was recorded as a function of distance by the cw laser probe-beam-deflection method. The Fourier transform of the transmitted SAW waveforms provides a stationary solution for any frequency. With this procedure the depth of the crack, produced in a separate experiment with a strongly nonlinear SAW pulse in a silica sample, was evaluated.

Interferometric surface-wave isolation and removal

The removal of surface waves (ground roll) from land seismic data is critical in seismic processing because these waves tend to mask informative body-wave arrivals. Removal becomes difficult when surface waves are scattered, and data quality is often impaired. We apply a method of seismic interferometry, using both sources and receivers at the surface, to estimate the surface-wave component of the Green’s function between any two points. These estimates are subtracted adaptively from seismic survey data, providing a new method of ground-roll removal that is not limited to nonscattering regions.

Nondestructive technique for estimating crack positions in a concrete structure by subtraction of the surface-wave component

In this paper, we describe a method for estimating a crack position in a concrete structure using several accelerometers. An array of accelerometers is attached to the concrete structure and an impact is made on the concrete surface using a small impulse hammer. A reflection wave is generated from the crack position if a crack exists. Conventional methods for estimating the reflection point might seem to be useful for the detection of cracks. Because the concrete structure is elastic, however, it has three wave-propagation modes: the surface-wave mode, the primary-wave mode, and the secondary-wave mode. We cannot estimate the position using conventional methods because the necessary primary-wave mode is weaker than the surface-wave mode. To estimate the crack position precisely, we have already proposed two methods for eliminating the surface-wave and side-wall reflections. However, elimination using those methods was insufficient because they sometimes indicated a peak at a position where no crack existed. Therefore, in this paper, we propose a new method for estimating the surface wave more precisely to suppress such peaks. Some experiments were carried out, yielding better results.

Spectral signature of wave breaking in surface wave components of intermediate-length scale

This paper investigates the length scale of ocean surface breaking waves in the spectral range of intermediate wavelength components a few centimeters to a few meters long. The spectral properties of wave breaking are examined first with the dissipation function of the wave action density conservation equation. The analysis reveals a strong breaking signature in wave components between 0.15 and 1.5 m long in the form of a quasi-singular behavior of the dissipation function using the present formulation of the wind-generation and breaking dissipation functions. Independent studies of more-direct breaking observations of radar tracking of sea spikes in the past have shown close correlation between sea spikes and scatterers traveling at the speed of surface waves a few meters long and much shorter than the dominant wavelength. This feature of sea-spike properties is consistent with the breaking signature of the dissipation function in similar wavelengths. The intermediate-scale waves are the primary contributor of the ocean surface mean-square slope. The close correlation between the gas transfer rate and the mean-square slope has been demonstrated repeatedly. A better understanding of the wave dynamics of intermediate-scale waves is important for clarification of various gas transfer mechanisms.

Acoustic field excited by a pulsed laser line source in a cylinder

The excitation and propagation of the acoustic waves in an elastic cylinder are studied by laser ultrasonics both theoretically and experimentally. The theoretical analysis of the two-dimensional acoustic field excited by a pulsed laser line source impacting on the generatrix of an elastic cylinder is presented. The dispersive properties for both cylindrical Rayleigh wave and the higher modes – whispering gallery (WG) modes are analyzed in detail. The numerical transient displacement waveforms for a detecting point located another terminal of the cylinder diameter opposite the source are calculated. The experimental excitation and detection of the acoustic waves in an aluminum cylinder are carried out on a laser ultrasonic system, which mainly consists of a Q-switched Nd:YAG laser and a laser interferometer. The wave components of bulk waves and surface waves (cylindrical Rayleigh waves and WG modes) are analyzed by comparing the numerical and experimental waveforms. The results are in good agreement.

Calculation of spatial-domain Green functions for multilayered media using DCIM with automatic handling of surface wave poles

The discrete complex image method (DCIM) has been used extensively for the calculation of spatial-domain Green functions for one-layer or two-layer media. However, one difficulty in applying DCIM to general multilayered media is the lack of a reliable procedure for the extraction of surface wave components. In the paper, the spectral-domain potential Green functions for a general multilayered medium are reformulated so that their denominators are obtained in an iterative form. A two-stage root-searching procedure is proposed for seeking the roots of these denominators. The residuals associated with the surface wave poles are calculated through contour integration. Special methods for handling the branch cuts close to the surface wave poles are discussed. Numerical examples of different geometries are given to reveal the behaviour of the surface wave poles and verify the method.