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2004, Computers & Structures
The changes in the seismic response due to the presence of an irregular elastic seabed, and/or the presence of a waterfilled inclusion located under the elastic seabed surface, in the presence of a dilatational spatially harmonic line source, is assessed. The seabed surface deformations and the water-filled inclusions are bi-dimensional. The solution is obtained using the Boundary Elements Method for a wide range of frequencies and spatially harmonic line sources, which are then used to compute the time series by means of fast inverse Fourier transforms.
Geophysical Journal International, 2006
We compare three numerical methods to model the sea surface interaction in a marine seismic reflection experiment (the frequencies considered are in the band 10-100 Hz): the finitedifference method (FDM), the spectral element method (SEM) and the Kirchhoff method (KM). A plane wave is incident at angles of 0 • and 30 • with respect to the vertical on a rough Pierson-Moskowitz surface with 2 m significant wave height and the response is synthesized at 6, 10 and 50 m below the average height of the sea surface. All three methods display an excellent agreement for the main reflected arrival. The FDM and SEM also agree very well all through the scattered coda. The KM shows some discrepancies, particularly in terms of amplitudes.
Coastal areas are generally characterized by human manufacturing; thus, seismic data analysis is necessary to characterize the properties of the subsoil, the main purpose of which is to clarify risk situations. In the case of very shallow water environments, seismic multiple attenuation becomes a challenge when the reflection of the seafloor is post-critical, so it is not recorded because of the acquisition parameters. We propose an approach to attenuate the multiples by using wave equation datuming that does not require the detection of seafloor reflection and avoids the seafloor reflection prediction and related approximations in the post-critical conditions. Moreover, this approach allows for the enhancement of higher frequencies, and, consequently, an increase in resolution, demonstrating that it is a powerful tool to attenuate multiples and reverberations, especially where other approaches are found to be inefficient. An example of the application of seismic data acquired in the continental shelf of South Chile is reported.
IEEE Journal of Oceanic Engineering, 2002
The problem of coherent reflection of an acoustic plane wave from a rough seabed with a randomly inhomogeneous sediment layer overlying a uniform elastic basement is considered in this analysis. The randomness of the sound field is attributable to the roughness of the seabed and the sound-speed perturbation in the sediment layer, resulting in a joint rough surface and volume scattering problem. An approach based upon perturbation theory, combined with a derived Green's function for a slab bounded above and below by a fluid and an elastic half-space, respectively, is employed to obtain an analytic solution for the coherent field in the sediment layer. Furthermore, a boundary perturbation theory developed by Kuperman and Schmidt [4] is applied to treat the problem of rough surface scattering. A linear system is then established to facilitate the computation of the coherent reflection field. The coherent reflection coefficients for various surface roughness, sediment randomness, frequency, sediment thickness, and basement elasticity have been generated numerically and analyzed. It was found that the higher/larger size of surface and/or medium randomness, frequency, thickness, and shear-wave speed, the lower the coherent reflection. Physical interpretations of the various results are provided.
Applied Ocean Research, 2009
In this study, a set of generalized analytical solutions are developed for the wave-induced response of a saturated porous seabed under plane strain condition. When considering the water waves originating in deep water and travelling towards the shore, their velocities, lengths and heights vary. Depending on the characteristics of the wave and the properties of the seabed, different formulations (fully dynamic, partly dynamic, quasi-static) for the wave-induced response of the seabed are possible. The solutions for the response with these formulations are established in terms of non-dimensional parameters. The results are presented in terms of pore pressure, shear stress and vertical effective stress distributions within the seabed. For typical values of wave period and seabed permeability, the regions of applicability of the three formulations are identified and plotted in parametric spaces. With given wave and seabed characteristics, these regions provide quick identification of the appropriate formulation for an adequate evaluation of the wave-induced seabed response.
Soil Dynamics and Earthquake Engineering, 1999
The indirect boundary element method (IBEM) is used to simulate wave propagation in two-dimensional irregularly layered elastic media for internal line sources. The method is based on the integral representation for scattered elastic waves using single layer boundary sources. Fulfillment of the boundary conditions leads to a system of integral equations. Results are obtained in the frequency domain and seismograins are computed through Fourier synthesis. In order to test and validate the method we present various comparisons between our results and the time series obtained analytically for a buried line source in a half-space and by using the recently developed spectral element method (SEM).
The necessary actions connected with interpretations of examining of physical properties of sea bottom structure is developing of proper procedure of modeling of the layered structure. The implementation of the acoustic waves for the determination of seabed stratification requires a precise diagnosis of the topic from both the theoretical and practical side. Little differences between the parameters of sediments, as well as irregular distance between layers make the interpretation of the images of cross sections uneasy. The paper presents the model of stratified structure of seabed in the form of a liquid on which falls the perpendicular elastic wave. Since the description of the seabed is made on the basis of reflected impulse shape, which provides the information about the structure and material properties such as density and speed of wave propagation, the important role among other things has the spatial resolution, because the pulse length determines the possibility of distingui...
Bulletin of the Seismological Society of America, 2012
Seismograms recorded on the seafloor are affected by reverberations in the overlying ocean layer. We examine the feasibility of removing these reverberations through a 1D wave-field decomposition based on a reflection-transmission formulation of the problem. Two decomposition schemes are presented. The first scheme involves a simple manipulation of the fundamental matrix relating stress and displacement of plane harmonic waves to upgoing and downgoing wavevector coefficients and requires measurement of both ocean bottom displacement and pressure. The second approach involves only displacement recordings and knowledge of the water column depth. This latter quantity will generally be known a priori from deployment logs, or, alternatively, it may be estimated using vertical displacement and pressure seismograms in a preprocessing step. Both approaches require prior information on seabed properties. In the case of P-wave incidence, the decomposition depends primarily on the seabed S velocity β 0 . This quantity can be determined by examining trial decompositions over a range of β 0 's and selecting the value that minimizes the energy of the upgoing S-wave component at the arrival time of the incident wave. We apply this approach to synthetic seismograms for a simple Earth structure and to recordings of two large events from the Central Oregon Locked Zone Array (COLZA) on the continental margin of Oregon. The real data indicate that the wave-field decompositions are largely successful at lower frequencies (< 0:1 Hz) but that 3D scattering, likely originating near the sediment-basement contact, is manifested at higher frequencies.
Journal of the Acoustical Society of America, 2001
A perturbation model is developed for sound scattering by a poroelastic seafloor having roughness small compared to the acoustic wavelength. The sediment is assumed to be homogeneous and isotropic with wave propagation described by Biot's equations. When applied to sandy sediments, the model predicts backscattering levels that are substantially lower than those of a fluid model having the same roughness, density, sound speed, and attenuation.
The Journal of the Acoustical Society of America, 2017
Bottom scattering is important for a number of underwater applications: it is a source of noise in target detection and a source of information for sediment classification and geoacoustic inversion. While current models can predict the effective interface scattering strength for layered sediments, these models cannot directly compute the ensemble averaged mean-square pressure. A model for bottom scattering due to a point source is introduced which provides a full-wave solution for mean-square scattered pressure as a function of time under first-order perturbation theory. Examples of backscatter time series from various types of seafloors will be shown, and the advantages and limitations of this model will be discussed.
Geophysical Prospecting, 1987
The space-time acoustic wave motion generated by an impulsive monopole source is calculated with the aid of the Cagniard-de Hoop technique. Two configurations with plane interfaces are discussed : an air/fluid/solid configuration with the source and the receiver located in the fluid layer; and a stack of n fluid layers between two acoustic half-spaces where the source and the receiver are located in the upper half-space. Synthetic seismograms are generated for the pressure of the reflected wavefield, using the source signature of an airgun.
GEOPHYSICS, 2003
Accurate knowledge of the seismic material properties in the immediate vicinity of the receivers represents a prerequisite for elastic wavefield decomposition. We present strategies for estimating the elastic material properties for both land and seabed multicomponent seismic data. The proposed scheme for land data requires dense multicomponent geophone configurations, which allow spatial wavefield derivatives to be explicitly calculated. The required information can be obtained with four three-component surface geophones positioned at the corners of a square, and a fifth geophone buried at a shallow depth below the center of the square. The technique yields local estimates of the near-surface P-and S-wave velocities, but the density cannot be constrained. Using a similar approach for four-component (three orthogonal components of particle velocity plus pressure) seabed recordings allows the P-and S-wave velocities as well as the density of the seafloor to be estimated. In this case, the proposed scheme does not require buried geophones, and it is applicable to multicomponent data recorded in routine seabed surveys. Compared to existing techniques, the new method allows the elastic seafloor properties to be more accurately determined, and it does not rely critically on the inclusion of large-offset data. Numerical tests indicate that the proposed schemes are robust and yield accurate results, provided that the signal used for the inversion contains sufficient horizontal energy and can be clearly identified and separated from other signals. Although the schemes are designed for application on the first arrivals, they are, in principle, applicable to any data window containing isolated P-or S-arrivals. The proposed scheme is successfully applied to a seabed data set acquired in the North Sea. In contrast, the application on a multicomponent land data set was unsuccessful, because of strong receiver-to-receiver variations in amplitude and phase, probably caused by differences in coupling and instrument response.
KSCE Journal of Civil Engineering, 2006
This study computed numerically wave damping passing over the porous seabed using a boundary element method. The fluid is assumed to be inviscid and incompressible, and the flow is assumed to be irrotational. The analytical domain is formulated mathematically as a non-linear problem in terms of velocity potential. The Darcy's law is applied to account for the seabed porosity. The pressure and vertical velocity potential in the interface boundary between seabed soil and fluid is assumed to be continuous. The comparison with other results in order to prove the analysis model shows a good agreement in spite of small amplitude wave theory, and also those results obtained by present numerical method show that wave height profiles and horizontal velocities passing over the porous seabed are damped due to porous seabed media. Therefore, numerical analysis method considering porous seabed conditions seems to be more useful.
IEEE Journal of Oceanic Engineering, 1988
A method is presented for estimating the elastic properties of the topmost layer of seafloor sediment by inversion of the amplitude versus range information in precritical acoustic reflection data. The amplitude of sound reflected from the seafloor depends on the range between the source and receiver and the physical properties of the seafloor sediment. It is assumed that the relationship between the reflected signal amplitude and the range can be described by the plane wave reflection coefficient. A simulation study using synthetic data for reflection from both fluid and elastic sediment models is used to demonstrate the use of the inversion scheme and to test the sensitivity of the technique to interference from subbottom reflections. Measurements obtained in an experiment to study the seafloor reflectivity at low frequencies in a deep-water basin are inverted to obtain estimates of the compressional and shear wave sound speeds and the density in the surficial sediments. The results are consistent with the values expected for the seafloor sediment at the experiment site.
GEOPHYSICS, 1994
We present an inversion method for determining the velocities, densities, and layer thicknesses of a horizontally stratified medium with an acoustic layer at the top and a stack of elastic layers below. The multioffset reflection response of the medium generated by a compressional point source is transformed from the time-space domain into the frequency-wavenumber domain where the inversion is performed by minimizing the difference between the reference data and the modeled data using a least-squares technique. The forward modeling is based on the reflectivity method where the solution for each frequency-wavenumber component is found by computing the generalized reflection and transmission matrices recursively. The gradient of the objective function is computed from analytical expressions of the Jacobian matrix derived
Journal of Marine Science and Engineering, 2016
Seismic interface waves generated by seabed impacts are believed to have biological importance. Various wave types are of interest to seismologists, who can minimize the unwanted, but often dominant, ground roll waves with suitable instrumentation. Waves made by dredging and piling have been measured using geophones and found to be of this interface type, which propagate much more slowly than the pressure waves in the water column above. Short interface wavelets of a few cycles were modeled using transient finite element analysis (FEA). Wavelets with low losses have been modeled using graded sediment data from the literature. They do not radiate energy away from the interface because the evanescent acoustic pressures they generate decay rapidly with distance from the seabed. Associated water particle velocities are much greater than would be expected from similar acoustic pressure measurements in a free field. This motion is significant to aquatic life which is dependent on inertial sensors (otoliths, etc.) to respond to the environment. Additional amplification of the horizontal seabed motion of the adjacent water is predicted for a short seismic wavelet modeled in a graded solid seabed. Further recent analysis studied the distribution of the energy flux within the sediment layers.
The Journal of the Acoustical Society of America, 1992
Acoustic scattering by a rough, possibly dynamic interface is experimentally studied by insonifying the seabed and the sea surface at high frequency at various incident angles. A directional source working at 300 kHz was placed at the top of a 3.5 m high tower deployed on the seabed. A vertical array of 3 omnidirectional hydrophones was suspended from a portable frame, which was deployed in bistatic configuration at a variable range between 30 and 70 m. A selection of the results is presented to evaluate the sea surface and seabed scattering amplitude in the nominal specular reflection direction. Scattering by the sea surface was measured during relatively long periods of time in order to correlate its value with the sea state. Model-data comparison was conducted between the scattering data and a time-domain, three-dimensional rough surface scattering model (BORIS-SSA). Model- based analysis allows for a better understanding of some aspects of high-frequency multipath reverberation ...
Applied Ocean Research, 2014
The problem of water wave scattering by a thin horizontal elastic plate (semi-infinite as well as finite) floating on an ocean of uniform finite depth in which the ocean bed is composed of porous material of a specific type is analyzed. The method of eigenfunction expansion is used in the mathematical analysis and the quantities of physical interest, namely the reflection and transmission coefficients, are obtained. Numerical estimates for these coefficients are obtained for different values of the parameter describing the porosity of the ocean bed and for different edge conditions of the elastic plate. The edge conditions considered here involve (i) a free edge, (ii) a simply supported edge and (iii) a built-in edge. From the numerical results it is observed that for free edge condition, the porosity of the ocean bed has little effect on the reflection and transmission coefficient for both the cases of semi-infinite and finite elastic plate. The energy identity related to reflection and transmission coefficients in a porous bed is derived and is used as a partial check on the correctness of the numerical results for the semi-infinite elastic plate.
Journal of Marine Science and Engineering, 2021
The emerging tasks of determining the features of bottom sediments, including the evolution of the seabed, require a significant improvement in the quality of data and methods for their processing. Marine seismic data has traditionally been perceived to be of high quality compared to land data. However, high quality is always a relative characteristic and is determined by the problem being solved. In a detailed study of complex processes, the interaction of waves with bottom sediments, as well as the processes of seabed evolution over short time intervals (not millions of years), we need very high accuracy of observations. If we also need significant volumes of research covering large areas, then a significant revision of questions about the quality of observations and methods of processing is required to improve the quality of data. The article provides an example of data obtained during high-precision marine surveys and containing a wide frequency range from hundreds of hertz to kilohertz. It is shown that these data, visually having a very high quality, have variations in wavelets at all analyzed frequencies. The corresponding variations reach tens of percent. The use of the method of factor decomposition in the spectral domain made it possible to significantly improve the quality of the data, reducing the variability of wavelets by several times.
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