Elastic interferometry for ocean bottom cable data: Theory and examples‡
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ABSTRACT Elastic imaging from ocean bottom cable (OBC) data can be challenging because it requires the prior estimation of both compressional‐wave (P‐wave) and shear‐wave (S‐wave) velocity fields. Seismic interferometry is an attractive technique for processing OBC data because it performs model‐independent redatuming; retrieving ‘pseudo‐sources’ at positions of the receivers. The purpose of this study is to investigate multicomponent applications of interferometry for processing OBC data. This translates into using interferometry to retrieve pseudo‐source data on the sea‐bed not only for multiple suppression but for obtaining P‐, converted P to S‐wave (PS‐wave) and possibly pure mode S‐waves. We discuss scattering‐based, elastic interferometry with synthetic and field OBC datasets. Conventional and scattering‐based interferometry integrands computed from a synthetic are compared to show that the latter yields little anti‐causal response. A four‐component (4C) pseudo‐source response retrieves pure‐mode S‐reflections as well at P‐ and PS‐reflections. Pseudo‐source responses observed in OBC data are related to P‐wave conversions at the seabed rather than to true horizontal or vertical point forces. From a Gulf of Mexico OBC data set, diagonal components from a nine‐component pseudo‐source response demonstrate that the P‐wave to S‐wave velocity ratio ( V P / V S ) at the sea‐bed is an important factor in the conversion of P to S for obtaining the pure‐mode S‐wave reflections.Keywords:
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Instability of seabed due to wave action may cause serious damages to coastal infrastructure. The wave-induced instantaneous liquefaction near and around a structure, may impact the stability and capacity of foundation elements. The wave-seabed interaction has been mostly studied based on decoupled analysis for the flat or slightly sloping seabed (slope less than five degrees). However, some of marine structures, near shore may be built on (or anchored to) seabed with considerable slopes. In this paper, the response and instability of the sloping seabed supporting a marine structure and subjected to surface waves is evaluated. The scope included developing a numerical model using a fully coupled approach. Biot's equations of the seabed in conjunction with governing equations for other domains (structure and fluid regions) are solved simultaneously using finite element method. The instability of seabed due to wave-induced instantaneous liquefaction around the marine structure is evaluated and the effect of slope on the extent of liquefied zone is examined. The results indicated that seabed response and the extent of liquefied zone near the structure are reduced with increasing steepness of seabed. The effect of various slope steepness and soil parameters on the extent of the liquefied zone is characterized and discussed.
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To provide a computational framework for simulating fluid-sediment-seabed interactions, a three-dimensional coupled fluid-sediment-seabed interaction model is developed and applied to seabed response under regular waves and local scouring from tsunami run-up. For the seabed response under regular waves, the computational capability of the model is demonstrated from comparison with analytical solutions in terms of pore-water pressure and the displacement of the seabed skeleton. For the local scouring from tsunami run-up, numerical results show that the model can compute the seabed response during the onset of the local scouring, and suggest that the model is a useful tool for analyzing and evaluating a general complex fluid-sediment-seabed interaction phenomenon in the marine environment.
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Wen, F. and Wang, J.H., 2015. Response of layered seabed under wave and current loading.Wave and other environmental loadings in the ocean are the main factors affecting seabed stability and the safety of marine structures. Most previous literature has considered seabed response in a layered seabed under wave loading or in a single soil layer only under combined wave and current loading. In this article, a two-layered seabed, subjected to combined wave and current loading, is considered. The influence of currents on the stratified seabed response is investigated initially. The results indicate that a following current will aggravate the instability of the seabed, either in the form of liquefaction or shear failure. The effects of stratification on the seabed response were also studied. Numerical results reveal that upper layer of the seabed will become more stable as the permeability ratio (k1 : k2) increases, where k1 and k2 are the permeability of upper and lower layer of the seabed, respectively. The thickness of each layer of the seabed also affects its stability. For example, the upper layer is apt to be more unstable when the thickness of the lower layer of the seabed (h2) is equal to about one-fifth of the wave length (L/5). Analysis of the seabed reformation in relation to engineering practice shows that replacing or adding a layer onto the original seabed can both prevent liquefaction effectively, and the approach of adding a layer has the better potential to prevent liquefaction, but those treatments have a lesser effect on the maximum depth of shear failure in the original seabed.
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