Effect of Free-surface Related Multiples on Near Surface Velocity Reconstruction with Acoustic Frequency Domain FWI

2014 ◽  
Author(s):  
K. Gadylshin ◽  
A. Bakulin ◽  
M. Dmitriev ◽  
P. Golikov ◽  
D. Neklyudov ◽  
...  
Keyword(s):  
Free Surface ◽  
Frequency Domain ◽  
Surface Velocity ◽  
Acoustic Frequency ◽  
Near Surface

Optimizing the finite-difference implementation of three-dimensional free-surface boundary in frequency-domain modeling of elastic waves

Geophysics ◽  
2019 ◽  
Vol 84 (6) ◽  
pp. T363-T379
Author(s):  
Jian Cao ◽  
Jing-Bo Chen
Keyword(s):  
Free Surface ◽  
Finite Difference ◽  
Frequency Domain ◽  
Computational Cost ◽  
Good Choice ◽  
Weighted Averaging ◽  
Seismic Exploration ◽  
Surface Boundary ◽  
Domain Modeling ◽  
Near Surface

The problem of modeling seismic wave propagation for multiple sources, such as in the solution of gradient-based elastic full-waveform inversion, is an important topic in seismic exploration. The frequency-domain finite-difference (FD) method is a good choice for this purpose, mainly because of its simple discretization and high computational efficiency. However, when it comes to modeling the complete elastic wavefields, this approach has limited surface-wave accuracy because, when modeling with the strong form of the wave equation, it is not always easy to implement an accurate stress-free boundary condition. Although a denser spatial sampling is helpful for overcoming this problem, the additional discrete points will significantly increase the computational cost in the resolution of its resulting discrete system, especially in 3D problems. Furthermore, sometimes, when modeling with optimized schemes, an inconsistency in the computation precision between the regions at the free surface and inside the model volume would happen and introduce numerical artifacts. To overcome these issues, we have considered optimizing the FD implementation of the free-surface boundary. In our method, the problem was formulated in terms of a novel system of partial differential equations satisfied at the free surface, and the weighted-averaging strategy was introduced to optimize its discretization. With this approach, we can impose FD schemes for the free surface and internal region consistently and improve their discretization precision simultaneously. Benchmark tests for Lamb’s problem indicate that the proposed free-surface implementation contributes to improving the simulation accuracy on surface waves, without increasing the number of grid points per wavelength. This reveals the potential of developing optimized schemes in the free-surface implementation. In particular, through the successful introduction of weighting coefficients, this free-surface FD implementation enables adaptation to the variation of Poisson’s ratio, which is very useful for modeling in heterogeneous near-surface weathered zones.

Surfactant Effects on the Free Surface Thermal Structure and Subsurface Flow in a Wind-Wave Tunnel

2006 ◽  
Vol 128 (5) ◽  
pp. 913-920 ◽  
Author(s):  
Supathorn Phongikaroon ◽  
K. Peter Judd
Keyword(s):  
Free Surface ◽  
Wind Speed ◽  
Surface Temperature ◽  
Thermal Structure ◽  
Subsurface Flow ◽  
Surface Velocity ◽  
Ir Detector ◽  
Near Surface ◽  
Wind Speeds ◽  
Surfactant Effects

In this study, the dynamic effects of surfactant (oleyl alcohol) on the surface temperature and the near surface velocity field of a wind driven free surface are investigated. Different surfactant concentrations and wind speeds were examined to elucidate the flow physics. The water surface was imaged with an infrared (IR) detector and the subsurface flow was interrogated utilizing digital particle image velocimetry (DPIV). The IR imagery reveals the presence of a Reynolds ridge that demarcates the boundary between clean (hot) fluid and contaminated (cold) fluid. The clean region was found to be composed of laminae structures known as fishscales. A “wake region” which is an intermediate temperature region resulting from mixing of the near surface fluid layers develops behind the ridge. Experimental results from infrared imagery indicate that the fishscales in the clean region become elongated and narrowed as the wind speed increases. In addition, the results reveal that higher wind speed is required to form a Reynolds ridge in the presence of higher surfactant concentration. The plots of the surface temperature probability density functions reveal that these thermal structures undergo the same evaporative process while the increase in wind speed enhances this process. DPIV results reveal that the growth of a subsurface boundary layer for the contaminated case is more pronounced than that for the clean case.

Surface and near-surface effects on seismic waves—theory and borehole seismometer results

1987 ◽  
Vol 77 (4) ◽  
pp. 1168-1196
Author(s):  
P. M. Shearer ◽  
J. A. Orcutt
Keyword(s):  
Free Surface ◽  
Plane Wave ◽  
Seismic Waves ◽  
Wave Model ◽  
Body Waves ◽  
P Wave ◽  
Surface Velocity ◽  
Ocean Bottom ◽  
Near Surface ◽  
Borehole Seismic

Abstract A simple plane wave model is adequate to explain many surface versus borehole seismometer data sets. Using such a model, we present a series of examples which demonstrate the effects of the free-surface, near-surface velocity gradients, and low impedance surface layers on the amplitudes of upcoming body waves. In some cases, these amplitudes are predictable from simple free-surface and impedance contrast expressions. However, in other cases these expressions are an unreliable guide to the complete response, and the full plane wave calculation must be performed. Large surface amplifications are possible, even without focusing due to lateral heterogeneities or nonlinear effects. Both surface and borehole seismometer site responses are almost always frequency-dependent. Ocean bottom versus borehole seismic data from the 1983 Ngendei Seismic Experiment in the southwest Pacific are consistent with both a simple plane wave model and a more complete synthetic seismogram calculation. The borehole seismic response to upcoming P waves is reduced at high frequencies because of interference between the upgoing P wave and downgoing P and SV waves reflected from the sediment-basement interface. However, because of much lower borehole noise levels, the borehole seismometer enjoys a P-wave signal-to-noise advantage of 3 to 7 dB over nearby ocean bottom instruments.

scholarly journals Physics-Based Relationship for Pore Pressure and Vertical Stress Monitoring Using Seismic Velocity Variations

2021 ◽  
Vol 13 (14) ◽  
pp. 2684
Author(s):  
Eldert Fokker ◽  
Elmer Ruigrok ◽  
Rhys Hawkins ◽  
Jeannot Trampert
Keyword(s):  
Pore Pressure ◽  
Seismic Velocity ◽  
Pressure Head ◽  
Groundwater Table ◽  
Surface Velocity ◽  
Seismic Velocities ◽  
Stress Monitoring ◽  
Near Surface ◽  
Velocity Variations ◽  
Velocity Changes

Previous studies examining the relationship between the groundwater table and seismic velocities have been guided by empirical relationships only. Here, we develop a physics-based model relating fluctuations in groundwater table and pore pressure with seismic velocity variations through changes in effective stress. This model justifies the use of seismic velocity variations for monitoring of the pore pressure. Using a subset of the Groningen seismic network, near-surface velocity changes are estimated over a four-year period, using passive image interferometry. The same velocity changes are predicted by applying the newly derived theory to pressure-head recordings. It is demonstrated that the theory provides a close match of the observed seismic velocity changes.

scholarly journals Mathematical modelling of the overflowing cylinder experiment

2003 ◽  
Vol 474 ◽  
pp. 275-298 ◽  
Author(s):  
P. D. HOWELL ◽  
C. J. W. BREWARD
Keyword(s):  
Free Surface ◽  
Surfactant Concentration ◽  
Dynamic Properties ◽  
Vertical Cylinder ◽  
Surfactant Solution ◽  
Surface Concentration ◽  
Experimental Results ◽  
Surface Velocity ◽  
Experimental Apparatus ◽  
Finite Distance

The overflowing cylinder (OFC) is an experimental apparatus designed to generate a controlled straining flow at a free surface, whose dynamic properties may then be investigated. Surfactant solution is pumped up slowly through a vertical cylinder. On reaching the top, the liquid forms a flat free surface which expands radially before over flowing down the side of the cylinder. The velocity, surface tension and surfactant concentration on the expanding free surface are measured using a variety of non-invasive techniques.A mathematical model for the OFC has been previously derived by Breward et al. (2001) and shown to give satisfactory agreement with experimental results. However, a puzzling indeterminacy in the model renders it unable to predict one scalar parameter (e.g. the surfactant concentration at the centre of the cylinder), which must be therefore be taken from the experiments.In this paper we analyse the OFC model asymptotically and numerically. We show that solutions typically develop one of two possible singularities. In the first, the surface concentration of surfactant reaches zero a finite distance from the cylinder axis, while the surface velocity tends to infinity there. In the second, the surfactant concentration is exponentially large and a stagnation point forms just inside the rim of the cylinder. We propose a criterion for selecting the free parameter, based on the elimination of both singularities, and show that it leads to good agreement with experimental results.

Influence of the Waterline Integral on the Solution of the Frequency-Domain Forward-Speed Radiation-Diffraction Problem of a Ship Advancing in Waves

2014 ◽  
Author(s):  
D. C. Hong ◽  
S. Y. Hong ◽  
G. J. Lee ◽  
M. S. Shin
Keyword(s):  
Integral Equation ◽  
Free Surface ◽  
Green Function ◽  
Frequency Domain ◽  
Surface Condition ◽  
Numerical Results ◽  
Forward Speed ◽  
Line Integral ◽  
Free Surface Condition ◽  
Surface Green Function

The radiation-diffraction potential of a ship advancing in waves is studied using the three-dimensional frequency-domain forward-speed free-surface Green function (Brard 1948) and the forward-speed Green integral equation (Hong 2000). Numerical solutions are obtained by making use of a second-order inner collocation boundary element method which makes it possible to take account of the line integral along the waterline in a rigorous manner (Hong et al. 2008). The present forward-speed Green integral equation includes not only the usual free surface condition for the potential but also the adjoint free surface condition for the forward-speed free-surface Green function as indicated by Brard (1972). Comparison of the present numerical results of the heave-heave wave damping coefficients and the experimental results for the Wigley ship models I, II and III (Journee 1992) has been presented. These coefficients are compared with those calculated without taking into account of the line integral along the waterline in order to show the forward speed effect represented by the waterline integral when it is properly included in the free-surface Green integral equation. Comparison of the present numerical results and the equivalent time-domain results (Hong et al. 2013) has also been presented.

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