Constrained deformable layer tomostatics

Geophysics ◽  
2009 ◽  
Vol 74 (6) ◽  
pp. WCB35-WCB46 ◽  
Author(s):  
Hua-wei Zhou ◽  
Peiming Li ◽  
Zhihui Yan ◽  
Hui Liu
Keyword(s):  
Field Data ◽  
Multiscale Model ◽  
Surface Velocity ◽  
Depth Range ◽  
Near Surface ◽  
Weathered Zone ◽  
Iterative Inversion ◽  
Static Corrections ◽  
Geometric Changes ◽  
Interface Geometry

Although first-arrival tomography provides an effective way to estimate near-surface velocities and static corrections, the undulation of velocity interfaces such as the base of the weathered zone may not be easily determined by this method. The main reason is that first arrivals are insensitive to small geometric changes in velocity interfaces because their raypaths tend to traverse along those interfaces. To improve the solution of interface geometry, we developed a deformable layer tomostatics method that approximates the near-surface velocity field as several layers of constant velocity and variable thickness that can be inverted for the geometry of the velocity interfaces. We use a multiscale model parameterization in the inversion for interface geometry. Synthetic and field data tests showed that the method can determine the interface geometry. Constraining the depth range of the basal boundary of the weathered zone increases the convergence rate of the iterative inversion process. Tests on field data showed greater reflection coherency in a stacked section based on constrained static corrections than in one from unconstrained static corrections. The method yielded a better match with statics computed from sand-dune curves than does a match obtained by using two commercial grid tomography packages.

Model and Experimental Estimates of Vertical Mixing Intensity in the Sea Upper Homogeneous Layer

2021 ◽  
Vol 37 (3) ◽  
Author(s):  
A. M. Chukharev ◽  
M. I. Pavlov ◽  
◽  
Keyword(s):  
Experimental Data ◽  
Turbulence Intensity ◽  
Multiscale Model ◽  
Turbulent Exchange ◽  
Stokes Drift ◽  
Homogeneous Layer ◽  
Depth Range ◽  
Near Surface ◽  
Langmuir Circulations ◽  
Turbulence Generation

Purpose. The study is aimed at qualitative and quantitative analysis (based on the updated previously proposed multiscale model) of the experimental data on turbulence intensity and their comparison with theoretical and semi-empirical relationships for the purpose of describing the contributions of various turbulence sources. Methods and Results. A comparative analysis of experimental data and model calculations of turbulence characteristics near the sea surface was performed. The methods of theoretical assessing generation of turbulence in the near-surface sea layer by various physical processes are considered. The results of calculations by the well-known models of turbulent exchange were compared with the experimental data collected by the scientists of the Turbulence Department of MHI, RAS, using the specialized equipment. The analysis results made it possible to determine the possibility of applying the considered models for calculating turbulence intensity under different hydrometeorological conditions. At light winds, none of the models yielded the results which matched the measurement data. At moderate winds, the simulation results showed quite satisfactory agreement with the experiment data; and for strong winds, the multiscale model results were the best. This model was modified to assess the contributions of two other mechanisms of turbulence generation: the Stokes drift and the Langmuir circulations. Conclusions. Objective assessment of the turbulent exchange intensity requires taking into account of three main mechanisms of turbulence generation, namely flow velocity shear, wave motions and wave breaking. Depending on the hydrometeorological situation, each of these mechanisms can dominate in a certain depth range. The calculations performed using the updated model showed that the Stokes drift added 2–17 % to the total dissipation in the upper 30-meter layer, whereas the contribution of the Langmuir circulations calculated through dependence of the vertical velocity of kinetic energy transfer upon the Langmuir number, can reach 15 % for small Langmuir numbers.

Estimation of Near-Surface Velocity for 3-D Complicated Topography and Seismic Tomographic Static Corrections

2001 ◽  
Vol 44 (2) ◽  
pp. 268-274 ◽  
Author(s):  
Yi-Ke LIU ◽  
Xu CHANG ◽  
Hui WANG ◽  
Fu-Zhong LI
Keyword(s):  
Surface Velocity ◽  
Near Surface ◽  
Static Corrections

Overthrust imaging with tomo‐datuming: A case study

Geophysics ◽  
1998 ◽  
Vol 63 (1) ◽  
pp. 25-38 ◽  
Author(s):  
Xianhuai Zhu ◽  
Burke G. Angstman ◽  
David P. Sixta
Keyword(s):  
Velocity Model ◽  
Surface Velocity ◽  
Time Shift ◽  
Lateral Velocity ◽  
Prestack Depth Migration ◽  
Near Surface ◽  
Depth Migration ◽  
Static Corrections ◽  
Velocity Variations ◽  
Kirchhoff Method

Through the use of iterative turning‐ray tomography followed by wave‐equation datuming (or tomo‐datuming) and prestack depth migration, we generate accurate prestack images of seismic data in overthrust areas containing both highly variable near‐surface velocities and rough topography. In tomo‐datuming, we downward continue shot records from the topography to a horizontal datum using velocities estimated from tomography. Turning‐ray tomography often provides a more accurate near‐surface velocity model than that from refraction statics. The main advantage of tomo‐datuming over tomo‐statics (tomography plus static corrections) or refraction statics is that instead of applying a vertical time‐shift to the data, tomo‐datuming propagates the recorded wavefield to the new datum. We find that tomo‐datuming better reconstructs diffractions and reflections, subsequently providing better images after migration. In the datuming process, we use a recursive finite‐difference (FD) scheme to extrapolate wavefield without applying the imaging condition, such that lateral velocity variations can be handled properly and approximations in traveltime calculations associated with the raypath distortions near the surface for migration are avoided. We follow the downward continuation step with a conventional Kirchhoff prestack depth migration. This results in better images than those migrated from the topography using the conventional Kirchhoff method with traveltime calculation in the complicated near surface. Since FD datuming is only applied to the shallow part of the section, its cost is much less than the whole volume FD migration. This is attractive because (1) prestack depth migration usually is used iteratively to build a velocity model, so both efficiency and accuracy are important factors to be considered; and (2) tomo‐datuming can improve the signal‐to‐noise (S/N) ratio of prestack gathers, leading to more accurate migration velocity analysis and better images after depth migration. Case studies with synthetic and field data examples show that tomo‐datuming is especially helpful when strong lateral velocity variations are present below the topography.

Novel strategies for complex foothills seismic imaging — Part 1: Mega-near-surface velocity estimation

2020 ◽  
Vol 8 (3) ◽  
pp. T651-T665
Author(s):  
Yalin Li ◽  
Xianhuai Zhu ◽  
Gengxin Peng ◽  
Liansheng Liu ◽  
Wensheng Duan
Keyword(s):  
Seismic Imaging ◽  
Velocity Model ◽  
Velocity Estimation ◽  
Innovative Approach ◽  
Surface Velocity ◽  
Near Surface ◽  
Depth Migration ◽  
Static Corrections ◽  
Over Time

Seismic imaging in foothills areas is challenging because of the complexity of the near-surface and subsurface structures. Single seismic surveys often are not adequate in a foothill-exploration area, and multiple phases with different acquisition designs within the same block are required over time to get desired sampling in space and azimuths for optimizing noise attenuation, velocity estimation, and migration. This is partly because of economic concerns, and it is partly because technology is progressing over time, creating the need for unified criteria in processing workflows and parameters at different blocks in a study area. Each block is defined as a function of not only location but also the acquisition and processing phase. An innovative idea for complex foothills seismic imaging is presented to solve a matrix of blocks and tasks. For each task, such as near-surface velocity estimation and static corrections, signal processing, prestack time migration, velocity-model building, and prestack depth migration, one or two best service companies are selected to work on all blocks. We have implemented streamlined processing efficiently so that Task-1 to Task-n progressed with good coordination. Application of this innovative approach to a mega-project containing 16 3D surveys covering more than [Formula: see text] in the Kelasu foothills, northwestern China, has demonstrated that this innovative approach is a current best practice in complex foothills imaging. To date, this is the largest foothills imaging project in the world. The case study in Kelasu successfully has delivered near-surface velocity models using first arrivals picked up to 3500 m offset for static corrections and 9000 m offset for prestack depth migration from topography. Most importantly, the present megaproject is a merge of several 3D surveys, with the merge performed in a coordinated, systematic fashion in contrast to most land megaprojects. The benefits of this approach and the strategies used in processing data from the various subsurveys are significant. The main achievement from the case study is that the depth images, after the application of the near-surface velocity model estimated from the megasurveys, are more continuous and geologically plausible, leading to more accurate seismic interpretation.

scholarly journals Mitigation of defocusing by statics and near-surface velocity errors by interferometric least-squares migration with a reference datum

Geophysics ◽  
2016 ◽  
Vol 81 (4) ◽  
pp. S195-S206 ◽  
Author(s):  
Mrinal Sinha ◽  
Gerard T. Schuster
Keyword(s):  
Least Squares ◽  
Prior Knowledge ◽  
Seismic Data ◽  
Field Data ◽  
Velocity Model ◽  
Well Logs ◽  
Surface Velocity ◽  
Near Surface ◽  
Reference Layer ◽  
Least Squares Migration

Imaging seismic data with an erroneous migration velocity can lead to defocused migration images. To mitigate this problem, we first choose a reference reflector whose topography is well-known from the well logs, for example. Reflections from this reference layer are correlated with the traces associated with reflections from deeper interfaces to get crosscorrelograms. Interferometric least-squares migration (ILSM) is then used to get the migration image that maximizes the crosscorrelation between the observed and the predicted crosscorrelograms. Deeper reference reflectors are used to image deeper parts of the subsurface with a greater accuracy. Results on synthetic and field data show that defocusing caused by velocity errors is largely suppressed by ILSM. We have also determined that ILSM can be used for 4D surveys in which environmental conditions and acquisition parameters are significantly different from one survey to the next. The limitations of ILSM are that it requires prior knowledge of a reference reflector in the subsurface and the velocity model below the reference reflector should be accurate.

Migration from 3D irregular surfaces: A prestack time migration approach

Geophysics ◽  
2012 ◽  
Vol 77 (5) ◽  
pp. S117-S129 ◽  
Author(s):  
Jianfeng Zhang ◽  
Jincheng Xu ◽  
Hao Zhang
Keyword(s):  
Wave Propagation ◽  
Computational Cost ◽  
Surface Velocity ◽  
Detailed Knowledge ◽  
Near Surface ◽  
Effective Velocity ◽  
Time Migration ◽  
Imaging Results ◽  
Static Corrections ◽  
Prestack Time Migration

We have developed a modified 3D prestack time migration (PSTM) scheme that can handle rugged topography as well as high near-surface velocities in land seismic imaging. The proposed topography PSTM can be applied to seismic data recorded on a 3D irregular surface without static corrections. Two effective velocity parameters were found to describe wave propagation through inhomogeneous media above and below a chosen datum. As a result, wave propagation phenomena in the complex near surface, such as near-vertical incidences through a weathering layer and raypaths bending away from vertical in the presence of high near-surface velocities, are correctly considered. The two effective velocity parameters can be estimated by flattening events in imaging gathers. Hence, it is not necessary to have detailed knowledge of the near-surface velocity model and velocity field below the datum when applying topography PSTM. We integrated residual static corrections into topography PSTM. This eliminated the distortions along the events better than conventional residual static corrections, which are usually applied before migration. The computational cost of the topography PSTM was only slightly higher than that of conventional PSTM due to the use of a table-driven algorithm. Three-dimensional synthetic and field data sets were used to test the proposed topography PSTM. High-quality imaging results were obtained.

The velocity structure of the upper ocean in the presence of surface forcing and mesoscale oceanic eddies

1983 ◽  
Vol 308 (1503) ◽  
pp. 327-340 ◽  
Keyword(s):  
Velocity Field ◽  
Wind Stress ◽  
Mixed Layer ◽  
Opposite Direction ◽  
Temporal Variability ◽  
Velocity Vector ◽  
Surface Velocity ◽  
Depth Range ◽  
Local Wind ◽  
Near Surface

One goal of the Joint Air-Sea Interaction Experiment (JASIN) was to investigate the structure of the near-surface velocity field and to attempt to quantify what fraction of that field was related to the local wind. Toward that end, in the late summer of 1978, two moorings were deployed in the northern Rockall Trough with oceanographic instrumentation concentrated in the upper 100 m of the ocean. Simultaneous observations were made of the surface winds at each mooring and, adjacent to one of the moorings, of the velocity field at depths from 79 to 1000 m. Energetic, eddy-like circulation dominated the velocity field in the JASIN area at depths shallower than approximately 800 m. However, both the velocity and the vertical shear of horizontal velocity showed variability that increased with proximity to the surface. Empirical orthogonal functions, computed to separate the velocity data into uncorrelated modes of variability, showed that over 97 % of the variability in the upper 300 m was distributed among only three vertical modes. The first function had little depth dependence; the second had strong depth-independent flow in the depth range of the mixed layer and weak flow in the opposite direction at all depths below; and the third had strong flow near the surface, strong flow in the opposite direction just below the base of the mixed layer, and weaker flow at all other depths. Function 1 alone provided a near-complete description of the velocity variability below 85 m, where the flows associated with the eddy-like circulation and the barotropic semidiurnal tide were the dominant components. At 85 m and above all three functions were necessary to provide a complete description. Temporal variability of function 2 was coherent with the local wind stress at the inertial frequency, but, at lower frequencies, resulted in transport in the mixed layer to the southeast that was not coherent with the local wind. Low frequency temporal variability of function 3 was coherent with the local wind stress; at these frequencies the velocity vector of function 3 nearest the surface was directed to the right of the wind stress vector and the velocity vector just below the base of the mixed layer was directed to the left of the wind stress. Thus, forcing by the local wind can account for some but not all of the increased variability found near the surface.

High‐resolution seismic traveltime tomography incorporating static corrections applied to a till‐covered bedrock environment

Geophysics ◽  
2004 ◽  
Vol 69 (4) ◽  
pp. 1082-1090 ◽  
Author(s):  
Björn Bergman ◽  
Ari Tryggvason ◽  
Christopher Juhlin
Keyword(s):  
High Resolution ◽  
Velocity Model ◽  
Surface Velocity ◽  
Data Set ◽  
Near Surface ◽  
Seismic Processing ◽  
Simultaneous Inversion ◽  
Reflection Seismic ◽  
Static Corrections ◽  
Velocity Variations

A major obstacle in tomographic inversion is near‐surface velocity variations. Such shallow velocity variations need to be known and correctly accounted for to obtain images of deeper structures with high resolution and quality. Bedrock cover in many areas consists of unconsolidated sediments and glacial till. To handle the problems associated with this cover, we present a tomographic method that solves for the 3D velocity structure and receiver static corrections simultaneously. We test the method on first‐arrival picks from deep seismic reflection data acquired in the mid‐ late to 1980s in the Siljan Ring area, central Sweden. To use this data set successfully, one needs to handle a number of problems, including time‐varying, near‐surface velocities from data recorded in winter and summer, several sources and receivers within each inversion cell, varying thickness of the cover layer in each inversion cell, and complex 3D geology. Simultaneous inversion for static corrections and velocity produces a much better image than standard tomography without statics. The velocity model from the simultaneous inversion is superior to the velocity model produced using refraction statics obtained from standard reflection seismic processing prior to inversion. Best results using the simultaneous inversion are obtained when the initial top velocity layer is set to the near‐surface bedrock velocity rather than the velocity of the cover. The resulting static calculations may, in the future, be compared to refraction static corrections in standard reflection seismic processing. The preferred final model shows a good correlation with the mapped geology and the airborne magneticmap.

Sign in / Sign up

Export Citation Format

Share Document