scholarly journals Reflection tomography by depth warping: A case study across the Java trench

2021 ◽  
Author(s):  
Yueyang Xia ◽  
Dirk Klaeschen ◽  
Heidrun Kopp ◽  
Michael Schnabel
Keyword(s):  
Displacement Field ◽  
Velocity Structure ◽  
Time Lapse ◽  
Velocity Model ◽  
Data Driven ◽  
Image Point ◽  
Seismic Line ◽  
Velocity Models ◽  
The Common ◽  
Reflection Tomography

Abstract. Accurate subsurface velocity models are crucial for geological interpretations based on seismic depth images. Seismic reflection tomography is an effective iterative method to update and refine a preliminary velocity model for depth imaging. Based on residual move-out analysis of reflectors in common image point gathers an update of the velocity is estimated by a ray-based tomography. To stabilize the tomography, several preconditioning strategies exist. Most critical is the estimation of the depth error to account for the residual move-out of the reflector in the common image point gathers. Because the depth errors for many closely spaced image gathers must be picked, manual picking is extremely time-consuming, human biased, and not reproducible. Data-driven picking algorithms based on coherence or semblance analysis are widely used for hyperbolic or linear events. However, for complex-shaped depth events, pure data-driven picking is difficult. To overcome this, the warping method named Non-Rigid Matching is used to estimate a depth error displacement field. Warping is used, e.g., to merge photographic images or to match two seismic images from time-lapse data. By calculating the displacements between an offset to its neighbouring offset in the common image point domain, a locally smooth-shaped displacement field is defined for each data sample. Depending on the complexity of the subsurface, sample tracking through the displacement field along predefined horizons or on a simple regular grid yields discrete depth error values for the tomography. The application to a multi-channel seismic line across the Sunda subduction zone offshore Lombok island, Indonesia, illustrates the approach and documents the advantages of the method to estimate a detailed velocity structure in a complex tectonic regime. By incorporating the warping scheme into the reflection tomography, we demonstrate an increase in the velocity resolution and precision by improving the data-driven accuracy of depth error picks with arbitrary shapes. This approach offers the possibility to use the full capacities of tomography and further leads to more accurate interpretations of complex geological structures.

Incorporating geologic information into reflection tomography

Geophysics ◽  
2004 ◽  
Vol 69 (2) ◽  
pp. 533-546 ◽  
Author(s):  
Robert G. Clapp ◽  
Biondo L. Biondi ◽  
Jon F. Claerbout
Keyword(s):  
Velocity Structure ◽  
Null Space ◽  
Velocity Model ◽  
Prestack Depth Migration ◽  
Depth Migration ◽  
Interval Velocity ◽  
Velocity Models ◽  
Regularized Inversion ◽  
Symmetric Operators ◽  
Reflection Tomography

In areas of complex geology, prestack depth migration is often necessary if we are to produce an accurate image of the subsurface. Prestack depth migration requires an accurate interval velocity model. With few exceptions, the subsurface velocities are not known beforehand and should be estimated. When the velocity structure is complex, with significant lateral variations, reflection‐tomography methods are often an effective tool for improving the velocity estimate. Unfortunately, reflection tomography often converges slowly, to a model that is geologically unreasonable, or it does not converge at all. The large null space of reflection‐tomography problems often forces us to add a sparse parameterization of the model and/or regularization criteria to the estimation. Standard tomography schemes tend to create isotropic features in velocity models that are inconsistent with geology. These isotropic features result, in large part, from using symmetric regularization operators or from choosing a poor model parameterization. If we replace the symmetric operators with nonstationary operators that tend to spread information along structural dips, the tomography will produce velocity models that are geologically more reasonable. In addition, by forming the operators in helical 1D space and performing polynomial division, we apply the inverse of these space‐varying anisotropic operators. The inverse operators can be used as a preconditioner to a standard tomography problem, thereby significantly improving the speed of convergence compared with the typical regularized inversion problem. Results from 2D synthetic and 2D field data are shown. In each case, the velocity obtained improves the focusing of the migrated image.

Depth-consistent reflection tomography using PP and PS seismic data

Geophysics ◽  
2005 ◽  
Vol 70 (5) ◽  
pp. U51-U65 ◽  
Author(s):  
Stig-Kyrre Foss ◽  
Bjørn Ursin ◽  
Maarten V. de Hoop
Keyword(s):  
Seismic Data ◽  
Optimization Procedure ◽  
Transversely Isotropic ◽  
Data Depth ◽  
Image Point ◽  
Ocean Bottom ◽  
Data Set ◽  
Scattering Angle ◽  
Velocity Models ◽  
Reflection Tomography

We present a method of reflection tomography for anisotropic elastic parameters from PP and PS reflection seismic data. The method is based upon the differential semblance misfit functional in scattering angle and azimuth (DSA) acting on common-image-point gathers (CIGs) to find fitting velocity models. The CIGs are amplitude corrected using a generalized Radon transform applied to the data. Depth consistency between the PP and PS images is enforced by penalizing any mis-tie between imaged key reflectors. The mis-tie is evaluated by means of map migration-demigration applied to the geometric information (times and slopes) contained in the data. In our implementation, we simplify the codepthing approach to zero-scattering-angle data only. The resulting measure is incorporated as a regularization in the DSA misfit functional. We then resort to an optimization procedure, restricting ourselves to transversely isotropic (TI) velocity models. In principle, depending on the available surface-offset range and orientation of reflectors in the subsurface, by combining the DSA with codepthing, the anisotropic parameters for TI models can be determined, provided the orientation of the symmetry axis is known. A proposed strategy is applied to an ocean-bottom-seismic field data set from the North Sea.

Earthquake locations by 3-D finite-difference travel times

1990 ◽  
Vol 80 (2) ◽  
pp. 395-410 ◽  
Author(s):  
Glenn D. Nelson ◽  
John E. Vidale
Keyword(s):  
Travel Time ◽  
Velocity Structure ◽  
Three Dimensional ◽  
Computation Time ◽  
Velocity Model ◽  
Model Complexity ◽  
Grid Spacing ◽  
Origin Time ◽  
Velocity Models ◽  
Fault Trace

Abstract We present a new method for locating earthquakes in a region with arbitrarily complex three-dimensional velocity structure, called QUAKE3D. Our method searches a gridded volume and finds the global minimum travel-time residual location within the volume. Any minimization criterion may be employed. The L1 criterion, which minimizes the sum of the absolute values of travel-time residuals, is especially useful when the station coverage is sparse and is more robust than the L2 criterion (which minimizes the RMS sum) employed by most earthquake location programs. On a UNIX workstation with 8 Mbytes memory, travel-time grids of size 150 by 150 by 50 are reasonably employed, with the actual geographic coverage dependent on the grid spacing. Location precision is finer than the grid spacing. Earthquake recordings at six stations in Bear Valley are located as an example, using various layered and laterally varying velocity models. Locations with QUAKE3D are nearly identical to HYPOINVERSE locations when the same flat-layered velocity model is used. For the examples presented, the computation time per event is approximately 4 times slower than HYPOINVERSE, but the computation time for QUAKE3D is dependent only on the grid size and number of stations, and independent of the velocity model complexity. Using QUAKE3D with a laterally varying velocity model results in locations that are physically more plausible and statistically more precise. Compared to flat-layered solutions, the earthquakes are more closely aligned with the surface fault trace, are more uniform in depth distribution, and the event and station travel-time residuals are much smaller. Hypocentral error bars computed by QUAKE3D are more realistic in that the trade-off of depth versus origin time is implicit in our error estimation, but ignored by HYPOINVERSE.

Improving seismic image using the common-horizon panel

Geophysics ◽  
2019 ◽  
Vol 84 (5) ◽  
pp. S449-S458
Author(s):  
Lu Liu
Keyword(s):  
Time Lag ◽  
Velocity Model ◽  
Migration Velocity ◽  
Quantitative Information ◽  
Time Shift ◽  
Velocity Models ◽  
Seismic Image ◽  
The Common ◽  
Seismic Images ◽  
Zero Time

Generating high-quality seismic images requires accurate velocity models. However, velocity errors are predictably brought into the models. To mitigate the influences of velocity errors, we have used the common-horizon panel (CHP) for migration velocity analysis. CHP provides quantitative information to adjust mispositioned interfaces or correct deformed wavefields, which leads to improved image quality. It is generated by extrapolating seismic gathers to a selected target horizon and applying the time-shift imaging condition. Compared with the commonly used common-image gathers, the events in CHPs are more trackable because geologic interfaces are typically continuous in space. For a correct velocity model, the panel indicates a flat event at zero time lag, whereas in the case of an erroneous velocity model, the event becomes kinematically oscillating. This distinguishing difference provides a practical criterion to verify whether the migration velocity model is correct and to estimate the velocity or wavefield errors based on how much the event deviates from zero time lag. Tests on synthetic and field data sets have shown that the seismic images are improved by using the proposed CHP technique.

Seismic modeling and expression of common fold-thrust structures

2020 ◽  
Vol 8 (1) ◽  
pp. T55-T65
Author(s):  
Jianjun Li ◽  
Shankar Mitra
Keyword(s):  
Velocity Model ◽  
Fault Geometry ◽  
Seismic Modeling ◽  
Fault Propagation ◽  
Lateral Velocity ◽  
Slip Ratio ◽  
Velocity Models ◽  
Time Migration ◽  
The Common

We have conducted seismic modeling of common fold-thrust structures to understand the common geologic parameters influencing seismic data and to understand the common pitfalls associated with interpreting prestack time migration (PSTM) and prestack depth migration (PSDM) data. Mode 1 fault-bend folds are generally well-imaged in PSTM data, provided the correct migration velocities are used for the dipping back and front limbs. Seismic pull-ups of the footwall related to lateral velocity variations can result in problems in interpreting the fault geometry and the subthrust area underlying the crest. Fault-tip fault-propagation folds also show significant footwall pull-ups and show poor to no imaging of the steep front limbs. The geometry of trishear fault-propagation folds is dependent on the maximum slip on the fault (S) and the fault propagation to slip ratio (P/S ratio). We found that the slip has a strong influence on the dip of the front limb and therefore the quality of imaging whereas the P/S ratio, which controls the degree of folding versus thrust faulting, has only a secondary effect. For the front limb, only the area near the synclinal axial plan is well-imaged, so that the fault geometry and extent of propagation are typically difficult to interpret. The front limb dips are also sensitive to the accuracy of the rms velocity model used for migration. Lower velocities result in steeper dipping reflectors, whereas higher velocities result in shallower dips. In general, PSDM provides better imaging of the structures; however, the accuracy and quality of the image are dependent on the velocity models and interpretation derived from the PSTM data.

scholarly journals Seismic velocity model of the crust in the northern Canadian Cordillera from Rayleigh wave dispersion data

2017 ◽  
Vol 54 (2) ◽  
pp. 163-172 ◽  
Author(s):  
Shutian Ma ◽  
Pascal Audet
Keyword(s):  
Rayleigh Wave ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Group Velocity Dispersion ◽  
Velocity Model ◽  
Canadian Cordillera ◽  
Seismic Velocities ◽  
Crustal Layer ◽  
Velocity Models ◽  
Moderate Earthquake

Models of the seismic velocity structure of the crust in the seismically active northern Canadian Cordillera remain poorly constrained, despite their importance in the accurate location and characterization of regional earthquakes. On 29 August 2014, a moderate earthquake with magnitude 5.0, which generated high-quality Rayleigh wave data, occurred in the Northwest Territories, Canada, ∼100 km to the east of the Cordilleran Deformation Front. We carefully selected 23 seismic stations that recorded the Rayleigh waves and divided them into 13 groups according to the azimuth angle between the earthquake and the stations; these groups mostly sample the Cordillera. In each group, we measured Rayleigh wave group velocity dispersion, which we inverted for one-dimensional shear-wave velocity models of the crust. We thus obtained 13 models that consistently show low seismic velocities with respect to reference models, with a slow upper and lower crust surrounding a relatively fast mid crustal layer. The average of the 13 models is consistent with receiver function data in the central portion of the Cordillera. Finally, we compared earthquake locations determined by the Geological Survey of Canada using a simple homogenous crust over a mantle half space with those estimated using the new crustal velocity model, and show that estimates can differ by as much as 10 km.

A case study for azimuthally anisotropic prestack depth imaging of an onshore Alaska prospect

Geophysics ◽  
2010 ◽  
Vol 75 (4) ◽  
pp. B177-B186 ◽  
Author(s):  
Jinming Zhu ◽  
John Mathewson ◽  
Gail Liebelt
Keyword(s):  
Model Building ◽  
Velocity Model ◽  
Image Point ◽  
Depth Imaging ◽  
Depth Migration ◽  
Isotropic Media ◽  
Well Data ◽  
Reflection Tomography ◽  
Very High

In a study of the Sterling-Triangle area of Alaska, U.S.A., we initiated prestack depth migration (PSDM) to improve imaging on a prospect initially identified on a prestack time-migrated (PSTM) volume. Under the isotropic media assumption, the first few iterations of the reflection tomography had difficulty in converging to the proper velocity model. Upon further investigation, a very-high-velocity conglomerate layer was identified in the middle of the section across the whole survey area. We adopted the salt-flood practice, routine in depth-imaging salt provinces such as the Gulf of Mexico. The strategy was to focus on the shallow section above the conglomerate first, followed by a constant-velocity flood for picking the conglomerate base. The finalisotropic PSDM result showed that significant residual moveout differences existed on gathers along different azimuths. The net anisotropic effect on the isotropic PSDM was a degraded final PSDM volume. In the subsequent anisotropic PSDM work, azimuthally variant horizontal velocities were allowed in the model building. Common-image-point (CIP) gathers were created along different azimuths using sectored input gathers. Residuals picked on the sectored CIP gathers were used in joint tomography to invert different horizontal velocities. Incorporating significant well information, we built an anisotropic velocity model such that the azimuthal moveout on the butterfly gathers was essentially flat. The resulting anisotropic PSDM was consistent with well data and could be interpreted with much higher confidence.

Crustal velocity structure beneath the NW Dinarides from 1-D hypocenter-velocity inversion

2021 ◽  
Author(s):  
Gregor Rajh ◽  
Josip Stipčević ◽  
Mladen Živčić ◽  
Marijan Herak ◽  
Andrej Gosar
Keyword(s):  
Velocity Structure ◽  
Velocity Model ◽  
P Wave ◽  
Depth Range ◽  
Velocity Inversion ◽  
Arrival Times ◽  
Simultaneous Inversion ◽  
Velocity Modeling ◽  
Velocity Models ◽  
Station Corrections

<p>The investigated area of the NW Dinarides is bordered by the Adriatic foreland, the Southern Alps, and the Pannonian basin at the NE corner of the Adriatic Sea. Its complex crustal structure is the result of interactions among different tectonic units. Despite numerous seismic studies taking place in this region, there still exists a need for a detailed, smaller scale study focusing mainly on the brittle part of the Earth's crust. Therefore, we decided to investigate the velocity structure of the crust using concepts of local earthquake tomography (LET) and minimum 1-D velocity model. Here, we present the results of the 1-D velocity modeling and the catalogue of the relocated seismicity. A minimum 1-D velocity model is computed by simultaneous inversion for hypocentral and velocity parameters together with seismic station corrections and represents the best fit to the observed arrival times.</p><p>We used 15,579 routinely picked P wave arrival times from 631 well-located earthquakes that occurred in Slovenia and in its immediate surroundings (mainly NW Croatia). Various initial 1-D velocity models, differing in velocity and layering, were used as input for velocity inversion in the VELEST program. We also varied several inversion parameters during the inversion runs. Most of the computed 1-D velocity models converged to a stable solution in the depth range between 0 and 25 km. We evaluated the inversion results using rigorous testing procedures and selected two best performing velocity models. Each of these models will be used independently as the initial model in the simultaneous hypocenter-velocity inversion for a 3-D velocity structure in LET. Based on the results of the 1-D velocity modeling, seismicity distribution, and tectonics, we divided the study area into three parts, redefined the earthquake-station geometry, and performed the inversion for each part separately. This way, we gained a better insight into the shallow velocity structure of each subregion and were able to demonstrate the differences among them.</p><p>Besides general structural implications and a potential to improve the results of LET, the new 1-D velocity models along with station corrections can also be used in fast routine earthquake location and to detect systematic travel time errors in seismological bulletins, as already shown by some studies using similar methods.</p>

scholarly journals High-precision relocation of seismic sequences above a dipping Moho: the case of the January–February 2014 seismic sequence on Cephalonia island (Greece)

Solid Earth ◽  
2015 ◽  
Vol 6 (1) ◽  
pp. 173-184 ◽  
Author(s):  
V. K. Karastathis ◽  
E. Mouzakiotis ◽  
A. Ganas ◽  
G. A. Papadopoulos
Keyword(s):  
High Precision ◽  
Velocity Structure ◽  
Plate Boundary ◽  
Aegean Sea ◽  
Velocity Model ◽  
Ionian Sea ◽  
Velocity Models ◽  
Time Period ◽  
Crustal Thinning ◽  
Back Arc

Abstract. Detailed velocity structure and Moho mapping is of crucial importance for a high precision relocation of seismicity occurring out of, or marginal to, the geometry of seismological networks. Usually the seismographic networks do not cover the boundaries of converging plates such as the Hellenic arc. The crustal thinning from the plate boundary towards the back-arc area creates significant errors in accurately locating the earthquake, especially when distant seismic phases are included in the analysis. The case of the Cephalonia (Ionian Sea, Greece) sequence of January–February 2014 provided an excellent example where the hypocentral precision was greatly affected by the crustal thinning from the plate boundary at the Ionian sea towards the Aegean sea. This effect was examined in detail by testing various velocity models of the region in order to determine an optimal model. Our tests resulted in the adoption of a velocity model that resembles the crustal thinning of the region. Then, a relocation procedure was performed in the Cephalonia sequence for the time period of 26 January to 15 May 2014 by applying probabilistic non-linear location algorithms. The high-precision relocation resulted in an improved spatial distribution of the seismicity with respect to the preliminary locations and provided a reliable basis to examine seismotectonic implications of the Cephalonia sequence.

Seismic imaging beyond depth migration

Geophysics ◽  
2001 ◽  
Vol 66 (6) ◽  
pp. 1895-1912 ◽  
Author(s):  
A. J. Berkhout ◽  
D. J. Verschuur
Keyword(s):  
Seismic Imaging ◽  
Time Lapse ◽  
Velocity Model ◽  
Surface Layers ◽  
Near Surface ◽  
Depth Migration ◽  
New Type ◽  
Complex Propagation ◽  
The Common ◽  
Focus Point

If seismic imaging is formulated in terms of two focusing steps—focusing in emission and focusing in detection (or vice versa)—the output of the first focusing step yields a new type of seismic gather, the common‐focus‐point (CFP) gather, which is available for data analysis and information extraction. One important consequence of this novel option is that the involved focusing operators can be updated without updating the underlying velocity model. Introducing the concept of “dynamic focusing,” it is proposed to verify the validity of focusing operators by comparing the “gather of focus‐point responses” with the “gather of focusing operators.” Compared with velocity‐driven time and depth migration, operator‐driven CFP migration can be considered as the most general approach to seismic imaging: it does not require a velocity model, and it automatically takes into account unknown complex propagation effects such as conversion, anisotropy, and dispersion. In addition, in CFP migration, the second focusing step can be extended to produce both angle‐averaged reflection information and angle‐dependent reflection information. The CFP approach to seismic migration allows new solutions in the situation of complex near‐surface layers, subsalt targets, multicomponent processing, and time lapse analysis.

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