Flooding the topography: Wave‐equation datuming of land data with rugged acquisition topography

Geophysics ◽  
1997 ◽  
Vol 62 (5) ◽  
pp. 1558-1569 ◽  
Author(s):  
Dimitri Bevc
Keyword(s):  
Wave Equation ◽  
A Priori ◽  
Surface Velocity ◽  
Static Shift ◽  
Near Surface ◽  
Static Correction ◽  
Structural Image ◽  
Rugged Topography ◽  
And Migration ◽  
Normal Moveout Correction

Wave‐equation datuming overcomes some of the problems that seismic data recorded on rugged surface topography present in routine image processing. The main problems are that (1) standard, optimized migration and processing algorithms assume data are recorded on a flat surface, and that (2) the static correction applied routinely to compensate for topography is inaccurate for waves that do not propagate vertically. Wave‐based processes such as stacking, dip‐moveout correction, normal‐moveout correction, velocity analysis, and migration after static shift can be severely affected by the nonhyperbolic character of the reflections. To alleviate these problems, I apply wave‐equation datuming early in the processing flow to upward continue the data to a flat datum, above the highest topography. This is what I refer to as “flooding the topography.” This approach does not require detailed a priori knowledge of the near‐surface velocity, and it streamlines subsequent processing because the data are regridded onto a regularly sampled datum. Wave‐equation datuming unravels the distortions caused by rugged topography, and unlike the static shift method, it does not adversely effect subsequent wave‐based processing. The image obtained after wave‐equation datuming exhibits better reflector continuity and more accurately represents the true structural image than the image obtained after static shift.

scholarly journals Full Waveform Inversion in generalized coordinates for zones of curved topography

2018 ◽  
Vol 8 (2) ◽  
Author(s):  
César Augusto Arias- Chica ◽  
David Abreo ◽  
Sergio Abreo ◽  
Luis Fernando Duque- Gómez ◽  
Ana Beatríz Ramírez- Silva
Keyword(s):  
Computing Time ◽  
Waveform Inversion ◽  
Surface Velocity ◽  
Full Waveform Inversion ◽  
Near Surface ◽  
Velocity Models ◽  
Full Waveform ◽  
Wide Range ◽  
Rugged Topography ◽  
Uniform Rectangular Grid

Full waveform inversion (FWI) has been recently used to estimate subsurface parameters, such as velocity models. This method, however, has a number of drawbacks when applied to zones with rugged topography due to the forced application of a Cartesian mesh on a curved surface. In this work, we present a simple coordinate transformation that enables the construction of a curved mesh. The proposed transformation is more suitable for rugged surfaces and it allows mapping a physical curved domain into a uniform rectangular grid, where acoustic FWI can be applied in the traditional way by introducing a modified Laplacian. We prove that the proposed approximation can have a wide range of applications, producing precise near-surface velocity models without increasing the computing time of the FWI.

A method of estimating near‐surface velocity models with rugged topography using refraction travel‐time

2009 ◽  
Author(s):  
Xiaoqiao Ren ◽  
Xingyuan Zhou ◽  
Hequn Li ◽  
Guangkai Ma ◽  
Jianlei Zhang
Keyword(s):  
Travel Time ◽  
Surface Velocity ◽  
Near Surface ◽  
Velocity Models ◽  
Rugged Topography

Use of refraction, reflection, and wave-equation-based tomography for imaging beneath shallow gas: A Trinidad field data example

Geophysics ◽  
2008 ◽  
Vol 73 (5) ◽  
pp. VE281-VE289 ◽  
Author(s):  
Nurul Kabir ◽  
Uwe Albertin ◽  
Min Zhou ◽  
Vishal Nagassar ◽  
Einar Kjos ◽  
...  
Keyword(s):  
Wave Equation ◽  
Velocity Field ◽  
Velocity Model ◽  
Surface Velocity ◽  
Low Velocity ◽  
Shallow Gas ◽  
Near Surface ◽  
Velocity Anomaly ◽  
Refraction Tomography ◽  
Reflection Tomography

Shallow localized gas pockets cause challenging problems in seismic imaging because of sags and wipe-out zones they produce on imaged reflectors deep in the section. In addition, the presence of shallow gas generates strong surface-related and interbed multiples, making velocity updating very difficult. When localized gas pockets are very shallow, we have limited information to build a near-surface velocity model using ray-based reflection tomography alone. Diving-wave refraction tomography successfully builds a starting model for the very shallow part. Usual ray-based reflection tomography using single-parameter hyperbolic moveout might need many iterations to update the deeper part of the velocity model. In addition, the method generates a low-velocity anomaly in the deeper part of the model. We present an innovative method for building the final velocity model by combining refraction, reflection, and wave-equation-based tomography. Wave-equation-based tomography effectively generates a detailed update of a shallow velocity field, resolving the gas-sag problem. When applied as the last step, following the refraction and reflection tomography, it resolves the gas-sag problem but fails to remove the low-velocity anomaly generated by the reflection tomography in the deeper part of the model. To improve the methodology, we update the shallow velocity field using refraction tomography followed by wave-equation tomography before updating the deeper part of the model. This step avoids generating the low-velocity anomaly. Refraction and wave-equation-based tomography followed by reflection tomography generates a simpler velocity model, giving better focusing in the deeper part of the image. We illustrate how the methodology successfully improves resolution of gas anomalies and significantly reduces gas sag from an imaged section in the Greater Cassia area, Trinidad.

The zero‐velocity layer: Migration from irregular surfaces

Geophysics ◽  
1992 ◽  
Vol 57 (11) ◽  
pp. 1435-1443 ◽  
Author(s):  
Craig Beasley ◽  
Walt Lynn
Keyword(s):  
Wave Equation ◽  
Seismic Data ◽  
Migration Velocity ◽  
Time Shift ◽  
Irregular Surfaces ◽  
Interval Velocity ◽  
Static Correction ◽  
Zero Velocity ◽  
And Migration ◽  
Velocity Layer

Seismic data acquired in areas with irregular topography are usually corrected to a flat datum before migration. A time‐honored technique for handling elevation changes is to time shift the data before application of migration. This simple time shift, or elevation‐static correction, cannot properly represent wide‐angle or dipping reflections as they would have been recorded at the datum. As a result, when elevation varies significantly, accuracy in event positioning may be compromised for migration and other wave‐equation processes, such as dip moveout processing (DMO). Traditionally, such over‐ and under‐migration artifacts have been dealt with by increasing or decreasing the migration velocity. However, simple adjustment of the migration velocity cannot undo the wave‐field distortions induced in seismic data acquired over varying elevations. More sophisticated and accurate solutions such as wave‐equation datuming are too computationally demanding for routine use. Here, we propose an efficient and accurate technique for doing migration from irregular surfaces using conventional migration algorithms. As in elevation‐static corrections, surface‐recorded data are time‐shifted to a horizontal datum; for our process, we choose to have that datum elevation lie at or above the highest elevation in the survey. After migration, the datum elevation can always be adjusted to any other level by means of a bulk time shift. In the migration step, the velocity is set to zero (or some very small value) in the layer between the surface and the datum; below the original surface, the interval velocity represents the best estimate of the subsurface geology. By adding a zero‐velocity layer, the migration algorithm is applied to the data from the flat datum and no lateral propagation is allowed until a nonzero velocity is encountered at the recording surface. Synthetic and field data examples demonstrate that use of the “zero‐velocity layer” significantly improves imaging accuracy relative to conventional migration from a flat datum. Moreover, the geologically derived migration‐velocity field need not be adjusted to compensate for shortcomings in the datum‐static procedure. The technique can be extended to prestack processes such as DMO, shot‐ and receiver‐gather downward extrapolation, and migration and thus suggests a unified approach to processing data from irregular surfaces.

Prestack processing of land data with complex topography

Geophysics ◽  
1995 ◽  
Vol 60 (6) ◽  
pp. 1875-1886 ◽  
Author(s):  
Sara Rajasekaran ◽  
George A. McMechan
Keyword(s):  
Wave Equation ◽  
Processing System ◽  
Essential Elements ◽  
Surface Velocity ◽  
Velocity Variation ◽  
Data Set ◽  
Near Surface ◽  
Depth Migration ◽  
Elevation Changes ◽  
Data Extrapolation

A new wave‐equation–based prestack seismic processing system is proposed. This system has only two essential elements; velocity analysis and depth migration. This approach applies truly surface‐consistent statics corrections, regardless of the amount of elevation, change or of near‐surface velocity variation. It uses tomography for estimating the details of shallow velocities and a finite‐difference solution of the two‐way wave‐equation both for computation of image times and for data extrapolation in migration. A field data set that violates most of the assumptions in conventional common midpoint (CMP) processing, because of severe elevation changes and near‐surface velocity variations, is successfully processed. The final depth section reveals a complicated fold‐thrust geometry that was not visible after CMP processing.

Where is the zero‐velocity layer?

Geophysics ◽  
1997 ◽  
Vol 62 (1) ◽  
pp. 266-269 ◽  
Author(s):  
Samuel H. Gray
Keyword(s):  
Finite Difference ◽  
Seismic Velocity ◽  
Surface Velocity ◽  
Time Shift ◽  
Near Surface ◽  
Depth Migration ◽  
Static Correction ◽  
Zero Velocity ◽  
Conventional Procedure ◽  
Velocity Layer

The zero‐velocity layer was introduced in Higginbotham et al. (1985) to increase the maximum dip imaging capability of finite‐difference depth migration. Beasley and Lynn (1992) adapted the idea to improve the imaging, again using finite‐difference depth migration, of seismic data acquired in areas of irregular topography. Beasley and Lynn's application improves upon the conventional method of processing, which is to time shift the data from the acquisition surface to a horizontal datum, and then migrate using the near‐surface velocity above the surface and the best estimate of seismic velocity below the surface. The conventional procedure typically produces artifacts in the shallow part of the section that are characteristic of overmigration. To reduce these artifacts, velocities are often reduced for the migration step. The use of the zero‐velocity layer overcomes the need to adjust the migration velocities. Here, a component of the migration velocity is set to zero in the layer between the datum and the surface. The function of the zero‐velocity layer in migration is to remove the elevation‐static correction applied in shifting the data to the flat datum. Only after the data have migrated through the zero‐velocity layer to the irregular recording surface does the migration begin to act in its customary sense, moving energy from trace to trace.

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