Iterative Prestack Depth Migration Research Articles

Seismic Velocity Structure and Improved Seismic Image of the Southern Depression of the Tainan Basin from Pre-Stack Depth Migration

In this paper, a velocity model of the Southern Depression of the Tainan Basin is obtained along with its migrated image from an iterative pre-stack depth migration approach. The Cenozoic strata are uniformly layered with velocities varying from similar to 1.8 to similar to 3.6 km s(-1) However, the general velocity is slightly lower in the NW segment than the SE. Both fractures and burial depth might be the controls of their seismic velocities. There is an unconformable contact between the Cenozoic and underlying Mesozoic strata with an abrupt velocity jump from similar to 3.2 to similar to 4.3 km s(-1). The Mesozoic strata are recognized with acoustically distinct reflection patterns (chaotic, deformed and discontinuous) and complex internal structures (uplift, folds and faults). Their interval velocities range from similar to 4.3 to similar to 4.7 km s(-1) within a depth from similar to 3.5 down to similar to 12.5 km, and the maximum depositional thickness reaches up to 6.5 km. Multiple tectonic events such as collision, subsidence and uplift might be responsible for the complexity of the Mesozoic strata.

Fast model-based migration velocity analysis and reflector shape estimation

Migration velocity analysis can be made more efficient by preselecting the traces that contribute to a series of common-reflection-point (CRP) gathers and migrating only those traces. The data traces that contribute to a CRP for one reflection point on one layer are defined in a two-step procedure. First, poststack parsimonious Kirchhoff depth migration of zero-offset (or stacked) traces defines approximate reflector positions and orientations. Then, ray tracing from the reflection points for nonzero reflection angles defines the source and receiver locations of the data traces in the CRP gather. These traces are then prestack depth migrated, and the interval velocity model adjustment is obtained by fitting the velocity that maximizes the stack amplitude over the predicted (nonhyperbolic) moveout. A small number (2–3) of iterations converge to a 2D model of layer shape and interval velocity. Further efficiency is obtained by implementing layer stripping. The computation time is greatly reduced by combining parsimonious migration with processing only the salient portions of the whole seismic data set. The algorithm can handle lateral velocity variation within each layer as well as constant velocity. The computation time of the proposed algorithm is of the same order as that of the standard rms velocity scan method, but it does not have the inherent assumptions of the velocity scan method and is faster than current iterative prestack depth migration velocity analysis methods for typical field data.

Advances in subsurface imaging using potential field technology: gravimetry sensors and applications

Recent advances in resolution and sensitivity of potential field sensors could fundamentally change subsurface risk management. Gravity gradiometer data have been fully integrated in the cycle of iterative pre-stack depth migration (PreSDM) model, updating to arrive at a high-fidelity subsurface model in complex geological settings where seismic data is inevitably of poor quality. Upon integrating potential field data, for example provided by new instruments currently available, it will be shown that significantly improved seismic imaging can be achieved near salt overhangs (sub-salt) as well as below hard layers. Based on results so far it is concluded that the potential of this technology could substantially change the role of gravity field measurements in the process of subsurface de-risking. The resolution and interpretation issues of existing gravity gradiometer data are explored and their utility with several case studies demonstrated.

Optimization and sensitivity analysis of seismic depth migrations from Hibernia Field

Accurate imaging of seismic reflectors with depth migration requires accurate velocity models. In frontier areas with few well constraints, velocity estimation generally involves the use of methods such as normal moveout analysis, seismic traveltime tomography, or iterative prestack depth migration. These techniques can be effective, but may also be expensive or time‐consuming. In situations where we have information on formation tops from a series of wells which intersect seismic reflectors, we use a least‐squares optimization method to estimate velocity models. This method produces velocity models that optimize depth migrations in terms of well constraints by using least‐squares inversion to match the depth migration images to formation tops. The well log information is used to optimize poststack migration, thereby eliminating some of the time and expense of velocity analysis. In addition to applying an inversion method which optimizes depth migration in terms of formation tops, we can use a sensitivity analysis method of “most‐squares inversion” to explore a range of velocity models which provide mathematically acceptable solutions. This sensitivity analysis quantifies the expected result that our velocity estimates are generally less reliable for thin beds than for thick beds. The proposed optimization method is shown to be successful on synthetic and real data cases from the Hibernia Field of offshore Newfoundland.

The case against porosity change: Seismic velocity decrease at the toe of the Oregon accretionary prism

As sediments are incorporated into the first thrust sheet of the landward-vergent Oregon accretionary prism they undergo a decrease in interval velocity. This trend is opposite that observed at other accretionary prisms, where an increase in velocity normally accompanies the loss of porosity during deformation. Velocities were obtained from focusing analysis and iterative prestack depth migration of multichannel seismic data for four stratigraphically defined intervals. The shallowest interval (

Imaging and velocity estimation with depth‐focusing analysis

Prestack depth migration uses two imaging conditions, zero time and zero offset, during downward continuation to form a migrated depth section. When the migration velocities are exact, the two imaging conditions act in a complementary fashion to yield a focused image. When the migration velocities are in error, reflected energy collapses to zero offset at depths that are inconsistent with the zero‐time imaging condition. The result is a deteriorated seismic image. However, by interpreting the nonzero times at which focusing actually occurs, the migration velocities can be updated iteratively in a process called depth‐focusing analysis. To produce a well‐focused seismic image, the goal of depth‐focusing analysis must be the elimination of focusing errors; however, practical considerations can prevent this goal from being achieved. Therefore, to relax the sensitivity of the migrated image to focusing errors, we introduce a nonzero‐time imaging condition by extracting the data along the interpreted surface of focusing from the depth‐focusing analysis volume. This method, called focal‐surface imaging, estimates the results of prestack depth migration using the updated velocities. Depth‐focusing analysis is shown to be a robust approach to velocity estimation and imaging. Limitations arising from constant‐velocity and low‐dip approximations are reduced in the presence of increasing velocities with depth. Lateral velocity errors, sources of exaggerated focusing errors and diverging velocity solutions, can also be addressed by applying a damping factor to the interpreted depth errors. Velocity estimation and focal‐surface imgaging, using iterative prestack depth migration, were applied to a southern North Sea data set. Starting with a regional velocity function, the first iteration provided an updated velocity field that more accurately conformed to the known lithologies. The focal‐surface image, formed from the same iteration, contained significantly more focused energy than the conventional section formed by prestack depth migration. However, structural differences between the two sections indicated the need for another iteration of migration using the updated velocities. The second iteration indicated smaller velocity errors and enough similarity between the migrated section and the new focal‐surface image to indicate that further iterations were unnecessary.

Applications of Pre-Stack Depth Migration

Complex shallow faulting is a common problem which can lead to poor results when using the CDP assumption. Wave-field distortion makes it impossible to select a stacking velocity that does not deteriorate stack response. The only way to achieve a high-quality stack is to predict and remove the wave-field distortion before stack, that is, by correctly migrating the pre-stack gather. Predicting the distortion is difficult, as it requires the estimation of the actual interval velocity field. Many methods have been proposed for estimating velocities, three of which are iterative pre-stack depth migration, ray-trace modelling, and interactive interval velocity analysis. Each of these has advantages and disadvantages, but iterative pre-stack depth migration is generally the most useful. Reflection tomography can also be useful. Once accurate velocities have been obtained, many algorithms are available to do the actual migration. Two methods are compared; a shot-record finite-difference algorithm and a common-offset, Kirchhoff algorithm. The latter algorithm has practical advantages that lends itself more to iterative pre-stack depth-migration velocity analysis and generally makes it more attractive. Pre-stack depth migration, with both algorithms, is demonstrated on a real data set from offshore north-western Australia. The examples show that iterative pre-stack depth migration can successfully improve seismic images in difficult areas.

Nature of the S reflector beneath the Galicia Banks rifted margin: Preliminary results from prestack depth migration

Research Article| December 01, 1992 Nature of the S reflector beneath the Galicia Banks rifted margin: Preliminary results from prestack depth migration H. -J. Hoffmann; H. -J. Hoffmann 1GEOMAR, Christian-Albrechts University, Wischhofstrasse 1-3, Kiel, Germany Search for other works by this author on: GSW Google Scholar T. J. Reston T. J. Reston 1GEOMAR, Christian-Albrechts University, Wischhofstrasse 1-3, Kiel, Germany Search for other works by this author on: GSW Google Scholar Author and Article Information H. -J. Hoffmann 1GEOMAR, Christian-Albrechts University, Wischhofstrasse 1-3, Kiel, Germany T. J. Reston 1GEOMAR, Christian-Albrechts University, Wischhofstrasse 1-3, Kiel, Germany Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1992) 20 (12): 1091–1094. https://doi.org/10.1130/0091-7613(1992)020<1091:NOTSRB>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation H. -J. Hoffmann, T. J. Reston; Nature of the S reflector beneath the Galicia Banks rifted margin: Preliminary results from prestack depth migration. Geology 1992;; 20 (12): 1091–1094. doi: https://doi.org/10.1130/0091-7613(1992)020<1091:NOTSRB>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Beneath the tilted fault blocks of the Galicia rifted margin, a bright reflection, termed S, has been interpreted as the master detachment fault controlling the extension of the margin. However, on time-migrated sections, S appears to be locally offset by the steep faults of the upper section. We investigate the true geometry of S and its relation to the overlying faults through iterative prestack depth migration. We conclude that it is not significantly offset by the faults, which instead appear to stop abruptly at S. Thus, we interpret S as the master detachment and consider its role during extension. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.