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Seismic migration
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Main page Seismic migration is the process by which seismic events are geometrically re-located in either space or time to the location the event occurred in the subsurface rather than
Contents the location that it was recorded at the surface, thereby creating a more accurate image of the subsurface. This process is necessary to overcome the limitations of geophysical
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methods imposed by areas of complex geology, such as: faults, salt bodies, folding, etc.[1][2][3]
Current events
Random article Migration moves dipping reflectors to their true subsurface positions and collapses diffractions,[4] resulting in a migrated image that typically has an increased spatial resolution
Donate to Wikipedia and resolves areas of complex geology much better than non-migrated images. A form of migration is one of the standard data processing techniques for reflection-based
Wikipedia store geophysical methods (seismic reflection and ground-penetrating radar)

Interaction The need for migration has been understood since the beginnings of seismic exploration and the very first seismic reflection data from 1921 were migrated.[5] Computational
Help migration algorithms have been around for many years but they have only entered wide usage in the past 20 years because they are extremely resource-intensive. Migration
About Wikipedia can lead to a dramatic uplift in image quality so algorithms are the subject of intense research, both within the geophysical industry as well as academic circles.
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Recent changes Contents [hide]
Contact page 1 Rationale
2 Use
Tools
3 Types of migration
What links here
3.1 Time migration
Related changes
Upload file 3.2 Depth migration
Special pages 4 Resolution
Permanent link 5 Graphical migration
Page information 6 Technical details
Wikidata item
7 See also
Cite this page
8 References
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Download as PDF Rationale [ edit ]
Printable version
Seismic waves are elastic waves that propagate through the Earth with a finite velocity, governed by the elastic properties of the rock in
Languages which they are travelling. At an interface between two rock types, with different acoustic impedances, the seismic energy is either
refracted, reflected back towards the surface or attenuated by the medium. The reflected energy arrives at the surface and is recorded
Deutsch
Français by geophones that are placed at a known distance away from the source of the waves. When a geophysicist views the recorded
Bahasa Indonesia energy from the geophone, they know both the travel time and the distance between the source and the receiver, but not the distance
Norsk Diagram showing the raypath for a
down to the reflector.
zero-offset reflection from a horizontal
Tiếng Việt
In the simplest geological setting, with a single horizontal reflector, a constant velocity and a source and receiver at the same location reflector.
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(referred to as zero-offset, where offset is the distance between the source and receiver), the geophysicist can determine the location
of the reflection event by using the relationship:

where d is the distance, v is the seismic velocity (or rate of travel) and t is the measured time from the source to the receiver.

In this case, the distance is halved because it can be assumed that it only took one-half of the total travel time to reach the reflector
Diagram showing the raypath for a
from the source, then the other half to return to the receiver. zero-offset reflection from a dipping
reflector and the resultant apparent dip.
The result gives us a single scalar value, which actually represents a half-sphere of distances, from the source/receiver, which the
reflection could have originated from. It is a half-sphere, and not a full sphere, because we can ignore all possibilities that occur above
the surface as unreasonable. In the simple case of a horizontal reflector, it can be assumed that the reflection is located vertically
below the source/receiver point (see diagram).

The situation is more complex in the case of a dipping reflector, as the first reflection originates from further up the direction of dip (see
diagram) and therefore the travel-time plot will show a reduced dip that is defined the “migrator’s equation” :[5]

where ξa is the apparent dip and ξ is the true dip.

Zero-offset data is important to a geophysicist because the migration operation is much simpler, and can be represented by spherical
surfaces. When data is acquired at non-zero offsets, the sphere becomes an ellipsoid and is much more complex to represent (both
geometrically, as well as computationally).

Use [ edit ]
A zero-offset non-migrated data set.
For a geophysicist, complex geology is defined as anywhere there is an abrupt or sharp contrast in lateral and/or vertical velocity (e.g. Raw zero-offset data for a simple
a sudden change in rock type or lithology which causes a sharp change in seismic wave velocity). syncline in a constant velocity world.
Notice the signature bow-tie effect in
Some examples of what a geophysicist considers complex geology are: faulting, folding, (some) fracturing, salt bodies, and the image. This is the result of
unconformities. In these situations a form of migration is used called pre-stack migration (PreSM), in which all traces are migrated reflections occurring from both sides of
before being moved to zero-offset. Consequently, much more information is used, which results in a much better image, along with the the syncline, and arriving at the same
receiver at different times. Migration
fact that PreSM honours velocity changes more accurately than post-stack migration.
can correct this effect.

Types of migration [ edit ]

Depending on budget, time restrictions and the subsurface geology, geophysicists can employ 1 of 2 fundamental types of migration
algorithms, defined by the domain in which they are applied: time migration and depth migration.

Time migration [ edit ]

Time migration is applied to seismic data in time coordinates. This type of migration makes the assumption of only mild lateral velocity
variations and this breaks down in the presence of most interesting and complex subsurface structures, particularly salt. Some
popularly used time migration algorithms are: Stolt migration, Gazdag and Finite-difference migration.

Depth migration [ edit ]

Depth Migration is applied to seismic data in depth (regular Cartesian) coordinates, which must be calculated from seismic data in time
coordinates. This method does therefore require a velocity model, making it resource-intensive because building a seismic velocity
model is a long and iterative process. The significant advantage to this migration method is that it can be successfully used in areas A zero-offset migrated data set of
with lateral velocity variations, which tend to be the areas that are most interesting to petroleum geologists. Some of the popularly used the File:SimpleSyncline.jpg data. This
depth migration algorithms are Kirchhoff depth migration, Reverse Time Migration (RTM),[6] Gaussian Beam Migration[7] and Wave- data was migrated using a time-
migration referred to as phase-shift
equation migration.[8] which operates in the Fourier domain.
The migration has replaced all events
Resolution [ edit ]
in their correct locations, successfully
reconstructing a syncline. However,
The goal of migration is to ultimately increase spatial resolution and one of the basic assumptions made about the seismic data is that there are erroneous events (swinging
arcs) throughout the image which are
it only shows primary reflections and all noise has been removed.[5] In order to ensure maximum resolution (and therefore maximum
migration induced noise.
uplift in image quality) the data should be sufficiently pre-processed before migration. Noise that may be easy to distinguish pre-
migration could be smeared across the entire aperture length during migration, reducing image sharpness and clarity.

A further basic consideration is whether to use 2D or 3D migration. If the seismic data has an element of cross-dip (a layer that dips perpendicular to the line of acquisition)
then the primary reflection will originate from out-of-plane and 2D migration cannot put the energy back to its origin. In this case, 3D migration is needed to attain the best
possible image.

Modern seismic processing computers are more capable of performing 3D migration, so the question of whether to allocate resources to performing 3D migration is less of a
concern.

Graphical migration [ edit ]

The simplest form of migration is that of graphical migration. Graphical migration assumes a constant velocity world and zero-offset
data, in which a geophysicist draws spheres or circles from the receiver to the event location for all events. The intersection of the
circles then form the reflector's "true" location in time or space. An example of such can be seen in the diagram.

Technical details [ edit ]

This section does not cite any sources. Please help improve this section by adding citations to
reliable sources. Unsourced material may be challenged and removed. (October 2015) (Learn how and
when to remove this template message) An example of simple graphical
migration. Until the advent of modern
Migration of seismic data is the correction of the flat-geological-layer assumption by a numerical, grid-based spatial convolution of the computers in the 1960s and 1970s this
seismic data to account for dipping events (where geological layers are not flat). There are many approaches, such as the popular was a method used by geophysicists to
Kirchhoff migration, but it is generally accepted that processing large spatial sections (apertures) of the data at a time introduces fewer primitively 'migrate' their data. This
method is obsolete with the advent of
errors, and that depth migration is far superior to time migration with large dips and with complex salt bodies. digital processors, but is useful for
Basically, it repositions/moves the energy (seismic data) from the recorded locations to the locations with the correct common midpoint understanding the basic principle
behind migration.
(CMP). While the seismic data is received at the proper locations originally (according to the laws of nature), these locations do not
correspond with the assumed CMP for that location. Though stacking the data without the migration corrections yields a somewhat
inaccurate picture of the subsurface, migration is preferred for better most imaging recorder to drill and maintain oilfields. This process is a central step in the creation of an
image of the subsurface from active source seismic data collected at the surface, seabed, boreholes, etc., and therefore is used on industrial scales by oil and gas companies
and their service providers on digital computers.

Explained in another way, this process attempts to account for wave dispersion from dipping reflectors and also for the spatial and directional seismic wave speed
(heterogeneity) variations, which cause wavefields (modelled by ray paths) to bend, wave fronts to cross (caustics), and waves to be recorded at positions different from those
that would be expected under straight ray or other simplifying assumptions. Finally, this process often attempts to also preserve and extract the formation interface reflectivity
information imbedded in the seismic data amplitudes, so that they can be used to reconstruct the elastic properties of the geological formations (amplitude preservation,
seismic inversion). There are a variety of migration algorithms, which can be classified by their output domain into the broad categories of time migration or depth migration,
and pre-stack migration or post-stack migration (orthogonal) techniques. Depth migration begins with time data converted to depth data by a spatial geological velocity profile.
Post-stack migration begins with seismic data which has already been stacked, and thus already lost valuable velocity analysis information.

See also [ edit ]

Reflection seismology
Seismic Unix, open source software for processing of seismic reflection data

References [ edit ]

1. ^ Chen, Yangkang; Yuan, Jiang; Zu, Shaohuan; Qu, Shan; Gan, Shuwei (2015). "Seismic imaging of simultaneous-source data using constrained least-squares reverse time migration".
Journal of Applied Geophysics. 114: 32–35. Bibcode:2015JAG...114...32C . doi:10.1016/j.jappgeo.2015.01.004 .
2. ^ Xue, Zhiguang; Chen, Yangkang; Fomel, Sergey; Sun, Junzhe (2016). "Seismic imaging of incomplete data and simultaneous-source data using least-squares reverse time migration
with shaping regularization". Geophysics. 81 (1): S11-S20. Bibcode:2016Geop...81S..11X . doi:10.1190/geo2014-0524.1 .
3. ^ Chen, Yangkang; Chen, Hanming; Xiang, Kui; Chen, Xiaohong (2017). "Preserving the discontinuities in least-squares reverse time migration of simultaneous-source data". Geophysics.
82 (3): S185-S196. Bibcode:2017Geop...82S.185C . doi:10.1190/geo2016-0456.1 .
4. ^ Yilmaz, Öz; Doherty, Stephen M., eds. (2000). "Migration". Seismic data analysis : processing, inversion, and interpretation of seismic data. 2 (2nd ed.). United States: Society of
Exploration Geophysicists. pp. 463–654. ISBN 9781560800941.
5. ^ a b c Sheriff, R. E.; Geldart, L. P. (1995). Exploration Seismology (2nd ed.). ISBN 9781139643115.
6. ^ "Reverse Time Migration" . Imaging. CGG. Retrieved 24 October 2015.
7. ^ "Gaussian Beam Migration" . Imaging. CGG. Retrieved 24 October 2015.
8. ^ Long, A. (October–November 2004). "What is Wave Equation Pre-Stack Depth Migration? An Overview" (PDF). PESA News. Archived from the original (pdf) on 5 November 2006.
Retrieved 24 October 2015.

Categories: Geophysics Seismology

This page was last edited on 13 August 2019, at 20:54 (UTC).

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