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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.
Some examples of what a geophysicist considers complex geology are: In these situations a form of migration is used called pre-stack migration PreSMin which all traces are migrated before being moved to zero-offset.
Consequently, much more Cause and effect of migration is used, which results in a much better image, along with the fact that PreSM honours velocity changes more accurately than post-stack migration.
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[ 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 with lateral velocity variations, which tend to be the areas that are most interesting to petroleum geologists.
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 ] An example of simple graphical migration. Until the advent of modern computers in the s and s this was a method used by geophysicists to primitively 'migrate' their data.
This method is obsolete with the advent of digital processors, but is useful for understanding the basic principle behind migration. 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. 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 Learn how and when to remove this template message Migration of seismic data is the correction of the flat-geological-layer assumption by a numerical, grid-based spatial convolution of the seismic data to account for dipping events where geological layers are not flat.
There are many approaches, such as the popular Kirchhoff migration, but it is generally accepted that processing large spatial sections apertures of the data at a time introduces fewer errors, and that depth migration is far superior to time migration with large dips and with complex salt bodies.
While the seismic data is received at the proper locations originally according to the laws of naturethese 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.
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 causticsand 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 preservationseismic 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 migrationand 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.The U.S.
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immigration system. Causes and effects of human migration Migration is the movement of people from one place to another with the intent to settle.
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