Some examples of what a geophysicist considers complex geology are: faulting, folding, (some) fracturing, salt bodies, and unconformities. a sudden change in rock type or lithology which causes a sharp change in seismic wave velocity). 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).įor a geophysicist, complex geology is defined as anywhere there is an abrupt or sharp contrast in lateral and/or vertical velocity (e.g. Zero-offset data is important to a geophysicist because the migration operation is much simpler, and can be represented by spherical surfaces. Where ξ a is the apparent dip and ξ is the true dip. In the simplest geological setting, with a single horizontal reflector, a constant velocity and a source and receiver at the same location (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:ĭ = v t 2, When a geophysicist views the recorded energy from the geophone, they know both the travel time and the distance between the source and the receiver, but not the distance down to the reflector. The reflected energy arrives at the surface and is recorded by geophones that are placed at a known distance away from the source of the waves. 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. Seismic waves are elastic waves that propagate through the Earth with a finite velocity, governed by the elastic properties of the rock in which they are travelling. However, there are erroneous events (swinging arcs) throughout the image which are migration induced noise. The migration has replaced all events in their correct locations, successfully reconstructing a syncline. This data was migrated using a time-migration referred to as phase-shift which operates in the Fourier domain. Migration 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.Ī zero-offset migrated data set of the File:SimpleSyncline.jpg data. Computational 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. The need for migration has been understood since the beginnings of seismic exploration and the very first seismic reflection data from 1921 were migrated. A form of migration is one of the standard data processing techniques for reflection-based geophysical methods ( seismic reflection and ground-penetrating radar) Migration moves dipping reflectors to their true subsurface positions and collapses diffractions, resulting in a migrated image that typically has an increased spatial resolution and resolves areas of complex geology much better than non-migrated images. This process is necessary to overcome the limitations of geophysical methods imposed by areas of complex geology, such as: faults, salt bodies, folding, etc. 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 the location that it was recorded at the surface, thereby creating a more accurate image of the subsurface.