Reflection seismology

The seismic reflection profiling is a method of seismic, which is used to determine the boundaries of layers within the Earth. Seismic reflection measurements aim of the reflected P- waves to obtain information about the subsurface structure and reconstruct geological or geophysical interfaces.

Basics

The reflection seismic investigated by seismic waves generated artificially by various methods ( seismic blasting, Vibroseis, percussion hammer ). The waves propagate in the subsurface and are reflected at interfaces and broken. A small portion of the reflected wave field can return to the earth's surface, its energy and the temporal application of the wave motion is registered there with geophones. After the processing of the recorded data there is a seismogram from which it is then possible to determine at what depth are layer boundaries.

At layer boundaries is the seismic wave as a ray of light at optical boundaries in transition from one layer to another partially broken and partially reflected and converted into other wave types.

The refraction of the sound beam obeys the Snell's law. Applies to the reflection, as in the optical system, the law of reflection:

The proportion of the reflected wave at an interface is dependent upon velocity and density differences between the adjacent layers of rock. With vertical wave incidence, the standard case of seismic reflection, is valid for the reflection coefficient:

A geological interface is therefore only apparent when the impedance, the product of the adjacent layers are different.

Applications

Oil and gas exploration
Geothermal power plants (explore)
Mapping of landfills
Groundwater exploration ( the water table is a hydrogeological barrier )
Engineering structures, especially tunneling and building foundations
Natural hazards ( such as landslides )

Methods of seismic reflection

Split - spread method

This method is used to get a first impression of the subsurface. The shot point (the point to be raised to the seismic waves ) is located in the center of the Geophonauslage. In seismogram layer boundaries are recognizable as hyperbolas and the wave velocity of the layers can be determined from the Hyperbelkrümmung.

Common- midpoint technique and Normal Move Out

The common- midpoint technique is the most common method of seismic processings. The Geophonauslage is shot from different points. Then the recorded tracks by common midpoints are (Common MidPoint CMP) sorted between shot point and geophone. In order to superimpose the traces constructive is made a term correction (Normal Move Out ). Thus, the stakes appear in the seismogram as if they were registered directly above the reflector. Subsequently, all traces of the seismogram can be added so as to improve the Nutz-/Stör-Verhältnis.

The result of CMP -processing is a Lotzeitsektion in which the time interval of a reflector is shown perpendicular to the surface. The CMP process is characterized in particular by its low computation time requirements. In the event that the ground is made up of flat, slightly inclined reflectors, the method usually provides a very useful image.

Migration

Inclined or curved reflectors are imaged by the CMP process distorted. Furthermore, the position of the reflectors in the time domain, that is, by the source -receiver midpoint ( CMP) and duration of the wave will be described. Thus can not be closed immediately on their spatial position in the underground. An imaging process which provides the transformation from the time domain to the depth range, the so-called depth migration. This method is the underlying concept that the subsoil is made up of many Diffraktionspunkten. According to Huygens' principle, every diffractor who is hit by a wave, the starting point of a new elementary wave. The objective of migration is to be calculated from the reflection seismogram inserts in the place of diffractor underground. It is calculated with a product obtained by other methods subsurface velocity model for each input seismogram in the place of all possible Diffractors that could have caused this application ( Isochronenfläche ). Then these places the observed amplitude value is assigned. We will proceed with any job in the entire seismogram. At the places where actually is a diffractor, the Amplitutenwerte stack constructively and thereby give an undistorted, positionally correct image of the subsurface again.