Reflection seismics

Reflection seismic profile and its geological interpretation

The seismic reflection is a method of seismic , which is used to determine bed boundaries in the earth's interior. Seismic reflection measurements aim to gain knowledge about the subsurface structure from reflected P waves and to reconstruct geological or geophysical interfaces.

Basics

Reflection seismics uses seismic waves that are artificially generated with various methods ( seismic blasting , vibroseis , hammer ) to examine the nature of the ground. The waves propagate underground and are reflected and refracted at interfaces. A small part of the reflected wave field comes back to the earth's surface, its energy and the temporal use of the wave movement is registered there with geophones . After processing the recorded data, a seismogram is available from which it can then be determined at what depth the layer boundaries are.

At layer boundaries, the seismic wave, like a light beam at optical boundaries, is partially refracted and partially reflected and converted into other wave types when passing from one layer to another.

The refraction of the sound beam obeys Snellius' law of refraction . For reflection applies, as well as in optics, the law of reflection :

Angle of incidence equals angle of reflection.

The proportion of the wave reflected at an interface depends on the speed and density differences between the adjacent rock layers. With perpendicular wave incidence, the standard case of seismic reflection, the following applies to the reflection coefficient:

{\ displaystyle R_ {PP} = {\ frac {\ rho _ {2} v_ {2} - \ rho _ {1} v_ {1}} {\ rho _ {2} v_ {2} + \ rho _ { 1} v_ {1}}}.}

A geological interface can therefore only be recognized if the impedance , the product , of the neighboring layers is different. ${\ displaystyle v \ cdot \ rho}$

Applications

Oil and gas exploration
Exploration of mineral deposits
Geothermal power plants (exploration)
Mapping of landfills
Groundwater exploration (the water table is a hydrogeological barrier)
Civil engineering, primarily tunnel construction and building foundations
Natural hazards (e.g. slope instabilities)

Methods of reflection seismics

Split spread method

This method is used to get a first impression of the subsurface. The shot point (the point at which seismic waves are triggered) is in the center of the geophone display. In the seismogram , layer boundaries can be seen as hyperbolas and the wave speed of the layers can be determined from the hyperbola curvature.

Common Midpoint Technique and Normal Move Out

Common Midpoint and Normal Move Out

The common midpoint technique is the most common method of seismic processing. The geophone display is shot from different points. Then, the recorded traces for common mid-points are ( C ommon M id P oint, CMP) by between shot and geophone point. In order to superimpose the trace design is a delay correction ( N ormal M ove O ut) made. The inserts appear in the seismogram as if they had been registered directly above the reflector. All traces of the seismogram can then be added to improve the useful / interference ratio.

The result of a CMP processing is a plumb time section in which the temporal distance of a reflector perpendicular to the surface is shown. The CMP method is particularly characterized by its low computing time requirement. In the event that the subsurface is made up of flat, slightly inclined reflectors, the method usually provides a very useful image.

migration

Inclined or curved reflectors are shown distorted by the CMP process. Furthermore, the position of the reflectors in the time domain, i.e. by the source-receiver center point (CMP) and the transit time of the waves, is described. It is therefore not possible to infer their spatial position in the underground directly. One mapping process that transforms the time domain into the depth domain is what is known as deep migration . This process is based on the idea that the subsurface is made up of many diffraction points . According to Huygens' principle , every diffractor that is hit by a wave is the starting point of a new elementary wave. The aim of the migration is to use the reflection inserts in the seismogram to calculate the location of the diffractor in the subsurface. With a subsurface speed model obtained by other methods, the location of all possible diffractors that could have caused this use (isochronous surface) is calculated for each use in the seismogram. The amplitude value under consideration is then assigned to these locations. This is the procedure for all missions in the entire seismogram. At the places where there is actually a diffractor, the amplitude values are constructively piled up and thus reproduce an undistorted, correctly positioned image of the subsurface.

literature

Dirk Gajewski: Lecture notes: Applied Geophysics II. (PDF; 32.5 MB) Hamburg 2010 Template: dead link /! ... nourl ( page no longer available )

Web links

Commons : Seismic reflection - collection of images, videos and audio files

Individual evidence

↑ Alireza Malehmir, Milovan Urosevic, Gilles Bellefleur, Christopher Juhlin, Bernd Milkereit: Seismic methods in mineral exploration and mine planning - Introduction . In: GEOPHYSICS . tape 77 , no. 5 , September 2012, ISSN 0016-8033 , p. WC1 – WC2 , doi : 10.1190 / 2012-0724-spsein.1 ( seg.org [accessed September 17, 2019]).

[1] Alireza Malehmir, Milovan Urosevic, Gilles Bellefleur, Christopher Juhlin, Bernd Milkereit: Seismic methods in mineral exploration and mine planning - Introduction . In: GEOPHYSICS . tape 77 , no. 5 , September 2012, ISSN 0016-8033 , p. WC1 – WC2 , doi : 10.1190 / 2012-0724-spsein.1 ( seg.org [accessed September 17, 2019]).