Converted-wave analysis
Encyclopedia
During seismic
exploration, P-waves (also known as primary or compressive waves) penetrate down into the earth. When a P-wave hits an interface (e.g., solid-liquid), it can reflect upwards as an S-wave
(also known as a secondary, shear or transverse wave). Other P-wave to S-wave (P-S) conversions can occur, but the down-up conversion is the primary focus. Unlike P-waves, converted shear waves are largely unaffected by fluids. By analyzing the original and converted waves, seismologists obtain additional subsurface information, especially due to (1) differential velocity (VP/VS), (2) asymmetry in the waves' angles of incidence
and reflection
and (3) amplitude variations
.
As opposed to analysis of P-wave to P-wave (P-P) reflection, c-wave (P-S) analysis is more complex. C-wave analysis requires at least three times as many measurement channels per station. Variations in reflection depths can cause significant analytic problems. Gathering, mapping, and binning
c-wave data is also more difficult than P-P data. However, c-wave analysis can provide additional information needed to create a three-dimensional depth image of rock type, structure, and saturant. For example, changes in VS with respect to VP suggest changing lithology
and pore geometry
.
Seismology
Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic,...
exploration, P-waves (also known as primary or compressive waves) penetrate down into the earth. When a P-wave hits an interface (e.g., solid-liquid), it can reflect upwards as an S-wave
S-wave
A type of seismic wave, the S-wave, secondary wave, or shear wave is one of the two main types of elastic body waves, so named because they move through the body of an object, unlike surface waves....
(also known as a secondary, shear or transverse wave). Other P-wave to S-wave (P-S) conversions can occur, but the down-up conversion is the primary focus. Unlike P-waves, converted shear waves are largely unaffected by fluids. By analyzing the original and converted waves, seismologists obtain additional subsurface information, especially due to (1) differential velocity (VP/VS), (2) asymmetry in the waves' angles of incidence
Angle of incidence
Angle of incidence is a measure of deviation of something from "straight on", for example:* in the approach of a ray to a surface, or* the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.-Optics:In geometric...
and reflection
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
and (3) amplitude variations
Amplitude Versus Offset
In geophysics, amplitude versus offset or amplitude variation with offset is a variation in seismic reflection amplitude with change in distance between shotpoint and receiver. It is also referred as AVA...
.
As opposed to analysis of P-wave to P-wave (P-P) reflection, c-wave (P-S) analysis is more complex. C-wave analysis requires at least three times as many measurement channels per station. Variations in reflection depths can cause significant analytic problems. Gathering, mapping, and binning
Data binning
Data binning is a data pre-processing technique used to reduce the effects of minor observation errors. The original data values which fall in a given small interval, a bin, are replaced by a value representative of that interval, often the central value...
c-wave data is also more difficult than P-P data. However, c-wave analysis can provide additional information needed to create a three-dimensional depth image of rock type, structure, and saturant. For example, changes in VS with respect to VP suggest changing lithology
Lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
and pore geometry
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
.