Changes

AVO has been a prominent technique in detecting hydrocarbons and reducing drilling risk. AVO can detect hydrocarbons because AVO shows the variation of the amplitude of the offset, which represents the amplitude of the wave energy as it passes through the layer which is influenced by the parameters of the speed and density of the coating, so that the density of the layer can be analyzed by analyzing the reflection coefficient. AVO means that amplitude change with offset caused by lithology of fluid. AVO is also known as AVA (amplitude variation with angle) because this phenomenon is based on the relationship between the reflection coefficient and the angle of incidence. But since the angle of incidence affecting the offset and the offset itself can be varied in order to change the angle of incidence (fig.1), so it is commonly known as AVO.

AVO has been a prominent technique in detecting hydrocarbons and reducing drilling risk. AVO can detect hydrocarbons because AVO shows the variation of the amplitude of the offset, which represents the amplitude of the wave energy as it passes through the layer which is influenced by the parameters of the speed and density of the coating, so that the density of the layer can be analyzed by analyzing the reflection coefficient. AVO means that amplitude change with offset caused by lithology of fluid. AVO is also known as AVA (amplitude variation with angle) because this phenomenon is based on the relationship between the reflection coefficient and the angle of incidence. But since the angle of incidence affecting the offset and the offset itself can be varied in order to change the angle of incidence (fig.1), so it is commonly known as AVO.

[[File:Fig.1.incident angle.png|framed|center|Fig.1.The incident angle is directly proportional to the offset (www.wikiwand.com)]]

[[File:Fig.1.incident angle.png|framed|center|Fig.1.The incident angle is directly proportional to the offset <ref> www.wikiwand.com </ref>]]

Variation of reflection coefficient is the primary thing in analyzing AVO. It starts when the P-Waves hit the interface, if it is normal incidence (fig.2), the coefficient reflections will be the difference of acoustic impedance (product of density and P velocity) over the sum.

Variation of reflection coefficient is the primary thing in analyzing AVO. It starts when the P-Waves hit the interface, if it is normal incidence (fig.2), the coefficient reflections will be the difference of acoustic impedance (product of density and P velocity) over the sum.

Where RNI is the reflection coefficient.

Where RNI is the reflection coefficient.

[[File:Fig.2.normal incidence.jpg|framed|center|Fig.2. Normal Incidence (www.ukm.my)]]

[[File:Fig.2.normal incidence.jpg|framed|center|Fig.2. Normal Incidence <ref> www.ukm.my </ref>]]

While in the non-normal incidence (fig.3), some of the P-Waves that come will transform into the S waves so that there will be a reflection coefficient variation as a result of different Vp and Vs value. For example, when wave penetrates into a gas layer, the Vp will be decreasing and the Vs will be constant. It means that in certain condition there will be an anomaly in Vp/Vs. This anomaly will cause variation in the reflection coefficient whereas the coefficient itself is the main focus in AVO analysis [1].  

While in the non-normal incidence (fig.3), some of the P-Waves that come will transform into the S waves so that there will be a reflection coefficient variation as a result of different Vp and Vs value. For example, when wave penetrates into a gas layer, the Vp will be decreasing and the Vs will be constant. It means that in certain condition there will be an anomaly in Vp/Vs. This anomaly will cause variation in the reflection coefficient whereas the coefficient itself is the main focus in AVO analysis <ref>Choliq, M. T. (2007). Pemodelan parameter elastic batuan degan inversi AVO untuk karakterisasi reservoir hidrokarbon karbonat formasi baturaja, Jawa Barat Utara.</ref>.  

[[File:Fig.3.Non-normal incidence.png|framed|center|Fig.3. Non-normal incidence (Lecture Note Mr. Scott)]]

[[File:Fig.3.Non-normal incidence.png|framed|center|Fig.3. Non-normal incidence <ref> Stephen,Scott.(2011). University of Indonesia </ref>]]

The relationship between the reflection coefficient and the angle of incidence in write down the Zoeppritz equations by Karl Zoeppritz since the early 20th century and then this equations was developed again by some figures such as Bortfeld (1961), Aki, Richard and Frasier (1976), Hilterman (1983), and Shuey (1985).  

The relationship between the reflection coefficient and the angle of incidence in write down the Zoeppritz equations by Karl Zoeppritz since the early 20th century and then this equations was developed again by some figures such as Bortfeld (1961), Aki, Richard and Frasier (1976), Hilterman (1983), and Shuey (1985).  

== AVO Classification ==

== AVO Classification ==

In 1989 Rutherford and Williams introduced a three-fold classification of AVO (amplitude versus offset) characteristics for seismic reflections from the interface between shales and underlying gas sands. The classification scheme they proposed is explicitly defined for gas sands and has become the industry standard; it has proven its validity and usefulness in countless exploration efforts. In 1997 Castagna and Swan proposed AVO crossplotting wherein an estimate of the normal-incidence reflectivity is plotted against a measure of the offset dependent reflectivity. Using this approach Castagna and Swan graphically illustrated the continuum between the classes and defined the characteristics of the classes using what they termed AVO Intercept and AVO Gradient. They also added a class 4 [2].

In 1989 Rutherford and Williams introduced a three-fold classification of AVO (amplitude versus offset) characteristics for seismic reflections from the interface between shales and underlying gas sands. The classification scheme they proposed is explicitly defined for gas sands and has become the industry standard; it has proven its validity and usefulness in countless exploration efforts. In 1997 Castagna and Swan proposed AVO crossplotting wherein an estimate of the normal-incidence reflectivity is plotted against a measure of the offset dependent reflectivity. Using this approach Castagna and Swan graphically illustrated the continuum between the classes and defined the characteristics of the classes using what they termed AVO Intercept and AVO Gradient. They also added a class 4 <ref> Young, Roger A. and LoPiccolo, Robert D. (2004). Conforming and Non-conforming Sands – An Organizing Framework for Seismic Rock Properties. </ref>.

[[File:Fig.4.classification avo.jpg|framed|center|Fig.4. Classifications of AVO (Lecture Note Mr. Scott)]]

[[File:Fig.4.classification avo.jpg|framed|center|Fig.4. Classifications of AVO <ref>Ahmed,Haseb. Institute of Geologi,University of the punjab </ref>]]

'''Class 1 : High impedance Gas-Sandstone'''

'''Class 1 : High impedance Gas-Sandstone'''

'''Class 4 '''

'''Class 4 '''

Class 4 has negative reflection coefficient at zero offset and amplitude that is decreasing against the offset. There is a change in polarity at a certain angle and then amplitude will increasing proportionally to the offset.

Class 4 has negative reflection coefficient at zero offset and lower impedance with amplitude that is decreasing against the offset. There is a change in polarity at a certain angle and then amplitude will increasing proportionally to the offset.  

== References ==

= Reference =

[1] ] Choliq, M. T. (2007). Pemodelan parameter elastic batuan degan inversi AVO untuk karakterisasi reservoir hidrokarbon karbonat formasi baturaja, Jawa Barat Utara.

 

*Rutherford,S.R. and Williams R.H. (1989). Amplitude-versus-offset Variations in Gas Sands.

[2] Young, Roger A. and LoPiccolo, Robert D. (2004). Conforming and Non-conforming Sands – An Organizing Framework for Seismic Rock Properties.

*C. Ecker, D. Lumley, S. Levin, T. Rekdal, A. Berlioux, R. Clapp, Y. Wang & J. Ji. (2001). An AVO Analysis Project.

*http://www.ipt.ntnu.no/pyrex/stash/avo_theory.pdf , accessed on June 2015.