Difference between revisions of "Amplitude variations with offset (AVO)"
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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|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 (www.wikiwand.com)]] |
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. | ||
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| + | Where RNI is the reflection coefficient. | ||
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| + | [[File:Fig.2.normal incidence.jpg|framed|center|Fig.2. Normal Incidence (www.ukm.my)]] | ||
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| + | 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]. | ||
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| + | [[File:Fig.3.Non-normal incidence.jpg|framed|center|Fig.3. Non-normal incidence (Lecture Note Mr. Scott)]] | ||
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| + | 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). | ||
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| + | == Zoeppritz Equations == | ||
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| + | Zoeppritz derived the amplitudes of the reflected and transmitted waves using the conservation of stress and displacement across the layer boundary. | ||
Revision as of 14:45, 30 June 2015
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.
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.
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].
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).
Zoeppritz Equations
Zoeppritz derived the amplitudes of the reflected and transmitted waves using the conservation of stress and displacement across the layer boundary.