40 Questions
What does 'A' represent in the linearized AVO equation?
Amplitude at zero offset or angle
Which term of the AVO approximations is often termed Curvature?
Higher order term
What is the basic function of 'B' in the linearized AVO equation?
Gradient of the line
How are reflection coefficients represented by the first order approximation of the linearized AVO equation?
As a function of angle
What are extracted and plotted on a graph as a function of sin^2(theta)?
Amplitudes for all time samples
Which term from the linearized AVO equation provides information about the amplitude at zero offset or angle?
$A$ coefficient
What is the main purpose of AVO crossplotting?
To reveal distinguishable AVO anomalies
How does partial gas saturation affect the intercept and gradient on an intercept-versus-gradient crossplot?
Results in both the intercept and gradient being more negative
What would happen if the interval chosen for crossplotting is too wide?
It would cause a variety of background trends to superpose
Which rocks exhibit a well-defined background trend in the intercept-gradient crossplot over a limited depth range?
Brine-saturated clastic rocks and shales
Why might it be challenging to identify background trends when working with large data volumes?
When a huge amount of points is plotted, anomalies can be lost
What determines the slope of the background trend on an intercept-gradient crossplot?
The background Vp/Vs ratio primarily
What characterizes AVO Class 1 gas sands in terms of reflection amplitude?
Decreases with offset
Which type of gas sands can be associated with the classic 'bright spot' anomaly?
Class 3
What distinguishes a Class 2p anomaly from a Class 2n anomaly according to Ross and Kinman (1995)?
Polarity reversal vs. no polarity change
Which type of gas sands exhibits a Strong trough at near offset that becomes more negative with increasing offsets?
Class 3
What is the primary distinguishing factor of AVO Class 4 gas sands among the AVO classes mentioned?
'Amplitude decreasing' with offset
What is the main relationship between AVO Class and compaction trend?
AVO Class depends on the contrast between gas sand and overlying shale
What is the main characteristic of AVO Class 2 gas sands in terms of impedance contrast?
'Near zero' impedance contrast relative to overlying rocks
Which type of sand has a specific example given in the text as being mature highly compacted with relatively low Vp/Vs compared to surrounding rocks?
'Highly compacted' gas sand
'The magnitude of reflection decreases with increasing offset' is a characteristic of which AVO class?
'Class 4'
What causes the lower impedance of hydrocarbon sand compared to water sand?
Replacement of water in sand with hydrocarbon
In which zone do we expect to see the AVO response for gas sand as Class 3?
Zone 1
How does increasing the porosity of the sand affect the Intercept-Gradient values on the crossplot?
Decreases both Intercept and Gradient values
What is the significance of a separate data cluster falling to the left of points for brine-saturated sands when fluid substitution is considered?
Lower impedance of the fluid-substituted sand
How can a change in lithology be modeled on the Intercept-Gradient crossplot?
Increasing the shale content
What type of AVO response is expected for gas sand in Zone 2?
Class 2
What does a point represent on the Intercept-Gradient crossplot for a single reflecting interface?
Class 1 response for water sand overlain by shale
How does increasing shale content influence the Intercept-Gradient values on the crossplot compared to increasing porosity?
Decreases both Intercept and Gradient values.
What trend appears on the crossplot signature when random noise is added to data points?
Oval distribution around a real location
Where are all AVO class points from anomalous hydrocarbon zones expected to fall on an AVO crossplot?
Lower left corner
What does the Intercept term A represent in seismic data analysis?
Reflection amplitude at zero angle of incidence
Why do brine-saturated sandstones and shales plot along a well-defined linear trend in AVO analysis?
Because they follow the AVO background trend or mudrock line
How are gas-saturated sand responses different from fully brine-saturated sand responses in terms of Intercept and Gradient?
Both A and B become more negative
Which quadrant on the Intercept-Gradient crossplot do Class 3 sands fall into?
Quadrant III
According to Castagna et al., what do deviations from the AVO background trend indicate?
Hydrocarbons or lithologies with anomalous elastic properties
In seismic data analysis, what can be used as an interpretation tool when plotting R(0) against Gradient for every time sample in every CMP gather?
AVO crossplot
What is the significance of the Roman numbers I-IV on the Intercept-Gradient crossplot?
Identification of different AVO classes
What is the relationship between A and B in 'background' rocks according to the text?
Negatively correlated
'Class 1 sands are high impedance relative to overlying shales.' In which quadrant on the Intercept-Gradient crossplot would you expect Class 1 sands to fall?
IV
Explore the characteristics of AVO Class 1 gas sands, including their higher impedance compared to the encasing medium, positive reflection at zero offset, and amplitude variation with offset. Test your knowledge on how the reflectivity of class 1 sands can change polarity at far offsets.
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