AVO Class 1 Gas Sands

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What does 'A' represent in the linearized AVO equation?

Change in amplitude with respect to sine squared of the angle

Reflector's depth

Amplitude at zero offset or angle (correct)

Gradient of the line

Which term of the AVO approximations is often termed Curvature?

Intercept

B

Higher order term (correct)

A

What is the basic function of 'B' in the linearized AVO equation?

Amplitude at zero offset or angle

Change in amplitude with respect to sine squared of the angle

Reflector's depth

Gradient of the line (correct)

How are reflection coefficients represented by the first order approximation of the linearized AVO equation?

As a function of angle Signup and view all the answers

What are extracted and plotted on a graph as a function of sin^2(theta)?

Amplitudes for all time samples Signup and view all the answers

Which term from the linearized AVO equation provides information about the amplitude at zero offset or angle?

$A$ coefficient Signup and view all the answers

What is the main purpose of AVO crossplotting?

To reveal distinguishable AVO anomalies Signup and view all the answers

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 Signup and view all the answers

What would happen if the interval chosen for crossplotting is too wide?

It would cause a variety of background trends to superpose Signup and view all the answers

Which rocks exhibit a well-defined background trend in the intercept-gradient crossplot over a limited depth range?

Brine-saturated clastic rocks and shales Signup and view all the answers

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 Signup and view all the answers

What determines the slope of the background trend on an intercept-gradient crossplot?

The background Vp/Vs ratio primarily Signup and view all the answers

What characterizes AVO Class 1 gas sands in terms of reflection amplitude?

Decreases with offset Signup and view all the answers

Which type of gas sands can be associated with the classic 'bright spot' anomaly?

Class 3 Signup and view all the answers

What distinguishes a Class 2p anomaly from a Class 2n anomaly according to Ross and Kinman (1995)?

Polarity reversal vs. no polarity change Signup and view all the answers

Which type of gas sands exhibits a Strong trough at near offset that becomes more negative with increasing offsets?

Class 3 Signup and view all the answers

What is the primary distinguishing factor of AVO Class 4 gas sands among the AVO classes mentioned?

'Amplitude decreasing' with offset Signup and view all the answers

What is the main relationship between AVO Class and compaction trend?

AVO Class depends on the contrast between gas sand and overlying shale Signup and view all the answers

What is the main characteristic of AVO Class 2 gas sands in terms of impedance contrast?

'Near zero' impedance contrast relative to overlying rocks Signup and view all the answers

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 Signup and view all the answers

'The magnitude of reflection decreases with increasing offset' is a characteristic of which AVO class?

'Class 4' Signup and view all the answers

What causes the lower impedance of hydrocarbon sand compared to water sand?

Replacement of water in sand with hydrocarbon Signup and view all the answers

In which zone do we expect to see the AVO response for gas sand as Class 3?

Zone 1 Signup and view all the answers

How does increasing the porosity of the sand affect the Intercept-Gradient values on the crossplot?

Decreases both Intercept and Gradient values Signup and view all the answers

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 Signup and view all the answers

How can a change in lithology be modeled on the Intercept-Gradient crossplot?

Increasing the shale content Signup and view all the answers

What type of AVO response is expected for gas sand in Zone 2?

Class 2 Signup and view all the answers

What does a point represent on the Intercept-Gradient crossplot for a single reflecting interface?

Class 1 response for water sand overlain by shale Signup and view all the answers

How does increasing shale content influence the Intercept-Gradient values on the crossplot compared to increasing porosity?

Decreases both Intercept and Gradient values. Signup and view all the answers

What trend appears on the crossplot signature when random noise is added to data points?

Oval distribution around a real location Signup and view all the answers

Where are all AVO class points from anomalous hydrocarbon zones expected to fall on an AVO crossplot?

Lower left corner Signup and view all the answers

What does the Intercept term A represent in seismic data analysis?

Reflection amplitude at zero angle of incidence Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

Which quadrant on the Intercept-Gradient crossplot do Class 3 sands fall into?

Quadrant III Signup and view all the answers

According to Castagna et al., what do deviations from the AVO background trend indicate?

Hydrocarbons or lithologies with anomalous elastic properties Signup and view all the answers

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 Signup and view all the answers

What is the significance of the Roman numbers I-IV on the Intercept-Gradient crossplot?

Identification of different AVO classes Signup and view all the answers

What is the relationship between A and B in 'background' rocks according to the text?

Negatively correlated Signup and view all the answers

'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 Signup and view all the answers