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# AVO Class 1 Gas Sands

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@MotivatedBoltzmann

### 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

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

<p>As a function of angle</p> Signup and view all the answers

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

<p>Amplitudes for all time samples</p> Signup and view all the answers

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

<p>$A$ coefficient</p> Signup and view all the answers

### What is the main purpose of AVO crossplotting?

<p>To reveal distinguishable AVO anomalies</p> Signup and view all the answers

### How does partial gas saturation affect the intercept and gradient on an intercept-versus-gradient crossplot?

<p>Results in both the intercept and gradient being more negative</p> Signup and view all the answers

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

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

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

<p>Brine-saturated clastic rocks and shales</p> Signup and view all the answers

### Why might it be challenging to identify background trends when working with large data volumes?

<p>When a huge amount of points is plotted, anomalies can be lost</p> Signup and view all the answers

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

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

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

<p>Decreases with offset</p> Signup and view all the answers

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

<p>Class 3</p> Signup and view all the answers

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

<p>Polarity reversal vs. no polarity change</p> 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?

<p>Class 3</p> Signup and view all the answers

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

<p>'Amplitude decreasing' with offset</p> Signup and view all the answers

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

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

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

<p>'Near zero' impedance contrast relative to overlying rocks</p> 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?

<p>'Highly compacted' gas sand</p> Signup and view all the answers

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

<p>'Class 4'</p> Signup and view all the answers

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

<p>Replacement of water in sand with hydrocarbon</p> Signup and view all the answers

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

<p>Zone 1</p> Signup and view all the answers

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

<p>Decreases both Intercept and Gradient values</p> 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?

<p>Lower impedance of the fluid-substituted sand</p> Signup and view all the answers

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

<p>Increasing the shale content</p> Signup and view all the answers

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

<p>Class 2</p> Signup and view all the answers

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

<p>Class 1 response for water sand overlain by shale</p> Signup and view all the answers

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

<p>Decreases both Intercept and Gradient values.</p> Signup and view all the answers

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

<p>Oval distribution around a real location</p> Signup and view all the answers

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

<p>Lower left corner</p> Signup and view all the answers

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

<p>Reflection amplitude at zero angle of incidence</p> Signup and view all the answers

### Why do brine-saturated sandstones and shales plot along a well-defined linear trend in AVO analysis?

<p>Because they follow the AVO background trend or mudrock line</p> 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?

<p>Both A and B become more negative</p> Signup and view all the answers

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

<p>Hydrocarbons or lithologies with anomalous elastic properties</p> 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?

<p>AVO crossplot</p> Signup and view all the answers

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

<p>Identification of different AVO classes</p> Signup and view all the answers

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

<p><strong>Negatively</strong> correlated</p> 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?

<p><strong>IV</strong></p> Signup and view all the answers

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