AVO

AVO stands for Amplitude Versus Offset, a technique in seismic exploration that analyzes how the amplitude of reflected seismic waves changes with source-receiver distance (offset) or equivalently, angle of incidence. It’s a powerful tool used primarily in hydrocarbon exploration to infer rock properties, fluids, and interfaces.

✅ Basic Concept of AVO Modeling:

When a seismic wave reflects off an interface between two layers (with different elastic properties), the amplitude of the reflected wave changes depending on the angle of incidence. AVO analysis models this amplitude variation to extract geological information.

🎯 Why AVO is Important:

To distinguish gas/oil/brine in reservoirs
To estimate elastic properties: acoustic impedance (AI), shear impedance (SI), Poisson's ratio
To detect lithology and fluid content
To identify bright spots or flat spots

📈 AVO Classifications:

Based on the behavior of amplitude with offset, anomalies are classified:

Class I: High impedance contrast (amplitude decreases)
Class II: Zero-crossing amplitude
Class III: Low impedance contrast, amplitude increases with offset (common gas sands)
Class IV: Phase reversal or unusual gradient

AVO modeling in well location shows how the amplitude would be increased with angles around 1000-1600ms.

AVO Attributes

AVO (Amplitude Versus Offset) attributes are key indicators derived from the analysis of how seismic reflection amplitudes change with increasing offset or angle of incidence. These attributes provide insights into the elastic properties of subsurface rocks and are essential for identifying potential hydrocarbon reservoirs.

🔹 1. Fundamentals

Definition: AVO is the variation of seismic reflection amplitude with changing source–receiver offset (or angle of incidence).
Purpose: Helps estimate subsurface rock and fluid properties beyond conventional reflection strength.
Based on Zoeppritz equations, which describe reflection/transmission coefficients as a function of angle, P-wave velocity, S-wave velocity, and density.

🔹 2. Approximations

Shuey’s 2-term/3-term approximation (common in practice):
- R(θ)=P+G sin(2θ)+C(tan(2θ)−sin(2θ))
  where R is reflection coefficient and θ is incident angle.
  - P → Intercept (zero-offset reflectivity, related to acoustic impedance contrast).
  - G → Gradient (change of amplitude with angle, sensitive to Poisson’s ratio/fluids).
  - C → Curvature (higher angles, useful in gas sands).

1. Intercept (P):

Description: The intercept represents the reflection amplitude at zero offset (or the near-offset trace). It is a measure of the baseline reflection strength and is primarily influenced by the contrast in acoustic impedance between two layers.

2. Gradient (G):

Description: The gradient measures the rate of change in reflection amplitude with offset. It is sensitive to the Poisson's ratio contrast between layers, which can indicate the presence of gas or other fluids.

3. Curvature (C):

Description: Curvature, or the second-order term, captures the non-linear changes in amplitude with offset. It is often used in more detailed AVO analyses to refine interpretations, especially in complex geological settings.

4. Fluid Factor (∆F):

Description: This attribute is derived from the combination of intercept and gradient, designed to highlight fluid effects (like gas saturation) in the subsurface. It helps differentiate between hydrocarbon-bearing and water-bearing formations.

By analyzing a typical cross-plot of Intercept (P) versus Gradient (G), the background trend (also called the mud rock line) can be defined as the line on which water-saturated sandstones, shale, and siltstones lie. The angle between the horizontal Intercept axis and this background line is here named Phi (φ). Then the fluid factor trace can be envisaged as a measure of the distance of each sample point (P, G) from the background line. The fluid factor trace is designed to be low amplitude for all reflectors in a clastic sedimentary sequence except for rocks that lie off the "mud rock line". In particular, gas sands will brighten up on the fluid factor trace. Since the fluid factor is a combination of Intercept and Gradient, it will produce a zero result for the background trend, and since: tanφ = -P/G.
For the background trend, then the fluid factor can be defined as: ∆F = P sinφ+G cosφ.

5. Product of Intercept and Gradient (P*G):

Description: This product emphasizes areas where both intercept and gradient are strong, which can be indicative of gas sands or other significant subsurface anomalies.

Applications of AVO Attributes:

ntercept and Gradient are useful as lithology and fluid indicators that give information regarding fluid content of the analyzed layers. For increasing incident angle, the product (P * G) is negative for decreasing amplitudes and positive in the case of increasing amplitudes. This gives a clear indication of the AVO scenario.

The primary AVO attributes are given by the parameterization of the amplitude variation with incident angle using the normal incidence P-wave reflection coefficient and the Gradient of a linear fit of amplitude versus the squared sine of the incident angle. The normal incidence P-wave reflection coefficient in this case is given by the Intercept of the fitted line with the amplitude axis, hence this attribute is usually called the Intercept. The Gradient (or Slope) is useful to tell whether we have an increase or decrease of amplitude with incident angle.

If, for example, amplitudes vary from high negative values to small negative values, the Gradient is positive. However, if amplitudes vary from high positive values to small positive values, the Gradient is negative. Therefore, it is much better to compute the product of both attributes.

This product has a negative sign in both the situations mentioned above, indicating decreasing amplitudes, and is positive in case of increasing amplitudes. Also, if either the intercept is nearly zero, or the gradient is zero, the product of intercept and gradient will be nearly zero.

Application of AI AVO analysis to extract AVO attributes on real seismic data.

AVO classification

Amplitude Versus Offset (AVO) analysis is a seismic interpretation technique used to examine how the amplitude of seismic reflections changes with the distance (offset) between the seismic source and receivers. This method is particularly useful in identifying subsurface fluid content, lithology, and porosity variations, making it a powerful tool in hydrocarbon exploration.

In AVO analysis, the variation in reflection amplitude with offset is analyzed to detect changes in rock properties, such as acoustic impedance and shear modulus. Different types of AVO responses (e.g., Class I, II, III, IV) can indicate the presence of hydrocarbons or distinguish between gas, oil, and water-bearing formations. For example, a strong increase in amplitude with offset might suggest a gas-filled reservoir, while other patterns can indicate different fluid types or lithologies.

By integrating AVO analysis with other seismic attributes and well data, geoscientists can improve the accuracy of reservoir characterization, enhance the identification of potential hydrocarbon zones, and reduce the risk of drilling non-productive wells. AVO analysis is a critical component of modern seismic interpretation, providing valuable insights into the subsurface that go beyond traditional amplitude-based methods.

AVO (Amplitude Versus Offset) classification is a method used to categorize the behaviour of seismic reflection amplitudes as a function of offset or angle. This classification helps geoscientists interpret subsurface properties and identify potential hydrocarbon reservoirs. AVO classification typically divides the responses into different classes based on the changes in amplitude with increasing offset, which can indicate different types of subsurface materials and fluid contents.

Common AVO Classes

Class 1: High Impedance Gas-Sandstone

Class 1 sandstone has higher impedance than its cover (shale). Interface between shale and this kind of sandstone will generate a high reflection coefficient and a positive zero offset, but has amplitude magnitude decreasing in order to offset. Class 1 has greater gradient than class 2 and class 3. Sandstone at class 1 is having a change in polarity in certain angle, and then the amplitude will be increasing proportionally to the offset.

Class 2: Near Zero Impedance Contrast Gas Sandstone

Class 2 sandstone has almost equally acoustic impedance with its cover (seal rock) and the amplitude, which is increasing proportionally to the offset. Class 2 sandstone divided into class 2 and class 2p. Class 2 sandstone has negative reflection coefficient at zero offset while class 2p has positive at zero offset.

Class 3: Low Impedance Gas Sandstone

Class 3 has lower acoustic impedance than its cover.

Class 4: Low Impedance Sand

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.

Applications of AVO Classification

After generation of all the required AVO attributes, a detailed interpretation by considering the result of stratigraphic interpretation will be performed in order to highlight the most important sweet zones as drilling targets. For doing that AVO classification will be done based on standard AVO plots. It should be noted depends to lithology of the field and the quality of the seismic data the uncertainty in AVO attribute study could be high.

Application of AI AVO classification in selected zone in the reservoir. It seems some suit purple point with class type I in chart.

AVO