AVO Classification
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
1. Class I:
- Description: Characterized by a strong positive reflection amplitude at near offsets, which decreases with increasing offset.
- Indication: Typically represents high-impedance contrasts, often associated with gas sands overlain by denser rocks like shales.
2. Class II:
- Description: Displays a near-zero or weak positive amplitude at near offsets that increases slightly or stays flat with increasing offset.
- Indication: Often indicates gas sands with minimal impedance contrast relative to the surrounding rock.
3. Class IIp (Polarized):
- Description: A variant of Class II, where the amplitude is negative at near offsets and becomes positive at far offsets.
- Indication: Suggests a small impedance contrast that reverses polarity with increasing offset, often seen in gas-filled sands.
4. Class III:
- Description: Characterized by a negative reflection amplitude at near offsets that becomes more negative with increasing offset.
- Indication: Typically indicates low-impedance contrasts, such as gas sands with significant contrast to the overlying shales.
5. Class IV:
- Description: Displays a negative reflection amplitude at near offsets that decreases (becomes less negative) with increasing offset.
- Indication: Represents a weak negative impedance contrast, often associated with certain types of wet sands or low gas saturation.
Applications of AVO Classification
AVO classification helps distinguish between different lithologies and fluid types in the subsurface. By analysing the AVO response, geoscientists can identify prospective hydrocarbon zones, differentiate between gas and oil reservoirs, and reduce the risk of drilling dry wells. It is particularly valuable in exploring subtle traps and unconventional reservoirs, where traditional seismic interpretation might not provide clear indicators.