Applications for Full-wave Imaging
Imaging through Gas Clouds
In certain environments, P-wave data completely fails, or at best, results in distorted images. In gas-saturated rocks, for example, P-wave transmission suffers absorption and delayed travel time, obscuring and distorting sub-gas targets. Shear waves however, maintain the host rock matrix properties and transmit without distortion. ION has had many global successes using converted-wave (C-wave) technology to reveal what was previously invisible in these environments.
Hydrocarbon Bright Spot Confirmation
Bright amplitudes on P-wave data have the potential to be hydrocarbon indicative. However, many bright amplitudes are false direct hydrocarbon indicator (DHI). Even with detailed pre-stack AVO analysis, the interpretation may be ambiguous. Introducing C-waves can positively identify hydrocarbon bright spots. In a recent study, we were able to distinguish between bright amplitudes associated with gas and those associated with water, eliminating any dry holes drilled on P-wave amplitude anomalies.
Over-Pressure Zone Confirmation
Identifying over-pressure zones due to gas saturation is important not only because these areas are highly prospective, but they may also pose a safety hazard during drilling. Although the P-wave response in gas over-pressure zones is a characteristic low velocity, it is not the only possible cause. Because shear velocities do not slow in the presence of gas, introducing C-waves removes any ambiguity. Combining the two velocities more accurately defines pressure compartments.
Multicomponent acquisition and processing have been used successfully to characterize prospective areas subject to azimuthally anisotropic velocity variations. Azimuthally-varying velocities are often observed in fractured carbonates and sandstone or in zones of unequal stress. The multi-component method measures the horizontal particle motion associated with vertically propagating shear waves. In fractured environments, shear waves split, polarize, and travel at different velocities. Provided the geometry, offset, and azimuth sampling is suitable, we can capture this shear-wave information and relate it to fracture density and magnitude.
The key to accurately unlocking this information is to resolve the birefringence (splitting) after imaging. To accomplish this, our workflows are appreciably more complex and compute-intensive than those offered by most multicomponent processing vendors. A typical process may include over one hundred pre-stack migrations of azimuth-sectored volumes.
Rock properties are the fastest growing area of full-wave interest and hold the greatest future potential for gathering more useful information. Traditionally, measurements such as VP/VS ratio, shear impedance, density, and porosity have been estimated from P-wave data using its AVO response. Using direct shear impedance measurements from the C-wave data vastly improves the reliability of the measurement. Joint inversion using combined PP & PS is now providing more accurate measurements of density and porosity.
- Waveform Inversion Including Well Constraints, Anisotropy, and Attenuation - The Leading Edge (PDF)