All PGS migration algorithms are Q-enabled. They utilize the tomographically derived model within the migration process to compensate directly for amplitude loss and phase distortion. This can have the benefit of resolving localized attenuating anomalies that cause the loss of energy from seismic waves.
Seismic waves are attenuated as they travel through the subsurface of the earth. Attenuation causes a loss of high frequency energy and a distortion to the wavelet's phase (dispersion). Attenuation refers to the exponential decay of wave amplitude with distance. Dispersion is a variation of propagation velocity with frequency.
Attenuation and dispersion can be caused by a variety of physical phenomena that can be divided broadly into elastic processes, where the total energy of the wavefield is conserved, and inelastic dissipation, where wave energy is converted into heat. Of particular interest to exploration geophysics, is inelastic attenuation and dispersion of body waves resulting from the presence of fluids in the pore space of rocks.
Seismic attenuation and dispersion are usually compensated for in the time domain, either pre-migration to correct for phase distortions or/and post-migration to correct for amplitude decay. These compensation techniques are typically one-dimensional and strictly valid if the earth attenuation, described by the quality factor (Q), is constant. When the Q model varies rapidly, the 1D compensation is not accurate especially in complex geology that causes complicated ray paths, like for example, shallow hydrates in the Gulf of Mexico or shallow gas clouds in the North Sea.
To create the most accurate images of complex geological structures, PGS employs a Q-velocity model building (QVMB) workflow to simultaneously build high resolution anisotropic velocity models and Q models.
The QVMB workflow consists of three modules:
- Q-migration (conducted by Kirchhoff, beam or reverse-time migration (RTM))
- Q-residual moveout analysis (Q-RMO)
- Joint tomography
The illustration below shows the QVMB workflow in detail. Initial seismic images are generated through Q-migration using velocity model V0 and background Q model Q0. After this Q-RMO is applied to Common Image Gathers (CIGs) to measure traveltime residuals for velocity updates and spectral decay for Q updates. Both residuals are back projected by a joint tomography engine in order to derive updated models Vi and Qi, which may be used as input to Q-migration on subsequent iterations of QVMB.
An accurate 3D Q model used in a Q-migration can assist survey wide amplitude analysis, particularly in the pre-stack sense, increasing the accuracy of measures such as amplitude with offset.