Least-squares Q migration: the path to improved seismic resolution and amplitude fidelity

Standard prestack depth migration (PSDM), e.g., Kirchhoff/RTM, is by nature unable to fully recover the reflectivity with desired amplitude and resolution due to factors such as inhomogeneous subsurface illumination and irregular acquisition geometry. This shortcoming is well recognized by the imaging community and has propelled the re-emergence of least-squares migration (LSM) in recent years. Another factor that degrades amplitude fidelity and image resolution is the attenuation of seismic waves induced by anelastic absorption and elastic scattering during its propagation inside the earth, quantified by the so-called quality factor (Q). Absorption causes frequency-dependent amplitude decay, phase distortion, and resolution reduction. This effect can be compensated through an inverse Q prestack depth migration (QPSDM). QPSDM has become an effective solution for seismic imaging in areas where strong shallow absorption anomalies exist. However, the excessive noise often accompanying QPSDM poses a big challenge to its application. We propose a least-squares Q migration (LSQM) method that combines the benefits of both LSM and QPSDM to improve the amplitude fidelity and image resolution of seismic data. Using an OBC data set acquired over the Angelin gas field offshore Trinidad, we demonstrate that LSQM not only retains the full benefits of QPSDM while mitigating its issue of over-boosted noise but also compensates for inhomogeneous illumination caused by overburden velocity variations and irregular acquisition geometry that standard PSDM suffers, leading to a final product with higher resolution and improved amplitude fidelity.