Simultaneous joint inversion of refracted and surface waves

The shallow subsurface often exhibits large and rapid vertical and horizontal variations due to structural and stratigraphic reasons. The low compaction and cementation of shallow formations can generate extreme velocity changes, especially in arid areas without a shallow water table. From a geological and geophysical point of view, the near surface can be extremely complex, with multiple velocity inversion and sharp lateral velocity variations. It is well known that the impact of near-surface perturbations on seismic reflection data must be removed to obtain the correct geometrical image of deeper horizons and representative reservoir attributes. Additionally, the near surface is an important, but challenging, portion of velocity model building in depth imaging. The near-surface characterization is, therefore, an important part of seismic data processing, in particular for land data. Conventional approaches involve the use of refracted waves, or diving waves, for P-wave velocity model estimation. The use of Rayleigh waves can be an alternative when data are acquired properly. Refraction tomography (RT) and surface-wave inversion (SWI) are based on different physical principles, make use of different components of the wavefield, and have different limitations and strengths. Refraction tomography often provides deeper models and estimates directly the compressional wave velocity. But, with land data, refraction techniques can be challenging in areas with a complex near surface. The data quality can be critical; for instance, picking the near offset can be difficult. Often, only a limited offset range can be picked reliably. Moreover, velocity inversions and hidden layers can produce ambiguities, and they might not be resolved uniquely. Surface waves have a very high resolution in the shallow near surface and are very robust versus model complexity and data quality. However, even if the low-frequency penetration reaches hundreds of meters, the vertical resolution decreases with depth and the resolution at the final investigation target may be insufficient in some areas. Finally, conversion from shear to compressional velocities requires calibration with P-wave events. When RT and SWI are used, integration and reconciliation of the two models can be done in different ways. A robust framework for this task is simultaneous joint inversion (SJI). The two measurements are input into a single inverse problem, where a unique model with multiple properties is estimated, minimizing the data prediction error and a link between the two domains. The synergies between the two techniques can be exploited. Different portions of the model are resolved by the interacting contributions of the measurements.