3D Elastic full-waveform inversion for tilted orthorhombic media

Oct 23, 2023 | CWP Blog

Posted by Ashish Kumar

Elastic full-waveform inversion (FWI) is one of the important techniques in exploration geophysics to produce high-resolution images of the subsurface. Due to advancement in computational resources and acquisition of wide-azimuth data in the last two decades, significant improvement, such as Transverse Isotropy (TI) and orthorhombic, has extended the FWI from acoustic to elastic media. However, it is still a challenging task to estimate elastic properties in fractured reservoirs and salt structures due to the presence of azimuthal anisotropy (e.g., Gulf of Mexico) through FWI.

We develop an elastic full-waveform inversion algorithm for orthorhombic media with tilted symmetry planes needed to image unconventional reservoirs associated with dipping fractured layers. Here, we consider tilted orthorhombic models, in which one of the symmetry planes is assumed to coincide with the underlying seismic reflector. Therefore, the orientation of that symmetry plane can be estimated from migrated images. We describe orthorhombic symmetry using nine velocity-based parameters: the P-wave (VP0, VP1, VP2) and the S-wave (VS0, VS1, VS2) velocities along the symmetry directions and the P-wave normal-moveout velocities (Vnmo1, Vnmo2, Vnmo3). We tested the developed FWI on a modified version of the 3D SEG-EAGE overthrust model. We employed a multiscale FWI approach, starting with low-frequency data and further incorporating the higher frequency data in the later stages of inversion to update the model parameters. The velocity-based parameters are updated iteratively in each stage, while the tilt angle is updated after each stage of the inversion. The developed elastic FWI framework can be efficiently used to estimate subsurface properties in complicated geological structures associated with azimuthal anisotropy, typical for fractured reservoirs and subsalt exploration.

Figure 1: Vertical P-wave velocity (VP0) parameter of the overthrust model: (a) actual model, (b) initial model, and (c) inverted model.

Figure 2: Vertical S-wave velocity (VS0) parameter of the overthrust model: (a) actual model, (b) initial model, and (c) inverted model.