2D PP/PS‐stereotomography: Application to a real 2D‐OBC dataset

Abstract

A B S T R A C TIt has been shown on an 'ideal' synthetic dataset that PP/PS-stereotomography can estimate an accurate velocity model without any pairing of PP-and PS-events. The P-wave velocity model is first estimated using PP data and then, fixing this velocity field, the S-wave velocity is estimated using the PS data. This method needed to be evaluated further and we present here the first application of PP/PS-stereotomography to a real dataset: the 2D East-West Mahogany OBC line (Gulf of Mexico). We are here confronted with data which do not fit our working assumptions: coherent noise (due to an approximate separation of PP-and PS-events and some remaining multiples), probably some anisotropy and 3D effects. With a careful selection of the stereotomographic picks, which allows one to decrease the effect of the picked coherent noise by the automatic picker, our application can demonstrate the relevance of our approach in the upper part of the profile, where anisotropy and 3D effects might be low. We can thus estimate, without any pairing of PP-and PS-events, a velocity field which provides not only flat common image gathers, but also PP-and PS-depth migrated images located at the same positions. For the deeper part of the profile, a significant shift in depth appears. In addition to possible anisotropy, 3D effects and a more complex velocity field ('salt body'), this is due to the quality of the PZ-and X-components profiles: The PZ-component profile where the PP-stereotomographic picking is performed, is polluted by conflicting converted or multiple events and the X-component profile, where the PS-stereotomographic picking is performed, is highly noisy. This study emphasizes the need to develop accurate selection criteria for the stereotomographic picks. I N T R O D U C T I O NThe use of converted waves in reflection seismic imaging can be a significant improvement. Indeed P-and S-reflected waves bear complementary information about the elastic properties of the medium. It concerns both the reference velocity model (P-and S-wave velocities) and the short wavelength components of the elastic parameters describing the model (reflectivity).

Towards a more robust input for stereotomography

“…Several improvements to stereotomography have been proposed in the literature. These include evaluation of the gradient with the adjoint‐state method (Plessix, 2006); application of a PP/PS formulation in a two‐dimensional ocean bottom cable seismic data (Alérini et al ., 2008); investigation of several regularization functions (Costa et al ., 2008); implementation of the picking stage after the data migration, in‐depth domain (Nguyen et al ., 2008); presentation of different formulations combining borehole and surface seismic data (Gosselet and Le Bégat, 2009) and seismic reflected and transmitted data (Prieux et al ., 2013); use of eikonal solvers to generate the modelled data (Tavakoli et al ., 2017); proposal of an extension to the tilted transversely isotropic acoustic media (Tavakoli et al ., 2019); a parsimonious formulation of stereotomography that uncouples the velocity and the ray attributes associated with a scattering point in the inversion process (Sambolian et al ., 2019); and a reformulation of the parsimonious stereotomography as a hypocentre–velocity method (Sambolian et al ., 2021). Furthermore, several of these works highlight the importance of providing accurate input data to stereotomography by high‐quality slope estimation and picking methods (Billette et al ., 2003; Lambaré et al ., 2004; Tavakoli et al ., 2017; Sambolian et al ., 2019, 2021).…”

PP/PSanisotropic stereotomography

“…An overview and status of the stereotomographic method has been given in Lambaré (2008). Stereotomography has shown its simplicity of use and its quality in different contexts; in particular, Alerini et al (2007, 2008) extended stereotomography for the estimation of P ‐ and S ‐wave isotropic velocity models. Although their results were very encouraging, the method should be extended to anisotropic media to improve processing of OBS data.…”

Contributions to the initialization of seismic slope tomography

“…In Lambaré [2008], the author presents an extensive overview, ten years after the creation of stereotomography. In Alérini et al [2008], the authors applied a PP/PS version of stereotomography in a 2D ocean bottom cable (OBC) seismic data. The work of Nguyen et al [2008] investigates how stereotomography would be influenced by making the picking after the data migration, in depth domain.…”