Ju-Won Oh

King Abdullah University of Science and Technology | KAUST · Department of Earth Science and Engineering (ErSE)

Full-waveform inversion (FWI) aims at building a high-resolution velocity model by fitting numerically computed seismic data to observed one. Considering both the kinematic and dynamic properties of all waves in seismic data makes FWI highly non-linear. To mitigate its non-linearity, one can preferentially build low-wavenumber background velocity f...

Because the magnetotelluric (MT) method uses natural sources, the electric and magnetic fields recorded in the field acquisition are not directly used but usually converted into other MT response functions for interpretation such as inversion. Considering that inversion results are dependent on types of input data, it can be helpful to analyse diff...

Full waveform inversion (FWI) is a highly nonlinear optimization problem that aims to reconstruct high-resolution subsurface structures. The success of FWI in reflection seismology relies on appropriate updates of low-wavenumber background velocity structures, which are generally driven by the diving waves in conventional FWI. On the other hand, th...

The pressure-based acoustic approximation of the elastic wave equations in anisotropic media has advantages corresponding to computational efficiency and numerical stability. However, the numerical scattering potentials from the anisotropic parameter perturbations for the pressure wavefield are not consistent with the elastic scattering theory. In...

In the early stage of acoustic full-waveform inversion (AFWI), it is important to exploit the long-wavelength features of the gradient and suppress its short-wavelength features to update the background velocity. However, due to strong near-offset PP reflections and multiples within the pressure data, conventional AFWI sometimes primarily reconstru...

Full-waveform inversion (FWI), which is among the most powerful seismic data processing techniques for imaging subsurface geological structures, has a huge computational cost in proportion to the number of sources. To increase the speed of FWI, we explored the use of common-receiver gathers (CRGs) as an alternative to common-shot gathers (CSGs) as...

Seismic full-waveform inversion (FWI) estimates the subsurface velocity structures by reducing data misfit between observed and modeled data. Simultaneous matching of transmitted and reflected waves in seismic FWI causes different updates of different wavenumber components of a given model depending on the diffraction angle between incident and dif...

To improve the computational efficiency of reverse-time migration (RTM) for vertically transverse isotropic (VTI) media, various acoustic approximations of the elastic wave equations have been presented. Among these, the pseudo-acoustic wave equation, which combines differential and scalar operators, has the advantage that it does not produce shear...

Seismic anisotropy is an important physical phenomenon that significantly affects wave propagation in complex sedimentary basins. When geological structures exhibit steep dips or severe folding, the symmetry axis of the transversely isotropic (TI) representation of the region can be rotated, leading to tilted transversely isotropic (TTI) media. We...

Reflection traveltime tomography (RTT) has been used to describe subsurface velocity structures in practice, which can be used as a background or initial model for prestack depth migration or full waveform inversion. Conventional RTT is performed by solving an optimization problem based on a ray‐tracing method. As a result, RTT requires heavy compu...

In multiparameter full-waveform inversion (FWI) with acoustic-approximation for vertically transverse isotropic (VTI) media, it is important to choose an appropriate modeling technique for computational efficiency and numerical stability. In addition, because the gradient is determined by the modeling algorithm used in the FWI process, we need to e...

The multi‐parameter full waveform inversion is an essential tool to estimate subsurface anisotropic properties in a reservoir that may have complex geological behavior requiring an elastic orthorhombic medium description. For such elastic orthorhombic media, finding a proper inversion strategy to mitigate parameter tradeoff and reduce the Null spac...

We have developed an efficient elastic full-waveform inversion (FWI) based on the P-wave excitation amplitude (maximum energy arrival) approximation in the source wavefields. Because, based on the P-wave excitation approximation (ExA), the gradient direction is approximated by the crosscorrelation of source and receiver wavefields at only excitatio...

Multiparameter full-waveform inversion (FWI) usually suffers from the inherent tradeoff in the multiparameter nature of the model space. In orthorhombic anisotropy, such tradeoff is magnified by the large number of parameters involved in representing the elastic or even the acoustic approximation of such a medium. However, using a new parameterizat...

The resolution of a multiparameter full-waveform inversion (FWI) is highly influenced by the parameterization used in the inversion algorithm, as well as the data quality and the sensitivity of the data to the elastic parameters because the scattering patterns of the partial derivative wavefields (PDWs) vary with parameterization. For this reason,...

We introduce a depth scaling strategy to improve the accuracy of frequency-domain elastic full waveform inversion (FWI) using the new pseudo-Hessian matrix for seismic data without low-frequency components. The depth scaling strategy is based on the fact that the damping factor in the Levenberg-Marquardt method controls the energy concentration in...

Multi-parameter full waveform inversion (FWI) applied to an elastic orthorhombic model description of the subsurface requires in theory a nine-parameter representation of each pixel of the model. Even with optimal acquisition on the Earth surface that includes large offsets, full azimuth, and multi component sensors, the potential for tradeoff betw...

We suggest the new elastic orthorhombic (ORT) parameterisation for the multi-parameter full waveform inversion (FWI). The new parameterisation includes 2 seismic velocities and 7 dimensionless parameters that allow for the continuity of the radiation patterns from isotropic to VTI to orthorhombic and elastic models. This continuation offers the pos...

In multi-parameter full waveform inversion (FWI), the success of recovering each parameter is dependent on characteristics of the partial derivative wavefields (or virtual sources), which differ according to parameterisation. Elastic FWIs based on the two conventional parameterisations (one uses Lamé constants and density; the other employs P- and...

To enhance the feasibility of the multi-parameter full waveform inversion (FWI) for field data, including various types of seismic waves, we introduce a new 2-stage elastic VTI FWI strategy. Because the influence of S-waves on FWI is dominant due to its strong amplitudes in partial derivative wavefields, some parameters may not be recovered well. I...

We propose a depth scaling method to mitigate the sensitivity of the elastic full waveform inversion (FWI) to random noise, which is designed introducing flexible damping factor in the Levenberg-Marquardt method. When the damping factor is constant over iterations, FWI can be severely affected by noise distributions over depths. In our depth scalin...

For elastic full waveform inversion, several kinds of parameterisation have been used. The resolution of inversion results is dependent on the parameterisation, because different parameterisations yield different radiation patterns of partial derivative wavefields for each parameter. Accordingly, it is important to find an optimal parameterisation...

The anisotropic elastic full waveform inversion (FWI) is important because many sedimentary rocks have anisotropic properties and seismic waves are partially affected by such properties. However, the multi-parametric FWI for anisotropic media has lots of difficulties due to the complexities induced by multi-component data and multi-mode conversions...

One of the factors influencing the accuracy of the seismic modeling is the boundary condition. Several boundary conditions have been developed and have their own advantages and disadvantages. One possible method to perfectly remove edge reflections is to extend the dimension of a given model so that the edge reflections cannot be recorded within th...

In this study, we analyse the problems of conventional scaling methods in frequency-domain FWI and propose a weighting method to compensate for these problems. The weighting method is applied to the conventional elastic FWI, where the gradient is scaled by the diagonal of the pseudo-Hessian matrix inside the frequency loop so that the effect of the...

To enhance the robustness of the l2-norm elastic full-waveform inversion (FWI), we propose a denoise function that is incorporated into single-frequency gradients. Because field data are noisy and modelled data are noise-free, the denoise function is designed based on the ratio of modelled data to field data summed over shots and receivers. We firs...

To make the l1-norm frequency-domain elastic full waveform inversion (FWI) less sensitive to initial guesses, we propose the weighting method that incorporates weighting functions to gradients at each frequency. The weighting function should be designed so that the final gradient can properly describe the differences between assumed (initial or inv...

One of the limitations in seismic waveform inversion is that inversion results are very sensitive to initial guesses, which may be because the gradients computed at each frequency are not properly weighted depending on given models.Analyzingthe conventional waveform inversion algorithms using the pseudo-Hessian matrix as a pre-conditioner shows tha...

In seismic waveform inversion using the adjoint operator, using the pseudo-Hessian matrix as a pre-conditioner can allow us to achieve computation efficiency. However, the former pseudo-Hessian matrices (i.e., the original and new pseudo-Hessian matrices) have a limitation to simulate the features of the approximate Hessian matrix for the deeper pa...

We simulate the propagation of earthquake waves in the continental margin of Antarctica using the elastic wave modeling algorithm, which is modified to be suitable for acoustic-elastic coupled media and earthquake source. To simulate the various types of earthquake source, the staggered-grid finite-difference method, which is composed of velocity-s...