Sung-Ryul Shin

Efficient calculation of a partial‐derivative wavefield using reciprocity for seismic imaging and inversion

Linearized inversion of surface seismic data for a model of the earth’s subsurface requires estimating the sensitivity of the seismic response to perturbations in the earth’s subsurface. This sensitivity, or Jacobian, matrix is usually quite expensive to estimate for all but the simplest model parameterizations. We exploit the numerical structure of the finite-element method, modern sparse matrix technology, and source–receiver reciprocity to develop an algorithm that explicitly calculates the Jacobian matrix at only the cost of a forward model solution. Furthermore, we show that we can achieve improved subsurface images using only one inversion iteration through proper scaling of the image by a diagonal approximation of the Hessian matrix, as predicted by the classical Gauss-Newton method. Our method is applicable to the full suite of wave scattering problems amenable to finiteelement forward modeling. We demonstrate our method through some simple 2-D synthetic examples.

Traveltime calculations from frequency‐domain downward‐continuation algorithms

We present a new, fast 3D traveltime calculation algorithm that employs existing frequency‐domain wave‐equation downward‐continuation software. By modifying such software to solve for a few complex (rather than real) frequencies, we are able to calculate not only the first arrival and the approximately most energetic traveltimes at each depth point but also their corresponding amplitudes. We compute traveltimes by either taking the logarithm of displacements obtained by the one‐way wave equation at a frequency or calculating derivatives of displacements numerically. Amplitudes are estimated from absolute value of the displacement at a frequency.By using the one‐way downgoing wave equation, we also circumvent generating traveltimes corresponding to near‐surface upcoming head waves not often needed in migration. We compare the traveltimes computed by our algorithm with those obtained by picking the most energetic arrivals from finite‐difference solutions of the one‐way wave equation, and show that our trave...

Non-uniqueness of waveform inversions in the Laplace domain

The non-uniqueness of the solution is one of the major obstacles for a successful full waveform inversion. This paper shows that there are at least two solutions for waveform inversions in the Laplace domain: the true velocity model; the long-wavelength background velocity model. The Laplace domain inversion can recover the true velocity model if provided with an accurate initial model, but it will instead result in a smooth long-wavelength velocity model if the initial model is inaccurate and smooth. In this case, the inverted long-wavelength velocity model can still be used for either migration or as an initial velocity model for frequency domain waveform inversion.

A proposal of seismic oceanography for temperature model inversion of the East Sea, Korea

Much attention has recently been paid to the East Sea because of the great influence it exerts on weather in the Korean peninsula as well as its peculiar oceanographic features. A large number of oceanographic observations have been made to probe the physical properties of the waters at the East Sea. However, until now, measuring the physical properties of a broad oceanic area simultaneously was impractical. To address this problem, this study presents a methodology to measure the physical properties of seawater by introducing seismic oceanography (SO) into marine seismic surveys. SO has been attracting attention as a new technology capable of concurrent analysis of a broad oceanic area; however, hardly any research of this type has been conducted in Korea to date. The technique proposed in this study involves derivation of a seawater temperature model based on the 1D seismic waveform inversion and Leroy equations using seismic data and non-stationary ARGO data. The proposed technique was validated through numerical model experiments using the seawater temperature information obtained at the East Sea in 2013, through which much insight into the ocean currents and seawater characteristics of the East Sea could be gained.

Anisotropy and Scattering Characteristics in Fracture Zone by Seismic Modeling

Fractures and cracks in carbonate rock cause secondary porosity, which affects fluid behavior and production of oil and gas. Therefore, evaluation of fracture properties is crucial to optimize oil and gas production. To this end, we investigated representative seismic response in fracture zone, seismic anisotropy and scattering energy, by physical modeling. We found that application of conventional processing under an assumption of isotropy to anisotropic dataset from physical and numerical modeling experiments, resulted in inaccurate imaging of the reflection interface. Furthermore, scattered energy was found to be most incoherent and irregular when the receiver of the model moved in the direction normal to the fracture strike.

Travel time calculation using mono‐chromatic one‐way wave equation

A new fast algorithm for travel time calculation using mono-chromatic one-way wave equation was developed based on the delta function and the logarithms of the single frequency wavefield in the frequency domain. We found an empirical relation between grid spacing and frequency by trial and error method such that we can minimize travel time error. In comparison with other methods, travel time contours obtained by solving eikonal equation and the wave front edge of the snapshot by the finite difference modeling solution agree with our algorithm. Compared to the other two methods, this algorithm computes travel time of directly transmitted wave. We demonstrated our algorithm on migration so that we obtained good section showing good agreement with original model. our results show that this new algorithm is a faster travel time calculation method of the directly transmitted wave for imaging the subsurface and the transmission tomography.

Spectral Method in Scalar Wave Equation Modeling

ABSTRACT A new numerical method is proposed for the simulation of wave phenomena by applying the forward modeling of the scalar wave equation. In the computation of wavefield, the wave equation transformed into the frequency- wavenumber domain is utilized. A linear system composed of coefficients obtained by a discrete Fourier transform of discretized model parameters is solved to obtain the wavefield at each frequency. The wavefield in the time-space domain is obtained by inverse Fourier transforms of the wavefield computed in the frequency-wavenumber domain. This scheme has no numerical dispersion problems in the homogeneous media and can remove truncation errors originated in the finite-difference approximations to spatial and temporal derivatives. The accuracy of this scheme is confirmed through forward modeling with the Nyquist sampling limit. However, for solving in the linear system, lots of computation time and computer memory are required due to the huge resultant complex impedance matrix size. T...