Finite difference method

Abstract: The finite‐difference method is a powerful technique for studying the propagation of elastic waves in boreholes. Even for the simple case of an open borehole with vertical homogeneity, the snapshot format of the method displays clearly the interaction between the borehole and the rock, and the origin and evolution of phases. We present an outline of the finite‐difference method applied to the acoustic logging problem, including a boundary condition formulation for liquid‐solid cylindrical interfaces which is correct to second order in the space increments. Absorbing boundaries based on the formulations of Reynolds (1978) and Clayton and Engquist (1977) were used to reduce reflections from the grid boundaries. Results for a vertically homogeneous sharp interface model are compared with the discrete‐wavenumber method and excellent agreement is obtained. The technique is also demonstrated by considering sharp and continuous transitions (damaged zones) at the borehole wall and by considering the effects of wa...

Abstract: One-pass three-dimensional (3-D) depth migration potentially offers more accurate imaging results than does conventional two-pass migration, in variable velocity media. Conventional one-pass 3-D migration, done with the method of finite-difference inline and crossline splitting, however, creates large errors in imaging complex structures due to paraxial wave-equation approximation of the one-way wave equation, inline-crossline splitting, and finite-difference grid dispersion. After analyzing the finite-difference errors in conventional 3-D poststack wave field extrapolation, the paper presents a method that compensates for the errors and yet still preserves the efficiency of the conventional finite-difference splitting method. For frequency-space 3-D finite-difference migration and modeling, the compensation operator is implemented using the phase-shift method, or phase-shift plus interpolation method, depending on the extent of lateral velocity variations. The compensation operator increases the accuracy of handling steep dips, suppresses the inline and crossline splitting error, and reduces finite-difference grid dispersions. Numerical calculations show that the quality of 3-D migration and 3-D modeling is improved significantly with the finite-difference error compensation method presented in this paper. 13 refs., 7 figs.

Abstract: A numerical model is presented allowing calculation of the dc, ac, and large-signal parameters of field-effect transistors (FET's). The numerical procedure is based on finite-difference approximations to the full time-dependent set of equations. The scheme presented uses centered difference quotients and an implicit treatment of the continuity equation. It is shown to be absolutely stable and accurate for time steps below 1 ps. A set of numerical data calculated for one typical example is compared systematically with experimental values. Excellent agreement between measured and computed values is found for the dc characteristics. Small-signal solutions, obtained by Fourier transform methods are also close to the empirical values. The good fit between experiment and numerical simulation is a thorough validation of both the physical model and the numerical procedure.