Grid Expansion and Fill

GM-SYS 3D utilizes Fast-Fourier Transform (FFT) calculations to compute the model response. All the grids must be expanded in size and “filled” so that they are “periodic” to eliminate edge effects. The amount of expansion required depends on the shape of the model horizons near the edges of the original grid. The final, expanded dimensions of the grid must also be “good FFT” dimensions. If you use %’s to specify expansion amounts, all of the routines described below and the Oasis automatic routines expand to “good FFT” dimensions.

There are two options with regard to the “expand and fill” preparation:

Automatic FFT Preparation

The simplest is to use the Automatic FFT Preparation function. With this, all grids are expanded inside the calculation routine each time it runs and then “masked” back to the original size at the end of the calculation. This option is easier to use but gives less control and requires significantly more computation time with larger grids.

Manual FFT Preparation

The second option is to prepare each grid separately before building the model using the procedures in the Manual FFT Preparation submenu. This option allows more control and saves time during the calculations.