**November 1, 2011**

**VMOD Flex Finite-Difference Grid Types**

VMOD Flex is changing how you model. You can now define multiple numerical grids of different types within your conceptual model, select the most appropriate grid for your project, and generate the numerical model files in minutes.

When designing the vertical numerical grid, several approaches can be used:

- Uniform model layers (a rectilinear grid)
- Deformed model layers
- Semi-uniform layers (deformed on top and bottom, with uniformly thick layers in between)

**Uniform Grid**

In a uniform grid, a number of layers with uniform thickness will be created. At the time of translating the conceptual model to the numerical model, the properties will be assigned to the appropriate grid cells to represent the geological structure. Grid cells above the topmost geological layer (and below the bottom-most layer) are set as inactive.

This grid is useful for transport or density-dependent simulations, where it is desirable to have fine vertical discretization.

**Advantages:**

- fully respects the finite-difference assumptions
- less disconnected grid cells

**Disadvantages:**

- requires larger number of cells
- time consuming to define manually (requires you to assign the correct properties to the appropriate grid cells, individually

**Semi-Uniform Grid**

In a semi-uniform grid, the top and bottom of the grid are deformed, following the top-most and bottom-most horizons respectively; in between, a set of uniformly thick layers will be generated. At the time of translating the conceptual model to the numerical model, the properties

will be assigned to the appropriate grid cells to represent the geological structure. This grid is useful where you have discontinuous layers

**Advantages:**

- almost fully respects the finite difference assumptions
- less disconnected grid cells
- could be more stable for layers that pinch-out
- top and bottom layers deform

**Disadvantages:**

- requires larger number of cells
- time consuming to define manually (requires you to assign the correct properties to the appropriate grid cells, individually)

**Deformed Grid**

In a deformed grid, the tops and bottoms of the model layers follow the horizons elevations (the geological layers). Each model layer corresponds to a hydrogeological unit. Property parameters are applied to the full layer thickness. You can refine the model layers, by dividing the zones into proportionately thick layers.

**Advantages:**

- requires a smaller number of layers
- easy to design and construct

**Disadvantages:**

- potential inaccuracy problems
- pinch out layers, requires you to manually change the properties in these areas

Using VMOD Flex, a numerical model with any and/or all of the grids above can be quickly constructed from your conceptual model. Since the property zones in your conceptual model are grid-independent, you can change your grid type and quickly design and populate multiple numerical models, and select the most appropriate one. You are no longer restricted to using the deformed grid type. Designing and populating a uniform or semi-uniform grid can be done in minutes, as opposed to hours.