The impact point is the first point encountered on the specified source object(s) along the specified shoot direction.
In some geologic formations (such as folded or thrusted surfaces), there can be more than one intersection between the shooting path and the specified source object(s) (see the left illustration in Figure 4–4, dir_Z = -1). Only the first of such intersections is the impact point. Actually, the command stops searching in that particular direction once it finds an intersection.
Multiple impact
points If the shoot direction is two-way, but only one impact point is needed, the command searches in both directions to find impact points. It will keep the closest one if it finds two (one in each direction).
Figure 4–4 Impact points and the geometry of the source Surface
No impact point If the command does not find an impact point along the specified shoot direction(s) for a given point in the model layer, the value of that particular variable at that point is null.
See the right illustration in Figure 4–4, dir_Z = 1.
The following are illustrations of how the command finds the impact points under different circumstances:
source surface (the surface with the property you want) impact points on the source surface
4.3 Defining Layer Property Values Directly 4-9 User Guide
4.3 Defining Layer Property Values Directly
For information, see:
• "Assigning Constant Values to Properties or Variables," page 4-9
• "Defining Properties or Variables by Using Linear Functions," page 4-9
• "Defining Properties or Variables by Using Interpolation," page 4-10
• "Defining Properties or Variables from Grid Properties," page 4-12
• "Defining Values from Surface or Layer Boundary Properties," page 4-13
4.3.1 Assigning Constant Values to Properties or Variables
To assign a constant value to a property or variable
1 In the property browser, click the property or variable that you want to define or edit.
2 In the Variable type box, select Constant.
3 In the Constant value box, type a numerical constant value to assign to the variable.
4 Click Update Variable Definition to apply your changes.
4.3.2 Defining Properties or Variables by Using Linear Functions
This definition option is similar to defining property variables by using interpolation (see
"Defining Properties or Variables by Using Interpolation," page 4-10), except that the top and bottom Surfaces are replaced by the upper and lower bounding Surfaces of a model layer.
This definition option can be useful when there is more than one top and/or more than one bottom bounding Surface for the layer of interest. This definition option ensures that at any given point in the selected layer, the variable will find two impact points to get an interpolated (extrapolated) Value, since a layer, by definition, is completely bounded.
(There is one exception: when part of the bounding surface of the given layer is the bounding box of the model, the command may not be able to locate an impact point.)
To define a property or variable by using a linear function
1 In the property browser, click the property or variable that you want to define or edit.
2 In the Variable type box, select Linear Function of Property.
3 In the Referenced layer box, select the name of the referenced layer whose bounding Surfaces will be the source Surfaces.
Important Ensure that the property you want is defined on all of those Surfaces.
4 In the Referenced property box, enter the name of the property on the bounding Surfaces of the selected referenced layer. (This does not have to be the same name as the name of the variable or the name of the layer property.)
5 If you want to add a constant value to the referenced property, type a number in the Value to add to referenced property box.
6 If you want to multiply the referenced property by a constant scaling factor, type a number in the Scaling factor (multiple) to apply to referenced property box.
7 Click Update Variable Definition to apply your changes.
4.3.3 Defining Properties or Variables by Using Interpolation
This definition option finds the property values at two impact points (see "Impact Point"
on page 4-8), one from each of the two specified source boundaries, and performs a linear interpolation (or extrapolation) to find a value for the variable at (x, y, z).
The command finds the top and bottom impact points by shooting two ways (see
"two_way" on page 4-6) to the top source boundary and the bottom boundary surface from the given point (see "from_inside" on page 4-7).
Figure 4–5 presents graphic examples of where impact points may be located. The terms
"top source Surface" and "bottom source Surface" do not imply relative positions to each other or to the selected model layer. The terms are only used to remind you that you need two source Surfaces.
4.3 Defining Layer Property Values Directly 4-11 User Guide
Figure 4–5 Finding the two impact points on the two source Surfaces for a given point in a model layer
Each boundary property is defined as a function:
Where P(x,y,z) is the linear property function, Z is the coordinate at a given set of (x,y,z) points, and Po is a variable. Therefore, this definition option performs an interpolation between two linear functions of two boundaries.
To define a property or variable by using interpolation
1 In the property browser, click the property or variable that you want to define or edit.
2 In the Variable type box, select Interpolate Property.
3 To define Ptop, the variable representing the top boundary, do one of the following:
• To assign a constant, click Constant, and then type a numerical constant in the box.
• To set Ptop to equal another variable in the model, click Variable, and then select the variable in the box.
4 To define Ztop, the Z value of the top boundary, do one of the following:
• To assign a constant, click Constant, and then type a numerical constant in the box.
• To set Ztop to equal the Z of another surface/layer, click Variable, and then select the variable in the box.
X X X
Top source surface Top source surface
Bottom source surface Bottom source surface
Model layer Model layer
P x y z = P
o+ Z Z –
o
5 To define Pbot, the variable at the origin of the bottom boundary, do one of the following:
• To assign a constant, click Constant, and then type a numerical constant in the box.
• To set Pbot to equal another variable in the model, click Variable, and then select the variable in the box.
6 To define Zbot, the Z value of the bottom boundary, do one of the following:
• To assign a constant, click Constant, and then type a numerical constant in the box.
• To set Zbot to equal the Z of another surface/layer, click Variable, and then select the variable in the box.
7 Click Update Variable Definition to apply your changes.
4.3.4 Defining Properties or Variables from Grid Properties
This definition option creates a variable whose value at a given point in the selected layer is defined by the value of a Voxet property at the same (X,Y,Z) position. The name of the Voxet property need not be the same as the name of the variable or the layer property that you are defining.
To use this definition option, you need a Voxet with the desired property. Ideally, choose a Voxet that has the same or greater spatial extent as your model in order to avoid a possible core dump.
To define a property or variable from a grid property
1 In the property browser, click the property or variable that you want to define or edit.
2 In the Variable type box, select From Grid Property.
3 In the Voxet box, enter the name of the Voxet that includes the property you want.
4 In the Property box, enter the name of the property.
5 If you want to extrapolate the property outside the Voxet, select the Extrapolate check box.
6 If you want to set the point where the values are not defined as the specified default value, select the Use the default value check box, and then type a number in the box.
7 Click Update Variable Definition to apply your changes.
4.3 Defining Layer Property Values Directly 4-13 User Guide
4.3.5 Defining Values from Surface or Layer Boundary Properties
For information, see:
• "Defining Values from Surface Boundaries," page 4-13
• "Defining Values from Layer Boundaries," page 4-14
• "Notes on Defining Values from Surfaces and Layers," page 4-18
This definition option creates a variable whose value at a point (x, y, z) is determined by a property value on the selected boundary at the same xy-position. The boundary can be either a Surface or a layer. In other words, the command does the following:
• Shoots from a point (x, y, z) inside the layer of the model directly upward or downward until it hits the selected source boundary.
• Sets the property value at that impact point as the value of the variable at the shooting point (x, y, z).