Implicit Modelling: Control Point Basics

Implicit control points can either be additional observations that aren’t contained within the input file, or simply user-defined points to help guide the implicit algorithm. At the end of the day, you know the geology better than the algorithm, so you may need to utilise control points to honour your interpretation of the data.

Each implicit modelling tool uses control points in a different way due to how the algorithm behaves. This article will explain control points in the context of each tool and also how to use them. Point data is primarily used as an example in this article for ease of illustration, however, it's important to note that string files can also be used if you would like to constrain the algorithm even further.

Intrusion

Value Field

The value field for the intrusion modeller is optional. This lets you control how each single point in your file affects the wireframe:

• Positive values (+1) will be inside the resulting wireframe
• Zero values (0) will be on the boundary of the resulting wireframe
• Negative values (-1) will be outside the resulting wireframe

If this field is left blank, the algorithm will assume that all points represent the boundary of the intrusion model.

A) Wireframe output with no control points, B) Wireframe output with a negative value point, C) Wireframe output with a zero value point, D) Wireframe output with a positive value point.

Orientation

The orientation of the generated wireframe is influenced by the Dip and Dip Direction of the points or strings in the Input file; think of this as a localised structural trend. 

The dip values in the Input file should be 0-90. Dip direction values should be 0-360. Any values outside of these ranges (i.e. negative dips) are ignored and a warning message is displayed after processing is complete.

Contact

Value Field

The value field for the contact modeller is optional. This lets you control how each single point in your file affects the wireframe:

• Positive values (+1) are the points representing the roof
• Zero values (0) are the points representing the mid-surface
• Negative values (-1) are the points representing the floor

If the field name is left blank, all points will be given a default value depending on the surface type you are modelling (roof, middle or floor). 

A) Wireframe output with no control points, B) Wireframe output with a negative value point while modelling a floor contact.

Orientation

Use the inputs to include structural information (dip and dip direction) to points, and the contact surface will honour that orientation. Enter an orientation that is normal to the surface where it coincides with the input points.

Vein

Value Field

Either the value field or the orientation fields are compulsory for the vein modeller. The value field explicitly tells the algorithm which side of the vein in represents:

• Positive values (+1) are the points representing the hanging wall surface.
• Negative values (-1) are the points representing the footwall surface.
• Zero (0) values are the points representing pinch points.
• If the field value is left BLANK for individual records in the input DAT file, then those input points will not control the location of the Vein surfaces.
  
  

A) Wireframe output with no control points, B) Wireframe output with a positive and negative control point, C) Wireframe output with a zero value point, D) Wireframe output with a positive and negative control string.

Orientation

Optionally, the orientation of the generated wireframe is influenced by the Dip and Dip Direction of the points or strings in the Input file. If the control point value field is left blank (or there are blank values in the input file), the points will only act as a localised structural trend.

Fault

Contrary to the other tools, control points in the fault modeller do not require a value field. This is because the algorithm assumes that every data point will lie directly on the fault plane. Dip and dip direction for the control points are optional; without this information, the algorithm will use an orientation to best fit the points.

Fault wireframe with no control points (green) and fault wireframe with extra control points (brown).

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