# Interpolate/Extrapolate (Horizon)

Use Insight's interpolation/extrapolation operation to fill gaps and holes in horizons.

Regular X/Y, 3D and 2D horizons can be interpolated. To interpolate or extrapolate sparse X/Y horizons, use the regrid operation to convert them to a regular grid.

Note: The Interpolate/Extrapolate button is disabled if the input horizon is waiting for another operation (such as propagation). Insight notifies you to complete the previous operation before continuing.

## Where is the Interpolate/Extrapolate operation?

The Interpolate/Extrapolate operation is found in the Map View, under the Operations tab (along with other horizon operations).

1. Open the Map View.
2. Expand the left panel and open the Operations tab.
3. At Operation, select Interpolate/Extrapolate.

## Configuring Interpolate/Extrapolate

### Interpolate

The default mode (interpolation) fills enclosed regions according to the settings.

1. In the navigation bar, select the horizon to interpolate.
2. Algorithm: Select one of the two available algorithms:

Natural Neighbour/IWD: This algorithm is a blend of natural neighbour interpolation and inverse distance weighted interpolation, using the natural neighbour set of input points for each output location. For more information, refer to Natural Neighbour/IWD Settings.

Splines in Tension: This is an implementation of the algorithm described in "Gridding with Continuous Curvature Splines in Tension" by Smith & Wessel (Geophysics, 3, 293-305, 1990). This algorithm requires a regular grid and cannot be used to interpolate or extrapolate onto a 2D survey. For more information, refer to Splines in Tension Settings.

1. At Maximum search (m), define the maximum distance.
• When finding a result at a location, points beyond this distance have no effect.
2. Interpolation Settings: (see How Interpolation Settings Work)
• Smoothing: inverse-distance weighting
• Strike/dip: define the influence in the strike direction.
• Direction (degrees): The azimuth of the strike axis, in degrees (clockwise from North).
3. At the Limit area, select a polygon/probe to constrain the operation.
4. Enable Faults to prevent the interpolation from crossing fault planes.
• Insight honours any enabled faults in the Control Panel. Unassigned fault sticks are not used.
5. Fault Polygon: Fault polygons are excluded from interpolation/extrapolation (if selected)
6. Click Interpolate to start the operation.

### Extrapolate

Enable extrapolate to completely fill a specified region according to the settings.

1. Enable the Extrapolate check box.
• The maximum search distance has no effect when extrapolating.
2. Constrain to area field changes to Fill area.
• (required) Select a polygon/probe to fill
3. Click Extrapolate to start the operation.

## Completing the operation

After running the operation:

• Click Discard to cancel this calculation and delete the results.
• Click Replace this horizon to save this horizon containing the extracted values and replace the previous one.
• Click Save as New Horizon to create a new horizon.

To assist reviewing the results in the 3D and Map Views, interpolated / extrapolated points are displayed in a solid colour. The original values appear semi-transparent. This lasts until the operation is completed.

The following examples show a horizon in the Map View before and after interpolation:

Note: By default, the new horizon will be named: “horizon_name (interp/extrap)”.  For example, the operation above will yield a new horizon called "Sample (interp)". To rename the horizon, see Horizon Details and Configuration.