OpendTect User Documentation version 3.2
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3.2. Interpret horizons

3.2.1. Creating new horizons

1. When creating a new horizon, right-click Horizon on the tree menu and select new. The tracking wizard appears.

Once the horizon has been loaded or created, two new options are available :

Display at sections allows the user to display the horizon at section only.
Show in full shows the full horizon.

2. Specify a new name for the new horizon or choose an existing horizon from the list (this will completely replace the existing horizon. To edit or extend an existing horizon, load the horizon, right-click on it, and chose one of the options under tracking). Press next to continue.

3. Now a window appears in which the tracking mode can be selected; "tracking in volume", "line tracking" or "line manual". Additionally, the color of the horizon can be adjusted.

4. Now, the tracking setup should be defined. The setup features the following options:

Seismic data: Select the data on which the tracking should be based. The use of the word "Seismic data" is a bit misleading since it is possible to select any input volume or attribute. For example, the "energy" attribute is very useful as input when the user wants to track and visualize anomalies like bright spots or channels.
Event type: Specify the event type you want to track. The tracker can track negative reflectors (Min), positive reflectors (Max), a Z-type zero-crossing (0+-), or a S-type zero-crossing (0-+).
Search window: See figure below, drawing A The tracker will search in a time window relative to the last tracked sample. The tracker searches on amplitude for the chosen event type.
Threshold type: Cut-off amplitude. Here, an absolute amplitude is used as the stopping criteria for the tracker. When the tracker encounters a value below this threshold value it stops tracking. (For a max-event the tracker stops if the value is below this threshold value, and for a min-event when it is above this threshold value). Tip: The amplitude value is displayed at the bottom of your screen.

Relative difference: The tracker will compare the amplitude value to the value of the last tracked sample. If the difference exceeds the chosen percentage, the tracker stops tracking.
If tracking fails.: If tracking fails the tracker can either stop tracking or extrapolate the horizon. (Tip: When the tracker stops tracking before you want it to, adjust the Threshold value and similarity threshold before choosing the extrapolate option.)
Similarity.: In the tracking wizard, it is possible to select "Similarity" instead of "Event". When similarity is used for tracking, the trace within the Compare window of the last tracked sample is compared to the trace within the Compare window of each sample within the Search window (See figure below, drawing B). The measure of similarity between the traces within the compare windows is returned as a value between 0 and 1. Subsequently, the tracker will snap to the chosen event type closest to the sample that has the highest similarity value (figure below, drawing C). When the similarity drops below the defined Similarity threshold the tracker stops tracking. Tracking with similarity is more accurate, but it takes more time to compute.

When all parameters are set, press the apply button followed by ok and save the Tracker setup.

5. Now that the tracker setup has been defined a seed point can be made on an element (inline or crossline). The tracker will track the event on this element. By clicking Edit.. in the tracking wizard, the tracking setup can be modified if needed. After the setup parameters have been changed the tracker will retrack the event on the element when the apply button is clicked. When satisfied with the tracker setup, one can add more seed points if needed. Then press next to continue. OpendTect now loads the seismic data in the tracking cube around the horizon and the tracking toolbar Section appears.

6. OpendTect has loaded the input data in the Tracking cube around the horizon. Expand the tracking cube using the green anchors on the edges of the cube, but do not exceed your computer's memory size or the size of your graphics card memory. If you run out of computer memory, OpendTect will crash. If you run out of graphics memory, the tracking plane (see plane-by-plane mode below) cannot display the seismic. Instead, you will see a black plane.

You are now ready to track using auto-tracker or plane-by-plane mode.

Auto-tracker mode. The horizon is interpreted automatically within the tracking cube.
Plane-by-plane mode. Move the tracker plane through the seedpoint(s) by clicking the move-tracker-plane buttons or by dragging the tracker plane. The orientation of the tracking plane can be changed by SHIFT-click on the tracker plane. The colorbar of the tracking plane can be changed by clicking the Set track plane colorbar button.

7. Now the first part of the horizon is tracked, move the tracker cube. A small part of the horizon should be inside the new position of the tracker cube. When the cube is at its desired position, click outside the tracker cube area, and the data is loaded. Now a next part of the horizon can be tracked in either auto-track or plane-by-plane mode.

3.2.2. Edit horizons

When (a part of) a horizon is tracked, the quality can be best checked by using the "display only at sections" option. The horizon is displayed as a line on inline and crossline display elements and on the tracker plane. Move the tracker plane over the tracked horizon to QC the tracking. SHIFT-click on the tracker plane to QC in another direction. If the horizon needs editing, there are several options:

Retrack. Move the tracker cube around the area you want to edit, and load the data. Then, click the retrack icon and move the tracker plane from a correctly interpreted part of the horizon towards the part that needs editing.
Polygonal tool. A second option is to remove a part of an horizon. Click on the horizon, then SHIFT-click elsewhere on the horizon. A line will be drawn between the two points. Encircle the part of the horizon you want to remove, by SHIFT-clicking around the area (a U-shaped area is sufficient). Then, right-click on the line and select remove nodes inside.

Position the tracker cube around the hole in the surface and move the tracker plane (without tracking) to the middle of the hole. A new seed can be made on the tracker plane by clicking the "create seed" button. The horizon can now be either auto-tracked or tracked in plane-by-plane mode.
Editing tools: Horizons can be edited by clicking on the horizon and dragging the plane upwards and downwards. The shape of the affected area can be set by right-clicking on the anchor and selecting editor settings. The shape of the area can either be an ellipse or a box, and the sizes can be set. Furthermore, the vertical shape can be chosen.

The options are as follows:
- Horizontal shape, which can be a box or an ellipse.
- Horizontal size two values need to be inputed in the inline and the crossline direction. Default values are 5 in each direction.
- Vertical shape which has three options: Sinus, Pyramid, and Box.

3.2.3. 2D Horizon

For 2D Horizon tracking, the workflow is somewhat different. Using the auto-tracker is not an option in this case. After right-clicking the option "2D Horizon", select "New" if you would like to track a new horizon. Now, the Tracking Wizard pops up. First, define a name for the 2D Horizon and press "Next". The next step is to define the Tracking Mode (see figure below).

The first mode, "Track from seeds", means that the tracker automatically tracks the picked event up to the defined threshold. The second mode, "Track between seeds", tracks the event in between two picks. The third option, "Draw between seeds", interpolates linearly between two picks (this option is useful in case of low quality seismic data).

As soon as a 2D Horizon is tracked, a 3D Horizon can be derived from this 2D Horizon by right-clicking the 2D Horizon in the tree, a window pops up in which you can select the algorithm (inverse distance interpolation or extension) and specify the search radius or extension steps.

The end results can be displayed after deriving. This horizon can be improved by clicking the "Algorithm" option in the pop up after right-clicking the horizon in the tree. You will have the options between "Fill holes", "Filter", and/or "Snap to event".

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