| OpendTect User Documentation version 4.2 |
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Horizons can be tracked in various modes:
- Autotracked
- Tracked on lines (in the 3D scene or 2D viewer)
- Manually picked on lines (in the 3D scene or 2D viewer)
Workflow:
Add an inline/crossline, right-click "Horizon" in the tree and select New ....
1. Define mode and tracker settings.
2. Pick seeds
3. Autotrack or interpret the horizon on lines.
4. QC and Edit the horizon.
1. Define mode and tracker settings
Right-click "Horizon" in the tree and select New ....This will launch a tracking setup which contains several tabs:
Mode Tab
Choose the tracking mode: tracking in Volume, Line tracking, or Line manual:
- Tracking in volume is used to auto-track a horizon inside a user defined tracking area (3D (sub)volume). The tracker area can be moved and adjusted inside the survey box. This mode is preferred for most horizon tracking.
- Line tracking is used to track a horizon on a line (inline or crossline). This mode gives the interpreter more control and is used in difficult areas. In between the seeds the horizon is tracked along the line. The result is a grid that needs to be filled, either by autotracking or interpolation.
- Line manual mode will manually pick a horizon (interpolated line) between two seeds on lines. The workflow is similar to line tracking, with the difference that between seeds a line is drawn. This mode is used to cross faults, push through noise zones or interpret unconformities.
Event Tab
When Tracking in volume or Line tracking is selected the user needs to define several tracking criteria:
Input data: The input data is automatically selected as being any seismic attribute active in the tree. This can be the original seismic volume, or a filtered seismic volume (preferred) or any other attribute.
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 for the chosen event type based on amplitude.
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: point your mouse at the event and the amplitude value is displayed at the bottom of your screen.
Relative difference: The tracker will compare the amplitude of the last tracked point to the amplitude of the point that is candidate for tracking. If the difference exceeds the chosen percentage, the tracker stops tracking.
Steps...: Step-wise tracking results in better tracked horizons. Good parts of the horizon should be tracked first, followed by more difficult areas. First, track the parts of the horizon that have a low difference to the seeds or parts that have a high amplitude. In subsequent steps the tracker settings become less strict. Therefore, the horizon will be of better quality and needs less editing.
Define percentage values (incremental: eg 1,2,5,10,20) , or subsequent amplitude values (decremental e.g. 2500,2000,1500,1000,500)
If tracking fails.: If the tracker can not find a neighbouring point (that complies with the specified relative difference or cut-off amplitude) it can either stop tracking or extrapolate the horizon. (Tip: When the tracker stops tracking before you want it to, adjust the Threshold value and/or Similarity threshold before choosing the extrapolate option.)
Similarity Tab
Similarity is a cross-correlation option. A trace around the last tracked point is compared to all the trace segments of the neighboring trace around the points that lie within the Search window (See figure below). The distance between the points is typically 1 ms. The number of comparisons is thus controlled by the search window, while the compare window controls the length of the trace segments. The measure of Similarity between the trace segments lies between 0 and 1. The tracker will chose the point that has the highest similarity drawing C). When the point with the highest similarity has a value below the defined threshold the tracker stops tracking.
Tracking with similarity is more accurate but takes more time to compute.
Properties Tab
Adjust the properties of the horizon i.e. colour, line style, seed shape, seed size etc.
For loading a horizon as color-blended in the scene, please look at dGB's How to...manuals
2. Tracking
After adjusting the parameters in the tracker setup (which can remain open during tracking), start picking seeds on a displayed inline/crossline. The corresponding tracking controls are described in the tracking toolbar Section.
Tracking in volume
Pick one or more seeds and display the tracking area 
. Resize the tracking area by dragging 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.
Picked seeds and resize the tracking area
Now, click the auto-track icon 
. After the input data is loaded inside the tracking area the horizon is tracked
Auto-track the horizon within the tracking area
After tracking a part of the 3D-horizon, move the tracking cube to next place (click the top of the tracking cube and drag). A small part of the horizon should be inside the new position of the tracking area.
Move the tracking cube to next location
When the Cube is at its desired position, click again.
Next location is auto-tracked
Line tracking and line manual:
Interpreting on lines creates a grid, which allows for greater user control. Line tracking and manually interpretation can be best done in the 2D viewer (Right click the seismic data in the tree and select Display in a 2D Viewer as > VD. Select the track/edit mode button and pick seeds, then move the line and repeat the process.
When (a part of) a horizon is tracked, the quality can best be checked by using the "display only at sections" option 
. The horizon is displayed as a line on inline and/or crossline and on the QC plane. Drag the QC plane over the tracked horizon to QC the tracking. SHIFT-click on the tracker plane to QC in another direction or select an other orientation in the tracker toolbar. If the horizon needs editing, there are several options:
Retrack All. When the horizon is tracked with the wrong settings, change the settings and click the 
icon. This will first remove all the autotracked parts of the horizon while keeping the seeds. Then, the horizon will be retracked with the new tracker settings in the tracking area.
Polygonal / Rectangular tool. 
Remove a part of a horizon with either at polygon or rectangular geometry and re-track it. First select the polygonal / rectangular selection tool from the tracker toolbar and select the area to remove. Click the remove icon to remove the part of the horizon inside the polygon 
. Please note that your polygon should not intersect itself, otherwise the remove icon will not work.
To fill the hole again:
- Autotrack will track the hole from the edges
- Pick a seed on the QC-plain and track from seeds only
- Interpolate
Algorithms:
Three options are possible: "Grid", "Filter", and "Snap to event".
For a 2D horizon tracking, the workflow is slightly different. The tracking wizard is similar to the 3D tracking wizard shown in the previous section. To start tracking a 2D horizon, click on 2D Horizon element of the tree and select New option from the sub-menu. This will launch the tracking Setup window. The tracking setup contains the tabs similar to that of a 3D horizon. See description of the tracking setup in the chapter: Interpret 3D horizon.
First, define the tracking mode. Go to the Event >> tab to define the tracking parameters (time gate, event, thresholds etc). In this tab, a user shall select a primary seismic line-set on which the horizon will have to be tracked according to the provided parameters. It may be noted that a horizon can be tracked on multi-linesets. In later case, a user will have to change the input lineset in the input data field of the Event tab.
After defining the event parameters, pick a seed on a displayed 2D line in a scene. If the corresponding settings for event are not appropriate, edit and change the parameters (e.g. time gate and threshold type) and press Retrack icon in the tracking toolbar. This will re-track the same horizon with new settings. After adjusting the appropriate settings for the horizon, the tracking wizard can be closed by pressing Dismiss button. This will also optionally save the tracking settings.
Note: In order to change the settings (for tracking), the wizard can be launched later on by right clicking on a horizon (in the tree) selecting Tracking > Change setup (click on the tracking setup icon). Also at the bottom of the window, the tracking toolbar appears that is routinely used to update the modes and switching between seeds (TAB-key).
For seed picking the options are:
* left mouse-click on plane = pick seed & local track
+ Ctrl-Shift = drop seed
* left mouse-click on seed = local retrack
+ Ctrl = remove seed & local retrack
+ Shift = remove seed & local erase
+ Ctrl-Shift = undrop seed
After finishing the horizon tracking on one 2D line, start tracking on next 2D line. The horizon can be displayed only at corresponding lines by right clicking on the horizon name in the tree, and selecting "Display only at sections". There are also several others options in the pop-up tree.
Tip: Always save the horizon routinely.