| OpendTect User Documentation version 4.2 |
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An existing horizon can be displayed in the scene by selecting Load option from the pop-up menu (see above). It will launch a horizon selector window from which multi horizons can be selected. New sub menu is used to start a new horizon interpretation. Display all or Sort menus are used if multi horizons are displayed in the scene. Display all menu contains three sub menus: only at sections, full or at sections and full. Only at sections results a horizon display (as a line) on the inline/crossline/timeslice. Full displays a 3D horizon grid. The Sort sub-menu is used to put horizons in a stratigraphic order (shallow to deep).
The popup menu from a displayed horizon has several options.
This allows choosing the data to display on the horizon from stored cubes, a calculated attribute from the current attribute set, or surface data that were included with the horizon already. For Surface data, a dialog will popup where you can select multiple data files. After loading you can browse through the data by pressing the 'Page Up' and 'Page Down' buttons on your keyboard. Note that the mouse pointer has to be in the scene.
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The load color blended sub-menu displays an RGBA (red-green-blue and alpha) blended horizon(s) in the scene. This is used to blend multi-attributes with similar spectral outputs. This is an interactive tool especially to color blend the iso-frequency grids (or attributes).
Color blended display:
RGBA* blending attribute display is used to create a normalized color-blended display that often show features with greater clarity and enhances a detail map view. Traditionally, it is used to blend the iso-frequency responses (Spectral Decomposition), but a user can blend three/four different attributes that define a spectrum that is comparable. For instance, spectral decomposition outputs the amplitude at discrete frequencies. So, it renders the same output (unit=amplitude). Depending upon a geological condition or the objective, FFT short window or CWT (continuous wavelet transform) can be chosen. Results are best displayed on time/horizon slices, volume.
A color blended map view (image on right) of the spectral decomposition (red-10hz, green-20Hz, blue-40hz). Compare the results with the coherency map (image on left). Note that the yellowish colored fault bounder region is thicker as compared to the surrounding regions. The faults throw (red-color) are also clearly observable. Semblance/similarity together with color blended spectral images can reveal better geological information.
Several processing algorithms may be applied to horizon and will be described here:
Adding picks to existing horizons by interpolation (grid)
Filtering interpretations
Snapping an existing interpretation to a given amplitude event.
The preferred order should be the following:
- Snap the interpretation to a given event (min/max/zero crossing).
- Grid the snapped grid since snapping can generate holes.
- Filter the output grid.
This utility is used to grid/interpolate a horizon having gaps/holes or to filter (average/median) a horizon grid. There are several gridding algorithms supported in OpendTect.
Gridding Parameters:
Geometry:
There are different types of geometries that are used to do interpolation. The Full survey is used to interpolate (in-/out-wards) the Horizon-Z values within the entire survey box. The Bounding Box defines the rectangle fitting the horizon geometry, which is generally smaller than the survey box. The Convex hull type of area fitting also restricts the gridding geometry within the horizon boundaries. To grid the gaps or holes in a horizons, the Only holes type of gridding geometry is used.
Inl/Crl step:
The default steps correspond to the sampling rate of the input horizon. The step can be decreased up to the survey sampling rate to get a higher resolution horizon.
Algorithm(s):
- Inverse distance algorithm uses an inverse distance method of interpolation. Inverse distance requires the search radius with optional parameters (step-size and number of steps). The step size of '1' means that one bin would be used in all directions to interpolate the horizon Z-values. Whereas the number of steps define the number of concentric circles for inverse distance computation. For these steps, the grid computation can be set to the corner points for the defined radius or not (default option).
- Triangulation algorithm is a fast gridding algorithm, which uses triangulation method of interpolation. The interpolation can also be defined by providing an optional maximum distance (radius) by setting interpolate option checked.
- Extension type of gridding uses a simple linear interpolation algorithm to extend the horizon Z-values outward using the number of steps (bins), which need to be defined in the following parameter field (Number of steps).
- Continuous Curvature (GMT) is a continuous curvature algorithm of interpolation, which is a part of the GMT Plugin of OpendTect. Please check the GMT website for further details. This algorithm only requires the tension parameter (ranges from 0-1), which controls the smoothing. The tension 0 gives minimum curvature type of surface interpolation, while the tension of 1 gives a harmonic surface.
- Nearest Neighbour (GMT) is also another interpolation algorithm coming from the GMT Plugin of OpendTect. This algorithm requires the search radius to be defined. It is mostly useful for a regularly spaced grid data. Please check the GMT website for further details.
The "filter" utility enables filtering of the horizon. There are two filtering types: Median and Average. The inline and crossline step-out should be defined. The larger the step-out, the smoother the result of the filter.
In case the horizon is not correctly snapped to a seismic event, this option can be used. The user should define the input data, the event type (peak or trough, zero-crossing etc.), the search gate relative to the original horizon, and whether the snapped horizon should be saved as new or overwrite the original horizon.
Add Contour Display: This option display the contour on the horizon. The contours properties can be manipulated by right-clicking on the Contour attribute in the horizon element. A user can adjust contour range/index, color and line thickness. It may be noted that the contour step (interval) is automatically calculated, which can be edited at any time.
Calculate isopach: This option will compute the time or depth difference between two horizons. The computed grid will be displayed as a new layer on this horizon and may be stored as a surface data. The output will always be in seconds, meters, or feet.
Write Flattened Cube: It creates the flattened seismic at specified time value of horizon. The output is stored as a new flattened cube. The user can choose the benefit of this option by flattening the cube at the horizon.
Create flattened scene: This option enables the user to create a second scene in which the data is displayed relative to the flattened horizon. This can be a very useful tool in specific situations. By flattening a horizon, the user gets an idea of the approximate section at the time of the deposition of this horizon. The tectonic history can be derived from the difference between the original section and the "restored" section. Another advantage of flattening the horizon is that it becomes easier to evaluate the depositional environments.
The Tracking pop-up menu is used to enable horizon tracking for the selected horizon. To enable horizon tracking for a horizon, a user may select Start Tracking option, which will pop-up the Tracking Setup dialog and will also activate the tracking toolbar. After this, the user may start horizon tracking for the selected horizon. Please read the Section 3.3 How to ... Intepret Horizons for further details.
Shift: The scrollbar allows the user to scroll the 3D horizon vertically. The shift range allows the user to define the upper and lower boundaries of the scrollbar range. The step size defines the distance between each possible horizon position. (e.g. A range of -100 to +100 with a step of 10 allows for the user to scroll through 20 possible horizon positions, centered about the original position.) Different attributes can be calculated for the horizon in this user defined shift range. The user can then use the scrollbar to move up and down and view the attribute as it would appear on that horizon at the various shift positions. This shifted horizon can be saved as surface data to be viewed later.
Calculate volume: It is used to calculate the volume between the two horizons. The volume is calculated within an existing polygon. Select the polygon and press Estimate Volume button to calculate the volume within the polygon. To read more about this, please go to the chapter-Pickset: Pop-up Menus
Properties: The Material window allows changing of the graphical settings like transparency, line style, and thickness.
Resolution: The resolution of a horizon can be changed for performance reasons. When using a low-end graphics card, performance during rotating and moving the scene may suffer. By showing the horizons in a lower resolution, scrolling and rotating becomes smoother. By default, the resolution is set to Automatic. This setting uses higher resolution in areas where a horizon has a complicated shape, and low resolution in relatively flat areas. Also, when rotating, the resolution will be set to low in order to enhance responsiveness of the rotation action. When released again, the resolution is set higher again. Attributes displayed will always have full color resolution, only the shape of the horizon surface is affected by this setting.
Quick UVQ: This option is related the Neural Network plugin license, if it is available. It is used to create a quick unsupervised facies map. For further information please refer to the plugin documentation.
Use single color: When this option is selected, the horizon is displayed in a single color, which can be chosen from a standard color selection window.
Display: The horizon can be displayed on the sections (inline/crossline/2Dline/timeslice) as a line, as a 3D surface or both.
Tracking: Horizons can be edited and tracked through the survey. The various tracking options are described in here.
Save: The save option gets highlighted when changes are made to the surface geometry. Save saves the new geometry of the horizon. If a horizon consists of patches, you can save a sub-selection of these patches.
Save as: Save a sub-area or the complete horizon using an other name.
Position: It is used to re-position (selected inline/crossline range) the displayed horizon. In the position dialog, set the ranges of the inline or crossline to sub-select the horizon display.
Lock: This will lock the selected object. It prevents accidental removing, moving, or displaying data on the object. After clicking unlock, all manipulations are possible again.
Remove: This option removes the horizon from the tree and the graphics area.