OpendTect dGB Plugins User Documentation version 4.2
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3.4. Create HorizonCube (2D/3D)

A HorizonCube is created from the main Processing menu (Processing > HorizonCube > Create > 3D...). It can also be created for the individual 2D lines.

Processing Menu

The HorizonCube(2D/3D) is calculated between the given horizons (2D/3D). These horizons are selected (minimum of 2, top and bottom) by clicking the "Read horizons" button. When the horizons are read, they are automatically placed in stratigraphic order and the corresponding packages are defined. Each package is defined by two horizons (a top and base of the package). In case of 2D Horizon Cube, the input lineset/linename are selected to get the input geometrical information. This 2D lines selection is made by pressing the Select button next to the LineSet/LineName field in the HorizonCube Creator 2D window.

HorizonCube Creator-3D

HorizonCube Creator-2D

HorizonCube Modes

Three models are based on interpolation and one model uses a data driven approach:

Proportional: The proportional model can also be used for 3D Stratal Slicing. In settings, the user can specify the spacing between two HorizonCube horizons.
Parallel to upper: This model best depicts the lapout patterns, such as onlaps. In settings, the user can specify vertical spacing between HorizonCube horizons.
Parallel to lower: This model relates to upward truncation patterns. In settings, the user can specify vertical spacing between HorizonCube horizons.
Data driven: This model is driven from data based upon steering information. The HorizonCube will follow the dip and azimuth information read from a SteeringCube.

Fault: 3D faults or 2D faultsticksets are selected from the main HorizonCube creator dialog. The user can select more than one fault / faultstickset.

Area Subselection An area sub selection can be made to restrict the HorizonCube calculation. Please note that the HorizonCube can also be calculated within a polygonal area of interest. To know how to create a polygon in OpendTect, please refer to OpendTect Help documentation. However, this feature is not supported for a 2D HorizonCube.

Output HorizonCube: This field is used to give an output name for the HorizonCube.

Analyze: There are several checks being designed to quality control the failure of a batch program running to create a HorizonCube. The cause of failure could be a bad start position e.g. edge of survey, a trace defining a fault location, issues with the framework horizons etc. Therefore, it is always suggested to Analyze the HorizonCube processing before starting the actual processing (Go button).

Go: It starts the HorizonCube processing in a pop-up window. The batch processing window will provide you the instantaneous progress of HorizonCube calculation. Once the batch program prompts "Finished batch processing", the output is ready to be visualized. Press "Show options" to get the possibility for remote processing (after having pressed "Go").

3.4.1. Model-driven settings

The spacing (in ms / m /ft) is the only parameter required to define the settings for a model driven HorizonCube.
Its only utility is to set the sampling rate of the data in the Wheeler domain, and the number of Chronostratic events that can be exported as horizons.

The option "Apply to all sequences" takes effect only when pressing "Ok" and cannot be undone.

3.4.2. Data-driven settings

A data driven HorizonCube requires a SteeringCube to be selected, that will provide the sole input data for tracking. Thus, the quality of the HorizonCube depends on the quality of SteeringCube itself. Another key parameter to control the quality of HorizonCube is the start position that is defined in the Start at option in the settings dialog. The start position also defines indirectly the number of events to be initiated in the first iteration / pass, since the package thickness varies laterally.

Start at maximum thickness: Ideally the start position should be at a maximum thickness defined by the given framework horizons. Sometimes the trace defining the maximum thickness is either at the edge of a survey or in a poor seismic quality area. In such cases, the start position should not be set to maximum thickness. [Tip] The best practices are to create the isochron (or isopach) maps for the given horizons and find out the thicker areas with good quality of seismic data.
Start at center: Center refers to a trace that lies in the middle of a survey.
Start at inline center / maximum thickness: It defines a trace position that lies at a location of maximum thickness but it uses the central trace of that particular inline.

Advanced options for Continuous Events and Truncated Events are available and explained in the followings sub-sections.
The option "Apply to all sequences" takes effect only when pressing "Ok" and cannot be undone.

3.4.2.1. Advanced options

Continuous / Truncated Events: The continuous events are fully mapped events in 2D/3D that converge / diverge with each other but are not allowed to cross each other. The truncated events are diachronous in 2D/3D i.e. when two horizons come close to each other, the tracking is stopped and a new horizon is initiated afterwards. The continuous HorizonCube is good for GeoModel building or for low frequency model building for seismic inversion. On the other hand, truncated HorizonCube is useful for Sequence Stratigraphic (SSIS) interpretation e.g. wheeler transformation.

The following most important parameters apply to both HorizonCube types:

Spacing at start position: Used at the start position only. Vertical spacing between the seeds from which the HorizonCube events will be initiated. Implies a regular sampling of the events at the start position. The continuous HorizonCube proposes an alternative mode (see corresponding section below).
Smallest spacing (e.g. 4) will result into a dense HorizonCube and a largest spacing (e.g. 16) will result into a coarse HorizonCube.

Stepouts: The stepout (inline : crossline) parameters control the spatial quality of horizon tracking in 3D. It defines the number of z-values (of an event) to be used to forecast the z-value at a new trace position. By default the inline steps are set to 1 (i.e. 3 z-values on a crossline plane) and the crossline steps are set to 4 (i.e. 9 z-values on inline plane). The smaller stepout mean faster and detailed dip field tracking and the largest stepout are preferable for a regionally continuous event. The best practices are to test it with asymmetric parameters (e.g. 1:4 or 4:8 or 1:12). The symmetric steps (e.g. 4:4, 8:8 or even higher 12:12) are useful to average-out very small details / noisy trails while tracking. This parameter is a key to be tested through this utility prior to creating any HorizonCube. It is recommended to find optimal stepout values by varying it for an individual horizon.

3.4.2.2. Continuous Events

At the start position (1) numerous horizons are initiated at a user defined interval (2). Normally, the sample rate is used here in order to initiate a horizon at every sample. These horizons are tracked from the start position outward within the whole extend of the survey (3). When the horizons diverge large vertical spaces between the horizons are created, which are filled in iterative runs (4). To prevent very small vertical spaces to be filled (with horizons that are present in the whole survey) the spaces are defined by a vertical setting as well as a horizontal (5). A vertical space is filled when the vertical distance between the horizons exceeds a user specified amount over a lateral extend -the user defined number of traces-.

Start at: The (Start at) radio box (for Continuous HorizonCube) is used to define the trace position from which the horizons will be tracked. By default, Fixed spacing is used and the corresponding constant value is filled in the Spacing at start position field. The Min/Max relates to a given seismic cube (positive / negative amplitudes) i.e. the HorizonCube events will be initiated at a start position defined by the Min/Max amplitudes and this will not yield an evenly spaced HorizonCube at start position.

Fill spaces larger than (ms) or by (traces): This is used to specify the minimum allowed gap vertical (in ms / m / ft) or by distance (m / ft) to be filled in the subsequent defined iterations.

Max. Nr. of iterations Depending on the geologic thickness variations within a defined package, often the gaps are found after the first iteration of a HorizonCube. To fill the gaps in a HorizonCube the initial iteration value should be defined (either 1 or 2). The best practice is to create a HorizonCube with 1 iteration initially and then at later stages the gaps could be filled using the HorizonCube tools (Add more iterations). This is suggested as a quality control step because the HorizonCube calculation is slower for subsequent iterations. For instance, the HorizonCube with 1 iterations and smaller step outs could be generated in 1 or two hours. However, if the iterations are 2, the HorizonCube calculation time exponentially increases.

3.4.2.3. Truncated Events

Min or Max. spacing (ms / m / ft): This parameter is defined for a HorizonCube with truncated events. Converging events cause one of the two events to be stopped if the vertical distance becomes smaller than the minimum thickness. Diverging events cause one additional event to be added between the two diverging events when their vertical distance becomes larger than the maximum thickness.

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Create/Edit Horizons from a SteeringCube		Display Properties for HorizonCube