Petrel TIPS&TRICKS from SCM Knowledge Worth Sharing Smoothing 2D Grids in Petrel 2D Grids are the foundation of much of the work done in Petrel. They are used for display, volume calculations, input for building structural frameworks, as trends to control geocellular modeling, and much more. Often the form of grids, when initially built, contains roughness related to noisy data and that roughness needs to be smoothed before the grid can be used. In Petrel there are a variety of tools for smoothing 2D Grids. These tools are found in a number of places and a thorough knowledge of their location and use will ensure that you create the best possible grid in the most efficient manner. Smoothing is done by algorithms called filters applied at grid nodes. Filters work in many ways. One type of filter tosses out the value of the node it is centered on and uses the nearest circle of node values around the center node to calculate a new value for the one discarded. To make the filter more aggressive, additional circles of node values farther from its center are used in the calculation. The filter is moved over all nodes selected for smoothing and the same operation is performed at each. Petrel s smoothing algorithm probably does not work exactly as described here, but the analogy will help you understand how the algorithm approaches the smoothing process. Four methods are commonly used to apply the smoothing filter to 2D Grids in Petrel. They are: Smoothing during grid construction Operation smoothing Polygon constrained operation smoothing Interactive smoothing These four methods are described below. Smoothing During Grid Construction As mentioned above, data are typically the source of grid roughness. Seismic interpretation data will often contain a high frequency noise that is passed to the grid, given a sufficiently fine grid increment. This noise is usually removed with a general smoother applied uniformly to the entire grid. In most mapping systems this is done as an additional operation step after the grid is built. However, in Petrel, this uniform smoothing can be done as a post processing step (Post proc tab) during grid construction (Make/edit surface process). Implement the Post Process smoothing initially with one iteration, display the resulting grid in a 3D Window, and then rerun the Make/edit surface process using different numbers of iterations until the desired amount of smoothing is achieved. 1
Figure: Seismic data gridded without (left) and with (right) post process smoothing. Figure: Post Processing step used to smooth a grid. It is possible to apply a hard clip in the same Post proc tab of Make/edit surface. This might be used to clip a N:G surface to a maximum value of 1.0 and minimum value of 0.0. Smoothing can be applied before and/or after the hard clip. If applied after the hard clip, the smoothing effect is to round the sharp truncation edge. The beauty of this post truncation smoothing is that the smoothed values still honor the truncation and do not project above or below the limits. Operation Smoothing Operations based smoothing is the most commonly used method for smoothing a 2D Grid. The 2D Grid already exists, and is displayed in a 3D Window so the effect of smoothing can be seen. The operation is reached by opening the Settings dialog for the 2D Grid and going to either the More tab or the Operations tab. In the More tab, smoothing is at the top of the dialog. In the Operations tab, smoothing is found in the Surface operations folder. Both offer the same functionality. Two parameters control smoothing, Number of Iterations and Filter width. Number of Iterations is the equivalent of hitting the Run button as many times as the number of iterations. Because there is only one Undo for an operation, this multiple execution is very useful. You can try one iteration, undo it, try two iterations, undo it, and so on until the surface has an acceptable form. The Filter width parameter controls the size of the feature that gets smoothed out. The feature size can be thought of in grid increments with the filter centered on the grid node. A width of 1 will extend out one grid increment from the node, 2 will extend two grid increments, and 2
so on. Remember that the filter moves from node to node on the grid, therefore, when the filter width is large it will have considerable overlap on adjacent nodes. This means that large filter widths can be very aggressive in the amount of smoothing they do. Again, display the grid in a 3D Window and use the Undo button to test a variety of filter widths until you reach the desired affect. Figure: Operations and More tabs showing where smoothing is found. Figure: Example of smoothing a surface with operations. The original surface (top left), Smoothed with 1 iteration and 1 filter width (top right), smoothed with 3 iterations and 1 filter width (bottom left), smoothed with 1 iteration and 3 filter width (bottom right). 3
Polygon Constrained Operation Smoothing Polygon Constrained Operation Smoothing is a smoothing method that should be used a lot more than it is. The technique smoothes only inside or outside a set of polygons and leaves the rest of the surface unaltered. It is extremely useful for cleaning isolated problem areas in grids or cleaning the suture line related to merging two grids together. Its advantage over interactive smoothing is that the polygons provide documentation of where the smoothing was done and ensure that the smoothing is only done within those polygons and only done once. The steps used in this method are: 1. Use the Make/edit polygons process to create a set of polygons that encircle areas needing more aggressive smoothing than the rest of the grid. Figure: Polygons within which smoothing is to be done. 2. Display the surface and the constraint polygons in a 3D or 2D Window. 3. Go to the Smooth Operation dialog, hold your cursor over the set filter icon in the lower left corner and click on the line Set or change the filter in the box that appears. 4. When the Project Settings dialog appears, go to the Misc settings 2 tab and insert the Polygon by highlighting the polygon name and clicking the blue arrow under Global surface filter. Be sure to click on 4
the Apply inside or Apply outside radio button to tell the program where to smooth. Click OK to apply the parameters and dismiss the dialog. Figure: Operation showing the Set or change the filter option (left) and the Project dialog that appears when the line is clicked, showing the polygon inserted and the Apply the surfaces inside button pushed (right). 5. Click on the set filter icon so that it turns orange ( ). This will make the operation use the inserted polygons as constraints when it executes. 6. Set the parameters for the smoothing operation. 7. Click the Run button to perform the operation. Figure: Original grid with polygons circling areas ready for filtering (left), grid filtered only inside the polygons (right). 5
Interactive Smoothing Interactive smoothing is commonly used to clean small isolated problems in a grid. It is not normally used to smooth large areas of a grid, as that is more efficiently and accurately done with Operation Smoothing or Polygon Constrained Operation Smoothing. Interactive Smoothing is done in the Make/edit surface process. Making this process active makes the 2D Grid editing icons appear on the right side of the display area. Two icons are used to smooth, the Smooth area icon and the Peak remover icon. Of the two, the authors have found the Peak remover to be most useful. The steps used to make either of the methods work are the same: 1. Display the surface to be smoothed in a 3D Window. 2. Click on either the Smooth area or Peak remover icon. 3. Click on the Use the influence radius icon. 4. Set influence radius to adjust the size of the area to be smoothed. This is in terms of grid increments. 5. Smooth the grid by either: - Clicking on the spot to be smoothed, or - Clicking and dragging the mouse over the area of the grid that is to be smoothed. Figure: Unsmoothed grid (left), smoothed grid after using the Peak remover (right). Remember you have full Undo functionality. 6