GIM Surfaces

Surface generation interface

The surface generation interface in opened up from GIM > GIM Data Generation > Surfaces > Based on 3D tracking. The interface has 4 different menus and works as a wizard with next and back buttons. The 4 menus are:

1. Input 
   This is where you decide which inline and crossline datasets to generate surfaces from. The settings can also be adjusted so that you only create surfaces for a subset of the cube. 
2. Surface generation options 
   Options for deleting surfaces smaller than a certain size and adjusting the memory handling during the surface generation process. 
3. Output 
   Lets you change the surface dataset name. 
4. Status 
   Start the generation process and get an overview of how the surface generation is progressing. 

Generate surfaces

Generating surfaces is a simple procedure. As input, select the cube and the inline and crossline datasets. In most cases we keep the inline and crossline interval to the same as the tracking datasets.

The surface generation options usually has the delete small surface option set to 100 points, but can be increased to limit the surface database if the cube is large. In the Time window size option you can set the amount of tracking data to be loaded into memory at one time. The larger the number, the faster the surface generation takes place. On regional dataset, it might be necessary to reduce this number from 200 to 100ms. On small prospect size areas, one can consider to increase it to 500ms.

Surface attributes

Each cell in each surface in the GIM surface database, can hold more than just the time value. It is possible to generate attributes on the GIM surfaces. After attributes have been generated for the GIM surfaces, it is possible to visualize surfaces with the selected attribute mapped onto them. To start the surface attribute generation, click GIM > GIM Data Generation > Seismic Attributes.

Amplitudes
When GIM tracks the seismic data, it captures both the time value and the amplitude of each peak or trough. For this reason, the amplitude surface attribute generation is fast. Normally, it only takes a few minutes to run through the thousands of surfaces in the GIM surface database to add the amplitude attribute to each grid cell. It is not necessary to set any parameters to run the GIM Amplitude attribute generation. Amplitude attributes can not be generated on surfaces that have been generated from external interpretation (there is no corresponding tracking data with amplitudes since these surfaces are generated from grids in the project manager).

RMS
To generate the RMS attribute for the surfaces in the GIM surface database, select Top and Bottom Time Window. The RMS value is then calculated between these values (zero is the surface grid cell value). It is recommended not to use zero as Top time window due to avoid too strong influence on the RMS from the amplitude of the surface (which often is very strong). Typically, RMS value should be calculated in a window between 10 and 50ms (below the surface) or -10 to -50 (above the surface). To run RMS on a surface database normally takes a few hours.

Facies
The Facies attribute in GIM does not look at the SEG-Y to derive the attribute values. The Facies attribute looks at a specified tracking dataset (inline or crossline) created by the user.

The user also specifies a window in which the facies analysis are to be performed. Note that negative numbers can be used to alternatively investigate the stratigraphy above each GIM surface. It is recommended to set the window of investigation from 0-50 or 0-100 ms (positive numbers). This will cause the facies algorithm to investigate the immediate stratigraphy just below each GIM surface looking at the environment of deposition prior to time of burial. The user also has to specify how much of the tracking data that is to be brought into memory at any one time. The higher this number is, the faster the algorithm works, but it depends on how much RAM that is available. It is recommended to use 200ms.

The facies filter counts the number of tracking lines below each GIM surface in the specified window. It is recommended that the user tracks the dataset one more time with the purpose of generating a dataset suitable for facies analysis. This tracking dataset is recommended generated with a d(dT) as low as 4 or 5 ms. Such a low d(dT) makes the tracker very sensitive, inducing breaks in the tracking at every irregularity. This tracking is not suited for surface generation, but for facies analysis. If the user in addition to making the tracker more sensitive, also uses the "Remove small" option, holes in the tracking data are introduced. Wherever the seismic data does not allow interpretation with longer lines than n (in the range 7-14 traces), holes are introduced. Typically, features as reefs, channels, salt, gas chimneys etc will tend to dissappear from the tracking data. When the facies attribute is generated with such a tracking dataset, all these features immediately underlaying a surface will be mapped.