In order to help with such tasks, a process called ``coherency evaluation'' has been developed Bahorich and Farmer (1995); Marfurt et al. (1998). The coherency evaluation calculates continuity of seismic events from a 3D seismic image cube and produces a so-called ``coherency cube'' that shows the distribution of the event continuity in a 3D volume. The output of coherency evaluation processing helps the interpreters to locate geologically meaningful discontuinities more easily. However, interpreting and mapping 3D discontinuity surfaces by hand is still a challenging job because of the difficulty in visualizing a 3D image and the complexity of surfaces in a 3D sense.
As a supplementary tool for mapping discontinuities, I introduce an automatic discontinuity extraction method. The proposed method starts with a coherency evaluation for a 3D seismic image and produces a discontinuity map that locates the event's discontinuities in the form of arbitarily shaped 3D surfaces. The output of the proposed method could be a good starting point for an interpreter to narrow down the various discontinuities into more meaningful geological features such as faults, unconformities, and buried channels.
In the following section I will shortly review the coherency evaluation method to clarify the meaning of a discontinuity in a seismic image. The subsequent two sections are devoted to explaining the following procedures. In each step, the algorithm is explained and also demonstrated with a real seismic image. The testing image used is from the Boonsville natural gas field located in north-central Texas and was acquired by the DOE and the Gas Research Institute as part of a secondary gas recovery technique development. According to Hardage Hardage (1996), the Boonsville seismic image is a full-fold time migrated section with in bin size and covers 5.5 km2 area. The data consists of 97 lines along the crossline and each line has 113 traces. Figure 1 shows three plane sections of the image volume which are orthogonal to each other.