The task of the seismic analyst is twofold: to select the proper imaging steps for a given data set, and the proper imaging parameters to produce an accurate image of the subsurface. The ideal imaging system would allow the analyst to inspect every datum for the effects of parameter selections and to adjust those parameters for best results. In conventional practice, such a task would be extremely cumbersome, requiring the generation of hundreds of plots and dozens of passes through the data. ISIS, however, provides an efficient mechanism for accomplishing this task. The system keeps the entire data volume on-line and randomly accessible; thus, any gather may be assembled and displayed on the monitor very rapidly. A sequential series of gathers may be displayed at a rate of several per second, a feature we refer to as a movie. Movies provide an opportunity to inspect and edit the data and to adjust the imaging parameters on the data groupings that most naturally display the effects of those parameters. For example, a movie of shot gathers enables the analyst to quickly identify bad shots or to inspect the accuracy of the ground roll mutes. A movie of the midpoint gathers allows for the inspection of the normal moveout correction and the stretch mutes. A movie of receiver gathers permits the analyst to detect problematic surface conditions, and a movie of constant-offset gathers allows the analyst to study various offset-dependent characteristics. In this way, the analyst may inspect the entire data volume in various groupings in a few minutes and may stop at any point to interactively adjust the imaging parameters.
Some parameters have effects that manifest themselves more clearly in the composite image than they do in raw data gathers. For instance, the effects of the migration velocity are only apparent in the migrated image. Ideally, the analyst would adjust the imaging parameters and immediately see the effect on the image. We refer to this ability as interactive focusing, an analogy to a photographer focusing a camera while viewing an image through the viewfinder. A typical focusing technique is to alternate an image back and forth between under-focus and over-focus in diminishing steps until the point of optimal focus is reached. Any seismic analyst can easily recognize an over- or under-migrated image, but the ability to smoothly pass from one to the other allows for the fine-tuning of the velocity model. This process also allows the analyst to test the robustness of the reflectors and their orientation in the image. Other parameters, such as those used in deconvolution, for instance, may also be tuned interactively.
Another task of the analyst is to diagnose problems in the seismic image and take corrective action. An image may be contaminated by a variety of artifacts; it is important to eliminate them if possible, or identify them if not. To aid the analyst in this task, ISIS provides a feature called image deconstruction. Consider an analyst studying a stacked section. Image deconstruction allows the analyst to point the cursor to a feature on the image and call up the midpoint gather(s) that produced it. In the same way the analyst may display any of the shot or receiver gathers that provided traces to the midpoint gather(s). At this point, the analyst may use movies of the gathers to study the features of interest. By tying the image points back to the raw data through image deconstruction, the analyst has an additional tool for distinguishing true reflectors from processing artifacts.