Whither Seismic Imaging?

Abstract

Seismic imaging has evolved to the point where, with sufficient knowledge of the propagation velocities, we can now obtain accurate images, before stack in three dimensions, of almost arbitrarily complicated geologic structures, and we can do this at reasonable cost. That is not to say that we can close the book on seismic imaging, which is linked with velocity estimation, velocity model building, visualization, computer science, anisotropy, seismic acquisition, seismic modeling, and other subjects. Also, the future of imaging will depend heavily on the importance of imaging technology as perceived by seismic interpreters. The role of seismic data processing in the duties of many seismic interpreters appears to be less prominent today than it has been historically, as a result, the full power of seismic imaging is in some danger of not being fully realized in the years to come.

Recent advances in seismic imaging technology

The real breakthroughs in seismic imaging technology were the CDP stack, DMO, digital migration, depth migration, and prestack migration. Recent advances have evolved from those breakthroughs. As always, the major recent advance has been the growing availability of powerful imaging techniques previously reserved for the major oil companies. Some of these techniques have resulted from algorithmic advances; others have resulted from advances in computational capability; still others have resulted from innovations in acquiring seismic data.

In recent years, algorithmic advances have contributed to steep-dip imaging, efficient depth migration methods, 3-D depth migration, prestack depth migration, velocity analysis, and methods to deal with multiple energy, anisotropy, and amplitudes. Increased computational (and associated data storage) capabilities have led directly to the widespread availability of prestack migration, 3-D migration (poststack and prestack), and 3-D velocity model building and visualization. Novel seismic acquisition techniques such as vertical cables, cables deployed on the sea floor, and low-fold 3-D acquisition of land data, have led to novel data processing and imaging techniques. Increased computational power, by greatly increasing our ability to generate synthetic seismic data, has also given us the ability to measure the effects of new acquisition techniques on seismic imaging quality.

Technical challenges for seismic imaging: The Road Ahead

Many of the advances listed in the previous paragraph also provide technical challenges for the future. For example, our ability to construct 3-D velocity models is very limited, and needs improvement. We need to be able to build 3-D models as easily as we currently build 2-D models (which wasn't very easy ten years ago!). Similarly, our 3-D seismic data visualization capability is primitive. We are only beginning to be able to peel away the layers of the seismic onion. When we can build 3-D velocity models and view 3-D seismic data efficiently, we will begin to perform 3-D prestack depth migrations routinely. Also, we can presently account for, and even estimate, a limited amount of some simple forms of seismic anisotropy in imaging. These abilities need to be refilled, most likely by determining the most common forms of anisotropy present in rocks, and the most important anisotropic parameters to tweak. Our large-scale synthetic modeling exercises will tell us much about how we should acquire 3-D seismic data in the future to optimize cost, image quality, and velocity estimation, but the bulk of this work remains to be done.

Non-technical challenges for seismic imaging: Potholes in the Road

Seismic imaging technology continues to improve on several fronts, and promises further improvement. The technical challenges facing the developers of imaging technology are formidable, and promise exciting times. In my opinion, however, the technical challenge, namely, that of bringing the seismic interpreters closer to seismic data processing in general, and seismic imaging in particular, than they are today. How and why have interpreters lost their focus on processing? To oversimplify the answer: Interpreters have gotten older, they have become scarce, and they have more work to do than they used to. A decade ago, an interpreter might be my age, might have recent seismic data processing experience, and might be responsible for producing maps of drillable horizons from geophysical data. Today, an interpreter might be my age, might not have had any processing experience for ten years, and might be responsible for producing maps as well as pursuing land capture, 3-D survey design, and other duties. A decade ago, "technology" might preferably mean "seismic data processing technology" to an interpreter, today, "technology" might mean "interpretation workstation (as opposed to data processing) technology". So interpreters are not becoming more involved with seismic data processing as, say, prestack depth migration velocity analysis requires. Rather, they are becoming less involved, and are increasingly constrained to accept a product for interpretation without checking its quality. This phenomenon is not universal; a certain percentage of interpreters work closely with processing geophysicists on special projects involving 3D prestack depth migration, 3-D visualization of seismic data and velocity fields, the effects of anisotropy on seismic images, and so on. But this percentage, the real customer base for the imaging community, appears to be shrinking.

What will happen? At worst, the imagers will continue developing more specialized tools that interpreters find intimidating and/or irrelevant, and these tools will see limited use. As a result, seismic interpretations of increasingly subtle target structures and stratigraphy will fail to honor all the required subtleties, resulting in reduced drilling success. At best, oil company management will foresee the worst case, and will prevent its occurrence by encouraging a closer working relationship between seismic interpreters and seismic data processors.