# Yu Zhang

**Yu Zhang** is mathematician and geophysicist known for his work in the field of seismic migration.
His groundbreaking work has earned him both the SEG's J. Clarence Karcher Award and the Virgil Kauffman Gold Medal Awards.

## Contents

## Biography Citation for the Virgil Kauffman Gold Medal 2012

Yu Zhang began his career in the geophysical exploration industry in 1999. His curiosity piqued by a series of inspiring lectures given in a mathematical geophysics summer school at Stanford, Yu decided to leave his postdoctoral position at Cal Tech and the world of academic mathematics to dirty his hands with the complexities of real seismic data. He joined CGGVeritas as a member of the data-processing center in Houston. By his own admission, the first four months were difficult, but he soon moved to imaging research and began his long string of significant contributions.

He first worked on Kirchhoff migration when 3D prestack depth migration was becoming commercially viable. His major contributions here included true-amplitude time migration and an accurate antialiasing technique. While these were topics of widespread interest, Yu considered them from a new perspective and applied a sophisticated mathematical analysis. This became a hallmark of all of Yu’s subsequent work: originality combined with deep mathematics. In this case, it provided the entire community with a clear and consistent understanding of the problems.

His next area of interest was one-way wave-equation migration (OWEM). He extended true-amplitude theory from ray-based to wave-based extrapolations. Using these true-amplitude extrapolators, he then developed a one-way turning-wave extrapolation which could be used to image complex geology beyond the 90° limit implicit in standard OWEM. Other contributions included a stable x-k extrapolator, de-aliasing interpolation, delayed-shot migration, and harmonic-shot migration. While novel and theoretically appealing, these applications were soon commercially overshadowed by the advent of efficient reverse time migration (RTM). With coworkers at CGGVeritas, he developed an efficient production RTM code.

Then, extending earlier work, he developed a theory of true-amplitude RTM and applied it to angle-domain common-image gathers. He extended the work of other researchers in TTI (transverse tilted isotropy) migration to produce a stable, energy-conserving extrapolator. He recently has demonstrated further extension of these stable RTM extrapolators to the challenging case of orthorhombic anisotropy as well. Other works include wave-equation illumination, forward modeling, and some recent (quite difficult) work on wavefield attenuation.

This is an impressive list of accomplishments. Both of us, like probably many others working at competing contractor companies, were happy to hear that he had been promoted to a managerial position at CGGVeritas. Maybe managerial duties will slow him down enough to give the rest of us a chance to catch up? But we know he is indefatigable and continues to make time for his research interests. He also makes time to help his coworkers and colleagues and is generous with his ideas.

We hope the above elucidates why Yu Zhang is the well deserved recipient of this year’s Virgil Kauffman Gold Medal. As a multifaceted discipline, geophysics requires many talents. Yu exemplifies for all of us an outstanding drive and ability to apply powerful and innovative mathematics to the real-world complications of actual seismic data. SEG’s recognition of his talents may provide an appropriate counterexample to one of Sven Treitel’s dicta: “Keep a mathematician on tap but not on top.”

## Biography Citation for the J. Clarence Karcher Award 2004

A mathematician by training, Yu Zhang insists that his math be useful in the real world. He has followed that dictum in contributing to areas of our profession that are both very mathematical and very high-impact. These are amplitude-preserved (or “true-amplitude”) seismic migration, and depth migration by wavefield extrapolation (so-called “wave-equation” migration). Following the inspiration of the scientists who initially developed these theories, Yu has taken them further, made the math better, and helped to apply them to our present-day imaging problems. For the last few years, Yu has been at the forefront of efforts to bring higher levels of mathematical rigor and physical understanding to our seismic imaging methods. As a result, our ability to use seismic data to unravel complex geologic structure and predict what fluids lie trapped inside the rocks is greater than it ever has been.

Growing up in a research institute environment in Beijing, China, Yu naturally gravitated toward the sciences and obtained a bachelor’s degree in applied math from Peking University. Interestingly, Yu felt that his degree made him “useless to the world.” Not until he saw the connections between his undergraduate math and the signal processing techniques being applied in production work at the Chinese Bureau of Geophysical Prospecting did Yu see a future in applied math. He performed his graduate work at the Chinese Academy of Sciences under the supervision of Professor Guanquan Zhang, who was investigating ways to combine wave-equation migration with inverse theory to derive credible rock property estimates from the seismic image. Although well regarded in China, Zhang’s work was almost unknown in the West until Yu and mathematical luminary Norm Bleistein brought it to the attention of Western scientists. But we are getting ahead of our story. After receiving his PhD in 1996, Yu spent a postdoctoral period at Caltech. While there, he attended a Stanford University Summer School in mathematical geophysics, which was that year an intensive course in seismic imaging. That course rekindled Yu’s interest in geophysics, and he joined our industry shortly thereafter. Yu’s comment on the Summer School: “I should work harder if I knew I would work on it (i.e., imaging) after six months.” My comment (as an instructor): “If I had been aware of the quality of the students, I should work much harder.”

Yu joined Veritas in 1999 and immediately began to make a difference in our industry. He arrived just as 3D prestack depth migration was starting to be commonly applied, and he contributed greatly to our understanding and implementation of migration antialiasing and migrated amplitudes, mathematical topics that lead directly to a better understanding of rock properties. Yu brought a high level of sophistication to these problems, as well as a fresh viewpoint, and he managed to find a unified framework for both of them.

More recently, Yu has directed his efforts to wave-equa tion migration. He has improved our imaging methods and our understanding of how the various methods work. With colleagues at Veritas, he has identified and solved problems of migration antialiasing, higher-quality wavefield extrapolators, delayed-shot migration, and anisotropic migration.

But perhaps his greatest contribution so far has been his recent work with Zhang and Bleistein in the area of wave equation migrated amplitudes. While it has been possible for some time to analyze Kirchhoff-migrated amplitudes for reflection coefficients, very few people have succeeded in producing reliable amplitudes from wave-equation migration. Knowing how to do this, and knowing the limitations of the theory, will prove invaluable in the years to come.

Our profession is an ideal melting pot for professionals of many stripes-geologists, mathematicians, physicists, engineers, even geophysicists. It provides enormous opportunity for cross-fertilization of ideas and cross-disciplinary communication. It provides fertile ground for people such as Yu, who want to see their work make a difference in the real world.

## Best Paper in Geophysics 2005

Zhang won the SEG award for Best Paper in Geophysics in 2005 with Guanquan Zhang and
Norman Bleistein.^{[1]}