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2012 Distinguished Lecturer

Samuel Gray

CGGVeritas, Calgary, Canada

A brief history of depth…and time seismic imaging


Samuel Gray

Please tell us a little bit about yourself (e.g., your educational and work experience, why you became geophysicist, etc.).

I decided at a very early age that I would be a mathematician, so I majored in math in college and got my PhD in math from the University of Denver many, many years ago. Along the way, I had an epiphany: I found out that applied math is more "fun" than pure math, more of a social math, if you can call any kind of math fun.

My graduate work (applied math) was related to seismic imaging, so the jump into geophysics was very natural. As a geophysicist, I have found out that math is an important tool in solving geophysical problems, but it is not the only tool. (It helps to know some geology, for example.) I have also found out that geophysicists are amazingly opportunistic, willingly picking up whatever tools are needed to solve geophysical problems. I haven't exactly sat still mathematically, but I know some geophysicists with less formal mathematical training than mine who are far better mathematicians than I am. So I am kind of a mathematician, but kind of a geophysicist too.

Would you like to mention anything about your personal attributes that helped you achieve the professional status you enjoy today?

I had a lot of self-belief when I was younger, which was not really a bad thing: if you don't believe in yourself, how are you ever going to accomplish anything? So, even though I might not be as smart as I thought I was, I thought I could get things done. And I did get a few things done, but it was as much based on advice from two mentors as anything that came from myself.

My first mentor was Norm Bleistein, who was my PhD thesis advisor. Norm told me two things. First, careful mathematical analysis of a physical problem can lead to fast, elegant solutions. He said that in the context of one of his fields, asymptotic analysis. (Actually, as I developed geophysically, I turned that principle around, and used physics to guide the math - but it was the same general principle. This often works well for thinking about seismic waves, but sometimes it backfires.)

Second, you can only work so many productive hours in a day. Sometimes I would use this advice as an excuse to take the rest of the day off, but Norm really meant that you need to pace yourself over the course of a career, which is a lot of years.

My second mentor was Sven Treitel, who is a mentor for all petroleum geophysicists. Sven told me that personal life is more important than work. I needed to be told that! Even though I was pretty lazy and could be counted on not to work too many hours, Sven's words taught me to keep my priorities straight. Another thing Sven taught us is that, even if we are giants, we are standing on the shoulders of even bigger giants.

Why did you choose this lecture topic? Why is it important?

When I was asked to be DL, without hesitation I chose my topic to be the history of seismic imaging. My choice was "instinctive," based on no reflection at all. At first, I thought my job would not be too hard because I knew as much about the history as just about anybody. Then I came to realize that, while I might know as much as anybody, I still did not know very much. So I've done a lot of learning over the last several months, and I've had great discussions with some of the giants in our field. I'm especially thankful to Bee Bednar, Jon Claerbout, and John Sherwood. The more I learn about the history of imaging, the more I think I made a good choice of topic.

Imaging has been the culmination of seismic processing for so long that we tend to forget where it came from; we just hit the button, look at the image, and interpret the results. But seismic imaging is based on some very fundamental principles of classical physics that should be ingrained in our very souls as geophysicists. Anyone who interprets or processes seismic data really needs to understand the physical principles that are ultimately responsible for producing the image. That way, she can understand what is wrong with the image, and what is less wrong. (It's never right, only less wrong.) A lot of this comes through in studying the history, and that is what I hope to get across to the audience. Of course, there are a lot of details, and there have been a lot of stopgap solutions to problems of the day (caused by slow computers, etc.), but even these are interesting because they show how much amazing ingenuity people have applied to get an image that, while not perfect, is a little or a lot better than before.   

What do you hope people will have learned after they attend your lecture? How is it different from other lectures?

My lecture is different from most, because it is about the history of our field. I think geophysicists are not a particularly introspective group, given to thinking about the past very much. But we have a past, and it is exciting and illustrious; it is also deeply entwined with the history of oil and gas exploration. I hope people who see my lecture will come away with some appreciation of the technical accomplishments they are a part of, and of the technical challenges that lie ahead.

You have quite a busy year ahead. Do you enjoy traveling? Will it be difficult to balance the tour with your work?

I hope I enjoy traveling! Or, to put it differently, I enjoy traveling up to now. This will be different from the usual isolated business trip. But I will get to see some places I haven't seen, and revisit many places that I enjoy. I'm looking forward to the opportunity. Balancing the tour with work won't be a particular problem, since CGGVeritas has very generously allowed me to concentrate on the tour. But I hope to keep my hand in at work at least to some degree.

Would you share with us one or two of your most exciting successes?

Not long after I received my PhD, I was dissatisfied with my main result, which had to do with a toy (one-dimensional) version of seismic migration. In the approximation I was making, the norm of the error was too big. So I was fiddling around with some equations and tried a slightly different formulation. Still an approximation, but the norm of the error was much smaller! It was slightly embarrassing that my PhD thesis was suddenly obsolete, but exciting nonetheless. This gave me a lot of confidence.

Later, in the early days of depth migration, I was writing a Kirchhoff migration program. I was trying to figure out an accurate way to compute traveltimes to build diffraction curves that would not take longer than the Kirchhoff summation. Believe it or not, this was a challenging problem. After many sleepless nights, it suddenly occurred to me that I needed to turn the problem upside down from the standard way of looking at it. When I did this, the pieces all fell into place.

It turns out, neither of these "successes" was the breakthrough I thought they were at the time. Other people had figured them out already, or were about to. That's all right: they were reasonably clever, they were advances, and they were not in common use at the time.

How about a couple of disappointments?

I'm disappointed that I didn't work hard enough on some of the numerical aspects of wave-equation migration. I thought, "Why should I work hard on building a migration operator with one degree more dip when I can already do steep-dip Kirchhoff migration?" That attitude was shortsighted. Most of the exciting work on seismic imaging has been on wave-equation migration, which is very accurate beneath complex overburden. With my background in math, I've been able to understand this work, but I've had to watch from the sidelines. I should have jumped in and helped out when I first had the chance.

I'm also disappointed that I haven't learned more geophysics. I know a lot about seismic processing and a little about interpretation, but there is far more to geophysics than those subjects, complicated as they are.

What advice would you give to geophysics students and professionals just starting out in the industry?

I'm not really full of wise advice, but I think you need to follow your instincts, try to find good mentors, and work on easy, important problems. (And hope that your instincts are good!)