Special Report: POINT OF VIEW: Science is touchstone for new SEG president

Oct. 26, 2009
The new president of the Society of Exploration Geophysicists tends to address geophysical progress in terms that go beyond practicalities such as identifying targets for oil and gas drilling.

The new president of the Society of Exploration Geophysicists tends to address geophysical progress in terms that go beyond practicalities such as identifying targets for oil and gas drilling.

For Stephen J. Hill, the touchstone is science.

The perspective well suits a PhD physicist who sandwiched 25 years with a major oil company between stints as a college professor and now consults.

Here, for example, is how Hill assesses time-lapse seismic, also called 4D, in which sequential 3D surveys over a target reservoir monitor subsurface changes during production of oil and gas: "The introduction of time-lapse has been a huge plus for us as a science," Hill says, pointing out that time-lapse surveys embody "repeatable experiments," the essence of science.

Typically, improvement in the precision of those experiments has enhanced what scientists learn from them. Geophysicists at first used time-lapse surveys to monitor the movement of fluid boundaries through a reservoir as production progressed. They did so by tracking changes in acoustic impedance, a measure of the resistance of a subsurface layer to sound travel.

An early challenge was to ensure consistency of survey parameters, mainly source and receiver locations. Because the aim was to monitor changes in a specific set of survey measurements, other conditions had to be constant. The experiment, in other words, had to be repeatable, a requirement complicated by the marine environment in which most time-lapse surveys were and still are conducted.

Since the early days of time-lapse work, Hill points out, increasingly precise survey geometry and other factors of repeatability have given rise to new information about the subsurface. Geophysicists now assess changes not only in boundaries between reservoir fluids but also, because production changes reservoir volumes, movement in rocks surrounding the reservoir.

"So we've learned some things about rock physics as a result of time-lapse that make us better scientists," Hill says.

He uses the same framework to judge the application of geophysical methods to an emerging technological frontier: subsurface sequestration of carbon dioxide as a way to mitigate climate change.

"It's going to be interesting to see where that technology goes in the future," he says. "But it's going to be good for science because we're going to learn stuff."

Blending disciplines

In fact, the blending of geoscientific and engineering practices needed to develop CO2 sequestration technology fits another Hill observation: "Advances in science will occur at the interface of what we previously thought of as separate disciplines."

He offers examples from both science in general and geophysics in particular.

Many years ago nuclear physics joined astrophysics to help explain the distribution of elements in the universe. Similarly, nuclear physics expanded into medicine to produce advances in health care. And in modern geophysical work, solid-state detectors are replacing mechanical geophones in land surveys, the result of a combination of electrical engineering with physics.

Similar mergers of geophysics with other disciplines have fostered geophysical progress in the past: with electrical engineering in the 1960s and 1970s to bring geophysical methods into the digital world, for example; with physics to integrate the wave equation into seismic processing and interpretation; and with computer science to supply the computational power and visualization essential to modern geophysical work.

It will happen again, Hill says, adding a warning: "My crystal ball only works in a time reverse mode. Unfortunately, I do not have a good guess as to which outside expertise is to come in the next 10 years."

'Dream algorithms'

Hill's own specialties are seismic processing and imaging. In those areas, the burgeoning power of computers enables geophysicist to apply what Hill says were just "dream algorithms" years ago.

A resulting benefit is the ability to describe propagation of the full sound wave through the subsurface with a mathematical expression known as the wave equation. The ability greatly improves migration, a processing step that moves indications of reflection points to their proper positions on seismic displays.

Previously, migration relied to varying degrees on ray-tracing, a computationally less intense technique that Hill describes as "pretty good in a constant-velocity world." Because of subsurface irregularities, sound travel seldom follows constant-velocity assumptions. A more-complete solution to the wave equation than is available with ray-tracing more accurately represents wave behavior and thus greatly enhances geophysical interpretation.

"Advances in science will occur at the interface of what we previously thought of as separate disciplines."
—Stephen J. Hill, consultant

Wave-equation migration is especially important in areas of "strong velocity discontinuities," such as around subsurface bodies of salt, Hill notes.

Improvement of migration algorithms also has helped seismic processors suppress multiples, which appear in seismic displays as reflections but really represent energy reflected more than once before detection by receivers. Multiples can be especially troublesome in earth volumes of complex sound velocity, such as those intruded by salt.

Geophysicists have long known that the migration algorithms that assumed all energy followed a ray path rather than a spreading wave front provided "just an approximation of what nature does," Hill says. "We in fact had the algorithms 20 years ago, but we did not have the computer power to implement them."

Seismic attributes

Hill, who becomes SEG's president at the group's annual convention this week, points to interpretation of seismic attributes as an area of great promise but one "still very much in its infancy."

Attributes can be basic signal parameters such as frequency, amplitude, or phase—or any other measurable qualities within a volume of seismic data that enable interpreters to compare traces with one another, looking for patterns and discontinuities. Traces are the lines on seismic records on which wiggles indicate sound reflections.

Hill says he didn't fully appreciate attribute interpretation until he edited a book on the subject by Satinder Chopra of Arcis Corp., Calgary, and Kurt Marfurt of the University of Oklahoma. "What's the common problem that they're trying to address with this?" he asked himself, eventually deciding it was "the challenge that we have as human beings in visualizing things in 3D."

In the 3D world, he explains, "we look at things in 2D surfaces." Seismic attributes "collapse the information that's in a 3D slab [of the subsurface] down to just 2D data."

Hill considers seismic attributes "an extremely creative front of activity" and says, "I think that quest will continue full bore."

In line with his view about how science advances, he adds: "Seismic attribute analysis is a technology that really stands at the juncture of seismic processing and seismic interpretation."

Another area of promise, in Hill's view, is reservoir characterization through the acquisition and interpretation of shear-wave (S-wave) data. With S-waves, particle movement caused by a sound pulse is perpendicular to the direction of sound travel. Most seismic work is conducted with compressional waves (P-waves), in which particle movement is parallel with sound movement.

Career highlights

Employment

Stephen J. Hill worked on the faculty of the Michigan State University Astronomy Department during 1971-78, earning the MSU Teacher-Scholar Award for public service, teaching, and programming work in telescope automation. During 1978-2003 he worked for Conoco, where at different times he managed the seismic processing function, served as an interpreter of Oklahoma and Gulf of Mexico seismic data, consulted on technology at the company's international and North American headquarters, and taught internal seismic processing and seismic analysis courses. More recently he has been a consultant, based in Boulder, Colo., and professor at the Colorado School of Mines.

Education

Hill holds a bachelor's degree in physics from Iowa State University and earned a doctorate in physics and astrophysics through the Joint Institute for Laboratory Astrophysics at the University of Colorado.

Affiliations

He has chaired several SEG committees and served as Geophysical Developments Series Editor and member of the Seismic Interpretation Pitfalls Subcommittee. At the Geophysical Society of Tulsa he was president, past-president, editor, editor-elect, and education chair.

Because P-waves and S-waves behave differently in the subsurface, they provide "two independent observations which hopefully you can play off against one another," Hill says.

At the Colorado School of Mines, where he has been a professor, the independently sponsored, graduate-level Reservoir Characterization Project is using S-waves for time-lapse investigations.

By comparing P-waves with S-waves, Hill says, "hopefully, you can unravel the intrinsic stiffness of the rock vs. what's in the pores." While geophysicists have been experimenting with S-waves for a number of years, he adds, "It seems to be a technology that's always just around the corner."

A possible reason: "It's an extra cost item on the menue."

Need for geophysicists

Asked about the employment prospects for geophysicists, especially in view of diminished industry activity and steady gains in computer power, Hill offers a mixed answer.

Handling increasing amounts of data, seismic processing centers must build much larger velocity models than they did in the past. The task requires heavy interpretation, which requires people. But companies stressing high-cost wells, such as those in deep water, may need fewer prospects to absorb drilling budgets—and therefore fewer geophysicists to handle the interpretation, Hill says.

Offsetting that trend in marine work, however, is the heavy emphasis on gas drilling onshore, most of which is based on geophysical data.

"When we think of interpretation work and the SEG, one thinks first of the petroleum industry," Hill adds. "That's not the only place that's employing geoscientists and geophysicists."

Environmental work and archaeology also use geophysical methods, he says, and neither area is sensitive to the price of oil.

Science progresses in nonpetroleum realms, too—usually in unpredictable ways.

"Everything I've read about great physicists says that they were led by their intuition and not by their math," Hill says. "Of course, they had to have their math, too."

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