The MIT Geophysical Analysis Group (GAG), 1954 and Beyond
Sven Treitel
ABSTRACT
The MIT Geophysical Analysis Group (GAG) laid the groundwork for the socalled “digital revolution” in exploration seismology. An earlier article by Robinson
(2005) traces its history from its earliest days till 1954. Here the story continues with
GAG’s subsequent evolution until its end in 1957. But that was just the beginning: during
the sixties and seventies the new digital technology spread throughout the oil and service
industries worldwide, making it possible to develop progressively more sophisticated
seismic processing and imaging algorithms that permanently changed the landscape of
geophysical exploration.
INTRODUCTION
I began graduate school in MIT’s Department of Geology and Geophysics in
September 1953. I was offered a teaching assistantship as a lab TA in mineralogy for
that fall, and became an official member of GAG at the beginning of the 1954 spring
semester. It would be impressive to claim that I had already developed a passionate
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interest in geophysical time series analysis, yet nothing could be further from the truth: I
simply went where those assistantships were offered. Before becoming an official GAG
member, my advisor thought I should take an introductory time series course taught by
Enders Robinson. As luck would have it, that semester Enders happened to be registered
in the mineralogy lab course in which I was the TA. John Burg of subsequent Maximum
Entropy Spectrum renown was also in the class. Both made commendable efforts to
remain awake which were not always successful.
Once I joined GAG I was taken under the wing of Steve Simpson, who took over as
director after Enders received his PhD in the spring of 1954. Steve taught me how to
program the MIT Whirlwind Computer and my first assignment was to code a series of
algorithms developed by the MIT statistician J.G. Bryan to test the hypothesis that a time
series is stationary. At that time Whirlwind’s memory (or “magnetic core,” as it was then
called) consisted of 1,024 16 bit addressable memory cells, later expanded to a
munificent 4,096 such locations. Luckily the algebraic compiler system Enders mentions
in the preceding article was already in use. Even so, we were light years away from what
is today taken for granted. Because Whirlwind was used by the US Air Force for defense
work, military personnel naturally had prime-time access and civilian users were
relegated to the graveyard shift. In any event, one was fortunate to have one daily crack
at the machine. Days might go by before one discovered some trivial coding error. On the
other hand, the Barta Building housing Whirlwind was then one of MIT’s few airconditioned buildings; during Boston’s torrid summer months frustrations with a nonworking program could at least be endured in air conditioned comfort.
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The sponsor-supplied seismograms were of course paper records and required
digitization by hand, a chore as painful as it was boring. For some reason the sampling
interval was chosen to be exactly 2.5 ms; use of a ruler and a magnifying glass made this
task just slightly more bearable.
The GAG was housed in Room 20-E-222 of MIT’s legendary Building 20 (see the
figure), a “temporary” barracks-like structure erected in 1943 during World War II and
home to many people already well known, or soon to be. Norbert Wiener was an
occasional visitor and could be seen roaming through its dreary corridors. His former
student and long time collaborator Y.W. Lee had an office there, as did the
mathematician Manuel Cerrillo whose specialty was the method of saddle point
integration. Both the MIT Research Lab of Electronics and the MIT Acoustics Lab were
just a few steps away from us and I often chatted with faculty and grad students trying to
learn what they were up to. The building was in a perpetual state of disrepair with
frequent breakdowns. On one occasion Steve Simpson decided that our quarters required
a new coat of paint, and who better qualified to wield those brushes than us grad
students---cheap labor readily on hand.
(Figure 1)
And speaking of pecuniary factors, our finances were so strained that for a while we
students owned but a single dress shirt and tie between us, for use whenever someone had
to go for a job interview. Enders advises me the same frayed shirt and tie were still being
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used in the sixties during Project VELA-UNIFORM days (see below). During its
lifetime, GAG produced eleven reports assembled, again for the sake of frugality, by
students circling a large table as each report copy grew to its proper thickness.
During its lifetime, some 18 grad students were associated with the GAG. Of these,
however, only three continued with geophysical time series work after leaving MIT:
Mark Smith, Enders Robinson, and the present author. But Freeman Gilbert, who was
with us from 1953-1957 and who later made his reputation as the founding father of
geophysical inverse theory, cut his teeth with GAG. Mark Smith joined Geophysical
Service Inc. (GSI) in 1954 and helped launch GSI into the seismic digital era. Another
MIT grad student was Milo Backus. He played a key role in these efforts, having joined
GSI in 1956. While not a GAG member, he was a frequent “guest” in Building 20 and
knew what we were up to.
GAG’s TECHNICAL LEGACY
Today it is generally recognized that the GAG launched the digital revolution in
exploration seismology. In fact, its efforts were among the very earliest successful
applications of digital signal processing. Enders’ preceding article presents a fine picture
of its growing pains. GAG’s high-water mark was reached on July 12, 1954 with the
appearance of GAG Report No. 7 entitled “Predictive Decomposition of Time Series
with Applications to Seismic Exploration,” the full-length version of Enders’ PhD thesis
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submitted to the Department of Geology and Geophysics. To quote from the first
paragraph of his abstract:
“This thesis presents in an expository manner a treatment of the theory of discrete
stationary time series as developed by Cramér, Doob. Khintchine, Kolmogorov, Wiener,
Wold, Yule and others. The central theme deals with the development of the concept of
the predictive decomposition of stationary time series from the point of view of
applications. The Predictive Decomposition Theorem of Herman Wold states that a
stationary time series (with an absolutely continuous spectral distribution) is additively
composed of many overlapping wavelets, or pulses, which arrive as time progresses.
These wavelets all have the same unique stable shape or form; and the arrival times and
strengths of these wavelets are random and uncorrelated with each other.”
The above model of a seismic time series, or trace, has continued to be one of the
industry’s workhorses to this day. All that has changed is that rather than speak of
“predictive decomposition” we now speak of “predictive deconvolution.” The thesis laid
the groundwork for much of what was to come later; its six beautifully written and
succinct chapters are as relevant today as they were fifty years ago. The figure below is
taken from the June 1967 issue of GEOPHYSICS republication of Enders’ thesis. It
sketches the basics of what today is known as predictive deconvolution.
(Figure 2)
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During GAG’s three remaining years, significant research was undertaken in the
design of linear least squares single and multi-channel operators in the presence of noise.
The groundwork for the design of stable inverse filters was laid at that time, and the
relationships between a digital filter’s physical realizability and its stability were
clarified. Much attention was given to the properties, origin, and treatment of different
kinds of seismic noise. A clearer understanding of the fundamental differences between
random noise and coherent noise led to early designs of least-squares multiple
attenuation filters.
By early 1957, the project began losing steam. With twenty-twenty hindsight, too
much attention had been given to the more arcane statistical aspects of time series
analysis at the expense of further work with real exploration seismograms. At the same
time, only a few of the sponsoring companies had people on their staffs with a good grasp
of the technical issues GAG had begun to address, so the sorely needed guidance from
industry was not forthcoming. Nobody in GAG then had any significant industrial
experience. Several sponsors had already dropped out and by June of 1957 GAG was
shut down. It is worth mentioning that this lesson has not been lost on later consortia in
our industry: those which survive tend to be directed by individuals well aware of the
importance of doing innovative science along with “sticking to the real data” and
spinning off practical results.
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GAG ‘s LATER IMPACT: THE MIT VELA-UNIFORM PROJECT
To the best of my knowledge, some four years elapsed between GAG’s end and the
reawakening of interest in digital seismic processing. In 1961 Steve Simpson, then still a
junior faculty member at MIT, obtained US Air Force funding through Project VELAUNIFORM, which allowed him to begin a new research program based on the recently
established need to develop underground nuclear detection technology. While the
emphasis of the new project had now shifted to nuclear surveillance, the techniques
Simpson brought to bear were direct outgrowths of what had been accomplished during
GAG days. In fact, the new project was housed in the famous (infamous?) Room 20-E222 in Building 20, now even more decrepit than before. The project ran for four years,
and ended in 1965. Enders, who by that time was on the faculty at the University of
Wisconsin, was brought in as a consultant. Innovative work was done with single and
multi-channel stochastic systems, as well as with novel designs of seismic arrays for the
detection of nuclear explosions. From a computational viewpoint, two very significant
breakthroughs materialized during those years. Enders and Ralphe Wiggins extended the
single channel Wiener-Levinson algorithm to the multi-channel case, and Steve Simpson
developed a Wiener-Levinson type technique leading to a highly efficient way to
calculate optimal filters for successively lagged desired outputs---he called this the
“Sideways” Iteration.
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The list of grad students working in the VELA-UNIFORM project contains names
from a “Who’s Who in Geophysics:” Jon Claerbout, the founder of the Stanford
Exploration Project (SEP), Ralphe Wiggins, now in the stock market business in New
York, Carl Wunsch, now a professor of Oceanography at MIT, and Jim Galbraith, who
had a very distinguished career with Mobil.
GAG’s LATER IMPACT: THE COLLABORATION BETWEEN ENDERS
ROBINSON AND SVEN TREITEL
Upon receipt of my PhD from MIT in June 1958 I accepted a position in geophysical
exploration in Havana, Cuba with what today is ChevronTexaco. Given that GAG came
to an end in 1957 and a dissertation dealing with time series was not an option, and I
ended up working with wave propagation in lossy media. In early 1959 Fidel Castro
came to power and US oil companies were soon declared non-grata. A year later I joined
the Tulsa Research Center of the Pan American Petroleum Corp., later known as Amoco.
Dan Silverman was then the Director of Geophysical Research, having headed GAG’s
Advisory Committee throughout its lifetime. Following a futile struggle with the
electrical DC response of a layered medium, I reminded Dan of the impressive amount
of unfinished business GAG left behind, and its significant potential for an oil company.
Since few people in science can work in isolation (and I am not one of them), I convinced
Dan to invite Enders as a half-time consultant. At that time he was visiting at the
University of Uppsala in Sweden and e-mail was still decades away. Following an initial
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visit by Enders to Tulsa, we began a joint project with the goal of adapting former GAG
results to the needs of an oil company, as well as to develop FORTRAN based software
for easy implementation of the signal processing theory. In so doing we were able to
remedy a weakness in the GAG reports: writing was frequently at a level not
understandable to most people working as exploration geophysicists. The plan was to
write a series of internal reports at a sufficiently basic level as to serve as self-teaching
tools. In this quest we were largely successful. Within a few years several of these reports
were published in GEOPHYSICS and other signal-processing oriented journals; they
became quite popular with our readers. At the same time we developed accompanying
software, largely in the form of well documented FORTRAN subroutines. As time went
by, these programs became the basis for Amoco’s early seismic processing system; I am
told (Paul Gutowski, personal communication) that remnants of these prehistoric
subroutines can still be identified in BP’s present processing system.
This first stage of our collaboration lasted some three years (1962-1965). A second
period of joint research took us from 1974 through the mid-eighties. We communicated
entirely via air mailed notes. The week-long intervals between receipt of each other’s
hand-written notes were actually a blessing since such delays enabled us to think before
responding, a luxury not always available in this age of instant communication. I also
recall that at one time an accountant at our downtown office called to say that he saw no
reason why the company should spend airmail postage for our correspondence, instead he
proposed surface mail via the slow boat to Sweden. I convinced him that just this once he
should splurge.
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Throughout this period Dan Silverman and his successor Sam Martner gave us their
enthusiastic support. We need not contend with rigid deadlines, PERT charts or goals
chiseled in granite---our charge was to do long-range research in signal processing and
then to see where such efforts might take us. Such freedom to follow one’s own ideas
wherever they might lead is almost inconceivable in today’s industrial environment with
its obsessive quest of the bottom line.
Our first joint paper, “Principles of Digital Filtering”, was submitted to
GEOPHYSICS in 1963 and rejected. Editor F.A. van Melle (“Van”) felt that our
contribution was too simple for the expert and too complicated for the novice. We then
corresponded with Van and convinced him that our work did in fact satisfy both needs. It
was published in 1964 and won the SEG Best Paper Award the following year. This was
also the beginning of a long friendship with Van.
The much-touted digital revolution overtook exploration geophysics during the early
and mid sixties. Many major oil companies as well as some of the leading seismic
contractors of the day entered the field. While many details remained secret, key papers
were nevertheless published in GEOPHYSICS by John Sherwood and Alan Trorey
(Chevron), Bob Rice (Marathon), Manus Foster and Bob Watson (Mobil), and Norm
Neidell (Gulf R&D), just to mention a few. Among the contractors, GSI and its parent
company TI had built the TIAC (Texas Instruments Automatic Computer). This 15,000
transistor machine was the first dedicated seismic digital computer. The machine was
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developed in part with support from two majors (Mobil and Texaco). In return, the two
companies enjoyed a two year period during which they had exclusive use of the new
technology (Bob Graebner, personal communication). Mark Smith, a GAG graduate, put
together a signal processing R&D group at GSI, headed by yet another MIT grad, Milo
Backus. This group was charged with the development of TIAC software. Bob Graebner
(loc.cit.) feels that GAG’s influence on these endeavors must have been strong. At
roughly the same time Carl Savit at Western Geophysical started his organization on the
road to digital geophysics, as did Tury Taner and Fulton Koehler at Seiscom.
In 1964 Enders and I proposed to the SEG Executive Committee that an edited and
abridged version of the GAG reports be published in GEOPHYSICS, an idea that was
accepted. The Committee also appointed the late Edward A. (Ted) Flinn to act as a
Special Editor of the June 1967 issue, which was devoted entirely to a subset of GAG
writings that were then considered to remain of interest to the exploration community.
The contents included Enders’ PhD thesis in its entirety, along with two articles dealing
with S/N Ratio and moveout averaging experiments, and which can be considered to be
the precursors of what today is known under “seismic semblance.” The issue contained a
paper by J.G. Bryan entitled “Statistical test of the hypothesis that a time series is
stationary” as well as two appendices listing the GAG staff members as well as the titles
and tables of contents of the eleven reports issued during GAG’s lifetime.
By the late sixties Enders and I had published about a dozen papers both in
GEOPHYSICS as well as in other journals, which we decided to submit to the MIT Press
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in support of a proposal for a volume dealing with geophysical signal analysis. However,
the chairman of the MIT Geology and Geophysics Dept. advised MIT Press that he saw
no market for this material. At that point Bob Geyer, then with Seismograph Service
Corp. (SSC), convinced his advertising department to publish our collected papers as the
“Robinson-Treitel Reader.” This reprint collection went through several editions and was
distributed free of charge by SSC. Thousands of copies were distributed in this manner.
By the late seventies Prof. A.V. Oppenheim in MIT’s Electrical Engineering
Department invited us to submit a book proposal to a Prentice-Hall series he was editing.
The result was our joint volume, “Geophysical Signal Analysis”, published in 1980 and
reprinted by the SEG in 2000.
The early seventies also saw the beginning of what might well be called the “Golden
Era” of industry sponsored university consortia. One of the most successful, the
“Stanford Exploration Project” (SEP) was started by Prof. Jon Claerbout in 1973 . Prof.
Nafi Toksoz founded the “Earth Resources Laboratory” (ERL) at MIT in 1982. Both
groups continue to thrive to this day, as do a number of others in the US, Canada, and
Europe. It is surely fair to say that these later academic consortia followed in the
footsteps first taken by GAG’s
founders just about half a century ago.
CONCLUDING REMARKS
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The MIT GAG set the pace for a research philosophy in our industry whose essential
aspects have survived to this day, as evidenced by the ongoing existence of a significant
number of innovative academic consortia. The GAG gave us an early appreciation of
what can be accomplished by a combination of faculty members with vision and an
enthusiastic group of students willing and eager to take off in novel directions. At a time
when most major oil and service companies have abandoned their basic R&D efforts, we
must look toward the academic consortia to provide the technology needed to explore for
those increasingly hard-to-find new reservoirs.
REFERENCES
Graebner, Robert, 2005, personal communication
Gutowski, Paul, R. 2005, personal communication
Robinson, Enders A. and Treitel, S., 2000, Geophysical signal analysis, SEG reprint.
Robinson, Enders A., 2005, The MIT Geophysical Analysis Group (GAG) from
Inception to 1954, this issue.
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FIGURE CAPTIONS
Figure 1 - MIT Building 20 shortly before its demolition in 1998 (courtesy of MIT
Archives)
Figure 2 - Figure 13 of Robinson’s 1954 PhD Thesis
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