Reflections on Geophysics in the Twentieth Century

The story of geophysics in Canada is a tale of many facets; the triumph of immense oil and gas discoveries and the tragedy of calamities. But most of all, it is a story of ingenuity, perseverance and inspiration.

Early Exploration

The earliest discoverers of oil in North America were Aboriginal Americans, who, from time immemorial, collected the black, viscous fluid from natural ground seeps and used it for medicinal purposes.

Likewise, when European settlers first began using oil for lamp fuel, they concentrated on drilling near oil seeps. In 1857, when J.M. Williams drilled a shallow well in Oil Springs, Ont., the wildcatter set off the world's first oil boom, resulting in production of around 2,000 barrels per month from the region around Sarnia,

By the turn-of-the-century, however, most oil seeps had been exploited. Having learned that the low-gravity substance tended to migrate toward the surface unless impeded by a physical barrier, geologists turned to mapping structural features.

One area that held great promise was Turner Valley, in the foothills of the Alberta Rockies. Many of the sediments that comprised the Rockies were porous enough to make excellent reservoirs and the thrust and anticlinal features readily apparent from mapping supplied the closure necessary for accumulation. In 1914, Dingman #1 well produced condensate so pure that it could be fed directly into the gas tank of a Model T Ford. The rolling countryside was soon dotted with rigs, and production soon peaked at 15,000 barrels per day.

At the advent of the first world war, however, demand began to exceed supply, especially as the automobile became popular. Oil companies needed a more sophisticated way to effectively and efficiently penetrate the ground and identify oil traps.

In 1907, building on extensive research in Europe and North America, German scientists E. Wiechert and K. Zoeppritz had outlined the basic principles of seismic wave transmission through the earth and derived equations to predict seismic refraction and reflection. Unfortunately, their theoretical work was well in advance of the capability of contemporary equipment to test their hypotheses. It would take a monumental disaster to spur scientists into making it happen.

Inspiration from Tragedy

On the night of April 14, 1912, the radio operator of the Cunard liner Carpathia received a distress call from the Titanic. The radio operator couldn't believe his ears; the Titanic, a British luxury liner, was thought to be unsinkable. Completed just that year, the 2,200-passenger vessel had been designed with a double-bottomed hull and 16 watertight compartments.

But, sink it did. Shortly after striking an iceberg 600 kilometres off the coast of Newfoundland, the Titanic disappeared beneath the waves, carrying over 1,500 passengers to a watery grave.

But their deaths were not to be in vain. The tragedy of the Titanic would inspire a young inventor to design a safety device that would eventually become the inspiration for the world's first working seismic system.

Reginald Aubrey Fessenden, born in Milton, Quebec, in 1866, was one of the world's most prolific inventors. As a young man, he worked with Thomas Edison, developing and perfecting a wide range of electrical and mechanical devices. He developed the idea of AM radio and made the world's first radio broadcast, in 1906, from Brant Rock, Mass.

The destruction of the Titanic inspired Fessenden, in 1914, to invent the Sound Navigation Ranging device, or sonar. Based on the reflection method, his "fathometer" emitted a sound wave that reflected off icebergs. A receiver picked up the reflected wave and determined the distance and bearing of the deadly obstructions. His fathometer was quickly adopted by civilian and military seamen, saving countless lives.

Fessenden also saw the potential of his device for exploration. In 1917, he filed an application for a patent on a 'method and apparatus for locating ore bodies'. Using a mechanical oscillator and microphones, the system measured deviations in reflection and refraction waves to identify anomalies.

Fessenden's patent drew a great deal of interest from researchers in the oil field. In 1921, J.C. Karcher, a student of Oklahoma University, used reflection techniques at a nearby site to map a shallow limestone as it dipped beneath an overlying shale bed. It was the first successful field test of Wiechert and Zoeppritz's basic principles of seismic wave transmission, and the site is marked by a monument erected 50 years later, in 1971.

The Birth of Modern Geophysics

At the time, in the early 1920s, oil companies were exploring for salt-dome plays in the Gulf-coast. The prolific traps could be worth as much as $1 million, but were difficult to pinpoint using geological prospecting methods.

Gravity exploration was one of the mainstays of the nascent industry. Although gravity at the Earth's surface is very nearly constant, it is slightly greater where dense rock formations lie close to the surface, and decreases over the tops of lower density bodies, such as salt domes.

During the first world war, Hungarian scientist Roland Von Eotvos refined the torsion balance instrument to the point where it could detect subtle variations in gravitational force. In 1924, American petroleum geologist Everette DeGolyer imported Eotvos' work and used a portable torsion-balance device to discover the Nash salt dome in Texas.

But gravity surveys were slow, and the data was often ambiguous. A more efficient method was needed, and oil companies began to look at the refraction method, which was best suited to salt domes (due to their high absorption, 15,000 ft ls velocity, and time leads of 300 milliseconds).

Refraction

In 1921, L. Mintrop, a German scientist, formed Seismos exploration and contracted with American companies to use refraction techniques to search for oil in the Gulf Coast. His system was crude. He used mechanical seismographs and measured the distance between shot and detector points by air waves and surveying-but effective; Seismos discovered the Orchard Dome in Texas, in 1924. A flurry of other salt dome discoveries quickly followed.

But prospectors were well aware that other oil traps existed, including anticlinal and synclinal structures, unconformities and dipping beds. Wiechert and Zoeppritz's work showed that, theoretically, reflection methods should pinpoint these finer features. All that was needed was the right equipment.

Reflection

The Geophysical Research Corporation was established in 1925 by DeGolyer, then vice president of Amerada. Well-acquainted with Karcher's reflection tests, DeGolyer hired the Oklahoma scientist specifically for the purpose of improving the acquisition capabilities of reflection surveying.

Most refraction recorders were designed to record first-break low frequencies, while filtering out higher frequencies. GRC reversed the order, so that low frequencies were filtered out and the higher frequencies were recorded.

Newly-designed electrical detectors and amplifiers also increased sensitivity, and the use of radio between shot point and detector point instantly established time of detonation.

GRC also improved the channel capacity of recording equipment through the use of the multiple element camera, or oscillograph, which allowed six recording channels. Detectors were placed at intervals of 200 feet in line with the shot, to a distance of around 1,000 feet.

The innovations allowed for the exploration of several million acres along the Gulf Coast, including swampy land unsuitable for other prospecting methods. By the beginning of the second world war, almost five billion barrels of oil had been discovered in the United States, thanks largely to the use of reflection seismic.

It wasn't until after the second world war, however, when reflection seismic work would set off an explosion in the Canadian oil patch.

Leduc

It was a chilly morning on March 8, 1948. The fields surrounding Leduc, Alberta, were covered with a thick layer of snow. The steel tower of General Petroleum's drill rig stood over Atlantic #3, a deep-hole, wild-cat well.

At the time, most of Canada's production came from the Turner Valley fields, southwest of Calgary. But Turner Valley's 15,000 bpd production was only a fraction of the country's 150,000 bpd needs.

International oil companies had drilled Canada's huge western sedimentary basin, but with few successes. Over 100 wildcat wells had turned up little more than natural gas and traces of oil.

Imperial and its parent corporation, Standard Oil (now Exxon), were contemplating downscaling their exploration programme. But legendary oil finder Ted Link recommended a test well be run on an anomaly that had been recorded on a single reconnaissance seismic line near Redwater, Alberta.

The well, Leduc #1, encountered oil-stained Devonian carbonate on February 13, 1947. But it wasn't until May 6 that a subsequent well, Leduc #3, hit pay-dirt, penetrating prolific Devonian reef.

One of the first oil companies to jump into the new play was the Atlantic Oil Company. The Alberta-based firm was owned and operated by the McMahon brothers, George and Frank. Both had years of experience in the oil business, and had incorporated the company in 1945 to explore for oil in the central Alberta regIon.

When word of the Leduc discovery reached the McMahon brothers, they quickly snapped up mineral rights near Leduc and hired a rig to test the prospects. Ace driller Cody Spencer managed the rig, and over the course of 1947, two wells, Atlantic #1 and #2, were completed and brought onto production without incident.

But Atlantic #3 proved different. Right from the beginning, it had trouble with circulation of drilling fluids, and when it cut in to the top of the Devonian reef's oil zone on that fateful day in March, 1948, the pressure of the reservoir forced oil up through the drill hole, bursting hundreds of feet into the air.

Within days, the entire lease was flooded with thousands of barrels of sticky crude. Several attempts were made to kill the well, but to no avail. On September 6, 1948, Atlantic #3 finally caught fire, spewing flames and smoke thousands of feet into the air. Photographs of the dense, black cloud appeared in newspapers around the world. It wasn't until over a month later that relief wells finally choked off the supply of oil, smothering the fire.

Ironically, because of the spectacular fire, word of the 200 million barrel discovery spread quickly around the world, and ignited a revolution in the Canadian oilpatch. Exploration crews began to flood into the province. A string of prolific reefs, including Redwater and Golden Spike, was eventually discovered, forming the legacy of Canada's oil production that continues to this day.

Improving the Signal

By the early 1950s, a host of modern seismic acquisition equipment was in place; multi-channel recorders, magnetic tape and compact geophones. With the development of high-quality acquisition equipment, the focus of the industry shifted to the problem of subsurface interference.

This interference, or noise, can be caused by multiple reflections, near surface weathering, ground water tables and other variables. The effect is to obscure structural and stratigraphic details of the subsurface. In order to improve the seismic signal, some method had to be devised to decrease noise.

One of the most promising methods was Common Depth Point stacking, or CDP. Although several companies were working independently on the technique, W.H. Mayne of Petty Geophysical is generally credited with its invention, in 1950.

With the CDP technique, geophones and shot holes are used in various combinations so that reflections are recorded from the same portion of the subsurface a number of times. Mayne theorized that, by changing the distance and angle between shot point and geophones, changes in the signal attributed to noise could be isolated, and the signal-to-noise ration improved.

Geophysicists quickly discovered that COP was especially useful in dealing with multiples. Experimentation proceeded throughout the decade, and by the mid 1960s, it was employed to great effect during the Rainbow-Zama Lake pinnacle reef play in Northern Alberta.

Deconvolution

Another signal-to-noise advance was under way in the 1950s, one that would elevate the value of seismic exploration by an order of magnitude in the following decade.

Enders Robinson was a mathematics graduate at the Massachusetts Institute of Technology, or MIT. In the summer of 1950, his thesis professor, G.P. Wadsworth, assigned the 20-year old the task of applying mathematician Norbert Wiener's ground-breaking theoretical work on predictive theory to seismic exploration.

Robinson knew nothing about oil exploration, but he was well-acquainted with Wiener's work. During the second world war, Wiener had adapted his predictive theory to the job of forecasting a moving target's future position by analysis of its observed positions in the immediate past. Could the same predictive theory be used to help interpret seismic signals?

Using an adaptation of Wiener's theories, fellow MIT mathematician Norman Levinson came up with an algorithm that worked on discrete data. Robinson then devised a numerical filter for the data.

There was one big drawback; no equipment existed to digitize the data. Using a T-square, ruler and pencil, Robinson painstakingly converted 32 seismic traces by hand. He then submitted the data to a battery of clerks who crunched the numbers by desk calculators.

The process took an entire summer to complete, but the results stunned Robinson. Not only did strong seismic reflections show through, but weaker ones, as well. It was proof positive that numerical filtering could separate data and noise as well as-or better-than electronic filtering.

Robinson had another lucky break-MIT had just opened up its Whirlwind digital computer to academic use. With the help of funding from the oil industry to form the Geophysical Analysis Group (GAG), Robinson developed the technology that made deconvolution possible on a commercial scale.

The Digital Revolution

In the early 1960s, digital field recording was jointly developed by Texas Instruments, Texaco and Mobil. By the end of the decade, manufacturers were offering field systems with up to 64 channels, and conversion to all-digital techniques was almost complete.

Advances in high-speed computers throughout the 1960s also allowed geophysical companies to apply a wide range of data processing, such as static correction and migration.

Migration is the repositioning of each reflecting event appropriate to its subsurface location. Formerly a tedious, hand-driven exercise, in 1971, J. Claerbout and A. Johnson first developed a finite-difference algorithm for migration based on the scalar wave equation that allowed large volumes of data to be correctly repositioned.

Fast Fourier Transformation, or FFT, greatly aided the transformation of seismic data from the time domain to the frequency domain. Invented by John Tukey, an American mathematician who had worked with Robinson 15 years earlier on deconvolution, FFT allowed for a wide application of filtering procedures.

As data processing improved resolution of the signal, processors were able to delineate bright spots. A bright spot is an increase in amplitude reflection due to a lateral change in reflectivity. When the phase material in the voids of a porous sand changes from water to gas, for instance, the reflectivity of the medium increases. Interpreters were able to use bright spots to map the extent of stratigraphic reservoirs, including the gasprone region of Northeast British Columbia.

By the mid-1970s, geophysical acquisition companies were offering up to 1,024 channels. The phenomenal recording power allowed for the introduction of the 3D survey. According to S.J. Allen, in 1973, GSI ran the world's first 3D field survey in New Mexico, using vibrators moving at right angles across the geophone lines to collect data in a true, 3D manner. The technique quickly caught on, and by the 1980s, Canada led the world in its appetite for 3D surveys, delineating a wide range of stratigraphic targets and pinnacle reefs.

Much of that 3D data was collected offshore, by vessels crisscrossing the Grand Banks of Newfoundland. The seismic work resulted in the mapping in huge offshore structures, including Hibernia. Major Canadian companies, including Petro-Canada, launched an expensive drilling program to prove up reserves.

The East Coast of Canada is one of the most difficult regions in the world to explore. Not only do winter storms whip up monster waves, but material calved off the glaciers of Greenland give the region the sobriquet of "iceberg alley" .

On the night of February 15, 1980, tragedy struck. The Ocean Ranger, a giant, semi-submersible drilling rig, was hit by 75-knot winds and 20-metre waves. The rig toppled, killing all 84 crew on board.

Recommendations from the resulting royal commission investigation led to the increased safety of offshore exploration and development equipment. When Hibernia came on stream in 1997, the production platform, standing on an 111-metre concrete pedestal, was designed to withstand a hit from a one-milliontonne iceberg. With production exceeding 150,000 bpd, the platform is slated to produce over 600 million barrels, forming a cornerstone of Canada's oil industry for the next 20 years.

In the meantime, the search for petroleum continues. The increase in 3D's accuracy, and the decrease in cost, has meant an explosion in data for the western Canada sedimentary basin, with the concomitant workload. Geophysical workstations are now the norm in the industry, allowing fault correlations, horizon tracking, map making and sophisticated processing to be accomplished in the interactive environment.

The century that is about to close marks not only the creation of modern geophysics, but the maturation of the discipline into the most important and vital component of petroleum exploration. Although many of the innovations have been spurred by profound tragedies, it is hoped that the ensuing 100 years will bring many more advances made through the triumphs of the science of geophysics.

Acknowledgements

The author gratefully acknowledges the work of S.J. Allen (Seismic Method, Geophysics, November, 1980), R.D. Clark (Enders Robinson, Geophysics, February, 1985), Aubrey Kerr (Atlantic 1948 no.3, self published, 1986), and R. Sheriff, Geophysical Exploration and Interpretation (University of Houston, 1978)

Atlantic 1948 no.3 and other books by historian Aubrey Kerr are available at DeMille Technical Books, 815 - 8th Ave S.W. Calgary, AB (403) 264-7411.