HEAT SEEKER
Seiichi Nagihara records the amount of heat escaping for the Earth's interior.
Written by Jeff Whitley
A calm day in the Gulf of Mexico, Seiichi Nagihara, now an assistant professor of geosciences at Texas Tech University, and his colleagues have spent a hard day aboard their scientific research vessel – steaming about 10 knots – to arrive at a heaving, dark blue spot of ocean, some 200 miles off the coast of Galveston, Texas. Out in the depths with 4,000 feet of seawater below, and a few feet of steel separating the researchers from the water, one gets a sense of his or her stature amidst the power of nature. A beautiful and mysterious place, but also a hostile one, the Gulf of Mexico is a seemingly peculiar destination for a group of geologists and geophysicists.
Seichi Nagihara, Ph.D., a geosciences scholar and leading contributor to deep water oil exploration, uses the marine heat flow probe to determine the amount of heat escaping from the earth's interior to the surface through halite and sedimentary rocks, located in the deep waters of the Gulf of Mexico.
When they arrive at 27 degrees north latitude and 95.5 degrees west longitude, Nagihara gives the signal to the captain to trim the motor, and the research team staggers into place to negotiate a gigantic steel thermometer off the side of the ship. Once overboard, the 3,000-pound, 15-foot, pipe-like instrument, which is tethered to the end of a steel cable, will make its controlled 45-miniute decent to the awaiting seabed. With 100 feet left to go, the crew releases the winch brake, and the heavy instrument plunges vertically into the mud of the sea floor.
“With this kind of heavy equipment and the tossing ocean, working on deck can be dangerous,” says Nagihara. “We get up at dawn and work all day gathering data. Then we retrieve all the data from the instrument, and after dinner, we process the information. We do it all again the next day.”
Taking subtle temperature measurements with this awkward, but highly sensitive, device that he designed himself, Nagihara can record temperature differentials as slight as one-thousandths of a degree Celsius. Recording the amount of heat escaping from the Earth’s interior to the surface through the tectonic plates, processing the data and making sense of that data is the reason why the 41-year-old scientist is considered a leading contributor to mid- and deep-water oil exploration.
“About 160 million years, during Jurassic time, the Gulf of Mexico just was being formed,” says Nagihara. “The Gulf was a big salt lake then, like the one in Utah, and water there was drying up. As the seawater evaporated, vast layers of salt were deposited in some places 2 miles thick. The salt beds began to be covered with younger sediments over time, and the salt became buried deeper underground. Increased pressure and warming mobilized the deposits of the salt beds, causing them to pierce through the overlying sediments and create hundreds of salt domes that bump up against the land surface along the Gulf coast and out beneath the sea floor. The mound at Spindletop, Texas, near Beaumont, is a good example of the salt domes and is where the first oil in Texas was discovered at the turn of the 20th century.”
The Gulf is a unique place, says Nagihara. “After the breakup of the super continent known as Pangaea, some 200 million years ago, this separation created massive gaps, including the Gulf. With the continued release of heat through the sea floor, the Gulf is getting deeper. The reason for this is that as the mantel releases heat and cools down, it goes through the process of thermal contractions. So, what the Gulf actually sits on – the plate – is shrinking. Rivers continually are dumping sediments into the Gulf, compressing existing layers”
The object of Nagihara’s seabed explorations is massive chunks of halite (NaCl), the same substance that table salt is made of. Salt domes, some as large as 20 miles across, are suspended several thousand feet in the crust of the planet, locked in the primordial sediments left from ages past. Geophysicists and petroleum engineers understand that salt domes often flag the presence of hydrocarbons – the oil and gas deposits forming in the deeply buried sediments. As the salt domes ooze toward the surface, oil will migrate along the faults and cracks created by movement of the salt. Oil companies and their research partners depend on geophysicists like Nagihara to determine the probability of oil and gas reserves associated with the salt domes. Nagihara, employing his own brand of heat flow detection, offers an alternative to the standard seismic method that is not very reliable in mapping and finding the deep root of salt domes.
A shift in oil and gas exploration has occurred over the past decade as the industry seeks new resources for economic and political reasons. Instead of focusing on the more accessible domestic onshore regions, the industry is spending increasing time and dollars exploring the offshore continental shelves and slopes. According to a 2003 article published in the journal Nature, hydrocarbon exploration of deep-water continental margins (500 to 3,500 meters below sea level) is risky and costly in these environmentally hostile areas. However, the prize potentially is enormous. The key to success is a quantitative understanding of the structure and evolution of the thinned crust and lithosphere that underlie these margins.
Diagrammatic development of salt stock. The primary peripheral sink increases in thickness away from the dome, where as the secondary peripheral sink increases in thickness toward the dome.
Although Nagihara no longer goes to sea himself, oil exploration companies, such as TDI-Brooks International Inc., continue to utilize his experience in interpreting the data they collect. Bernie B. Bernard, Ph.D., vice president and laboratory director at TDI-Brooks, a Texas corporation headquartered in College Station, Texas, enlisted Nagihara to help publish data that he was gathering in the Gulf. The company, says Bernard, looks for oil and gas off shore and sells information of its whereabouts to major oil companies.
“Our measurements of the seabed are likened to meteorology, only upside down,” Bernard says. “Atmospheric scientists can predict cold fronts based on models created from data, such as temperatures and pressures. The same approach is used to examine the Earth’s crust. Geoscientists make mathematical models of the subsurface to predict what has happened in the past. Oil companies seek models of the Earth’s crust to determine if conditions are favorable for oil to be ‘cooked up’ in a locale,” explains Bernard.
“Seiichi Nagihara is a modeler,” Bernard says. “His careful data analyses attempt to see if potential hydrocarbons have been cooked hot enough or if they’ve been overcooked, so to speak. From our perspective, having the correct information of how hot the ‘oven’ has been helps us agree on the possibility of oil formation, providing us with that one piece of the puzzle – knowing where to drill.”
In 1987, John Sclater, Ph.D., professor of geophysics at Scripps Institution of Oceanography at the University of California, San Diego, and Nagihara’s graduate supervisor at the University of Texas at Austin, extended a research assistantship to Nagihara.
Sclater was a colleague of the renowned geologist Harry Hess (1906-1969) who set the stage for the emerging plate tectonics theory, which describes the surface of the Earth as being divided into huge plates whose movements carry the continents on a slow drift around the globe. Sclater is known for proving the relationship between the age and depth of the ocean floor, which led to his concepts of the stretching continental crust creating continental shelves and basins.
As a former Massachusetts Institute of Technology Professor, Sclater arrived in Texas about the time the oil industry was starting to move to offshore deepwater. When Nagihara joined his research team, the timing was excellent. “It was a good fit; deepwater oil exploration was beginning, and our first problem was the depth of the Gulf,” says Sclater. “It’s one of the deepest ocean basins in the world, and we didn’t understand why. When Seiichi arrived, we already were setting up heat flow measurements in the Gulf, and that’s when we began to see a large thermal anomaly associated with salt domes.”
Nagihara was more than ready for the chance to apply his knowledge to Sclater’s initial interactions with the puzzling salt domes. “I wrote a handful of well-known professors, and I was jumping up and down when I got the offer to join Dr. Sclater in Austin,” Nagihara says. “I didn’t have much money, but I bought a one-way airplane ticket to the United States and planned to pursue this topic related to plate tectonics. Dr. Sclater suggested that I go into a research topic that is more related to oil and gas exploration. Here was a chance to use my background in heat transfer to solve this problem in the Gulf of Mexico.”
Nagihara, grew up in suburban Tokyo, and like many Japanese geologists, was enthralled with the endlessly changing physical Earth. “In Japan, there are earthquakes and volcanoes – you grow up with them. So one has a natural tendency to be interested in geology,” he says. “In the field of geophysics, we solve these geological problems as a physics problem. We try to apply knowledge of physics to understand what really happens in the deep Earth. Geosciences basically borrow knowledge from physics, chemistry, biology and other sciences. The field is enjoyable to me because it helps to satisfy the many interests I have.”
Before coming to the United States, Nagihara completed his bachelor’s and master’s degrees in geophysics at Chiba University, one of the largest state universities near Tokyo. A book by a professor at the University of Tokyo fueled his interest in the geosciences, providing concepts for the natural occurrences he observed around him while growing up – like the fact that the Earth becomes elongated by the gravity force of the moon, which also causes the tidal changes of oceans. The text also first introduced Nagihara to the concept of plate tectonics.
The topic of heat transfer in the Earth’s tectonic plates became Nagihara’s primary interest by the time he began his master’s thesis. He realized that the leading professors in the field were all working in the United States.
“Essentially, we know that the inside of Earth is very hot, and the surface is not,” he explains. “The heat always is coming out of Earth to the surface. Along the margins of the Pacific Ocean, where Japan is, a band of so-called subduction zones exist where the sea floor subsides under the continents. For my master’s thesis research, I picked a place called Yap Island, a major battlefield during World War II. I was given the opportunity to go on a research cruise with other Japanese scientists to measure the heat coming out of the sea floor. We basically would deploy a lot of instruments onto the sea floor – about 18,000 feet deep – and make all sorts of measurements and then try to make sense of the data.”
By the time Nagihara began his thesis work in the mid 1980s, many geophysicists already had documented various heat distribution patterns in tectonic plates that form near mid-ocean ridges, which are massive underwater mountain ranges. The hot mantle wells up from the deep Earth, spills out to the sea floor, and then is cooled by seawater to form plates. As the plates migrate away from the mantle source, they become cooler over time.
“Of course we are talking about a time scale of 100 million years, but this is how the Earth gradually loses its heat from the inside to the surface,” he says. “At the other end of the ocean, subduction zones exist, where the plates usually become the coldest. So, by mapping how the amount of heat changes relative to the distance from the ridges, we can see the mechanics of mantle convection inside the Earth.” Under Sclater’s tutelage, Nagihara continued his research in the Gulf of Mexico, mapping heat transport through the salt domes buried under the sea floor. Sailing out of Galveston on a Texas A&M research vessel, Nagihara worked alongside members of the University of Texas Institute for Geophysics.
Bernard compares the formation of salt domes to a giant underground lava lamp, in which a heater is at the bottom and cold water is at the top. The salt, like the blob in the lamp, is highly conductive and brings the heat with it, as it rises.
“In some cases, the plume of salt breaks off from the base so that heat cannot transfer to the top of the dome,” he says. “So, measuring the temperatures at the sea floor above the dome helps us to predict the dynamics of the dome, including for example, whether the dome is connected at the bottom. Simply no techniques out there allow geophysicists to see around and under the dome. Seismic methods will not allow us to see deep enough,” Bernard says.
What Nagihara’s models allow, says Bernard, is a way to “backwards predict” using measurements at the surface. In other words, by making temperature measurements at the surface, scientists understand through Nagihara’s models, what produces these data.
Seiichi Nagihara has used GIS technology (Geographic Information Systems) to render a map of the Gulf of Mexico with data from the Geophysical Database of the national Oceanographic and Atmospheric Administration.
Nagihara says the process can be compared to burying a lump of hot charcoal in the ground and asking someone to locate the source of heat. “In our case, the salt dome is our piece of charcoal. We might feel our way around the surface and eventually find where the salt dome is, but we would not know how deep, how big and what shape it is,” says Nagihara. “The highly sophisticated computation algorithm that we use to interpret measurements of geothermal heat coming out of the surface of the ground is an effective way we know to find our charcoal.”
According to Nagihara, the problem takes levels of computation far above standard formulas to come up with the size and shape of a malleable salt formation in the Earth’s crust. Geographic Information Systems, the software for managing and analyzing geologic and geographic data, is Nagihara’s true medium of sorts. Sclater and other scientists he collaborates with point to his adeptness with the system. “Of course you can’t find this type of thing in a book,” says Nagihara. “I had to write my own computer program to accomplish this work. Basically, I use a combination of heat conduction theory and probability theory to come up with these models.”
Michael Smith, Ph.D., a geologist with the Minerals Management Service (MMS), a federal agency based in Herndon, Virginia, which oversees all of the offshore drilling activities by oil companies on the United States outer continental shelf (OCS) beyond the three-mile limit of state waters, is another collaborative partner. MMS co-sponsored a research project conducted by Nagihara and his research assistant, Mike Jones, to construct regional models for the sedimentary thermal history and hydrocarbon maturation in the deep water Gulf of Mexico.
“Our technical assistance research budget allows us to engage a few people like Nagihara and Jones for federal research partnerships and specific studies in deep water areas in the Gulf. We are a small agency, but we still recruit for new geosciences and engineering employees at about a half dozen core schools. We are happy that Texas Tech is one of them.”
In 2000, Smith notes, the amount of oil produced in deep water surpassed the amount being produced from the shelf areas, marking a strong shift toward the exploration and development of deep-water continental margins that are characterized by water depths of 1,000 to 10,000 feet or deeper. In the shelf areas with less than 650 feet of water, MMS recently added incentives of royalty suspension for as much as 35 billion cubic feet of gas to encourage deep gas exploration and development at depths greater than 15,000 feet beneath the sea floor. According to Bernard, a relatively small number of oil companies operate at the level of sophistication needed to conduct deep water oil and gas exploration. A growing number of smaller players are getting involved, but what is becoming clear is the fact that every major player is being forced to advance to the next level, technologically speaking. “The bottom line is, each country leases ocean acreage – blocks of sea floor for sale just like on land – to the highest bidder,” he says. “The more you know about what’s under your piece of ocean, the more capable you are of bidding the right price. It’s all about doing your homework. Drilling in these places is very expensive; the stakes couldn’t be higher, and Dr. Nagihara’s thermal models serve as a valuable part of the risk-reward equation,” Bernard says.
Multi outrigger bow heat flow probe.
Nagihara tends not to associate himself with predictions of world energy depletion, but those who study trends in oil production, supply and demand generally agree that world oil reserves eventually will tap out. Estimates of just when this will occur vary greatly. One U.S. Geological Survey estimate has total oil reserves peaking in the year 2036. Still, others believe that oil and gas will be the world’s primary energy source for the foreseeable future.
As a scientist, Nagihara’s vantage point is pragmatic in light of this bleak outlook in the globe’s energy supply. The problem and the solution depend, he says, on whether one has the long or the short view in mind. “In the short-term, we have a need to use every technological means available to discover safe and effective ways of exploiting the available natural resources. We appear to be consuming oil much faster than the natural processes can create it,” he says. “But our methods are helping for the time being.”
When at sea, humans are utterly vulnerable. But the enigmatic ocean continues to beckon those possessed with a spirit of adventure. Standing on the deck of a ship or offshore platform in the great aquatic rolling hills and plunging valleys of the ocean, one senses the greatness of the planet. The ocean form might shroud a landform that humankind may never see with the naked eye. One can only imagine a time and place more than 150 million years removed –– an age when dinosaurs moved about and a place where a shallow sea left the Louann Salt. After ages, the since-buried Louann has crept upward toward the ancient sea floor to unravel part of her mystery, and a few gifted geological explorers are standing at the gangway of knowledge and enterprise to record the moment for posterity.
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