Why Drill Down So Deep?
By Awake! correspondent in Germany
DID you know that just over five and a half miles [9 km] from your home, the temperature is a sizzling 570 degrees Fahrenheit [300° C.]? But don’t worry, the heat is far below you, at a depth of 30,000 feet [9,000 m]! And to make sure that your feet don’t get scorched, you are safeguarded by a protective shield called the earth’s crust.
This crust is the center of attraction at the Continental Deep Drilling Program, located near Windischeschenbach, a German village not far from the border of Czechia. The purpose of this program was to drill a hole over six miles [10 km] deep to investigate that protective shield. However, the drilling had to stop at 5.6 miles [9 km] because of the heat, as we shall see. But why go to the trouble of digging such a deep hole?
Deep drilling is not new. The Chinese reportedly bored to a depth of over 500 yards [500 m] in 600 B.C.E. in search of brine. Since the Industrial Revolution, the voracious appetite for raw materials in the West has meant that drilling technology has progressed rapidly. Recently, however, drilling has been motivated by something more pressing than commercial interest: Human life is at stake. How so? And how can drilling into the earth be of help?
Why Is Deep Drilling Important?
First, some of earth’s mineral resources are being consumed so quickly that they may be exhausted. Can these same minerals be found farther down in the earth, perhaps still in their developing stages? That is a question that deep drilling may answer.
Second, as the world’s population increases, earthquakes take an ever-rising toll. About half the world’s citizens live in areas threatened by earthquakes. That includes residents of more than a third of the largest cities on earth. What do earthquakes have to do with drilling? “Study of the lithosphere [the earth’s outer shell] ought to make forecasting more accurate,” reports the booklet Das Loch (The Hole). Yes, man has every reason to try to learn earth’s secrets.
The cost of deep drilling is high, though. The price tag for the German project is $350 million. Aren’t there other ways of unearthing our planet’s secrets? Yes and no. Science deduces much about the makeup of the earth by using instruments located on the surface. But a superdeep borehole is the only way to verify such deductions and to examine rocks that have remained until now under extremes of pressure and temperature. You could say that deep drilling tries to get to the bottom of things.
So much for drilling generally. Why don’t we visit the site at Windischeschenbach? Afraid of scientific terminology? No need. The guide, a geologist, has promised to keep everything simple.
An Amazing Drilling Rig
We are amazed to see the drilling rig towering over the borehole to the height of a 20-story building. The rig is one of the features that give this project special appeal even to the nonexpert. And there is more to come.
Take, for instance, the location. When planning the superdeep hole, scientists didn’t choose to drill just anywhere. The newspaper Die Zeit wrote about the project: “If you want to find out how earthquakes occur, concentrate on those places where [underground] plates bump together or drift apart.” Windischeschenbach is such a place, as it lies directly above the borderline of two subterranean continental plates or slow-moving sections of the earth’s crust.
It is thought that in the past, these two plates came together with such force that they thrust portions of the lower crust upward toward the surface, to within reach of modern technology. Drilling through different formations of rock produces what our guide calls a geological shish kebab. How deep is the hole?
On October 12, 1994, a flashing sign on the information building heralded the maximum depth: “9,101 meters” (29,859 feet). How deep is that? Well, if there were an elevator to transport us to the bottom, the descent would take almost one and a half hours. It would, however, be a journey we would never forget. Why? Because as we went down, we would feel the temperature rising between 14 and 17 degrees Fahrenheit every thousand feet [25-30° C. per 1,000 m]. So at the present depth, we would encounter a torrid 570 degrees Fahrenheit [300° C.]. How glad we are that our visit does not include an excursion to the bottom! But the question of temperature brings us to another interesting aspect of this project.
At about 5.6 miles [9,000 m], the borehole crosses the critical 570-degree-Fahrenheit [300° C.] threshold. Why critical? Because when rocks are subjected to such heat and pressure, they change from being rigid to being flexible. This change has never been examined in a natural environment.
Also noteworthy is the system that navigates the drill. To scale the operation down, picture yourself holding the end of a rod about a hundred yards [100 m] long and 0.0788 inch [2 mm] in diameter, the width of a thick sewing needle. Now imagine trying to steer a miniature drill at the other end. In no time at all, you would have a crooked hole, broken pieces, or both.
Equipment was developed to keep the hole vertical by automatically correcting the course taken by the drill. This navigation system proved so successful that at over 6,000 yards [6,000 m], the bottom of the hole deviates only 26 feet [8 m] from perpendicular. Quite an achievement, boring what our guide tells us is “presumably the straightest hole in the world”!
A Round-Trip to Change the Drill Bit
The motor that drives the drill is located “down hole,” not at the surface. Consequently, the whole length of drill pipe does not rotate when drilling. Nevertheless, drilling at such depths is a tiresome process. Toiling downward at one or two yards [meters] per hour, each bit labors through about 150 feet [50 m] of rock before being replaced. As our guide leads us closer to the rig, we observe the drill pipe being hoisted out of the hole for just that purpose, to change the bit.
Huge robot hands grasp and disconnect each 130-foot [40-m] section of pipe. The pipe-handling system constitutes another fascinating feature of the project. The system was newly designed to speed up the tedious process of raising and lowering the pipe, or making a round-trip, as drilling experts describe it. There is no shortcut. A beaming face peers from beneath a yellow helmet and explains: “To change the bit, we have to pull everything out!”
What Can We Learn From the Samples?
We inspect the laboratory and are staggered to see row upon row of shelves filled with samples of rock. How are the samples extracted from the earth? In two different ways.
One means is by coring, whereby cylinders of rock are retrieved. No time is lost in observing the behavior of these cores in the laboratory. Why the hurry? Because in the crust, rock is under intense pressure. Geophysicists deduce much about this pressure by noting how each core “unwinds” during its first few days above ground.
The more common method of retrieving samples is during normal drilling. Fluid is pumped down the drill pipe to cool the bit and flush out cuttings. Pressure forces the fluid and the cuttings to the surface to be separated by a filter. The fluid is reused, and the cuttings are analyzed. What do these analyses reveal?
Tests identify the type of rock and determine its electrical and magnetic properties. Data is gathered on the location of ore deposits. The density of the rock indicates how quickly a quake travels through the earth.
Tests also reveal a constant movement of water in both directions between the surface and depths of 13,000 feet [4,000 m] and beyond. “This throws completely new light upon the problems of disposal of harmful substances in mines and shafts,” comments the science magazine Naturwissenschaftliche Rundschau (Natural Science Review).
Our tour concludes with a hearty farewell to our guide. His unpretentious description of the project had the hallmark of an expert for whom the outstanding has become routine. To scientists, Windischeschenbach may seem down-to-earth, but for us, our visit was something quite special.
[Pictures on page 10]
Above: Measuring cores taken from the drill
Left: Model of the earth’s crust
[Credit Line]
KTB-Neuber